T
treez
Guest
Hi
Why is it that PFC controller datasheets don’t tell you that they all have a facility to damp the LC filter ringing near the mains zero crossing?
The PFC controllers all do this because PFC controllers don’t draw much current near the zero cross, and so when the mains suddenly forward biases the bridge diodes just after the zero cross, current then suddenly flows in to the converter, and the LC filter starts ringing which puts unwanted harmonics into the mains input current.
Most PFC converters deal with this by switching the FET on for a long period at the zero cross. However, if the PFC’d converter is a PFC’d Buck, then this doesn’t work, and an external FET as in the attached example (M3) is needed, to provide the necessary damping.
(LTspice simulation and pdf schematic attached)
Why are there no off the shelf PFC’d Buck controllers which have a pin to switch on the said FET (M3)?
The attached waveforms show the PFC'd Buck (mains input current) with and without the damping of the FET M3.
Why is it that PFC controller datasheets don’t tell you that they all have a facility to damp the LC filter ringing near the mains zero crossing?
The PFC controllers all do this because PFC controllers don’t draw much current near the zero cross, and so when the mains suddenly forward biases the bridge diodes just after the zero cross, current then suddenly flows in to the converter, and the LC filter starts ringing which puts unwanted harmonics into the mains input current.
Most PFC converters deal with this by switching the FET on for a long period at the zero cross. However, if the PFC’d converter is a PFC’d Buck, then this doesn’t work, and an external FET as in the attached example (M3) is needed, to provide the necessary damping.
(LTspice simulation and pdf schematic attached)
Why are there no off the shelf PFC’d Buck controllers which have a pin to switch on the said FET (M3)?
The attached waveforms show the PFC'd Buck (mains input current) with and without the damping of the FET M3.