JP4401010B2 - Synchronous rectification type forward converter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a synchronous rectification type forward converter, and more particularly to a synchronous rectification type forward converter suitable for parallel connection operation.
[0002]
[Prior art]
Conventionally, as a DC-DC converter using this type of synchronous rectifier circuit, as shown in FIG. 5, a DC voltage of a DC input power source is converted into a rectangular wave pulse voltage by a switching operation of the semiconductor switch 2, After this rectangular wave pulse voltage is converted to a desired voltage by the transformer 3, a rectifier circuit of bidirectional switch elements (synchronous rectification FET) 4 and (commutation FET) 5, a smoothing circuit by a choke coil 10 and a capacitor 11 Is rectified and smoothed to obtain the average voltage.
The switching operation of the semiconductor switch 2 is PWM-controlled based on the detection status by the voltage detection control circuit 12 that detects the output voltage of the synchronous rectification converter.
[0003]
In general, in the case of a synchronous rectifier circuit, there are a case where a synchronous operation is performed using the switch elements 4 and 5 as shown in FIG. 5 and a case where a synchronous operation is performed using only the switch element 4 using an FET. The former circuit in which improvement of conversion efficiency is emphasized, that is, a circuit using a semiconductor switch (FET) for both switch elements 4 and 5 is targeted. Therefore, the bidirectional switch element 4 is expressed as a synchronous rectification FET, and the element 5 is expressed as a commutation FET.
Since the commutation FET 5 cannot obtain a gate signal in the entire commutation period, a commutation diode 9 is usually added. Such synchronous rectification converters have a small capacity to a large capacity, and a synchronous rectification converter corresponding to the load capacity is selected and used.
[0004]
[Problems to be solved by the invention]
However, preparing a synchronous rectifier converter according to the load capacity means increasing the number of models and requiring inventory for each model. Therefore, it is desired to reduce the number of models, and to cope with a large capacity load by parallel connection operation of converter modules (CONV1 to CONV3) of the same model. This parallel operation has the following problems.
[0005]
That is, when such modules are operated in parallel, if there is a difference in the output voltage, a current flows from the module with the higher output voltage to the other lower module, and the gate of the switching element 2 in the module with the lower output voltage. The synchronous rectifier circuit on the output side starts self-oscillation despite the signal being narrowed down.
By starting self-oscillation, power is regenerated from the secondary winding side to the primary winding side, and when the output voltage difference increases, the regenerated power also increases, resulting in power loss and power supply damage. Also.
[0006]
Therefore, in the conventional circuit, a method of controlling the maximum power regenerated on the primary side without stopping the self-oscillation of the synchronous rectifier circuit has been performed. The method utilizes the fact that the self-oscillation frequency decreases when the regenerative power is large, and controls the oscillation frequency so as not to be lower than a certain frequency.
However, since self-oscillation is not stopped, there is a disadvantage that a certain amount of power is regenerated to the primary side, resulting in wasted power.
Therefore, the present invention is efficient by stopping the self-oscillation phenomenon of the synchronous rectification circuit that occurs when the synchronous rectification type switching power supply is operated in parallel and eliminating the power regenerated from the output side of the power supply to the input side. Propose a power supply system.
[0007]
[Means for solving the problems]
In order to solve the above-mentioned problem, the invention of claim 1 converts a DC input voltage into a rectangular wave pulse voltage by a switching element and applies it to a primary winding of an output transformer, and outputs an output on the secondary winding side of the output transformer. In a synchronous rectification type forward converter that outputs a DC voltage by rectifying and smoothing an output side synchronous rectification circuit composed of a synchronous rectification FET, a commutation FET, a choke coil, a capacitor, etc. An auxiliary winding is provided and a switch is provided in parallel with the auxiliary winding, and a self-oscillation detecting circuit of the output side synchronous rectifier circuit is provided, and the auxiliary winding is controlled by the detection signal of the self-oscillation detecting circuit. The wire is short-circuited or opened.
[0008]
In order to solve the above problems, the invention of claim 2 comprises a plurality of the synchronous rectification type forward converters of claim 1, wherein each converter has a common input terminal and output terminal in parallel. The synchronous rectification type forward converter is characterized in that it is connected.
[0009]
[Outline of Implementation]
FIG. 1 is a circuit diagram of an embodiment of the present invention, where 1 is an input capacitor, 2 is a switching element, 3 is an output transformer for power conversion, 4 is a bidirectional switching element for rectification (synchronous rectification FET), and 5 is regenerative. Bidirectional switching element (commutation FET), 6 is a driving capacitor of the switching element 4, 7 is a driving resistance of the switching element 4, 8 is a driving capacitor of the switching element 5, 9 is a driving resistance of the switching element 5, 10 is An output choke, 11 is a smoothing capacitor, 14 is a switch element, and 12 is an auxiliary winding (can also be used as an auxiliary power supply winding).
[0010]
In this circuit, when the modules operate in parallel, if the output voltage of the other party is higher than their own output voltage, power regeneration starts from the output side to the input side, and self oscillation occurs. When the winding voltage rises and regeneration starts, the self-oscillation is reduced by the control circuit 28 to reduce the duty of its own switching element 2, and the self-oscillation starts and the power regeneration increases. Then, utilizing the fact that the oscillation frequency is lowered, the self-oscillation detection circuit (29) turns on the switch (14) to short-circuit the auxiliary winding 12 of the power conversion transformer 3 and stop the self-oscillation. It is.
Since the current at the time of the short circuit is limited by the output choke 10, an excessive current does not flow, and the winding short circuit transistor (14) does not need to have a large current resistance.
[0011]
In FIG. 1, it is assumed that when a voltage higher than the output is applied to the output and the duty of the pulse by the control circuit 28 becomes zero, the regenerative bidirectional switching element 5 is ON. The regenerative bidirectional switching element 5 continues to be turned on while the built-in gate-source capacitor voltage is discharged and falls to the threshold voltage, and energy is stored in the output choke 10.
When the element 5 is turned off, the energy of the output choke 10 is released, the rectifying bidirectional switching element 4 is turned on, and the energy is transmitted to the primary side by the power conversion transformer 3 and transmitted to the primary side. The energy is then regenerated to the input through the built-in diode of the switching element 2.
[0012]
When the energy of the output choke 10 has been regenerated to the input side, a reverse kick voltage is generated from the power conversion transformer 3 and the regenerative bidirectional switching element 5 is turned on again. By repeating this, self-oscillation is continued.
In order to stop the self-oscillation in this way, it is only necessary to stop the operation of the power conversion transformer 3, and it is understood that the self-oscillation stops if the winding with the transformer continues to be short-circuited for one cycle or more.
[0013]
FIG. 2 shows another embodiment of the present invention. When the modules operate in parallel and the output voltage of the other party is higher than their own output voltage, the control circuit 28 operates to reduce their duty. When the duty becomes zero, the auxiliary winding 12 is short-circuited by turning on the switch (14) to prevent self-oscillation.
Briefly explaining the operation, the drive waveform of the switching element 2 is integrated by the resistor 18 and the capacitor 19, and when the duty becomes zero and the drive waveform disappears, the transistor 30 is turned off and the switch (14) is turned on to stop the self-oscillation. To do.
[0014]
FIG. 3 shows a third embodiment of the present invention. When power regeneration starts from the secondary side, the input winding and other winding voltages rise, so that the voltage exceeds a preset voltage value. When the switch (14) is turned on, the winding is short-circuited and self-oscillation is stopped.
Briefly explaining the operation, when the voltage of the auxiliary winding 12 becomes larger than a certain value by the IC 25, the IC 25 is turned on, the transistor 22 is also turned on, and the switch (14) is also turned on to short-circuit the auxiliary winding 12, and self It stops oscillation.
[0015]
FIG. 4 shows a fourth embodiment of the present invention, which utilizes the fact that the higher the output voltage of the other modules operated in parallel and the higher the regenerative power, the lower the oscillation frequency of self-oscillation. This change is read by the frequency monitoring circuit, and when it becomes lower than a certain value, the switch 14 is turned on to prevent self-oscillation. Briefly, in a time constant circuit composed of resistors 30, 33 and a capacitor 31 when self-oscillation continues and the frequency decreases, the voltage of the capacitor 31 increases when the frequency decreases, and the switch (14) is turned on. It is turned on to short-circuit the auxiliary winding 12 and stop self-oscillation.
[0016]
【The invention's effect】
As is apparent from the above description, according to the present invention, the synchronous rectification switching power supply can be connected in parallel to stop the self-oscillation of the synchronous rectifier circuit during parallel operation.
By the way, in the power supply of output 10w, in the conventional circuit, the regenerative power from the output side to the input side was about 8w. It becomes.
[Brief description of the drawings]
FIG. 1 Basic circuit of the present invention FIG. 2 Example of the present invention (gate signal monitoring type)
FIG. 3 shows an embodiment of the present invention (overvoltage monitoring type).
FIG. 4 shows an embodiment of the present invention (oscillation frequency monitoring type).
[Figure 5] Conventional circuit [Explanation of symbols]
1: Input capacitor 2: Switching element 3: Transformer for power conversion 4: Bidirectional switching element for rectification (synchronous rectification FET)
5: Bidirectional switching element for regeneration (commutation FET)
10: Output choke 11: Smoothing capacitor 12: Auxiliary winding (short-circuit winding)
14: Switch 28: Control circuit 29: Self oscillation detection circuit

