WO2014021470A1

WO2014021470A1 - Isolated switched-mode power supply apparatus and control method for the same

Info

Publication number: WO2014021470A1
Application number: PCT/JP2013/071091
Authority: WO
Inventors: Shohtaroh Sohma
Original assignee: Ricoh Company, Ltd.
Priority date: 2012-07-31
Filing date: 2013-07-30
Publication date: 2014-02-06
Also published as: JP2014030316A

Abstract

A flyback isolated switched-mode power supply apparatus to convert an input voltage to an output voltage, the isolated switched-mode power supply apparatus includes a transformer having a primary winding and a secondary winding, a primary side switch configured to control a current flowing in the primary winding of the transformer, a primary side control circuit configured to measure an on-time of the primary side switch and perform on/off control of the primary side switch, a secondary side switch configured to control a current flowing in the secondary winding of the transformer, and a secondary side control circuit configured to measure an on-time of the secondary side switch and perform on/off control of the secondary side switch.

Description

[Description]

[Title of Invention] ISOLATED SWITCHED-MODE POWER SUPPLY APPARATUS AND CONTROL METHOD FOR THE SAME

[Technical Field]

[0001]

The present invention relates to an isolated switched-mode power supply apparatus and a control method for the same, and particularly relates to a flyback converter and a control method for the same.

[Background Art]

[0002]

Patent Literatures 1 to 3 (Patent Literature 1: JP- A 2008-182794, Patent Literature 2: JP-A 2005-525069, Patent Literature 3: JP-B 4442226) disclose inventions of conventional isolated switched-mode power supply apparatuses.

[0003]

A DC-DC converter of Patent Literature 1 includes a power transmission transformer at least including a primary winding and a secondary winding, at least one power switch to switch a current flowing in the primary winding, and a rectifier circuit and a smoothing circuit which are connected to the secondary wining. The DC-DC converter regulates an output voltage to obtain a desired value by performing on/off control of the power switch. The DC-DC converter of Patent Literature 1 further includes: a timing signal output unit provided on the secondary side and configured to output a timing signal based on a variation in the output voltage, the tuning signal causing the power switch to perform a switching operation, i.e., any one of turn-on and turn-off operations; a timing signal transmission unit configured to transmit the timing signal from the secondary side to the primary side in an electrically isolated manner; and a power switch control unit provided on the primary side and configured to cause the power switch to perform the switching operation in accordance with the timing signal.

[0004]

In general, the isolated switched-mode power supply apparatus uses a photocoupler for transmitting an error signal from the secondary side circuit to the primary side circuit. However, the photocoupler generally has a low absolute maximum rated temperature. In consideration of dirating, it is not suitable to use the photocoupler in a switching power source which has a wide operating temperature range. The photocoupler is also subject to a decrease in reliability due to deterioration of its CTR (current transfer ratio) over time. In addition, the error signal is generally generated by a shunt regulator, and the properties of the shunt regulator vary depending on a bias current. A feedback circuit including a photocoupler and a shunt regulator has large part-to-part variation and property fluctuations, and therefore needs to be designed with a large margin. This brings design difficulties. According to the DC-DC converter of Patent Literature 1, an isolated switched-mode power supply can be configured without using a photocoupler or shunt regulator.

[0005]

A power converter of Patent Literature 2 includes: an input terrninal; an output teirninal; a transformer having a primary side and a secondary side, the transformer further including a primary side winding connected to the input teniiinal and a secondary side winding connected to the output teirninal; a primary side switch; a secondary side switch; a slave control unit controlling switching of the secondary side switch; a master control unit controlling switching of the primary side switch; and a secondary current sense detecting a secondary current in the secondary side switch. The primary side switch and the secondary side switch are switched on and off alternately. The switching of the secondary side switch is accomplished depending on the secondary current detected in the secondary side switch. The primary side switch and the secondary side switch are switched on under a zero voltage switching condition. The slave control unit further includes a comparator and a set/reset flip-flop. The secondary current sense conveys an output signal to the comparator when the secondary current becomes negative. The comparator compares the output signal of the secondary current sense with a current reference level and outputs a switching signal to the set/reset flip-flop when the output signal of the secondary current sense exceeds the current reference level. Upon receipt of the switching signal, the set/reset flip-flop switches the secondary side switch.

[0006]

The current reference level (I_reverse) is given by the following formula:

[0007] [Formula 1]

[0008]

where Luc is an inductance of the secondary side winding of the transformer, Cds_t is an mtrinsic capacitance of the primary side switch, Cds₂ is an intrinsic capacitance of the secondary side switch, Vin is an input voltage, Vo is an output voltage and N is a winding ratio between the primary and secondary side windings.

[0009]

The power converter of Patent Literature 2 is able to perform zero-voltage switching (ZVS) and thereby reduce a switching loss.

[0010]

A synchronous rectifier circuit of Patent Literature 3 includes a MOSFET for synchronous rectification, provided on a secondary side of a transformer for voltage conversion and configured to charge an output capacitor with a rectified secondary current. The synchronous rectifier circuit further includes a comparator circuit including a non-inverting input terminal connected to both of a drain and a source of the MOSFET, a resistance through which an inverting input terminal of the comparator circuit is connected to the source of the MOSFET, a driving control unit connected between a gate and the source of the MOSFET and configured to control the gate of the MOSFET, and a switching unit provided in parallel with the driving control unit and configured to be switched off in response to an output of comparator circuit when the comparator circuit detects a reverse current to the output capacitor. In the synchronous rectifier circuit, the MOSFET is turned on to allow a rectified current to flow from the source to the drain usually. However, when the reverse current flows from the drain to the source of the MOSFET, the switching unit causes a short circuit between the gate and the source of the MOSFET to block the current flow between the source and the drain of the MOSFET, and thereby prevents the rectified current from flowing as the reverse current.

[0011]

The isolated switched-mode power supply apparatuses of the conventional techniques have several drawbacks. [0012]

For example, the DC-DC converter of Patent Literature 1 needs to include the timing signal output unit and the timing signal transmission unit additionally. Although the isolated switched-mode power supply apparatus usually uses a transformer as a signal transmission unit, the transformer is large in size, and also raises costs due to its expensiveness.

[0013]

In the power converter of Patent Literature 2, the secondary side switch is switched when the output signal exceeds the current reference level. Thus, the bottom of the magnetizing current of the transformer is fixed. In this case, the switching frequency needs to be modulated depending on a load. If a load is heavy, the switching frequency becomes low, so that the output voltage largely ripples. Accordingly, a transformer having a high current rating is required, which results in an increase in component size. In contrast, if a load is light, the switching frequency becomes high, so that the efficiency is reduced. Further, due to such variations in the switching frequency, noise-filtering is difficult and thereby costs for a noise filter are increased.

[Summary of Invention]

[0014]

An objective of the present invention is to solve the foregoing problems and to provide an isolated switched-mode power supply apparatus and a control method for the same, which are highly efficient and capable of regulating an output voltage without transmitting the output voltage on a secondary side to a primary side.

[0015]

According to an aspect of the present invention, provided is a flyback isolated switched-mode power supply apparatus to convert an input voltage to an output voltage, the isolated switched-mode power supply apparatus including:

a transformer having a primary winding and a secondary winding;

a primary side switch configured to control a current flowing in the primary winding of the transformer;

a primary side control circuit configured to measure an on-time of the primary side switch and perform on/off control of the primary side switch;

a secondary side switch configured to control a current flowing in the secondary winding of the transformer; and

a secondary side control circuit configured to measure an on-time of the secondary side switch and perform on/off control of the secondary side switch, wherein

the secondary side control circuit adjusts the on-time of the secondary side switch such that the output voltage becomes a desired voltage,

the primary side control circuit

turns on the primary side switch when a potential difference across the primary side switch is smaller than a first threshold, and

turns off the primary side switch under conditions where the on-time of the primary side switch expires and where a magnetizing current flowing in the primary winding of the transformer has a magnitude large enough to make a potential difference across the secondary side switch smaller than a second threshold by a counter electromotive force of the transformer if the primary side switch is turned off, and

the secondary side control circuit

turns on the secondary side switch when the potential difference across the secondary side switch is smaller than the second threshold, and

turns off the secondary side switch under conditions where the on-time of the secondary side switch expires and where a magnetizing current flowing in the secondary winding of the transformer has a magnitude large enough to make the potential difference across the primary side switch smaller than the first threshold by a counter electromotive force of the transformer if the secondary side switch is turned off.

