Transient response of boost PFC?

[grizedale]

Hello,

may i just confirm that a transition mode boost PFC stage would have a faster transient response than a continuous mode boost PFC stage.?

...i do appreciate that talking about transient response with a PFC stage is a little off target, but i do believe that a transition mode boost would respond quicker to transients....

...or rather , it would be easier to set up a transition mode boost pfc with faster transient repsonse than a ccm boost pfc?

 

[mtwieg]

Are you talking about faster current loop response or voltage loop response?

Transient response on the output is essentially related to the distortion seen on the input current. Higher voltage bandwidth will always result in more distortion and lower PF. Normally you set the bandwidth far below that of what an equivalent non-PFC converter would be. So voltage bandwidth is practically always limited by your desired input THD or PF. The limitation is never in the topology of the converter itself.

In the case of the current control loop, the CCM should be a bit faster, since its high frequency ripple is less, thus requiring less filtering of the current signal and allowing a higher crossover frequency.

 

[grizedale]

hi

i am speaking of the output voltage...so the voltage loop.

a ccm boost has a much bigger inductor than an equivalent power Transition mode boost, so surely the TM boost will be quicker at responding to load transients.......and at the same time giving a good-enough power factor(?)

 

[mtwieg]

a ccm boost has a much bigger inductor than an equivalent power Transition mode boost, so surely the TM boost will be quicker at responding to load transients.......and at the same time giving a good-enough power factor(?)

The size of the inductor only matters for the current control loop. To the voltage control loop, within its bandwidth, the inductor will appear as a current source due to the action of the current loop. And from the input's perspective, the inductor will look like a resistor. That has nothing to do with inductor sizing.

Changing the inductor would only change the way the current control loop is compensated. But so long as its bandwidth is high enough that it is able to track the multiplier output, the inductance will not affect the voltage control loop.

 

[Orson Cart]

One has to limit the speed of the voltage loop to keep the input THD down, a fast voltage loop will give a distorted input waveshape

Quite often non-linear add ons are used to speed up the booster when a sudden load is applied to the psu, and an overvolt ckt is used to stop the converter when a load is suddenly taken away to stop the volts going destructively high - Regards, Orson Cart