A switch mode power supply includes a flyback power converter and a control circuit. The flyback power converter includes an input, an output, a transformer coupled between the input and the output, a power switch coupled between the input and the transformer, and a clamp circuit coupled between the input and the transformer. The clamp circuit includes a capacitor and a clamp switch coupled in series with the capacitor. The control circuit is configured to control the power switch and the clamp switch. The switch mode power supply further includes at least one additional capacitor coupled in parallel with the capacitor of the clamp circuit to facilitate selection of a combination of capacitors to adjust a resonant frequency of the clamp switch for optimizing efficiency of the power supply. Other examples switch mode power supplies and/or methods for adjusting a resonant frequency of flyback power converters are also disclosed.
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