UC3525A: When I used UC3525 as a push-pull converter and adopted full bridge rectification output, I encountered some problems。

Iiujun,

I'm not sure what is the purpose of the parallel resonant circuit formed between (D2+C64)//L3? Consider to remover D2+C63. Also for the two HV output capacitors stacked in series, consider to add balancing resistors which means high value resistor in parallel with C63 and another of the same value in parallel with C58. Check dead-time directly at OUTA, OUTB from the UC3525 and compare to the dead-time observed at VGS for each corresponding MOSFET in bridge. Measure the leakage inductance of the transformer, when out of the circuit, and make sure it is reasonable (rule if thumb is less than 1% Lmag).

For the compensation, measure the open loop response of the converter with a network analyzer and compensate accordingly. For VMC you should be considering a Type 3 comp network. For help to model the control and verify your push-pull power stage I recommend to download the free TI Power Stage Designer and read the accompanying application notes.

Regards,

Steve

Hi,

D22 oscillation is due to the transformer leakage inductance and parasitic capacitance. D2 can also join the D22 oscillation so if adding some resistance in series with D2 can help attenuate the oscillation. But I am not understanding why you need D2 and C64 if L3 is for output capacitor. I think D2 and C64 should be removed. You can add R-C in parallel with D22 to reduce its ringing. Your layout should be improved to help if you can shorten the parasitic capacitance and shorted the length from the transformer to D22.

The secondary rectifier snubber can be done on each diode or on an arm, i.e., on the left of L3 to GND.

The snubber on the primary side to attenuate the Vds ringing of the MOSFETs such as Q10 can also use R-C. You can also try to add a diode from transformer 3 (anode) to 4, and 5 (anode) to 4 to see if the ringing can be reduced.

The first critical step is to reduce your transformer leakage, and also improve your power stage loop area, make thicker traces to help reduce the ringings..

Hi,

D2 and C64 will introduce high frequency noise to the output as C64 provide low impedance to high frequency. So it is not good to use D2 and C64 for the rectifier snubber.

Hi,

Typical applications are when each push-pull MOSFET duty cycle setup at 50%, and fixed input voltage and fixed output voltage. The output capacitors are mainly depending on the dead time during which how much Vout dip your application can tolerate based on your load condition since during the dead time the capacitors will keep providing energy to the load so discharge the capacitors then causing dip. So you need to know what dip is acceptable then estimate the energy to the load to decide the capacitors.