JP2882472B2 - Power supply circuit using power insulated gate type FET - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit using a power insulated gate type FET, and more particularly to a synchronous rectification circuit in a single-switch forward type switching power supply using a voltage conversion transformer and a power MOSFET as a switching element.

[0002]

2. Description of the Related Art Conventionally, as a switching power supply of this kind, various types of switching power supplies and power MOSFETs, which are employed in DC-DC converters, have been devised to stabilize the output voltage. A circuit has been proposed.

FIG. 5 shows a conventional power MOSFE.
It is a power supply circuit diagram using T. As shown in FIG. 5, the power supply circuit using the power MOSFET of the monolithic forward system includes an electrolytic capacitor 8 connected between a pair of input terminals 1a and 1b, a primary winding and a secondary winding, and A voltage conversion transformer 4 having one end of the next winding connected to one input terminal 1a; a switching power MOSFET 3 connected between the other end of the primary winding of the voltage conversion transformer 4 and the other input terminal 1b; A choke coil 9 connected between one end of the secondary winding of the voltage converting transformer 4 and the output terminal 2a, and a rectifier connected between the other end of the secondary winding of the voltage converting transformer 4 and the output terminal 2b. Power MO for
SFET5, one end of secondary winding and rectifying power MO
A capacitor 11 connected between the gate of the SFET 5, a resistor 13 connected between the gate and the source of the rectifying power MOSFET 5, a commutation power MOSFET 6 connected in parallel between one end of the secondary winding and the output terminal 2b, and a high-speed Rectifier diode 7 and capacitor 1 connected between the gate of commutation power MOSFET 6 and the other end of the secondary winding
2, a resistor 14 connected between the gate and source of the commutation power MOSFET 6, a capacitor 10 connected between the output terminals 2a and 2b, and a voltage between the output terminal 2a and the other end of the secondary winding. The error amplifier 25 is connected via the photocoupler 16 driven by the output of the error amplifier 25, and the output is connected to the switching power M
And a feedback control unit 15 that feeds back to the gate of the OSFET 3. Diodes 3 ', 5', and 6 'connected between the drains (D) and the sources (S) of the power MOSFETs 3, 5, and 6 are internal diodes of the respective FETs. 13, the capacitor 12 and the resistor 14 are power MOSFEs, respectively.
This element determines the time constant of the rise or fall of T5 and T6.

Although the feedback control unit 15 is shown in a simplified form here, this feedback control unit 15 is normally used.
Is formed by a control IC dedicated to a power supply, and internally includes an oscillation circuit, a PWM comparator, a power supply unit, and the like. The input terminals 1a, 1b, and has a function of controlling on / off of the switching power MOSFET 3 by a comparator output for comparing the output of the photocoupler 16 and the output of the oscillation circuit. That is, the input level of the comparator is changed by the output voltage on the secondary side of the transformer 4. For example, when the output voltage increases, the on-width of the power MOSFET 3 is controlled to be narrowed, thereby decreasing the output voltage. Has been stabilized.

In short, the switching power MOSFE
T3 repeats the on / off operation when a continuous rectangular wave is supplied as its gate signal.

First, a DC voltage (+ on the 1a side,-on the 1b side) is always applied to the input terminals 1a and 1b, but when the switching power MOSFET 3 on the primary side is turned on,
The rectifying power MOSFET 5 on the secondary side is biased in the forward direction between its gate and source electrodes, and is turned on. Therefore, a current flows through a closed circuit including the voltage conversion transformer 4, the choke coil 9, the capacitor 10, and the rectifying power MOSFET 5, and supplies a DC voltage to the outside through the output terminals 2a and 2b.

Next, when the primary-side power MOSFET 3 is turned off, the secondary-side rectifying power MOSFET 5 is turned off because its gate and source electrodes are biased in opposite directions. At this time, the power MOSFET 6 for commutation
Is forward biased between its gate and source electrodes,
Turn on. Therefore, the energy stored in the choke coil 9 when the rectifying power MOSFET 5 is turned on is
Choke coil 9, capacitor 10, commutation power MO
The output terminal 2a, which is regenerated to a closed circuit composed of the SFET 6,
A DC voltage is supplied to the outside via 2b.

As described above, a constant power is supplied to the external circuit by repeatedly turning on and off the power MOSFET 3.

