# PFC output overvoltage limit too close to nominal output voltage level?

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#### treez

##### Guest
Hello,
We are designing a 3kw PFC stage (part of a battery charger) which must have its vout variable so that it can feed the downstream LLC converter with that voltage which allows the LLC stage to operate at its resonant frequency.

This means we would like to be able to set the PFC output voltage all the way up to 435V. however, the PFC output caps are 450V rated so we would have to set the PFC overvoltage limit at 445V.
But 445V is far too close to the PFC’s nominal output voltage of 435V. PFC’s are naturally “slow” and wandering as they attempt to get into regulation, and if the PFC was set up for 435VDC output nominal , then the PFC would in fact never get properly into regulation, -instead it would just keep “bouncing” up and down from the 445V overvoltage limit.

Therefore, with an overvoltage limit of 445VDC, the highest that we would be able to set the nomnal PFC output voltage to would be 405VC.

We could possibly go a little higher than 405VDC , but then that places more constraints on the setting of the feedback loop, ie it has to be faster if the nominal output voltage is too near to the overvoltage limit.

So do you agree, that if the PFC overvoltage limit is set to 445V, then it is very unadviseable to have the nominal PFC output voltage set to 435VDC?

By the way in the above I am assuming enough output capacitance to ensure no more than 12V pk-to-pk output voltage ripple.

#### Velkarn

##### Full Member level 3
yes, i think limit too close to nominal
maybe make capacitor bank for 500V, connecting two 250v caps in series (with resistors to align voltages)

treez

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#### treez

##### Guest
thanks, what about setting PFC to 425VDC nominal, with a 445VDC overvoltage clamp? Is that too close?
(we'll look into the series caps for 500v, thanks)

#### Velkarn

##### Full Member level 3
i do not have experience in design high power pfc, but i just measured transient responce of my 100W PFC. pfc supplying LLCconverter and i apply load step at its output.

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you see more than 20v overshoot, so all depends of what load steps is possible in your system and what is this system transient responce

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the transients at 400V nominal output pfc

treez

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#### treez

##### Guest
Thanks,
I was wondering if your PFC hit its overvoltage protect limit during the transient?..Is your overvoltage clamp limit set to more than 20V above the nominal regulation value?

From your kindly supplied oscillograms, the PFC voltage goes back into regulation smartly after the initial disturbance, suggesting that it would not have mattered even if it had hit the overvoltage limit. The worry is that if the overvoltage clamp limit is set to close to the nominal vout, then the PFC will simply just keep “bouncing” its vout up and down off the overvoltage limit level, even when the load is steady (ie, after transient activity has ceased).

As you say, one can simply increase the bandwidth, within reason. Our load is constant (battery charger), but as you know, it has to get started up.

#### mtwieg

Depends on the dynamics of your control loop and the behavior of the overvoltage monitor. If you voltage control loop is critically damped, then it may not be an issue on power up or recovery from a fault, but a load or line transient would be more likely to cause the overvoltage to trip once.

Personally I would always overrate the cap voltage by at least 10-20%, but I don't know what your MTBF requirement is.

treez

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#### treez

##### Guest
thanks, yes, though it will be a hassle to get electrolytics of greater than 450v.....Velkarn said to use 250v ones in series to get 500v...but the investors are already moaning enough about the component cost and it will be difficult to get the extra room for higher voltage/series caps.

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A competitior product has the dual pfc under total microcontroller control…and they manage to get it doing 3kw with the output voltage at 435v and the voltage never goes above 445v.
There must be some special software algorithm that they have used for this. They only have 810uf of output capacitance on the pfc output.

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