[Brief Description of Drawings]

[0016]

[FIG 1] FIG 1 is a circuit diagram illustrating a configuration of an isolated switched-mode power supply apparatus according to a first embodiment of the present invention.

[FIG 2] FIG 2 is a timing chart illustrating waveforms of voltages and currents in the isolated switched-mode power supply apparatus in FIG 1.

[FIG 3] FIG 3 is a diagram illustrating a waveform of a current flowing in a secondary winding 4b when a load of the isolated switched-mode power supply apparatus in FIG 1 varies.

[FIG 4] FIG 4 is a circuit diagram illustrating a configuration of an isolated switched-mode power supply apparatus according to a second embodiment of the present invention.

[FIG 5] FIG 5 is a circuit diagram illustrating a configuration of an isolated switched-mode power supply apparatus according to a third embodiment of the present invention.

[FIG 6] FIG 6 is a circuit diagram illustrating a configuration of an isolated switched-mode power supply apparatus according to a fourth embodiment of the present invention.

[FIG 7] FIG 7 is a timing chart illustrating waveforms of voltages and a current in the isolated switched-mode power supply apparatus in FIG 6.

[FIG 8] FIG 8 is a circuit diagram illustrating a configuration of an isolated switched-mode power supply apparatus according to a fifth embodiment of the present invention.

[Description of Embodiments]

[0017]

Hereinafter, isolated switched-mode power supply apparatuses according to embodiments of the present invention are described with reference to the drawings. The same reference signs are given to the same or similar components throughout the drawings.

[0018]

First Embodiment.

FIG 1 is a circuit diagram illustrating a configuration of an isolated switched-mode power supply apparatus according to a first embodiment of the present invention. The isolated switched-mode power supply apparatus in FIG 1 is a flyback converter that converts an input voltage Vin supplied from an input voltage source 3 to an output voltage Vout, and outputs the out ut voltage Vout from an output voltage terminal VOUT. The isolated switched-mode power supply apparatus includes an isolation transformer 4 having a primary winding 4a and a secondary winding 4b, a primary side switch 5 configured to control a current flowing in the primary winding 4a, a primary side control circuit 1 configured to measure an on-time of the primary side switch 5 and to perform on/off control of the primary side switch 5, a secondary side switch 6 configured to control a current flowing in the secondary winding 4b, and a secondary side control circuit 2 configured to measure an on-time of the secondary side switch 6 and to perform on/off control of the secondary side switch 6. Moreover, the isolated switched-mode power supply apparatus is provided with a capacitor CI for smoothing the current in the secondary winding 4b.

[0019]

Each of the primary side switch 5 and the secondary side switch 6 is a MOSFET. The primary side switch 5 has a drain connected to the primary winding 4a of the isolation transformer 4, a source grounded, and a gate to which a control signal is applied by the primary side control circuit 1. The secondary side switch 6 has a drain connected to the secondary winding 4b of the isolation transformer 4, a source grounded, and a gate to which a control signal is applied by the secondary side control circuit 2. The primary side switch 5 has an on-resistance Ron5, whereas the secondary side switch 6 has an on-resistance Ron6. The primary side control circuit 1 is able to measure a current flowing in the primary winding 4a of the isolation transformer 4 by measuring a voltage (drain voltage) at a node Nl between the primary side switch 5 and the primary winding 4a. Likewise, the secondary side control circuit 2 is able to measure a current flowing in the secondary winding 4b of the isolation transformer 4 by measuring a voltage at a node N2 between the secondary side switch 6 and the secondary winding 4b.

[0020]

The primary side control circuit 1 includes reference voltage sources 11, 15, a flip-flop 12, a constant current source 13, a switch 14, an AND circuit 16, a drive buffer 17, comparators CMP11, CMP 12, inverters INV 11 , INV 12 and a capacitor C 11. The primary side control circuit 1 includes an internal voltage source VDD 1. The reference voltage source 11 generates a reference voltage Vrefl 1 , and the reference voltage source 15 generates a reference voltage Vrefl 5. In the comparator CMP11 , a drain voltage of the primary side switch 5 is inputted to an inverting input terminal and the reference voltage Vrefl 1 is inputted to a non-inverting input terminal. An output signal of the comparator CMP 11 is inputted to a CLK input terminal of the flip-flop 12, and then is inputted to a first input terminal of the AND circuit 16 via the inverter INV11. A D input terminal of the flip-flop 12 is connected to the voltage source VDD1. At the rising edge of the signal inputted to the CLK input terminal, the flip-flop 12 outputs a voltage inputted in the D input terminal as an output signal from a Q output terminal. When a reset signal is inputted to an R input terminal of the flip-flop 12, the output signal of the flip-flop 12 is set to a low level. The output signal of the flip-flop 12 drives the primary side switch 5 via the drive buffer 17 and is also transmitted to the switch 14 via the inverter INV12. The switch 14 is a MOSFET, and has a drain connected to the constant current source 13, a source grounded, and a gate to which the output signal of the flip-flop 12 is inputted via the inverter INV12. A node between the switch 14 and the constant current source 13 is grounded via the smoothing capacitor Cll, and is connected to a non-inverting input terminal of the comparator CMP 12. At this node, a slope voltage Vslopel is generated. The reference voltage Vrefl5 is inputted to an inverting input terminal of the comparator CMP 12. An output signal of the comparator CMP 12 is inputted to a second input terminal of the AND circuit 16. An output signal of the AND circuit 16 is inputted as a reset signal to the R input terminal of the flip-flop 12.

[0021]

The constant current source 13, the switch 14, the capacitor Cll, the reference voltage source 15, and the comparator CMP12 function as an on-time adjuster circuit to adjust the on-time of the primary side switch 5. This on-time adjuster circuit holds a predetermined on-time of the primary side switch 5, i.e., the on-time determined depending on the capacitor Cll and the reference voltage Vrefl5. Thus, the on-time of the primary side switch 5 is fixed in a steady operation of the isolated switched-mode power supply apparatus. The slope voltage Vslopel generated at the node between the switch 14 and the constant current source 13 has a waveform illustrated in FIG 2. The slope voltage Vslopel is 0V when the switch 14 is turned on, and increases from 0V to the reference voltage Vrefl5 by charging the capacitor Cll when the switch 14 is turned off. When the slope voltage Vslopel does not exceed the reference voltage Vrefl5, the reset signal inputted to the R input terminal of the flip-flop 12 is at the low level, and therefore the primary side switch 5 is not turned off.

[0022]

When the drain voltage of the primary side switch 5 (i.e., a potential difference across the primary side switch 5) is smaller than the reference voltage Vrefll, the primary side control circuit 1 turns on the primary side switch 5 (zero-voltage switching). In addition, the primary side control circuit 1 turns off the primary side switch 5 under conditions where the on-time of the primary side switch 5 expires (i.e., the output signal of the comparator CMP 12 is at a high level) and where the product of a magnetizing current flowing in the primary winding 4a and the on-resistance Ron5 of the primary side switch 5 becomes larger than the reference voltage Vrefl 1 (i.e., the output signal of the comparator CMP11 is at a low level). When the product of the magnetizing current flowing in the primary winding 4a and the on-resistance Ron5 of the primary side switch 5 becomes larger than the reference voltage Vrefll, the primary side control circuit 1 determines that the magnetizing current flowing in the primary winding 4a has a magnitude large enough to make the potential difference across the secondary side switch 6 smaller than a predetermined threshold (a reference voltage VrefZl to be later described) (i.e., to practically perform zero-voltage switching of the secondary side switch 6) by a counter electromotive force of the isolation transformer 4 if the primary side switch 5 is turned off. The reference voltage Vrefll is set to a value which corresponds to this "magnitude large enough" and is small enough to practically perform zero-voltage svvitching to turn on the primary side switch 5.

