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[FAQ] PMP21479: What is the advantage of the active clamp flyback over a quasi-resonant (QR) flyback?

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Part Number: PMP21479

What is the advantage of the active clamp flyback over a quasi-resonant (QR) flyback?

  • There are two main differences between the active clamp flyback and QR flyback. The first is that the clamp capacitor in the active clamp flyback recycles the leakage energy of the transformer, whereas the dissipative clamp in the QR flyback burns the leakage energy as power loss. This results in lower efficiency in the QR flyback compared to the active clamp flyback. The leakage losses are also frequency dependent, so this can also limit the maximum switching frequency in a QR flyback. The second major difference is that the active clamp flyback operated in transition mode can provide zero voltage switching (ZVS) across all input conditions. The QR flyback provides near-ZVS at the minimum input condition, but suffers from higher switching loss at higher input voltages. Because of ZVS, the active clamp flyback can be operated at much higher switching frequencies than the QR flyback, resulting in a smaller design and higher power density. The UCC28780 controller provides all of the functionality to control active clamp flyback power supply in transition mode resulting in high power density designs like the PMP21479 65W adapter reference design.

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