FIGS. 6A and 6B are a timing chart for explaining the current characteristics of the power MOSFET shown in FIG. 5 and a partially enlarged view of a portion B thereof. FIG.
As shown in (a), these current characteristics show the drain current characteristics of the rectifying power MOSFET 5 and the commutating power MOSFET 6, and in the switching power supply circuit of the single-pole forward type, the secondary side of the switching rectifier circuit constitutes the synchronous rectifier circuit. When the diversion power MOSFET 6 changes from the on state to the off state, the commutation power MOSFET is turned on despite the reverse bias state between the drain and source electrodes of the FET 6.
The internal diode 6 ′, the choke coil 9,
Since a closed circuit of the capacitor 10 is formed, there is a reverse recovery period. That is, the rectification power MOSFET 5 and the commutation power MOSFET 6 are turned on at the same time. This, as shown by a dotted line frame B, the drain current I _D of the drain current I _D and FET6 the FET 5 (opposite directions) is meant to flow simultaneously.

As shown in FIG. 6B, the relationship between the source / drain current I _SD and the source / drain voltage V _SD of the FET 6 during the simultaneous ON period B is opposite to each other because the reverse recovery period T2 exists. In a relationship. Although the current-voltage characteristics of the FET 5 are omitted, the same is true except that the current-voltage characteristics of the FET 6 are reversed. In other words, it means that the overlap period in which the FETs 5 and 6 are simultaneously turned on becomes longer.

For this purpose, the power MOSFET 6 for commutation is used.
Connects a high-speed rectifier diode 7 between its source and drain in a forward direction and in parallel in order to reduce the loss of T2 at the time of reverse recovery.
During the ON period of 6, a current flows through the internal diode 6 '.
However, even when the internal diode 6 'is used, the power loss during the reverse recovery period increases because the reverse recovery period T2 is long,
The efficiency of the circuit is reduced. This is shown in FIG.
Means that the overlap period of the current I _SD flowing from the source to the drain and the voltage V _SD applied between the source and the drain becomes longer.

[0012]

The power supply circuit using the conventional power MOSFET described above, as is apparent from FIG. 6B, has two rectifiers provided on the secondary side of the voltage conversion transformer. And commutation power MOSFETs
Is on for a long time. Therefore, a synchronous rectifier circuit using two power MOSFETs has a drawback that power loss increases and circuit efficiency decreases.

As a known example for solving such a problem, a switching regulator provided with a control circuit 25 "for simultaneously driving the secondary power MOSFETs 5" and 6 "and the primary power MOSFET 3" shown in FIG. 7 is proposed. (JP-A-60-59973).

In this example, the simultaneous ON period of the rectifying and smoothing power MOSFETs 5 ″ and 6 ″ on the secondary side is controlled by the drive circuit 25 ″. The control method and timing chart are concretely described. Not known.

Further, although it is specified in the known example that the short-circuit period of the MOSFET is almost eliminated, there is actually a reverse recovery period of the internal diode, and how to shorten the reverse recovery period is a major problem. No remedies for this have been mentioned.

In FIG. 7, 8 ″ is an input terminal 1a.
, A rectifying element (diode), 26 ″ a pulse width modulation circuit,
An oscillation circuit 7 "determines the repetition period of the pulse width modulation circuit 26".

An object of the present invention is to provide a transformer for voltage conversion.
It is an object of the present invention to provide a power supply circuit using a power insulated gate type FET which shortens a simultaneous ON period of two power insulated gate type FETs provided on the next side and reduces power loss and circuit efficiency.

[0018]

According to the present invention, there is provided an electrolytic capacitor connected between a pair of input terminals, a primary winding and a secondary winding, and one end of the primary winding is connected to the pair of input terminals. A voltage conversion transformer connected to one of the input terminals; a switching power insulated gate FET connected between the other end of the primary winding of the voltage conversion transformer and the other of the pair of input terminals; A choke coil connected between one end of the secondary winding of the voltage conversion transformer and one of the pair of output terminals, and the other end of the secondary winding of the voltage conversion transformer and the other of the pair of output terminals A power insulated gate FET for rectification connected between
A first capacitor connected between the one end of the secondary winding and the gate of the rectifying power insulated gate FET; and a commutation power connected in parallel between one end of the secondary winding and the other of the output terminals. A second insulated gate FET and a high-speed rectifier diode connected between the pair of output terminals;
And an error amplifier connected to the one of the pair of output terminals and the other end of the secondary winding, and an output thereof connected via a photocoupler driven by the output of the error amplifier. A feedback control unit that feeds back to the gate of the switching power insulated gate type FET, and a power supply circuit using a power insulated gate type FET that supplies a constant voltage between the output terminals. An on-period for detecting a voltage change on the secondary winding side and controlling an on-period of the commutation power insulated gate FET between both ends of the secondary winding and a gate of the commutation power insulated gate FET. A power supply circuit using a power insulated gate FET, to which a control circuit is connected, is obtained.