[0023]

The secondary side control circuit 2 includes reference voltage sources 21, 25, 27, a flip-flop 22, a constant current source 23, a switch 24, AND circuits 26, 28, comparators CMP21, CMP22, CMP23, inverters INV21,INV22, an error amplifier AMP21, capacitors C21,C22, and a resistance R21. The secondary side control circuit 2 uses the output voltage Vout as a voltage source VDD2. The reference voltage source 21 generates the reference voltage Vref21, the reference voltage source 25 generates a reference voltage Vref25, and the reference voltage source 27 generates a reference voltage Vref27. In the comparator CMP23, the output voltage Vout is inputted to a non-inverting input terminal and the reference voltage Vref27 is inputted to an inverting input terminal. An output signal of the comparator CMP23 is inputted to a first input terminal of the AND circuit 28. In the comparator CMP21, a drain voltage of the secondary side switch 6 is inputted to an inverting input terminal and the reference voltage Vref21 is inputted to a non-inverting input terminal. An output signal of the comparator CMP21 is inputted to a CLK input terminal of the flip-flop 22, and then is inputted to a first input terminal of the AND circuit 26 via the inverter INV21. A D input terminal of the flip-flop 22 is connected to the voltage source VDD2. The flip-flop 22 operates in the same manner as the flip-flop 12. An output signal of the flip-flop 22 is inputted to a second input terminal of the AND circuit 28, and then is transmitted to the switch 24 via the inverter INV22. The AND circuit 28 functions as a drive buffer and drives the secondary side switch 6 by using the output signal therefrom. The switch 24 is a MOSFET, and includes a drain connected to the constant current source 23, a source grounded, and a gate to which the output signal of the flip-flop 22 is inputted via the inverter INV22. A node between the switch 24 and the constant current source 23 is grounded via the smoothing capacitor C21, and is connected to a non-mverting input terminal of the comparator CMP22. At this node, a slope voltage Vslope2 is generated. In the error amplifier AMP21, a voltage fraction of the output voltage Vout divided by resistances Rl, R2 is inputted to a non-inverting input terminal and the reference voltage Vref25 is inputted to an inverting input tenninal. An error voltage Verror outputted from the error amplifier AMP21 is subjected to phase compensation by the capacitor C22 and the resistance R21, and then is inputted to an inverting input terminal of the comparator CMP22. An output signal of the comparator CMP22 is inputted to a second input terminal of the AND circuit 26, and an output signal of the AND circuit 26 is inputted as a reset signal to an R input terminal of the flip-flop 22.

[0024]

The reference voltage source 27, the comparator CMP23 and the AND circuit 28 function as a UVLO (under voltage lock out) circuit, and prevents the isolated switched-mode power supply apparatus from performing a misoperation when the output voltage Vout is very low. Specifically, when the output voltage Vout does not exceed the reference voltage Vref27, the output signal of the comparator CMP23 is at a low level and therefore the secondary side switch 6 is turned off.

[0025]

The constant current source 23, the switch 24, the capacitors C21, C22, the reference voltage source 25, the comparator CMP22, the resistance R21, and the error amplifier AMP21 function as an on-time adjuster circuit to adjust the on-time of the secondary side switch 6. This on-time adjuster circuit adjusts the on-time of the secondary side switch 6 based on the error voltage Verror such that the output voltage Vout can become a desired voltage. Thus, the on-time of the secondary side switch 6 is _ variable. The reference voltage Vref25 is set to a voltage fraction of the desired voltage divided by the resistances Rl, R2. The error voltage Verror indicates an error between the reference voltage Vref25 and the voltage fraction of the output voltage Vout divided by the resistances Rl, R2. The slope voltage Vslope2 generated at the node between the switch 24 and the constant current source 23 has a waveform illustrated in FIG 2. The slope voltage Vslope2 is 0V when the switch 24 is turned on, and increases from 0V to the error voltage Verror by charging the capacitor C21 when the switch 14 is turned off. When the slope voltage Vslope2 does not exceed the reference voltage Vrefl 5, the reset signal inputted to the R input terminal of the flip-flop 22 is at the low level and therefore the secondary side switch 6 is not turned off.

[0026]

When the drain voltage of the secondary side switch 6 (i.e., a potential difference across the secondary side switch 6) is smaller than the reference voltage Vref21 , the secondary side control circuit 2 turns on the secondary side switch 6 (zero-voltage switching). In addition, the secondary side control circuit 2 turns off the secondary side switch 6 under conditions where the on-time of the secondary side switch 6 expires (i.e., the output signal of the comparator CMP22 is at the high level) and where the product of the magnetizing current flowing in the secondary winding 4b and the on-resistance Ron6 of the secondary side switch 6 becomes larger than the reference voltage Vre£21 (i.e., the output signal of the comparator CMP21 is at the low level). When the product of the magnetizing current flowing in the secondary winding 4b and the on-resistance Ron6 of the secondary side switch 6 becomes larger than the reference voltage Vref21, the secondary side control circuit 2 determines that the magnetizing current flowing in the secondary winding 4b has a magnitude large enough to make the potential difference across the primary side switch 5 smaller than the reference voltage Vrefl 1 (i.e., to practically perform zero-voltage switching of the primary side switch 5) by a counter electromotive force of the isolation transformer 4 if the secondary side switch 6 is turned off. The reference voltage Vref21 is set to a value which corresponds to this "magnitude large enough" and is small enough to practically perform zero- voltage switching to turn on the secondary side switch 6.

[0027]

Next, an operation of the isolated switched-mode power supply apparatus in FIG 1 is explained with reference to FIG 2.

[0028]

FIG 2 is a timing chart illustrating waveforms of voltages and currents in the isolated switched-mode power supply apparatus in FIG 1. FIG 2 illustrates waveforms of the drain voltage of the primary side switch 5, the drain voltage of the secondary side switch 6, the current flowing in the primary winding 4a, the current flowing in the secondary winding 4b, the slope voltage Vslopel of the primary side control circuit 1, and the slope voltage Vslope2 of the secondary side control circuit 2. The current flowing in the primary winding 4a is defined such that the direction from the drain to the source of the primary side switch 5 is a positive direction, while the current flowing in the secondary winding 4b is defined such that the direction from the source to the drain of the secondary side switch 6 is a positive direction. The following description is provided on the assumption that the output voltage Vout is higher than the reference voltage Vref27 and accordingly the secondary side switch 6 is not turned off by the UVLO circuit.

[0029]

FIG 2 illustrates six exemplary switching cycles (A) to (F).

[0030]

In the switching cycle (A) in FIG 2, the secondary side control circuit 2 operates as follows.

[0031]

When the primary side switch 5 is turned off, a forward current starts flowing into the secondary winding 4b due to the counter electromotive force of the isolation transformer 4, and the drain voltage of the secondary side switch 6 is lowered. When the drain voltage of the secondary side switch 6 becomes lower than the reference voltage Vref21, the output signal of the comparator CMP21 becomes at the high level, the output signal of the flip-flop 22 becomes at the high level, the output signal of the AND circuit 28 becomes at the high level, and thus the secondary side switch 6 is turned on. At this time, the output signal of the inverter INV22 becomes at the low level, and thereby the slope voltage Vslope2 starts increasing from 0V. The error voltage Verror becomes high when the divided voltage fraction of the output voltage Vout becomes higher than the reference voltage Vref25 (that is, the output voltage Vout becomes higher than the desired voltage), while the error voltage _

Verror becomes low when the divided voltage fraction becomes lower than the reference voltage Vre£25. When the slope voltage Vslope2 becomes higher than the error voltage Verror, the output signal of the comparator CMP22 becomes at the high level. At this time, the product of the current flowing in the secondary winding 4 and the on-resistance Ron6 of the secondary side switch 6 exceeds the reference voltage Vref21, and accordingly the output signal of the comparator CMP21 becomes at the low level and the output signal of the inverter INV21 becomes at the high level. Thus, the output signal of the AND circuit 26 becomes at the high level, the output signal of the flip-flop 22 becomes at the low level, the output signal of the AND circuit 28 becomes at the low level, and consequently the secondary side switch 6 is turned off. At the same time, the output signal of the inverter INV22 becomes at the high level, the node between the switch 24 and the constant current source 23 is grounded, and thereby the slope voltage Vslope2 becomes 0.

[0032]

In the switching cycle (A), the primary side control circuit 1 operates as follows.

