PFC of high power battery chargers

"Jamie Morken"

** Silly question.

....... Phil

I guess that it must be active power factor correction, using a boost converter,

To get a power factor of 1, I think the boost circuit has to run at constant dutycycle, and then change the constant dutycycle pulse width depending on the load to maintain the same output voltage, but what is the most efficient voltage to have the boost converter output from a

240VAC input?

cheers, Jamie

** Eeeee-yep.

** The PWM chopper's duty cycle TRACKS the instantaneous AC voltage - so the current drawn is in proportion to it.

Mimics Mr Ohm's famous law.

** Does that too.

** Just a little over the peak AC voltage or 340 volts.

It is a "boost" converter after all.

....... Phil

For a wide input voltage supply, ie. 120VAC to 240VAC, would the "optimal" output of the boost converter still be 340VDC?

cheers, Jamie

** Silly question.

...... Phil

For designing the transformer primary of a battery charger it is useful to know what the DC bus voltage will be out of the PFC output section. Using 340Volts as the transformer primary, and 100V as the secondary, to charge a 48V battery pack through an LC filter would work well at 240VAC input but I'm not sure how well it would work at 120VAC input, thus maybe it would be better to use a lower transformer primary voltage as a compromise I was thinking.

cheers, Jamie

"Jamie Morken"

** Huh ????????????

All far too silly for me.

.... Phil

it could also be a PFC flyback. but straight boost PFC is more common.

some do, some dont. most PFC chips (eg UC3854) have a multiplier, forcing the peak current to be proportional to the full-wave rectified input voltage (many actually normalise this voltage, using A*B/C).

Others use constant on-time control, which only really works for DCM boost & flyback PFCs. when you crank thru the maths of a conventional PFC controller, you see that the actual effect of the normalised multiplier is to give a constant on-time.

in either case the error-amp comes thru and scales the setpoint to provide output voltage control.

for universal input one generally considers 80-265Vac, and sets Vout =

380V - 400Vdc

380Vdc, but yes it has to be, as the PFC doesnt know whether it will be operating from 120Vac or 230Vac. one could add circuitry to scale Vout as a function of Vac - the net result would be higher duty cycle and lower stress on the PFC switch - but I havent seen it done. it also takes more components.

Cheers Terry

The constant duty cycle discontinuous mode only nears unity power factor at critical conduction, which requires a variable operating frequency. There are fairly simple dedicated controllers that can do this by detecting the zero current condition in the flyback period. This isn't something that a fixed frequency controller or a PIC would be inherently good at.

The error introduced with fixed frequency might not be large enough to prevent it's use over a fixed voltage range or unvarying load conditons.

There are also correction factors that can be introduced in digital control to minimize the error, while maintaining a fixed frequency.

The ripple in critical conduction oor discontinuous mode has implications on the size and cost of input filtering, and semiconductor sizing, as well.

RL

Hi Rob,

indeed. but for low powers, where the nasty current waveform isnt too much of a problem, DCM can be cheaper, even with the requisite filtering.

there are also some controllers which maintain critical conduction, thus giving variable frequency operation.

having a brain in a smps is indeed a wonderful thing. I seriously toyed with an MSP430 in this particular application, but I have a tight time frame, and pretty severe budgetary constraints.

Cheers Terry