JPH07170734A - Flyback dc-dc converter - Google Patents

Abstract

PURPOSE:To lower noises, and to reduce loss by connecting the parallel circuit of a diode in the reverse direction to a second switching element and a second capacitor and a series circuit consisting of a first capacitor in parallel with a primary coil for a transformer and tuning the second switching element on for a period when the winding voltage of the transformer has negative polarity. CONSTITUTION:A first switching element S1 is turned on and excitation currents are made to flow through a transformer T, the first switching element S2 is tuned off, and a first capacitor C2 is charged. When the voltage of a primary coil np for the transformer T reaches negative polarity, the stored energy of the transformer T is discharged to an output while a second switch S2 is turned on and the first capacitor C2 is charged. When energy is discharged completely to the output and a diode D0 is turned off, a ringing is generated, and the voltage of the primary coil np for the transformer T reaches positive voltage. When the second switching element S2 is turned off at that time, the voltage VS1 of the first switching element S1 reaches zero, thus attaining soft switching.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to reduction of switching loss and noise of a flyback type DC-DC converter having a discontinuous transformer current.

[0002]

2. Description of the Related Art Recently, a compact and highly efficient switching type DC-DC converter has been widely used for a computer, a communication device and the like. In particular, the flyback type DC-DC converter of which the transformer current is discontinuous, an example of which is shown in FIG. 4, is used in a field requiring a relatively low cost such as a power supply for a television because of its simple circuit configuration.

The operation of this circuit will be described with reference to FIGS. In FIG. 4, Q is a main switch element, and the input voltage Vi is intermittently turned on and off by the main switch element Q.
Apply to. During the period (T1) when the main switch element Q is ON, voltage is applied to the primary coil np of the transformer T so that the "." Mark has (+) polarity. (3) is the secondary of the transformer T Coil ns is output rectification diode D
Transformer 1 because 0 has a reverse bias polarity
The exciting current ip (see FIG. 5) of the transformer core is applied to the next coil np.
(B)) flows. While the main switch element Q is off, the flyback voltage of the transformer T turns on the rectifying diode D0, the secondary current is (FIG. 5 (c)) is discharged to the output, is smoothed by the output capacitor C0, and the DC power is supplied to the load. Supplied. In a steady state in which the main switch element Q is repeatedly turned on and off, the output voltage V0 is shown as follows.

V0 = Vi · ns / np · TON / TOFF (1) where ns: number of turns of secondary winding np: number of turns of primary winding TON: ON period of main switch element TOFF: Off period of main switch element

When the secondary current is (FIG. 5 (c)) becomes zero again, the output diode D0 is turned off and the main switching element Q is turned on.
Since this is also off, parasitic oscillation (ringing) begins with the exciting inductance of the transformer T, the output capacitance Cak of the output diode D0, the parasitic capacitance Cp of the transformer T, and the output capacitance Cds of the main switch element. In the self-excited flyback type DC-DC converter, when the voltage Vds of the main switching element Q becomes lower than the input voltage Vi, that is, the control winding voltage Vf becomes positive, The switch element Q is turned on. Therefore, the spike current ip as shown in FIG. 5B is (Cds, Cp, Cak).
Charge flow, which increases switching loss and becomes a source of switching noise.

As a countermeasure against this, there is a method of adding a bottom turn-on control circuit (a circuit for turning on the main switching element at the valley point of the ringing of Vds) shown in the circuit example of FIG. 6 (4). In this case, as shown in FIG. 7A, the voltage νds of the main switching element Q is an example voltage, the primary coil voltage of the transformer is Vi, the voltage of Cak is Vins / np, and the main switching element Q is turned on. Therefore, spike current does not flow and switching loss and noise are reduced.

However, when the ringing is considered as Vds, if νds is νds' at the time when is becomes zero, νds'-Vi, that is, V0.np/ns-Vi is the initial value and the resonance waveform is centered on Vi. And the valley point of ringing is Vi-Vds', that is, Vi-V0.
It only goes down to np / ns. V0 ・ np / ns> Vi
7B, the spike current at the time of turning on the main switching element Q does not flow, but V0.np/n
When s <Vi, spike current flows as shown in FIG. 8B, and reduction of loss and noise cannot be expected.