[0033]

When the secondary side switch 6 is turned off, a negative current starts flowing into the primary winding 4a due to the counter electromotive force of the isolation transformer 4, and the drain voltage of the primary side switch 5 is lowered. When the drain voltage of the primary side switch 5 becomes lower than the reference voltage Vrefl 1 , the output signal of the comparator CMP 11 becomes at the high level, the output signal of the flip-flop 12 becomes at the high level, and thereby the primary side switch 5 is turned on. At this time, the output signal of the inverter INV12 becomes at the low level, and the slope voltage Vslopel starts increasing from 0V. When the slope voltage Vslopel becomes higher than the reference voltage Vrefl5, the output signal of the comparator CMP12 becomes at the high level. At this time, the product of the current flowing in the primary winding 4a and the on-resistance Ron5 of the primary side switch 5 exceeds the reference voltage Vrefl 1. Thus, the output signal of the comparator CMP 11 becomes at the low level, and the output signal of the inverter INV11 becomes at the high level. Accordingly, the output signal of the AND circuit 16 becomes at the high level, the output signal of the flip-flop 12 becomes at the low level, the output signal of the drive buffer 17 becomes at the low level, and thus the primary side switch 5 is turned off. At the same time, the output signal of the inverter INV12 becomes at the high level, the node between the switch 14 and the constant current source 13 is grounded, and thereby the slope voltage Vslopel becomes 0.

[0034]

In the switching cycle (B), the secondary side control circuit 2 operates as follows.

[0035]

After the primary side switch 5 is turned off, the secondary side control circuit 2 operates in the same manner as in the switching cycle (A). When the slope voltage Vslope2 becomes higher than the error voltage Verror, the output signal of the comparator CMP22 becomes at the high level. At this time, the product of the current flowing in the secondary winding 4b and the on-resistance Ron6 of the secondary side switch 6 does not exceed the reference voltage Vref21. Accordingly, the output signal of the comparator CMP21 is at the high level, and the output signal of the inverter INV21 is at the low level. Thus, the output signal of the AND circuit 26 is at the low level, the output signal of the flip-flop 22 is kept at the high level, and consequently the secondary side switch 6 is not turned off. Thereafter, the secondary side control circuit 2 stays in standby until the product of the current flowing in the secondary winding 4b and the on-resistance Ron6 of the secondary side switch 6 exceeds the reference voltage Vref21, and operates in the same manner as in the switching cycle (A) after the product exceeds.

[0036]

In the switching cycle (B), the primary side control circuit 1 operates in the same manner as in the switching cycle (A).

[0037]

In the switching cycle (C), the primary side control circuit 1 and the secondary side control circuit 2 operate in the same manner as in the switching cycle (A).

[0038]

In the switching cycle (D), the secondary side control circuit 2 operates in the same manner as in the switching cycle (A).

[0039] In the switching cycle (D), the primary side control circuit 1 operates as follows.

[0040]

After the secondary switch 6 is turned off, the primary side control circuit 1 operates in the same manner as in Ihe switching cycle (A). When the slope voltage Vslopel becomes higher man the reference voltage Vrefl5, the output signal of the comparator CMP12 becomes at the high level. At this time, the product of the current flowing in the primary winding 4a and the on-resistance Ron5 of the primary side switch 5 does not exceed the reference voltage Vrefll. Accordingly, the output signal of the comparator CMP 11 is at the high level, and the output signal of the inverter INV11 is at the low level. Thus, the output signal of the AND circuit 16 is at the low level, the output signal of the flip-flop 12 is kept at the high level, and consequently the primary side switch 5 is not turned off. Thereafter, the primary side control circuit 1 stays in standby until the product of the current flowing in the primary winding 4a and the on-resistance Ron5 of the primary side switch 5 exceeds the reference voltage Vrefll, and operates in the same manner as in the switching cycle (A) after the product exceeds.

[0041]

In the switching cycle (E), the primary side control circuit 1 and the secondary side control circuit 2 operate in the same manner as in the switching cycle (A).

[0042]

In the switching cycle (F), the primary side control circuit 1 and the secondary side control circuit 2 operate in the same manner as in the switching cycle (A).

[0043]

In reference to FIG 2, it is understood that the isolated switched-mode power supply apparatus in FIG 1 achieves the following things.

[0044]

Firstly, the zero- voltage switching is always performed by: turning on the primary side switch 5 when a low voltage is applied to the primary side switch 5; turning off the primary side switch 5 when a current having a magnitude large enough to perform zero-voltage switching of the secondary side switch 6 flows in the primary winding 4a; turning on the secondary side switch 6 when a low voltage is applied to the secondary side switch 6; and turning off the secondary side switch 6 when a current having a magnitude large enough to perform zero-voltage switching of the primary side switch 5 flows in the secondary winding 4b.

[0045]

Secondary, the primary side control circuit 1 is able to control the primary side switch 5 on the basis of the fixed on-time which is determined depending on the capacitance of the capacitor Cll and the reference voltage Vrefl5. Here, the "fixed on-time" means that the on-time is determined by a time during which the slope voltage Vslopel increases from 0V to the reference voltage Vrefl5. Although the on-time becomes longer temporarily in the switching cycle (D) in FIG 2, the other switching cycles in the steady operation can be repeated at the fixed frequency. What is a problem in the converter are variations in the switching frequency in the steady operation. An instantaneous or transient variation in the switching frequency is not a problem.

[0046]

Thirdly, by use of the error amplifier AMP21 of the secondary side control circuit 2, the current flowing in the secondary winding 4b is controlled and the current flowing in the primary winding is also indirectly controlled. Thereby, the isolated switched-mode power supply apparatus is capable of regulating the output voltage to obtain the desired voltage.

[0047]

FIG 3 is a diagram illustrating a waveform of the current flowing in the secondary winding 4b when the load of the isolated switched-mode power supply apparatus in FIG 1 varies. A solid line represents the current flowing in the secondary winding 4b, a dotted line represents OA, and a dot-dash line represents a load current at the output voltage terminal VOUT. The bottom current in the secondary winding 4b needs to be always negative due to the necessity to generate a negative current at the primary winding 4a by the counter electromotive force of the isolation transformer 4. Then, the maximum value of the load current depends on a ripple current in the isolation transformer 4. The ripple current can be increased as the inductance of the isolation transformer 4 becomes smaller, or as the on-time of the primary side control circuit 1 becomes longer. Here, the fixed on-time is used for the primary side switch 5. Accordingly, given an on-time of the primary side switch 5, an input voltage Vin and an output voltage Vout, the switching frequency can be made almost constant.

[0048]

Under particular conditions, however, the switching frequency temporarily varies as described above. For example, when a current flowing in the primary winding 4a or the secondary winding 4b falls short of the magnitude large enough to perform zero- voltage switching, the current needs to be increased. In the switching cycle (D) of FIG 2, if the primary side switch 5 is turned off at the instant when the slope voltage Vslopel reaches the reference voltage Vrefl5, the drain voltage of the secondary side switch 6 does not drop and consequently the secondary side switch 6 cannot be turned on. For this reason, in the switching cycle (D), after the slope voltage Vslopel reaches the reference voltage Vrefl5, there is a standby time until the product of the current flowing in the primary winding 4a and the on-resistance Ron5 of the primary side switch 5 exceeds the reference voltage Vrefl 1. As a result, the switching cycle (D) has a longer length than the other switching cycles. However, an increase in the length of the switching cycle is just a temporary phenomenon. Note that, when the switching cycle having a varied length is repeated, for example, the current flowing in the secondary winding 4b varies following the load current (see FIG 3) and a new steady state is created. The switching cycle having an increased length as in the switching cycle (D) in FIG 2 is repeated in the case where taking electric charge from the output voltage terminal VOUT needs to be continued (i.e., in a steady state in which an integrated value of the current flowing in the secondary winding 4b is negative). The isolated switched-mode power supply apparatus generally operates as a power supply apparatus that supplies electric charge to the output voltage terminal VOUT. In this operation, the power supply apparatus is a steady state in which a non-zero load current flows (see FIG 3) and the integrated value of the current flowing in the secondary winding 4b is positive. Hence, the switching cycle having an increased length is repeated, if at all, only while the integrated value of the current flowing in the secondary winding 4b is 0.

