JPH0318430B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention controls on/off switching elements connected in series to the primary winding of a transformer to generate direct current.
This relates to a device that converts direct current (DC-DC converter).

[Prior art] A series circuit of a transformer primary winding and a transistor is connected to a DC power supply, an output rectifying and smoothing circuit is connected to the transformer secondary winding, and a pulse width modulation (PWM) circuit that responds to the output voltage is installed. The method of controlling the transistors on and off using the output of this pulse width modulation circuit is well known.

[Problems to be Solved by the Invention] When a transformer is connected to a DC circuit, it must be used so as not to cause magnetic saturation of the transformer. Therefore, the residual magnetism of the transformer is reset by a snubber circuit. If an ON control signal is sent to the switching transistor before resetting, magnetic saturation of the transformer will occur and an overcurrent will flow through the switching transistor. Therefore,
Generally, the off period is set to be long enough so that an on control signal is sent to the switching transistor after the transformer is reset. With this configuration, magnetic saturation of the transformer does not occur during steady operation, but the duty ratio decreases and the power capacity decreases.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a DC converter that can prevent magnetic saturation of a transformer and widen the on-pulse width.

[Means for Solving the Problems] In order to solve the above problems and achieve the above objects, the present invention is directed to a series circuit of a winding and a switching element; a secondary winding of a transformer electromagnetically coupled to the primary winding;
an output rectifying and smoothing circuit connected to the next winding; an output voltage detecting circuit that detects the output voltage of the output rectifying and smoothing circuit; a reference voltage source; and a voltage detected by the output voltage detecting circuit and the reference voltage source. A differential amplifier that forms a voltage corresponding to the difference from a reference voltage, a triangular wave generating circuit that generates a triangular wave, and a pulse width modulated wave that is generated by comparing the triangular wave generated from the triangular wave generating circuit and the output of the differential amplifier. and a voltage comparator that controls on/off of the switching element by detecting a flyback voltage generated in the transformer during an off period of the switching element or a voltage corresponding thereto. a flyback voltage detection circuit that generates a signal indicative of a flyback voltage generation period, and prohibiting generation of the triangular wave during the flyback voltage generation period in response to a signal indicative of the flyback voltage generation period obtained from the flyback voltage detection circuit; The present invention relates to a DC converter, further comprising a delay control circuit that controls the triangular wave generation circuit so as to generate the triangular wave in synchronization with the end of the flyback voltage generation period.

[Function] In the above invention, the triangular wave generation circuit does not generate a triangular wave at a constant period, but generates a triangular wave under the control of the delay control circuit. Since the delay control circuit triggers the generation of the triangular wave in synchronization with the end of the flyback voltage generation period, it is possible to completely prevent the switching element from turning on before the transformer is reset.

[Example] Next, a DC converter according to an example of the present invention shown in FIG. 1 will be described. 1 and 2 in FIG. 1 are one and the other DC power supply terminals. A series circuit including a primary winding 4 of a transformer 3 and a transistor 5 as a switching element is connected between the pair of power supply terminals 1 and 2. An output rectifying and smoothing circuit 11 consisting of diodes 7 and 8, a reactor 9, and a capacitor 10 is connected to the secondary winding 6 of the transformer 3. Therefore, by controlling the transistor 5 on and off, a stabilized DC output voltage can be obtained between the output terminals 12 and 13 based on the DC power supply voltage E _S between the power supply terminals 1 and 2.

14 is a voltage control circuit, and output terminals 12, 1
Voltage detection voltage dividing resistors 15 and 16 connected between 3
, one input terminal is connected to the reference voltage source 17 and the voltage dividing point of the voltage dividing resistors 15 and 16, and the reference voltage source 1
A differential amplifier 18 whose other input terminal is connected to 7
, triangular wave generating circuit 19 , and triangular wave generating circuit 19
a voltage comparator 20 having one input terminal connected to the differential amplifier 18 and the other input terminal connected to the differential amplifier 18;
It consists of a drive amplifier 21 connected to a comparator 20 and a delay control circuit 32. This control circuit 14 receives an error output from a differential amplifier 18 and a triangular wave generation circuit 19.
A pulse width modulation (PWM) wave is formed based on the comparison with the triangular wave voltage obtained from the pulse width modulation (PWM) wave, thereby controlling the transistor 5 on and off.

Diode 28, resistor 29, and capacitor 3
0 constitutes a snubber circuit, a resistor 29 is connected in parallel to the primary winding 4 via a reverse current blocking diode 28, and a capacitor 30 as a voltage suppressing element is connected in parallel to the resistor 29. ing.

A flyback voltage detection circuit 31 connected to the secondary winding 6 detects the flyback voltage generated in the transformer 3 during the off period of the transistor 5, and
It is sent to the delay control circuit 32.

