# Relation between PFC and Load power demand in SMPS (flyback) converter

#### polashd

##### Member
I tried to understand PFC in an SMPS voltage converter. I read from many websites but still confused very much about the following things.

Situation: it’s an SMPS flyback (for example) converter, to output (for example) 24v dc to an inductive load of 100W (average). Input 230v AC mains 50Hz.

AC mains............ rectifier ...................PFC .......................flyback primary ....................flyback secondary

Confusions:
1) When output demand (voltage, current, power) is constant how input current (ac) be in phase with input voltage. Don’t we need to draw more current when input voltage is low and vice versa (as P=V x I)? Otherwise we would need very big storage (Inductor or capacitor) to provide same power when AC input is low.

2) How PFC current draw is determined. Does it vary depending on output power demand (which may be constant or change at any point of input AC (of rectified DC) sin wave)?

3) When primary circuit of the flyback continuously adjust (PWM) according to the feedback from secondary – does PFC operation have anything to do with that.

4) In some websites PFC has been described as a boost converter the role of which is to provide a constant voltage to the main flyback input (which should be , usually, couple of voltage more than maximum ac input). If so, then the same question comes again (relation of P, V & I).

I’m not an expert (just a hobbyist). I apologize for my ignorance, if I’ve asked something stupid.

#### rjenkinsgb

##### Well-Known Member
It's not something I've even needed to be involved with, at anything beyond casual interest, as we do not design any direct mains connected equipment.
Other people on here will likely have more detailed info.

But:
Don’t we need to draw more current when input voltage is low and vice versa (as P=V x I)? Otherwise we would need very big storage (Inductor or capacitor) to provide same power when AC input is low.
Yes - but: Firstly, think about a basic non-PFC supply - just a rectifier and capacitor input.

That will only take current for a relatively small part of each AC half cycle, when the rectifier output voltage exceeds the reservoir capacitor voltage; a short "recharge" several times higher than the load output current.

The capacitor has to provide the load for the rest of the half cycle.

If the PFC stage can provide current for a larger percentage of the cycle, the capacitor is needed for less of each half cycle so can presumably be smaller.

Also, the input current to the capacitor will be lower as it is available longer.

I'm not sure on the feedback part of things; from bits of info I remember, I believe there are both independent PFC stages and combined systems in use in different designs.

4) In some websites PFC has been described as a boost converter the role of which is to provide a constant voltage to the main flyback input (which should be , usually, couple of voltage more than maximum ac input). If so, then the same question comes again (relation of P, V & I).

That sounds about right, in concept.

#### ronsimpson

##### Well-Known Member
1) When output demand (voltage, current, power) is constant how input current (ac) be in phase with input voltage. Don’t we need to draw more current when input voltage is low and vice versa (as P=V x I)? Otherwise we would need very big storage (Inductor or capacitor) to provide same power when AC input is low.
"Don’t we need to draw more current when input voltage is low"
If the input voltage is low because the power line voltage is low for an hour, yes we need more power.
If you are talking about "input low during the 50/60hz cycle", there are times when the power line is at zero volts and the PFC pulls zero power for that very short time. There are storage capacitors to cover that time.

2) How PFC current draw is determined. Does it vary depending on output power demand (which may be constant or change at any point of input AC (of rectified DC) sin wave)?
PFC current is a sine wave, as seen from the power line side. A PFC reacts slowly. If the load changed 2X it will take about 10 power line cycles to get the current to 2X.

3) When primary circuit of the flyback continuously adjust (PWM) according to the feedback from secondary – does PFC operation have anything to do with that.
The PWM that makes your 24V reacts fast. Its job is to keep the 24V stable.

4) In some websites PFC has been described as a boost converter the role of which is to provide a constant voltage to the main flyback input (which should be , usually, couple of voltage more than maximum ac input). If so, then the same question comes again (relation of P, V & I).
My last power supply: Input is 85 to 250 vac, 50 to 60hz, or it will work on DC. A boost PFC circuit makes 400VDC. The 400V has ripple and is not well regulated. It main job is to take sine wave current from the power line.
Next is the 400V to 24V power supply. It has an easy life because it does not see the wide input voltage range. It works in the 350 to 425V range.

Hope this helps
ron simpson

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