[0049]

According to the isolated switched-mode power supply apparatus in FIG 1, there is no need for an extra signal transmission unit (for example, a transformer) to transmit a timing signal from the secondary side to the primary side, which leads to downsizing and cost-cutting. [0050]

Moreover, the isolated switched-mode power supply apparatus in FIG 1 is able to cause a sufficient negative current to flow in the secondary winding 4b for the purpose of performing zero-voltage switching of the primary side switch 5, and to cause a sufficient positive current to flow in the primary winding 4a for the purpose of performing zero-voltage switching of the secondary side switch 6. The isolated switched-mode power supply apparatus of the conventional technique requires a transformer having a high current rating because the output voltage Vout largely ripples due to a decrease in the switching frequency in operation under heavy load, and thereby has a problem of an increase in component size. However, according to the isolated switched-mode power supply apparatus in FIG 1, the switching frequency can be fixed and thereby the problem of the conventional technique can be solved. In addition, the isolated switched-mode power supply apparatus of the conventional technique has a problem that the efficiency is lowered due to an increase in the switching frequency in operation under light load. However, according to the isolated switched-mode power supply apparatus in FIG 1, the switching frequency can be fixed and thereby the problem of the conventional technique can be solved. Further, the isolated switched-mode power supply apparatus of the conventional technique has difficulty in noise-filtering because of variations in the switching frequency and has a problem of an increase in costs for a noise filter. Nevertheless, according to the isolated switched-mode power supply apparatus in FIG 1, the switching frequency can be fixed and thereby the problem of the conventional technique can be solved.

[0051]

Since the output voltage Vout can be regulated by changing the on-time of the secondary side switch 6 based on the error voltage Verror, the isolated switched-mode power supply apparatus in FIG 1 does not need a circuit for feedback to the primary side, such as a photocoupler, which is used in the conventional isolated switched-mode power supply apparatus. Moreover, the isolated switched-mode power supply apparatus in FIG 1 is able to always perform the zero-voltage switching and thereby to reduce a switching loss, which leads to efficiency improvement. This also enables an increase in the switching frequency and downsizing of components.

[0052] Further, since the secondary side control circuit 2 includes the error amplifier AMP21 and the reference voltage source 25, the stabile regulation and the highly-accurate output voltage Vout can be easily achieved.

[0053]

In addition, since the output voltage Vout is used as the voltage source of the secondary side control circuit 2, a loss due to a current consumption in the secondary side control circuit 2 can be kept low.

[0054]

Furthermore, the secondary side control circuit 2 is provided with the UVLO circuit, and the UVLO circuit turns off the secondary side switch 6 when the power supply voltage is equal to or lower than the predetermined voltage. Thus, a misoperation under a condition where the power supply voltage of the secondary side control circuit 2 is low can be prevented.

[0055]

Hence, according to the isolated switched-mode power supply apparatus in FIG 1, provided is an isolated switched-mode power supply apparatus which is highly efficient and capable of regulating the output voltage Vout without transrm^'tting the output voltage Vout to the primary side control circuit 1.

[0056]

Second Embodiment.

FIG 4 is a circuit diagram illustrating a configuration of an isolated switched-mode power supply apparatus according to a second embodiment of the present invention. The isolated switched-mode power supply apparatus in FIG 4 is the isolated switched-mode power supply apparatus in FIG 1 additionally including sense resistances and overcurrent detector circuits. The description common with the isolated switched-mode power supply apparatus in FIG 1 is omitted herein.

[0057]

The source of the primary side switch 5 is grounded via a resistance R3 which serves as a primary side sense resistance. Thus, the resistance R3 is connected to the primary winding 4a in series. Likewise, the source of the secondary side switch 6 is grounded via a resistance R4 which serves as a secondary side sense resistance. Thus, the resistance R4 is connected to the secondary winding 4b in series.

[0058]

The primary side control circuit 1 A includes comparators CMP 13, CMP 14, reference voltage sources 31, 32, and an OR circuit 33 in addition to the component elements in the primary side control circuit 1 in FIG 1. The reference voltage source 31 generates a reference voltage Vref31, and the reference voltage source 32 generates a reference voltage Vref32. In the comparator CMP13, the reference voltage Vref31 is inputted to a non-inverting input terminal, and a voltage at a node N3 between the primary side switch 5 and the resistance R3 is inputted to an inverting input terminal. The input terminal of the inverter INV11 is connected to an output terminal of the comparator CMP 13 instead of the output terminal of the comparator CMP11. In the comparator CMP 14, the reference voltage Vref32 is inputted to an inverting input terminal and the voltage at the node N3 between the primary side switch 5 and the resistance R3 is inputted to a non-inverting input terminal. In the OR circuit 33, an output signal of the AND circuit 16 is inputted to a first input terminal and an output signal of the comparator CMP 14 is inputted to a second input terminal.

[0059]

The primary side control circuit 1 A turns off the primary side switch 5 under conditions where the on-time of the primary side switch 5 expires (i.e., the output signal of the comparator CMP 12 is at the high level) and where the product of the magnetizing current flowing in the primary winding 4a and the resistance R3 exceeds the reference voltage Vref31 (i.e., the output signal of the comparator CMP13 is at the low level). When the product of the magnetizing current flowing in the primary winding 4a and the resistance R3 exceeds the reference voltage Vref31, the primary side control circuit 1 A determines that the magnetizing current flowing in the primary winding 4a has a magnitude large enough to make the potential difference across the secondary side switch 6 smaller than the reference voltage Vref21 (i.e., to practically perform zero-voltage switching of the secondary side switch 6) by the counter electromotive force of the isolation transformer 4 if the primary side switch 5 is turned off. The reference voltage Vref31 is set to a value corresponding to this "magnitude large enough." [0060]

The secondary side control circuit 2 A includes comparators CMP24, CMP25, reference voltage sources 41, 42, and an OR circuit 43 in addition to the component elements in the secondary side control circuit 2 in FIG 2. The reference voltage source 41 generates a reference voltage Vref41 and the reference voltage source 42 generates a reference voltage Vref42. In the comparator CMP24, the reference voltage Vref41 is inputted to a non-inverting input terminal, and a voltage at a node N4 between the secondary side switch 6 and the resistance R4 is inputted to an inverting input terminal. The input terminal of the inverter INV21 is connected to an output terminal of the comparator CMP24 instead of the output terminal of the comparator CMP21. In the comparator CMP25, the reference voltage Vref42 is inputted to an inverting input terminal, and the voltage at the node N4 between the secondary side switch 6 and the resistance R4 is inputted to a non-inverting input terminal. In the OR circuit 43, an output signal of the AND circuit 26 is inputted to a first input terminal and an output signal of the comparator CMP25 is inputted to a second input terminal.

[0061]

The secondary side control circuit 2A turns off the secondary side switch 6 under conditions where the on-time of the secondary side switch 6 expires (i.e., the output signal of the comparator CMP22 is at the high level) and where the product of the magnetizing current flowing in the secondary winding 4b and the resistance R4 exceeds the reference voltage Vref41 (i.e., the output signal of the comparator CMP24 is at the low level). When the product of the magnetizing current flowing in the secondary winding 4b and the resistance R4 exceeds the reference voltage Vref41, the secondary side control circuit 2A determines that the magnetizing current flowing in the secondary winding 4b has a magnitude large enough to make the potential difference across the primary side switch 5 smaller than the reference voltage Vrefll (i.e., to practically perform zero-voltage switching of the primary side switch 5) by the counter electromotive force of the isolation transformer 4 if the secondary side switch 6 is turned off. The reference voltage Vref41 is set to a value corresponding to this "magnitude large enough."

[0062]

The comparator CMP 14 and the reference voltage source 32 function as a forward overcurrent detector circuit for the primary winding 4a. After the primary side switch 5 is turned on, when the voltage at the node N3 between the primary side switch 5 and the resistance R3 exceeds the reference voltage Vref32 (i.e., the product of the positive magnetizing current flowing in the primary winding 4a and the resistance R3 becomes larger than the reference voltage Vref32), the output signal of the comparator CMP 14 becomes at the high level. Thus, the output signal of the OR circuit 33 becomes at the high level, the output signal of the flip-flop 12 becomes at the low level, the output signal of the drive buffer 17 becomes at the low level, and consequently the primary side switch 5 is turned off. The reference voltage Vref32 is set such that a desired overcurrent can be detected.

[0063]

Likewise, the comparator CMP25 and the reference voltage source 42 function as a reverse overcurrent detector circuit for the secondary winding 4b. After the secondary side switch 6 is turned on, when the voltage at the node N4 between the secondary side switch 6 and the resistance R4 exceeds the reference voltage Vref42 (i.e., the product of the magnetizing current flowing in the secondary winding 4b and the resistance R4 becomes larger than the reference voltage Vref42), the output signal of the comparator CMP25 becomes at the high level. Accordingly, the output signal of the OR circuit 43 becomes at the high level, the output signal of the flip-flop 22 becomes at the low level, the output signal of the AND circuit 28 becomes at the low level, and consequently the secondary side switch 6 is turned off. The reference voltage Vref42 is set such that a desired overcurrent can be detected.