[Operation] The operation of the circuit shown in FIG. 1 will be explained with reference to the waveforms shown in FIG. 2. As shown in FIG. 2D, when the base current I _B is passed from the control circuit 14 to the transistor 5 during the period t2 to t3, the transistor 5 is turned on, and as shown in FIG.
As shown in FIG. 2, the collector-emitter voltage of the transistor 5 becomes low, and almost all of the power supply voltage is applied to the primary winding 4. As a result, Figure 2B
A positive voltage is obtained in the secondary winding 6 as shown in FIG.

When transistor 5 is controlled from on to off at time _t3 , the flyback voltage (return pulse voltage)
occurs. Therefore, the collector-emitter voltage of the transistor 5 becomes higher than the power supply voltage E _S as shown in FIG. 2C. The flyback voltage of the primary winding 4 is applied to the resistor 29 through the diode 28 and acts on the primary winding 4 as a residual magnetic reset voltage. Note that the capacitor 30 suppresses the flyback voltage from increasing abnormally. At time t4, when the residual magnetism in the transformer 3 disappears and the transformer 3 enters the magnetically reset state, the flyback voltage also disappears, and each winding 4, 6 of the transformer 3
voltage becomes zero. Therefore, the voltage between the collector and emitter of the transistor 5 becomes the power supply voltage _ES .

Next, voltage control using a triangular wave and control of the triangular wave by the delay control circuit 32 will be explained. t1 in Figure 2
A triangular wave voltage V _S is generated at ~t3, and the differential amplifier 18
The comparator 20 compares the error voltage V _R obtained from
When the base current I _B shown in FIG. 4D is supplied to the base of the transistor 5, the transistor 5 is turned on and turned off at t3, and a flyback voltage is generated between t3 and t4 as shown in FIG. 2B. This voltage is detected by a voltage detection circuit 31 and supplied to a delay control circuit 32. The delay control circuit 32 delays the time point at which the ramp voltage of the triangular wave voltage V _S starts to be generated until the time point t4 when the flyback voltage disappears. That is, the time point at which charging of the capacitor built in the triangular wave generating circuit 19 starts coincides with the time point at which the flyback voltage disappears. This type of control is achieved by configuring the delay control circuit 32 to be substantially identical to the holdoff and sweep gate circuits of an oscilloscope.

As shown in the period t5 to t6, as the error output voltage V _R becomes lower and the on period becomes longer, the flyback voltage generation period t6 to t7 changes. However, the next triangular wave voltage does not occur before the flyback voltage disappears. Therefore, saturation of the transformer 3 is prevented.

[Modifications] The present invention is not limited to the above-mentioned embodiments, and for example, the following modifications are possible.

(a) The transistor 5 in FIG. 1 may be a field effect transistor. Further, the transistor 5 may be replaced with a circuit in which a plurality of transistors are connected in series and/or in parallel.

(b) It is also applicable when connecting a varistor instead of the capacitor 30.

(c) The voltage corresponding to the flyback voltage may be detected based on the voltage across the transistor 5 or the voltage across the resistor 29.

[Effects of the Invention] As is clear from the above, according to the present invention, it is possible to easily prevent the switching element from being turned on during the flyback voltage generation period.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a DC converter according to an embodiment of the present invention, and FIG. 2 is a waveform diagram of each part of FIG. 1. 1, 2...Power terminal, 3...Transformer, 4...
Primary winding, 5...Transistor, 6...Secondary winding, 11...Output rectification and smoothing circuit, 14...Control circuit, 19...Triangular wave generation circuit, 32...Delay control circuit.

Claims (1)

[Claims] 1. A series circuit of a primary winding of a transformer and a switching element connected between one DC power terminal and the other DC power terminal, and a series circuit of a switching element, which is electromagnetically coupled to the primary winding. a secondary winding of a transformer; an output rectifying and smoothing circuit connected to the secondary winding; an output voltage detecting circuit that detects the output voltage of the output rectifying and smoothing circuit; a reference voltage source; and the output voltage detecting circuit. a differential amplifier that forms a voltage corresponding to the difference between the voltage detected by the reference voltage source and the reference voltage of the reference voltage source; a triangular wave generating circuit that generates a triangular wave; and a triangular wave generated from the triangular wave generating circuit and the differential amplifier. In the DC-DC converter, the DC-DC converter includes a voltage comparator that compares the voltage with the output to form a pulse width modulated wave to control on/off of the switching element, wherein flyback generated in the transformer during the off period of the switching element a flyback voltage detection circuit that detects a voltage or a voltage corresponding thereto and generates a signal indicating a flyback voltage generation period; and a flyback voltage detection circuit that detects a voltage or a voltage corresponding thereto and generates a signal indicating a flyback voltage generation period; A delay control circuit is provided for controlling the triangular wave generation circuit to prohibit generation of the triangular wave during the voltage generation period and to generate the triangular wave in synchronization with the end of the flyback voltage generation period. DC converter.