• TI Thinks Resolved

# Flyback Quasi Resonant Converter Switching & Transformer Selection

Hi all,

As I know, the major difference between normal flyback converter & flyback quasi resonant converter is for flyback quasi resonant converter, the switching frequency is variable. This means that the switching frequency is based on the output load. When the load decrease, the switching frequency will increase, and when the load is increase, the switching frequency will decrease.

My question as below:

1. Since the switching frequency is always changing, how do I select the switching frequency as my initial value to start the transformer design?

2. Since the switching frequency is always changing, how do I know that my transformer is capable for that switching frequency range. Let say I choose a transformer A with value of 300uH @ 55kHz, and my switching frequency for quasi resonant converter is range from 35kHz to 130kHz. Will this transformer A is capable for that frequency range? As I know, as the frequency increase, the inductance will be smaller & as the frequency decrease, the inductance will be bigger. Is it means that at lower frequency, this transformer A (300uH,55kHz) will change its inductance value at lower frequency(suddenly will increase inductance) & high frequency(suddenly will decrease inductance)? Or this transformer A will have fix inductance value of 300uH at all frequency?

Thanks

• Lutz

For a QR flyback like UCC28600 or UCC28704, the switching frequency range is set by the magnetizing inductance of the transformer.  Typically the maximum frequency is at full power minimum input, so an inductance is selected to limit the maximum switching frequency.  The minimum frequency is set by the operating range of the power supply (input voltage range, minimum power level).

If you would like to better understand how a QR flyback operates, I recommend that you review the following Power Supply Design Seminar paper https://www.ti.com/seclit/ml/slup302/slup302.pdf

Best Regards,

Eric