[0064]

The isolated switched-mode power supply apparatus in FIG 4 is provided with the sense resistances, and thereby is capable of detecting the currents flowing in the primary winding 4a and the secondary winding 4b with high accuracy. In addition, the isolated switched-mode power supply apparatus in FIG 4 is provided with the overcurrent detector circuits, and thereby is capable of avoiding an overcurrent due to an anomaly such as a short circuit.

[0065]

Third Embodiment.

FIG 5 is a circuit diagram illustrating a configuration of an isolated switched-mode power supply apparatus according to a third embodiment of the present invention. In place of the reference voltage sources 11, 15 in the primary side control circuit 1 in FIG 1, the isolated switched-mode power supply apparatus in FIG 5 includes reference voltage sources in which the reference voltages Vrefll, Vrefl5 are adjustable by connecting external elements thereto.

[0066]

A primary side control circuit IB includes, in place of the reference voltage source 11 in FIG 1, a constant current source 51 and an external resistance Rll to adjust a minimum forward current. In addition, the primary side control circuit IB includes, in place of the reference voltage source 15 in FIG 1, a constant current source 52 and an external resistance R12 to adjust an on-time of the primary side switch 5. The resistances Rll, R12 serve as resistors each having a variable resistance value, a semi-fixed resistance value or a predetermined resistance value. The user of the isolated switched-mode power supply apparatus can adjust the reference voltages Vrefll, Vrefl5 by changing the resistance values of the resistances Rl 1 , Rl 2.

[0067]

When the primary side control circuit IB is configured as an integrated circuit, the resistances Rl 1, R12 are connected as external elements to the isolated switched-mode power supply apparatus.

[0068]

The reference voltage sources 21, 25, 27 of the secondary side control circuit 2 in FIG 1 may be also configured in the same way as the reference voltage sources of the primary side control circuit IB in FIG 5.

[0069]

In the case where any one of the isolation transformer 4, the primary side switch 5, and the secondary side switch 6 is replaced or modified, the magnitude of a forward current flowing in the primary winding 4a, the magnitude of a negative current following in the secondary winding 4b, and the like can be adjusted by changing the resistance values of the resistances Rll, R12. With this configuration, the user of the isolated switched-mode power supply apparatus can operate the isolated switched-mode power supply apparatus at a desired switching frequency.

[0070]

Similarly, the isolated switched-mode power supply apparatus in FIG 4 may be configured such that the reference voltages VreG 1, Vref32, Vref41, Vref42 can be adjusted by connecting external elements thereto. With this configuration, a desired and optimal counter electromotive force can be generated at the isolation transformer 4 during operation of the isolated switched-mode power supply apparatus.

[0071]

Fourth Embodiment.

FIG 6 is a circuit diagram illustrating a configuration of an isolated switched-mode power supply apparatus according to a fourth embodiment of the present invention. The isolated switched-mode power supply apparatus in FIG 6 is capable of causing a reverse current to flow in the primary winding 4a over a desired setup time, by using a variable reference voltage as the reference voltage Vrefl 5 of the isolated switched-mode power supply apparatus in FIG 1.

[0072]

A primary side control circuit 1C includes, in place of the reference voltage source 15 in FIG 1, a reference voltage source 61, a phase comparator (PFD) 62, comparators CMP15, CMP16, a capacitor CI 2, and a resistance R13. The reference voltage source 61 generates a reference voltage Vref61. In the comparator CMP 15, the drain voltage of the primary side switch 5 is inputted to a non-mverting input terminal and an inverting input terminal is grounded. In the comparator CMP 16, the slope voltage Vslopel is inputted to a non-inverting input terminal and the reference voltage Vref61 is inputted to an inverting input terminal. The phase comparator 62 is a rising edge detection type of phase comparator. In the phase comparator 62, an output signal of the comparator CMP 15 is inputted to an A input terminal and an output signal of the comparator CMP 16 is inputted to a B input terminal. An output signal of the phase comparator 62 is subjected to phase compensation by the capacitor C12 and the resistance R13, and then is inputted to an inverting input terminal of the comparator CMP12.

[0073]

FIG 7 is a timing chart illustrating waveforms of voltages and a current in the isolated switched-mode power supply apparatus in FIG 6. The output signal of the comparator CMP 15 is inverted from the low level to the high level when the comparator CMP 15 detects that the drain voltage of the primary side switch 5 becomes 0V and that the current flowing in the primary winding 4a is changed from negative (reverse current) to positive (forward current). The output signal of the comparator CMP 16 is inverted from the low level to the high level when the slope voltage Vslopel exceeds the reference voltage Vref61 (i.e., when a desired setup time for the reverse current to flow in the primary winding 4a expires). The reference voltage Vref61 is set to cause the reverse current to flow in the primary winding 4a over the desired setup time. The phase comparator 62 is configured to pull up its output signal when the A input receives an edge before the B input does, and pull down its output signal when the A input receives an edge after the B input does. In FIG. 7, since a rising edge of the output signal (A input) of the comparator CMP 15 comes after a rising edge of the output signal (B input) of the comparator CMP 16, the output signal of the phase comparator 62 is lowered and thereby the on-time of the primary side switch 5 is made shorter. The off-time of the primary side switch 5 is shortened according to a load current so that the output can be stabilized by the secondary side control circuit 2. Thus, a rising edge of the output signal of the comparator CMP 15 and a rising edge of the output signal of the comparator CMP 16 converge so as to coincide with each other.

[0074]

The primary side control circuit 1C adjusts the on-time of the primary side switch 5 such that the negative magnetizing current can flow in the primary winding 4a over a predetermined time when the primary side switch 5 is turned on.

[0075]

According to the isolated switched-mode power supply apparatus in FIG 6, since a time when the magnetizing current is negative can be adjusted to a desired time, the ripples of the currents flowing in the isolation transformer 4 can be kept small, and the efficiency can be improved by reduction of a core loss and a conduction loss. In addition, the isolation transformer 4 can have a small rated current, which enables downsizing of components.

[0076]

Fifth Embodiment.

FIG 8 is a circuit diagram illustrating a configuration of an isolated switched-mode power supply apparatus according to a fifth embodiment of the present invention. The isolated switched-mode power supply apparatus in FIG 8 synchronizes timing of the on-time of the primary side switch 5 with an oscillator 71 of a primary side control circuit ID.

[0077]

The primary side control circuit ID includes, in place of the reference voltage source 15 in FIG 1, an oscillator (OSC) 71, a phase comparator (PFD) 62, a capacitor C12, and a resistance R13. The oscillator 71 generates a signal at a fixed reference frequency. In the isolated switched-mode power supply apparatus in FIG 1, the switching frequency slightly varies due to an on-to-off time ratio variation attributed to a loss. In contrast, the isolated switched-mode power supply apparatus in FIG 8 is capable of adjusting the on-time of the primary side switch 5 by using the phase comparator 62 such that the primary side switch 5 can be turned on and off in synchronization with the signal at the reference frequency.

[0078]

The isolated switched-mode power supply apparatus in FIG 8 is able to always operate at the fixed switching frequency, because the primary side switch 5 can be turned on and off in synchronization with the fixed frequency. ^"With this configuration, noise filtering such as EMI becomes easier and the property fluctuation due to variations in the switching frequency can be kept low.

[0079]

In the embodiments described above, the primary side switch 5 and the secondary side switch 6 do not have to be MOSFETs, but may be bipolar transistors, a MOSFET and a rectifier diode connected in parallel, or a bipolar transistor and a rectifier diode connected in parallel. Use of such primary side switch 5 and secondary side switch 6 leads to reductions in a conduction loss and a loss due to dead time, and thereby achievement of higher efficiency.

[0080]

In the embodiments described above, the secondary side control circuit is configured as a current mode controller circuit. Heretofore, there have been no isolated switched-mode power supply apparatuses which perform current mode control on the secondary side. According to the embodiments of the present invention having the foregoing configuration, it is possible to obtain high stability and responsiveness. [0081]

In the embodiments described above, the UVLO circuit (the reference voltage source 27, the comparator CMP23, and the AND circuit 28) in the secondary side control circuit may be omitted.

[0082]

In the embodiments described above, the internal voltage source VDD2 of the secondary side control circuit may be provided separately from the output voltage Vout.

[0083]

The embodiments described above may be implemented as a control method for an isolated switched-mode power supply apparatus.