[0019]

Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows a power supply circuit using a power MOSFET according to a first embodiment of the present invention. As shown in FIG. 1, a power supply circuit using this power MOSFET has a switching power MOSFET 3 between input terminals 1a and 1b, similarly to the above-described conventional power supply circuit of FIG.
And the primary winding of the voltage conversion transformer 4 and the electrolytic capacitor 8, and the rectifying power MOS is similarly connected to the secondary side.
The FET 5, the commutation power MOSFET 6, the high-speed diode 7, and the capacitor 10 are connected. From the secondary side including the output, the error amplifier 25, the photocoupler 1
6, switching power MOS provided on the input side via
A feedback control unit 15 for controlling the FET 3 is provided. Note that these are the same as the above-described conventional example, and a specific description thereof will be omitted.

In the present embodiment, in addition to these, a voltage change on the secondary winding side is detected between both ends of the secondary winding of the voltage conversion transformer 4 and the gate of the commutation power MOSFET 6 and the commutation is performed. The ON period control circuit 24 for controlling the ON period of the power MOSFET 6 is connected.
The on-period control circuit 24 mainly includes resistors 19 and 20 for detecting a voltage change on the secondary winding side,
A commutation control unit 18 for changing an output voltage according to a voltage at a connection point of the connection 20; resistors 19 and 20;
And a positive power supply 17 for regulating the potential of the reference numeral 8 and further include resistors 21 and 22 and a capacitor 23 for adjusting a time constant. The resistor 22 and the capacitor 23 are elements that define the output voltage of the commutation control unit 18. Further, the commutation control unit 18 includes an inverter INV that inverts the potential of the output terminal 2b, an AND circuit that takes the logical product of the potential at the junction of the resistors 19 and 20, and the output of the inverter INV,
Commutation power MOSFET by the output of this AND circuit
And a flip-flop FF for outputting a control potential of 6 and a CMOS logic IC is used in this embodiment.

In short, the present embodiment is based on the power MO for commutation.
Components 19 and 20 for detecting, by resistance division, a voltage change on the secondary side of the voltage conversion transformer 4 at the preceding stage, and a commutation control unit 18 for controlling the ON period of the commutation power MOSFET 6 are provided on the gate electrode of the SFET 6. That is, the commutation control unit 18 is connected to the positive driving power supply 17.

FIGS. 2 (a) and 2 (b) are a timing chart for explaining the current characteristics of the power MOSFET and the potential at each point in FIG. As shown in FIG. 2A, these characteristics are the power MOSF
Represents a drain current I _D of ET5,6, the potential VA at the connection point of the resistors 19, 20, and a gate potential VB of the power MOSFET 6, also in FIG. 2 (b), an enlarged portion A, the reverse recovery period T1A and power MOSFET 5,
Reference numeral 6 denotes a simultaneous ON period in which the ON is performed simultaneously.

2B, the current I _SD represents the source / drain current of the power MOSFET 6 for commutation, and the voltage V _SD represents the source / drain voltage of the power MOSFET 6 for commutation. By doing so, the overlap time of the current I _SD and the voltage V _SD is reduced, and the simultaneous ON period is shortened.

Hereinafter, the circuit operation will be described more specifically with reference to FIGS.

First, when the power MOSFET 3 as a switching element provided on the primary side of the transformer 4 is turned on, a rectifying power MOSFET connected to its secondary side
5 is biased in the forward direction between the gate and source electrodes, and is turned on. Therefore, the voltage conversion transformer 4, the choke coil 9, the capacitor 10, the rectifying power MOSFET 5
A current flows through a closed circuit composed of, and supplies power to an external circuit via output terminals 2a and 2b. At this time, the resistance 19,
Although the potential at the connection point VA at 20 goes high, the output (VB) of the commutation control section 18 goes low (0) because it passes through the AND circuit and the flip-flop FF. Therefore, the drive circuit 24 including the commutation control unit 18
Does not drive the commutation power MOSFET 6.

