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Compensation Pin on UCC28950

Peter Baxter
Intellectual 390 points
Other Parts Discussed in Thread: UCC28950

UCC28950 Full-Bridge PWM Converter

Error Amplifier "Compensation" Pin (Output) (Pin-4)

Can you please describe the output stage of the error amplifier?

Is it an "open collector with light pull-up resistor" type output (which could be pulled to ground via the COMP pin)?

Or is it a typical opamp "push-pull" type output?

Can I digitally drive (74HCxx 5V TTL levels) the COMP pin (which internally feeds the PWM Comparator)?

Is there a prefered opto-isolator topology to use with this part for an isolated output (isolated from the UCC28950's ground).

I'm considering ways to add an additional 40V isolated output to the UCC28950 Evaluation Module circuit (The 12V circuitry could remain the same). I'd like to be able to give that 40V output better regulation than might typically occur with basic un-monitored windings.

Might you have application information that shows how to do this properly? I'm trying to avoid synchronising two UCC28950s together to do this - as that feature might be required for another aspect of the project. 

I'm thing of taking the UCC28950's secondary Synchronous Recitfier output OUTE and OUTF and then paralleling another gate drive IC's with ENABLE controls to drive Synchronous Recitfiers on the 40V winding (OUTG and OUTH). While I won't be extending the "ON" time, I can reduce/disable synchronous rectification should the 40V output exceed a set voltage. 

Considering that the opto-coupler's output in this situation is essentially an on/off digital signal, I'd like to be able to drive the UCC28950's COMP pin directly.

The Error Amplifier Type II or Type III circuit would be on the isolated output side.

Thanks   

  • 0
    Prodigy 155 points

    I have the same problem, regarding the COMP pin, I'd like to add an external average current loop to obtain a CV/CC type of behaviour before reaching the current limit, the only thing in the datasheet is the "maximum current" at the op amp output these values are -8 to 5.75mA.