[0084]

An isolated switched-mode power supply apparatus in an aspect of the present invention is configured as follows.

[0085]

A flyback isolated switched-mode power supply apparatus to convert an input voltage to an output voltage includes: a transformer having a primary winding and a secondary winding; a primary side switch configured to control a current flowing in the primary winding of the transformer; a primary side control circuit configured to measure an on-time of the primary side switch and perform on/off control of the primary side switch; a secondary side switch configured to control a current flowing in the secondary winding of the transformer; and a secondary side control circuit configured to measure an on-time of the secondary side switch and perform on/off control of the secondary side switch. The secondary side control circuit adjusts the on-time of the secondary side switch such that the output voltage can be a desired voltage. The primary side control circuit turns on the primary side switch when a potential difference across the primary side switch is smaller than a first threshold, and turns off the primary side switch under conditions where the on-time of the primary side switch expires and where a magnetizing current flowing in the primary winding of the transformer has a magnitude large enough to make a potential difference across the secondary side switch smaller than a second threshold by a counter electromotive force of the transformer if the primary side switch is turned off. The secondary side control circuit turns on the secondary side switch when the potential difference across the secondary side switch is smaller than the second threshold, and turns off the secondary side switch under conditions where the on-time of the secondary side switch expires and where a magnetizing current flowing in the secondary winding of the transformer has a magnitude large enough to make the potential difference across the primary side switch smaller than the first threshold by a counter electromotive force of the transformer if the secondary side switch is turned off.

[0086]

In the isolated switched-mode power supply apparatus, the primary side control circuit holds the on-time of the primary side control circuit, the on-time being determined in advance.

[0087]

In the isolated switched-mode power supply apparatus, the on-time of the primary side switch is adjusted by connecting an external element to the isolated switched-mode power supply apparatus.

[0088]

In the isolated switched-mode power supply apparatus, the primary side control circuit adjusts the on-time of the primary side switch such that a negative magnetizing current can flow in the primary wmding of the transformer over a predetermined time when the primary side switch is turned on.

[0089]

In the isolated switched-mode power supply apparatus, the primary side control circuit includes an oscillator configured to generate a signal at a fixed frequency, and adjusts the on-time of the primary side switch such that the primary side switch can be turned on or off in synchronization with the signal at the fixed frequency.

[0090]

In the isolated switched-mode power supply apparatus, the secondary side control circuit includes a reference voltage source configured to generate a reference voltage corresponding to the desired voltage, and an error amplifier configured to generate an error voltage indicating an error between the reference voltage and a voltage corresponding to the output voltage. The secondary side control circuit adjusts the on-time of the secondary side switch on the basis of the error voltage such that the output voltage can be the desired voltage.

[0091] In the isolated switched-mode power supply apparatus, at least one of the first and second thresholds is adjusted by connecting an external element to the isolated switched-mode power supply apparatus.

[0092]

In the isolated switched-mode power supply apparatus, when a product of the magnetizing current flowing in the primary winding of the transformer and an on-resistance of the primary side switch exceeds the first threshold, the primary side control circuit determines that the magnetizing current flowing in the primary winding of the transformer has the magnitude large enough to make the potential difference across the secondary side switch smaller than the second threshold by the counter electromotive force of the transformer if the primary side switch is turned off.

[0093]

The isolated switched-mode power supply apparatus further includes a first sense resistance connected in series to the primary winding of the transformer. When a product of the magnetizing current flowing in the primary winding of the transformer and the first sense resistance exceeds a third threshold, the primary side control circuit determines that the magnetizing current flowing in the primary winding of the transformer has the magnitude large enough to make the potential difference across the secondary side switch smaller than the second threshold by the counter electromotive force of the transformer if the primary side switch is turned off.

[0094]

In the isolated switched-mode power supply apparatus, the third threshold is adjusted by connecting an external element to the isolated switched-mode power supply apparatus.

[0095]

In the isolated switched-mode power supply apparatus, when a product of the magnetizing current flowing in the secondary winding of the transformer and an on-resistance of the secondary side switch exceeds the second threshold, the secondary side control circuit determines that the magnetizing current flowing in the secondary winding of the transformer has the magnitude large enough to make the potential difference across the primary side switch smaller than the first threshold by the counter electromotive force of the transformer if the secondary side switch is turned off. [0096]

The isolated switched-mode power supply apparatus further includes a second sense resistance connected in series to the secondary winding of the transformer. When a product of the magnetizing current flowing in the secondary winding of the transformer and the second sense resistance exceeds a fourth threshold, the secondary side control circuit determines that the magnetizing current flowing in the secondary winding of the transformer has the magnitude large enough to make the potential difference across the primary side switch smaller than the first threshold by the counter electromotive force of the transformer if the secondary side switch is turned off.

[0097]

In the isolated switched-mode power supply apparatus, the fourth threshold is adjusted by connecting an external element to the isolated switched-mode power supply apparatus.

[0098]

In the isolated switched-mode power supply apparatus, the primary side control circuit turns off the primary side switch when a positive magnetizing current flowing in the primary winding of the transformer exceeds a fifth threshold.

[0099]

In the isolated switched-mode power supply apparatus, the secondary side control circuit turns off the secondary side switch when the absolute value of a negative magnetizing current flowing in the secondary winding of the transformer exceeds a sixth threshold.

[0100]

In the isolated switched-mode power supply apparatus, the secondary side control circuit uses the output voltage as a power supply source.

[0101]

In the isolated switched-mode power supply apparatus, the secondary side control circuit turns off the secondary side switch when the output voltage does not exceed a seventh threshold.

[0102]

In the isolated switched-mode power supply apparatus, the primary side switch and the secondary side switch are MOSFETs, bipolar transistors, a MOSFET and a rectifier diode connected in parallel, or a bipolar transistor and a rectifier diode connected in parallel.

[0103]

In the isolated switched-mode power supply apparatus, the secondary side control circuit is a current mode controller circuit.

[0104]

A control method for an isolated switched-mode power supply apparatus according to an aspect of the present invention is configured as follows.

[0105]

In a control method for a flyback isolated switched-mode power supply apparatus to convert an input voltage to an output voltage, the isolated switched-mode power supply apparatus includes: a transformer having a primary winding and a secondary winding; a primary side switch configured to control a current flowing in the primary winding of the transformer; a primary side control circuit configured to measure an on-time of the primary side switch and perform on/off control of the primary side switch; a secondary side switch configured to control a current flowing in the secondary winding of the transformer; and a secondary side control circuit configured to measure an on-time of the secondary side switch and perform on/off control of the secondary side switch, the secondary side control circuit adjusting the on-time of the secondary side switch such that the output voltage can be a desired voltage. The control method includes the steps of: turning on the primary side switch by the primary side control circuit when a potential difference across the primary side switch is smaller than a first threshold; turning off the primary side switch by the primary side control circuit under conditions where the on-time of the primary side switch expires and where a magnetizing current flowing in the primary winding of the transformer has a magnitude large enough to make a potential difference across the secondary side switch smaller than a second threshold by a counter electromotive force of the transformer if the primary side switch is turned off; turning on the secondary side switch by the secondary side control circuit when the potential difference across the secondary side switch is smaller than the second threshold; and turning off the secondary side switch by the secondary side control circuit under conditions where the on-time of the secondary side switch expires and where a magnetizing current flowing in the secondary winding of the transformer has a magnitude large enough to make the potential difference across the primary side switch smaller than the first threshold by a counter electromotive force of the transformer if the secondary side switch is turned off.

[0106]

The isolated switched-mode power supply apparatus and the control method for the same in the aspects of the present invention produce the following effects.

[0107]

According to the isolated switched-mode power supply apparatus in the aspect of the present invention, since the output voltage can be regulated by varying the on-time of the secondary side switch, there is no need for a circuit for feedback to the primary side, such as a photocoupler, which is used in the conventional isolated switched-mode power supply apparatus. Moreover, the isolated switched-mode power supply apparatus is able to always perform the zero-voltage switching and thereby to reduce a switching loss, which leads to efficiency improvement. In addition, this enables use of a higher frequency as the switching frequency, and downsizing of components.

[0108]

According to the isolated switched-mode power supply apparatus, when the on-time of the primary control circuit is adjusted by connecting the external element to the isolated switched-mode power supply apparatus, the isolated switched-mode power supply apparatus can be operated at a desired switching frequency.