Therefore, in this measure, the transformer primary 2
If there is a limit to the design of the next turns ratio, V0 · np /
Since ns> Vi, there is a drawback that the applied voltage to the main switch element becomes high as shown in FIG.

[0009]

An object of the present invention is to provide a primary coil of a transformer or a main switching element with a parallel circuit of a second switching element, a diode in the reverse direction and a second capacitor, and a series circuit composed of a first capacitor. By connecting in parallel and turning on the second switch element while the winding voltage of the transformer is negative, spike current does not occur even under the condition of V0 · np / ns <Vi, and low loss and low noise are achieved. It measures.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The basic circuit of the present invention is shown in FIGS. 1 (a) and 1 (b), and its operation explanatory diagram is shown in FIG. (5) In FIG. 1, S1 is a main switching element, which turns on / off the input voltage Vi to turn on and off the primary coil np of the transformer T1.
Apply a pulsating voltage to. The Do connected to the secondary coil ns of the transformer T supplies a DC output to the load via the smoothing capacitor C0 for the energy stored in the transformer by the rectifying diode. A primary coil np of the transformer is connected in parallel with a series circuit composed of a second capacitor C3, a second switch element S2, a parallel circuit of a diode D2 and a first capacitor C2. Second switch element S2
Is a period in which the voltage of the transformer T has a negative polarity (Vs in FIG. 2A).
It is controlled so that it is turned on for a period in which 1 is higher than Vi. The capacitors C1, CP and Cak are respectively the first switch element S1 and
It is the parasitic capacitance of the transformer T and the diode D0.

Next, the operation of FIG. 1 will be described with reference to FIG. In FIG. 2, when the first switch element S1 is turned on at time t1, the input voltage Vi is applied to the primary coil np of the transformer T so that the "." Mark has a positive polarity. The voltage of the secondary coil of the transformer T is the exciting current ip of the transformer T because the rectifying diode D0 has the reverse bias polarity (FIG. 2 (b)).
Only flowing. When the first switch element S1 is turned off at time t2, the primary current ip of the transformer T flows to the route of np → C2 → D2 → np, the first capacitor C2 is charged, and the primary coil np of the transformer T is charged. The voltage decreases toward zero and the voltage υs1 of the first switch element S1 increases.

When the voltage of the primary coil np of the transformer T exceeds zero and becomes negative, the rectifying diode D0 is turned on and the energy stored in the transformer T is discharged to the output. At the same time, the second switch element S2 is turned on and np → C2 → S2 → np
, The first capacitor C2 is charged to V0.ns / np.

When the release of energy to the output ends at time t5 and the diode D0 is turned off, the exciting inductance of the transformer T and the parasitic capacitance of the first capacitor C2 and each part (6) The quantities C1, Cp, Cak, etc. Ringing occurs between them, the voltage of the primary coil np of the transformer T turns toward zero, and the voltage υs1 of the first switch element S1 also starts to decrease. When the voltage of the primary coil np of the transformer T exceeds zero and becomes a positive voltage at time tb, the second switch element S2 is turned off and the second capacitor C3 is equivalently connected to the second capacitor C3 in series. It For this reason, the capacitance of the capacitor that contributes to ringing decreases and the voltage change of ringing increases, and the voltage of the transformer T increases to V0.
Even if it oscillates more than np / ns and Vo · np / ns <Vi, the voltage υs1 of the first switch element S1 becomes zero volt at time t7, and it becomes possible to oscillate as shown by the broken line thereafter. The reverse voltage of Vs1 shown by the broken line is the diode D1.
Is clamped at.

By turning on the first switch element S1 during the period of time t7 to t8, the spike current stops flowing.

As described above, the first switch element S1 is so-called soft switching. In the second switch element S2 as well, the diode D2 is conducting when the turn-on is performed, and the voltage between the terminals is zero volt when the turn-off is performed, so that the zero-voltage switching is performed.

Regarding ringing at time t5 to t8, FIG.
This will be described in detail with reference to 11. FIG. 9 shows the equivalent circuit of FIG. 1 at time t5. Here, Lp is the exciting inductance of the transformer T, and np and ns are the primary and secondary turns of the transformer. An AC equivalent circuit is shown in FIG. 10 by considering the initial value of each capacitor and further eliminating the DC voltage source.

