Electronic – Resonant Choke Filter Revisited

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The Resonant Choke filter in its traditional form looks like an orthodox choke input filter but it has a cap across the choke tuning it to twice the power frequency. This means 100 or 120 Hz depending on what part of the world. Intuitively you can get lower output ripple for a given output cap value compared to the normal choke input filter. In fact old audiophiles would sometimes "tune the choke" to reduce hum. Quite simply the choke and resonant cap form a high impedence parallel tuned circuit that blocks the 2F ripple. The higher order terms of the full wave rectified sine wave are of much lower amplitude and are dealt with by the output cap. An interesting aspect of this postwar circuit is that it has the potential for a good power factor but the old choke input filter can have a power factor that beats the cap input filter anyway. Could the choke in a resonant choke be smaller than the choke in a traditional choke input filter because less inductance is needed ? What would be a sensible L/C ratio? Would the power factor compete with the valley fill? When this was originally done loads tended to have less variation. How would the resonant choke perform under changing loading?Is this circuit easy on the diode recovery making it a contender in a sine wave SMPS? How would it perform if it was slightly off resonance ? Would the component tolerances be reasonably broad for easy production ? Would the low THD input current waveform be useful for a quiet SMPS ?

Best Answer

I just simulated a simple parallel inductor capacitor circuit. A 4H choke (common size for a guitar amp) requires a 440n cap to get a notch at exactly 120Hz. But with a 470n cap for example, the notch would be at 116Hz and so you would only get about 12dB attenuation (as opposed to more like 40dB if it really is tuned). Load has no effect on the resonance of the circuit in the simulation. I don't think it would work with SMPS because those chokes are usually pretty small. For example, a 100uH choke and 101.3n cap would resonate at 50kHz. But because the choke is so small, the notch is very narrow. I had to use 10ths of nano-farads to really get the 50kHz.