Next, when the primary-side power MOSFET 3 is turned off, a bias is applied in the reverse direction between the gate and source electrodes of the secondary-side rectifying power MOSFET 5, so that the rectifying power MOSFET 5 is turned off and connected to the connection point. VA
Also drops to the 0 level. That is, this point is a trigger point for the commutation control unit 18. In contrast to the rectifying power MOSFET 5, the commutating power MOSFET is
Since the gate-source electrode of T6 is biased in the forward direction by the output of the commutation control unit 18, the commutation power MO
The SFET 6 turns on. Therefore, rectifying power MOSF
The energy stored in the choke coil 9 when the ET 5 is turned on is regenerated to a closed circuit including the choke coil 9, the capacitor 10, and the power MOSFET 6 for commutation, and the power is supplied to the external circuit from the output terminals 2a and 2b.

During this regeneration period, the rectifying power MOSFET 5 is used by utilizing the voltage change of the voltage converting transformer 4.
Is detected from the voltage across the voltage conversion transformer 4. That is, by inputting the potential at the VA point (low level) to the commutation control unit 18 as a trigger signal, the commutation power MOSFET 6 is turned on. During the ON period of the commutation power MOSFET 6, this MOS
The constant of the resistor 22 and the capacitor 23 connected to the commutation control unit 18 is set so that the “turn-off time” of the FET 6 (the time when the FET 6 is completely turned off) <“dead time”, and the reverse recovery period T1 Time (ie, power M
When the OSFETs 5 and 6 are simultaneously turned on), the commutation power M
OSFET 6 is turned off. Therefore, no current flows through the internal diode 6 'of the commutation power MOSFET 6, and the high-speed rectifier diode 7 is made conductive,
Power loss during this period can be reduced, and the efficiency of the circuit can be improved.

FIG. 3 shows a power supply circuit using a power MOSFET according to a second embodiment of the present invention. In FIG.
The first embodiment differs from the first embodiment in that an on-period control circuit (delay circuit) 24 for controlling the on-period of the commutation power MOSFET 6 includes a resistor 20, a capacitor 24 ', and a DIAC (Diode AC).
Switch) 25 'and SCR (Silicon)
(Controlled Rectifier) 26.

Next, the operation of the embodiment of FIG. 3 will be described with reference to FIG.

The basic operation on the secondary side is the same as that of the first embodiment, and the operation of the ON period control circuit (delay circuit) 24 will be described.

During the regenerative period in which the rectifying power MOSFET 5 is off and the commutating power MOSFET 6 is on, point b on the secondary side of the voltage conversion transformer 4 has a positive voltage, and charges are accumulated in the capacitor 24 ′ through the resistor 20. Is done. The voltage at the middle point c of the resistor 20 and the capacitor 24 is DI
When the voltage exceeds the breakover voltage of AC25 ', DIA
C25 'is turned on to supply a trigger current to the gate of the SCR 26, and the SCR 26 is turned on. The anode and cathode of the SCR 26 are connected to the gate and source of the power MOSFET 6 for commutation. When the SCR 26 is turned on, the voltage between the gate and the source of the commutation power MOSFET 6 becomes almost 0 V, and the commutation power MOSFET 6 is turned off. As described above, the ON period of the commutation power MOSFET 6 can be set by the time constant of the resistor 20 and the capacitor 24 ', and the reverse recovery time can be shortened as in the first embodiment.

This embodiment is different from the first embodiment in that a resistor 20, a capacitor 24 ', a DIAC 25', an SCR 26
In particular, since no IC is used for control as in the first embodiment, a stabilizing power supply circuit such as a three-terminal regulator is not required, and the size can be further reduced.

[0034]

As described above, according to the present invention, the power M
A power supply circuit using an OSFET turns off a commutation power MOSFET and turns on a high-speed rectifier diode before a simultaneous ON period of a rectification power MOSFET and a commutation power MOSFET connected to the secondary side of a voltage conversion transformer. Providing the control unit has the effects of reducing power loss during this period and improving circuit efficiency.

When the on-period control circuit is composed of a resistor, a capacitor, a DIAC, and an SCR, a stabilizing power supply circuit is not required, and the size can be further reduced.