FIG. 11 shows the switch SW of FIG.
It shows the voltage and current of the capacitor C when turned on. Before and after t6, the capacitance of the capacitor C changes from (7) C to C ', but the energy of the entire circuit wants to change, and the following equation holds at the point where the current i is zero.

1/2 · CVco2 = 1 / 2C′VP ··· (2) ∴VP = √ · (C / C ′) · VCO ··· (3) However, VCO = V0.nP / ns VP: Peak voltage of load C = C2 + CP + C1 + Cak (ns / np) 2 C '= C2.C3 / C2 + C3 + CP + C1 + Cak (ns / np) 2 VP> Vi is VS1 in FIG. Since the condition is zero crossing at t7, C / C '> (Vi / Vco) 2 ∴C / C'> (Vi / (V0.np/ns)) 2 ...・ (4) is a necessary condition.

FIG. 1B shows another basic circuit diagram of the present invention. The principle of operation is the same as that of FIG.

FIG. 3 shows an embodiment of the present invention 1, and the same reference numerals as those in FIG. 1 indicate the same functions. The first switch element S1 is an N-channel MOSFET, and the second switch element S2 is P
The channel MOSFET is used. R1 is a starting resistor. When the first switch element S1 is turned on, a voltage having a positive polarity is applied to the primary coil np of the transformer T, and the first switch element S1 is kept on via the feedback winding nf and the feedback resistor R2. To do.

When the first switch element S1 is turned off, the energy stored in the transformer T is rectified by the rectifying diode D0,
It is supplied to the load via the smoothing capacitor C0. The output voltage is compared with the reference voltage Vr by the amplifier A, and the error is transmitted to the photocoupler PC (8) to control the gate voltage of the first switch element S1. Since the current when the first switch element S1 is on is a triangular wave as shown in FIG. 2, the first switch element S1
When the current of the first switch element S1 increases to a current value determined by the gate voltage of 1 and the characteristics of the first switch element S1, positive feedback is applied so that the first switch element S1 is turned off. Switch element S1 turns off. Therefore, the peak current of the triangular wave is controlled and the energy stored in the transformer T is controlled, so that the output voltage becomes a constant value. The second switch element S2 is turned on by the transformer winding nc when the voltage of the transformer winding is negative, so that the ringing occurs during ringing as shown in the operation description of FIG. 1 (a). The voltage of the first switch element S1 easily drops to zero.

The capacitor C4 is connected to delay the turn-on timing. As a result, the first switch element S1 turns on when the voltage of the first switch element S1 surely drops to zero volt due to ringing.

[0023]

[Explanation of Effect] As described above, according to the present invention, V0
・ Even under the condition of (np / ns) <Vi, the voltage at the time of switching of all switch elements can be changed softly without generating spike current, and a flyback converter with low switching loss and low noise can be obtained. can get. Since the heat generation is small and the noise filter can be miniaturized, it is possible to supply an inexpensive and compact power supply device for televisions and communication devices.

[Brief description of drawings]

FIG. 1 (a) is a basic circuit diagram of the present invention.

FIG. 1 (b) is another basic circuit diagram of the present invention.

FIG. 2 (9) Operation explanatory diagram of the basic circuit of the present invention

FIG. 3 The present invention

FIG. 1 (a) embodiment

FIG. 4 is a conventional flyback DC-DC converter.

FIG. 5 is an operation explanatory diagram of a conventional circuit.

FIG. 6 is another conventional circuit diagram.

FIG. 7 is a waveform example of each part of another conventional circuit (part 1)

FIG. 8 is a waveform example of each part of another conventional circuit (part 2).

9 is a basic circuit of the present invention at time t5 of the operation waveform of FIG. 2;

FIG. 1 is an equivalent circuit of FIG.

[Figure 10]

FIG. 9 is an equivalent circuit in which the DC voltage source in FIG.

FIG. 11

FIG. 10 is an equivalent circuit of switch SW turned on at time t5.
The voltage and current waveforms of the capacitor C when you do.

