RCD reset forward converter circuit diagram

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Figure (a) shows the RCD reset forward converter, that is, a diode D, capacitor C, and resistor R are connected in parallel on the transformer. The induced potential of the magnetizing inductance and leakage inductance when the switch S is turned off. Diode D is turned on and the transformer is reset by the voltage across capacitor C. Figure (b) shows the main operating waveform of the RCD reset forward converter. The voltage across capacitor C is approximately DC voltage during one switching cycle, and the RCD reset voltage is a square wave. Similarly, when the switch is turned off, there is a voltage spike on the transformer, which is caused by the leakage inductance of the transformer and the resonance of the junction junction capacitance.

(a) RCD reset forward converter

(b) RCD reset forward converter operating waveform

Both resonant reset and RCD reset have their own advantages and disadvantages, and the advantages and disadvantages of the two reset methods are basically complementary.
1) According to the volt-second balance principle, the average value of VT in one cycle should be equal to zero. The reset voltage of the resonant reset is a sine wave, so the platform of the reset voltage is relatively high, that is, the VDS voltage platform of the switch S is relatively high, and the reset voltage of the RCD reset is a square wave, so the platform for resetting the voltage is relatively low, that is, The VDS voltage platform of the switch S is relatively low.
2) Resonant reset The voltage spike on the forward converter transformer (finally reflected to the voltage spike of the vDS) is caused by the transformer leakage inductance LS and capacitor C resonance. The voltage spike on the RCD reset forward converter transformer is leaked by the transformer. The sense LS is caused by the resonance of the junction capacitance of the switch S. Since the capacity of the capacitor C is much larger than the junction capacitance of the switching transistor S, the resonance period of the resonance reset voltage spike is much larger than the resonance period of the RCD reset voltage spike. Therefore, in the case where the load current energy of the transformer leakage inductance LS is constant, The voltage spike amplitude of the resonant reset is much lower than the voltage spike of the RCD reset. From another point of view, it can be considered that the capacitance C of the resonant reset forward converter connected in parallel between the switching tubes D-S functions to absorb voltage spikes.
3) The excitation energy and the leakage inductance energy of the RCD reset forward converter are all consumed on the resistor R, and the excitation energy and the leakage inductance energy of the resonance reset forward converter are substantially not consumed, as shown in Fig. (b). However, since the resonant reset forward converter has a voltage across the capacitor C before the switch is turned on, the energy consumption of the CVin2 is during the turn-on of the switch.
4) From the waveforms of (b) and (b) iLm, it can be seen that the magnetic bias of the transformer of the resonant reset forward converter is relatively small, while the magnetic bias of the transformer of the RCD reset forward converter is large.
The above analysis can be known that the forward converters of the two reset modes have their own advantages, but the disadvantages are also obvious, and in some cases, there are large bottlenecks in design. It is not difficult to combine the two reset methods to soften their respective shortcomings, but also bring a new advantage, that is, the resonant RCD reset forward converter.