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Boost PFC controller with fast transient load capability?


Advanced Member level 5
Jun 13, 2021
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We seek a Boost PFC controller which bypass's its normal slow voltage feedback loop and has a very fast feedback loop whenever vout falls below
a certain settable threshold.
This is not a digikey searchable feature unfortunately.

The UCC28070 has it , but only in mild , partial form...(a mere 100uA current source charges up the error amplifier feedback caps when the vout falls below x volts.)

We want one that , as soon as vout falls below the (settable) threshold voltage, then it suddenly cracks into its maximum possible power delivery mode, and restores the vout
rapidly into the range above the threshold.

Do you know of such a controller?
Install a brownout capacitor close to the output stage. Arrange a switching circuit so it charges (from the output stage) to your desired threshold voltage while everything is running normally. Prevent the cap from discharging and allow it to hold its voltage in reserve.

Then as soon as you detect a voltage droop, switch the cap over instantly so it substitutes for the output stage. This gives time for the control circuit to resume production.

A desired reference voltage is not readily available in the boost converter. The control circuit needs to build its voltage, then self-regulate the output. It's difficult to make a feedback circuit which has high gain to correct its voltage, yet never overshoots the target value. The blackout capacitor always has the target value at the ready.
Usually this is implemented as a separate analog circuit - that puts a defined short signal into the main control loop - to reduce volt sag under sudden load application,

we were designing such " speed up " ckts way back in 1992 to go with the common UPF IC's of the time, e.g. UC3854, UC3854A, and a myriad of others.

also useful to perform the complement - that is, reduce pwm to zero as the Vout approaches a maximum tolerable on the main bus caps.
Usually this is implemented as a separate analog circuit
Thanks, i see what you mean, yes, the offtheshelf ones only have a weak speed up , if at all, by the looks of things.

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