[Explanation of symbols]

T transformer np primary winding ns secondary winding nf feedback winding S1 first switch element eg N channel MO
SFET S2 second switch element, eg P channel MO
SFET (10) C1 Parasitic capacitance of first switching element C2 First capacitor C3 Second capacitor Cp Parasitic capacitance of transformer winding Cak Parasitic capacitance of diode Cds Parasitic capacitance of main switching element C0 Smoothing capacitor D0 Rectification diode -D D, D2 Diode Vi Input voltage Vo Output voltage Q Main switching element Lp Transformer exciting inductance R1, R2, R3 Resistance PC Photo coupler A Amplifier νas, υs1 Main switching element voltage ip Transformer exciting current is2 Next current

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[Procedure amendment]

[Submission date] February 9, 1995

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Change

[Correction content]

[Brief description of drawings]

FIG. 1A is a basic circuit diagram of the present invention. FIG. 1B is another basic circuit diagram of the present invention.

FIG. 2 is an operation explanatory diagram of a basic circuit diagram of the present invention.

FIG. 3 The present invention

FIG. 1 (a) embodiment

FIG. 4 is a conventional flyback DC-DC converter.

FIG. 5 is an operation explanatory diagram of a conventional circuit.

FIG. 6 is another conventional circuit diagram.

FIG. 7 is a waveform example of each part of another conventional circuit (part 1)

FIG. 8 is a waveform example of each part of another conventional circuit (part 2).

9 is a basic circuit of the present invention at time t5 of the operation waveform of FIG. 2;

FIG. 1 is an equivalent circuit of FIG.

[Figure 10]

FIG. 9 is an equivalent circuit in which the DC voltage source in FIG.

FIG. 11

FIG. 10 is an equivalent circuit of switch SW turned on at time t5.
The voltage and current waveforms of the capacitor C when you do.

[Procedure Amendment 2]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

[Figure 9]

[Figure 1]

[Figure 4]

[Fig. 2]

[Figure 3]

[Figure 5]

[Figure 8]

FIG. 11

[Figure 6]

[Figure 7]

[Figure 10]

Claims (5)

[Claims] 1. A transformer (T) having a primary winding (n) and one or more secondary windings (ns), and the transformer (T).
A first switch element (S1) that repeatedly turns on and off an input voltage to the primary winding (np) of the same, and is stored in the transformer (T) when the first switch element (S1) is off. Energy is converted into a DC output through the secondary winding (ns), the rectifying diode (D0) and the smoothing capacitor (C0), and the ON period or OFF period of the first switch element (S1) is converted. In a flyback type DC-DC converter that controls the DC output voltage by changing the second switching element (S2) and the second switching element (S2) in the primary winding (np) of the transformer (T). A parallel circuit of a diode (D2) and a second capacitor (C3) in antiparallel with the switching element (S2) of FIG. Element (S2) is the winding voltage of the transformer (T) Flyback DC-DC converter, characterized in that the negative polarity period on the - data. 2. A transformer (T) having a primary winding (np) and one or more secondary windings (ns), and the transformer (T).
First switch element (S1) for repeatedly applying the input voltage to the primary winding (np) by repeatedly turning it on and off, and the energy stored in the transformer (T) when the first switch element (S1) is off. -Is the secondary winding (ns) and rectifying diode-
Flyback DC for converting the DC output voltage through a switch (D0) and a smoothing capacitor (C0) and changing the ON period or OFF period of the first switch element (S1) to control the DC output voltage. -In the DC converter, the second capacitor (C) is connected to the first switch element (S1).
3) and the second switch element (S2) and the second switch element (S2) and the parallel circuit of the diode (D2) anti-parallel to the second switch element (S2) and the series circuit composed of the first capacitor (C2) The flyback (2) type DC-DC converter, wherein the second switch element (S2) is turned on while the winding voltage of the transformer (T) is negative. 3. A claim in which the second capacitor (C2) is the parasitic capacitance of the second switch element (S2) and the anti-parallel diode (D2) of the second switch element (S2). Flyback type DC-according to the first and second aspects
DC converter. 4. The target of the first switch element (S1).
The turn-on timing is the first switch element (S1).
The flyback type DC-DC converter according to claim 1 or 2, wherein the terminal voltage of the switch is switched to zero volt or at a time closest to zero volt.
Ta. 5. The flyback type DC-DC converter according to claim 1 or 2, wherein the current of the transformer (T) is discontinuous.