[Brief description of the drawings]

FIG. 1 shows a power MOSFET according to a first embodiment of the present invention.
It is a block diagram of a power supply circuit using T.

FIG. 2 is a timing chart for explaining current characteristics of the power MOSFET and potentials at respective points in FIG. 1;

FIG. 3 shows a power MOSFET according to a second embodiment of the present invention;
It is a block diagram of a power supply circuit using T.

FIG. 4 is a timing chart for explaining the operation of the power supply circuit of FIG. 3;

FIG. 5 is a block diagram of a power supply circuit using a conventional power MOSFET.

FIG. 6 is a timing chart for explaining current characteristics of the power MOSFET in FIG. 5;

FIG. 7 is a block diagram of another conventional example.

[Explanation of symbols]

 1a, 1b Input terminals 2a, 2b Output terminals 3 Switching power MOSFET 3 ', 5', 6 'Internal diode 4 Voltage conversion transformer 5 Rectification power MOSFET 6 Commutation power MOSFET 7 High-speed rectifier diode 8 Electrolytic capacitor 9 Choke coil 10, 11 Capacitor 15 Feedback control unit 16 Photocoupler 17 Positive power supply 18 Commutation control unit 19, 20 Voltage detection resistor 24 ON period control circuit 24 'Capacitor 25 Error amplifier 25' DIAC 26 SCR

Claims (4)

(57) [Claims] An electrolytic capacitor connected between a pair of input terminals, a voltage conversion device having a primary winding and a secondary winding, and one end of the primary winding connected to one of the pair of input terminals. A transformer, a switching power insulated gate FET connected between the other end of the primary winding of the voltage conversion transformer and the other of the pair of input terminals, and the secondary winding of the voltage conversion transformer And a choke coil connected between one end of the output terminal and one of the pair of output terminals, and a rectifying power insulation gate connected between the other end of the secondary winding of the voltage conversion transformer and the other of the pair of output terminals. And a first FET connected between the one end of the secondary winding and the gate of the rectifying power insulated gate FET.
And a high-speed rectifying diode and a power insulated gate FET for commutation connected in parallel between one end of the secondary winding and the other of the output terminals, and a second capacitor connected between the pair of output terminals An error amplifier connected to the one of the pair of output terminals and the other end of the secondary winding, and an output connected to a photocoupler driven by an output of the error amplifier, and A feedback control unit that feeds back to the gate of the switching power insulated gate FET, wherein the power conversion circuit uses a power insulated gate FET that supplies a constant voltage between the output terminals. A voltage change on the secondary winding is detected between both ends of a secondary winding and a gate of the power insulated gate FET for commutation, and the power insulated gate for commutation is detected. Power circuit using a power insulated gate FET, characterized in that connecting the on-period control circuit for controlling the ON period of the FET. 2. The on-period control circuit includes: a resistor for detecting a voltage change on the secondary winding side; a commutation controller for changing an output voltage according to a voltage of the resistor; 2. A power supply circuit using a power insulated gate type FET according to claim 1, comprising a positive power supply for regulating a potential of the commutation control unit. 3. The commutation control unit includes: an inverter that inverts a potential at the other end of the output terminal; and an AND that calculates a logical product of a coupling potential of the resistance unit and an output of the inverter.
3. The power insulated gate type F according to claim 2, comprising: a D circuit; and a flip-flop which outputs a control potential of the power insulated gate type FET for commutation based on an output of the AND circuit.
Power supply circuit using ET. 4. The on-period control circuit is connected to a series connection circuit of a resistor and a capacitor connected between both ends of the secondary winding of the voltage conversion transformer, and to a connection point of the resistor and the capacitor. A first terminal and a second terminal, and have a breakover voltage. When the voltage of the first terminal becomes equal to or higher than the breakover voltage, the first terminal is turned on, and a trigger current is supplied to the second terminal. D to output
An IAC, a gate connected to the second terminal, a cathode connected to the other of the pair of output terminals, and an anode connected to a gate of the power insulated gate FET for commutation; When a trigger current is supplied to the gate, the transistor is turned on, and the power insulated gate FET for commutation is turned on.
2. The power insulated gate according to claim 1, further comprising an SCR for turning off the power supply, and wherein the ON period of the power insulated gate FET for commutation is determined by a time constant of the series connection circuit of the resistor and the capacitor. Power supply circuit using FET.