Claims (5)

A DC input voltage is converted into a rectangular wave pulse voltage by a switching element and applied to the primary winding of the output transformer, and the output on the secondary winding side of the output transformer is converted into a synchronous rectification FET, a commutation FET, and a choke coil. In a synchronous rectification forward converter that outputs a DC voltage by rectifying and smoothing by an output side synchronous rectifier circuit constituted by a capacitor or the like, an auxiliary winding is provided in the output transformer and in parallel with the auxiliary winding A switch is provided, and a self-oscillation detection circuit of the output side synchronous rectification circuit is provided, and the switch is controlled by a detection signal of the self-oscillation detection circuit to short-circuit or open the auxiliary winding. Synchronous rectification type forward converter. 2. A synchronous rectification forward converter according to claim 1, comprising a plurality of synchronous rectification forward converters, wherein each converter is connected in parallel with a common input terminal and output terminal. -Data. 3. A synchronous rectifying forward converter according to claim 1, wherein the self-oscillation detecting circuit detects that the PWM signal for controlling the switching element is lost, and uses this as a self-oscillation detecting signal. 3. The synchronous rectification forward converter according to claim 1, wherein the self-oscillation detection circuit detects an increase in the winding voltage of the output transformer and uses this as a self-oscillation detection signal. 3. The synchronous rectification type forward converter according to claim 1, wherein the self-oscillation detecting circuit detects a self-oscillation frequency and uses it as a self-oscillation detection signal.

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