[0109]

According to the isolated switched-mode power supply apparatus, since the time when the magnetizing current is negative can be adjusted to a desired time, the ripples of the currents flowing in the transformer can be kept small, and the efficiency can be improved by reduction of a core loss and a conduction loss. In addition, the transformer can have a small rated current that enables downsizing of components.

[0110]

The isolated switched-mode power supply apparatus is able to always operate at the fixed switching frequency, because the primary side switch can be turned on and off in synchronization with the fixed frequency. With this configuration, noise filtering such as EMI becomes easier and the property fluctuation due to variations in the switching frequency can be kept low.

[0111]

According to the isolated switched-mode power supply apparatus, the secondary side control circuit includes the reference voltage source and the error amplifier, whereby the stabile regulation and the highly-accurate output voltage can be easily achieved.

[0112]

According to the isolated switched-mode power supply apparatus, any one of the first and second thresholds is adjusted by connecting the external element to the isolated switched-mode power supply apparatus. Thus, the current following in the transformer can be detected with high accuracy.

[0113]

The isolated switched-mode power supply apparatus detects that the product of the magnetizing current flowing in the primary winding of the transformer and the on-resistance of the primary side switch exceeds the first threshold. This configuration does not need a sense resistance and can eliminate a loss due to such a sense resistance.

[0114]

The isolated switched-mode power supply apparatus is capable of detecting the current flowing in the transformer with high accuracy, by detecting that the product of the magnetizing current flowing in the primary winding of the transformer and the first sense resistance exceeds the third threshold.

[0115]

According to the isolated switched-mode power supply apparatus, the third threshold is adjusted by connecting the external element to the isolated switched-mode power supply apparatus, and thereby the transformer is enabled to generate a desired and optimal counter electromotive force during operation of the isolated switched-mode power supply apparatus.

[0116]

The isolated switched-mode power supply apparatus detects that the product of the magnetizing current flowing in the secondary winding of the transformer and the on-resistance of the secondary side switch exceeds the second threshold. This configuration does not need a sense resistance and can eliminate a loss due to such a sense resistance.

[0117]

The isolated switched-mode power supply apparatus can detect the current flowing in the transformer can with high accuracy by detecting that the product of the magnetizing current flowing in the secondary winding of the transformer and the second sense resistance exceeds the fourth threshold.

[0118]

According to the isolated switched-mode power supply apparatus, the fourth threshold is adjusted by connecting the external element to the isolated switched-mode power supply apparatus, and thereby the transformer is enabled to generate a desired and optimal counter electromotive force during operation of the isolated switched-mode power supply apparatus.

[0119]

The isolated switched-mode power supply apparatus is provided with the overcurrent detector circuits, and thereby is capable of avoiding an overcurrent of the transformer due to an anomaly such as a short circuit.

[0120]

According to the isolated switched-mode power supply apparatus, since the output voltage is used as the power supply source of the secondary side control circuit, a loss due to a current consumption in the control circuit can be kept low.

[0121]

According to the isolated switched-mode power supply apparatus, the secondary side control circuit is provided with the UVLO circuit, and turns off the secondary side switch when the power supply voltage is equal to or lower than the predetermined voltage. Thus, a misoperation under a condition where the power supply voltage of the secondary side control circuit is low can be prevented.

[0122]

According to the isolated switched-mode power supply apparatus, MOSFETs, bipolar transistors, a MOSFET and a rectifier diode connected in parallel, or a bipolar transistor and a rectifier diode connected in parallel are used as the primary side switch and the secondary side switch, which leads to reductions in a conduction loss and a loss due to dead time, and thereby achievement of higher efficiency.

[0123]

According to the isolated switched-mode power supply apparatus, the secondary side control circuit is configured as a current mode controller circuit, which results in achievement of high stability and responsiveness.

[0124]

The control method for an isolated switched-mode power supply apparatus according to the aspect of the present invention produces effects similar to the above ones.

[0125]

According to the embodiments of the present invention, provided is a highly-efficient isolated switched-mode power supply apparatus capable of regulating an output voltage without transrmtting the output voltage on a secondary side to a primary side.

Although the embodiments of the present disclosure have been described above, the present disclosure is not limited thereto. It should be appreciated that variations may be made in the embodiment described by persons skilled in the art without departing from the scope of the present disclosure.

The present application is based on and claims priority from Japanese Patent Application No. 2012-170019, filed on July 31, 2012, the disclosure of which is hereby incorporated by reference in its entirety.

Claims

[Claims]

[Claim 1]

A flyback isolated switched-mode power supply apparatus to convert an input voltage to an output voltage, the isolated switched-mode power supply apparatus comprising:

a transformer having a primary winding and a secondary winding;

the primary side control circuit

the secondary side control circuit

[Claim 2]

The isolated switched-mode power supply apparatus according to claim 1, wherein the primary side control circuit holds the on-time of the primary side switch, the on-time being determined in advance.

[Claim 3]

The isolated switched-mode power supply apparatus according to claim 2, wherein the on-time of the primary side switch is adjusted by connecting an external element to the isolated switched-mode power supply apparatus.

[Claim 4]

The isolated switched-mode power supply apparatus according to claim 1, wherein the primary side control circuit adjusts the on-time of the primary side switch such that a negative magnetizing current flows in the primary winding of the transformer over a predetermined time when the primary side switch is turned on.

[Claim 5]

The isolated switched-mode power supply apparatus according to claim 1, wherein the primary side control circuit includes an oscillator configured to generate a signal at a fixed frequency, and adjusts the on-time of the primary side switch such that the primary side switch is turned on or off in synchronization with the signal at the fixed frequency.

[Claim 6]

The isolated switched-mode power supply apparatus according to any one of claims 1 to 5, wherein

the secondary side control circuit includes:

a reference voltage source configured to generate a reference voltage corresponding to the desired voltage; and

an error amplifier configured to generate an error voltage indicating an error between the reference voltage and a voltage corresponding to the output voltage, and

the secondary side control circuit adjusts the on-time of the secondary side switch on the basis of the error voltage such that the output voltage becomes the desired voltage.

[Claim 7]

The isolated switched-mode power supply apparatus according to any one of claims 1 to 6, wherein

at least one of the first and second thresholds is adjusted by connecting an external element to the isolated switched-mode power supply apparatus.

[Claim 8]

The isolated switched-mode power supply apparatus according to any one of claims 1 to 7, wherein

when a product of the magnetizing current flowing in the primary winding of the transformer and an on-resistance of the primary side switch becomes larger than the first threshold, the primary side control circuit determines that the magnetizing current flowing in the primary winding of the transformer has the magnitude large enough to make the potential difference across the secondary side switch smaller than the second threshold by the counter electromotive force of the transformer if the primary side switch is turned off.

[Claim 9]

The isolated switched-mode power supply apparatus according to any one of claims 1 to 7, further comprising a first sense resistance connected in series to the primary winding of the transformer, wherein

when a product of the magnetizing current flowing in the primary winding of the transformer and the first sense resistance becomes larger than a third threshold, the primary side control circuit determines that the magnetizing current flowing in the primary winding of the transformer has the magnitude large enough to make the potential difference across the secondary side switch smaller than the second threshold by the counter electromotive force of the transformer if the primary side switch is turned off.

[Claim 10]

A control method for a flyback isolated switched-mode power supply apparatus to convert an input voltage to an output voltage, the isolated switched-mode power supply apparatus including: a transformer having a primary winding and a secondary winding;

a secondary side control circuit configured to measure an on-time of the secondary side switch and perform on/off control of the secondary side switch, the secondary side control circuit adjusting the oh-time of the secondary side switch such that the output voltage becomes a desired voltage,

the control method comprising the steps of:

turning on the primary side switch by the primary side control circuit when a potential difference across the primary side switch is smaller than a first threshold;

turning off the primary side switch by the primary side control circuit under conditions where the on-time of the primary side switch expires and where a magnetizing current flowing in the primary winding of the transformer has a magnitude large enough to make a potential difference across the secondary side switch smaller than a second threshold by a counter electromotive force of the transformer if the primary side switch is turned off;

turning on the secondary side switch by the secondary side control circuit when the potential difference across the secondary side switch is smaller than the second threshold; and

turning off the secondary side switch by the secondary side control circuit under conditions where the on-time of the secondary side switch expires and where a magnetizing current flowing in the secondary winding of the transformer has a magnitude large enough to make the potential difference across the primary side switch smaller than the first threshold by a counter electromotive force of the transformer if the secondary side switch is turned off.