Other Parts Discussed in Thread: UCC29910, UCC28180
Dear support,
after some tests with the evaluation kit and the first design, which worked in generall, i have some questions and hopefully you can help me with this.
First of all, the initial project data:
- Vin: 190 Vrms up to 530V rms
- Vout: 150V
- Pout: 70W (load variation during operation)
- The design helper (sluc265a) was used
Major design changes du to the evaluation board:
- V_bulk measurment:
due to the high Voltage, the PNP transistor on its own is not an option. We are going with a differential amplifier and the second idea is a Voltage reduction (resistive or Emitter follower) in combination with a PNP transistor, like the referenced design.
Issues:
- V_bulk measuring with a differential amplifier .
I found some issues in the output stability and the PFC-Current at the AC-Voltage of 260-275VAC and 360-380VAC (Load: 20-60W). To me it looks like the „light load / Burst mode“ because some pulses are missing and than the device starting with the typical burst mode puls procedure (only with lower PFC inductance <= 300µH). Between 190-260VAC / 275- 360VAC and above 380V with 20W load it runs without any issue.
Is it possible that something in the logic, maybe in terms of measurment values will be switched inside the UCC29910A? Or is there any other explanation for this behavior?
It would be very helpful to me, if you provide some information about the needed accuracy and values for the Vbulk input (what kind of ripple is recommended etc.). Because in the simulation is no difference between the evaluation kit and our design, but if i use our design in the evaluation board, at some AC values the PCF-Buck can't hold the output (there is an Bulk-overload by 2-3V) and it starts using the "light load / burst mode" even with a 14W load. (Your reference design gets in "light load / burst mode" with a load lower than 4W). On the other operation points the output voltage is stable with different loads.
- Reducing the PFC inductor value
For the first device we used a smaller input range, wich allows us to use a higher PFC inductance. We haven’t seen any issue with this design, all issues startet with lowering the inductence for the higher input range.
@ 500µH
If we go with the higher inductence, the new design has no issuses. Of course we can‘t reach the 70W at lower input voltages.
@ 300µH
With 300µH we saw some issus at several AC-Voltages (see description in VBulk measureing)
@ smaller than 250µH
The PFC won‘t run stable
I saw that there is a saturation current in the PFC Choke at the reference board, we try to avoid that in the design. Is there a reason for the saturation current or is it needed?
3. UCC29910A sets the PFCDRV Pin to always high.
We found a operation Point where the UCC29910A sets the PFCDRV to always high until the bias Voltage drops.
We replaced the UCC29910A device several times but we ended up in this point again.(separated / external 10V bias)
If the fault gets "activated" and set to low, the UCC29910A will not react and the PFCDRV stays on high.
I couln'd find any description for that behavior in the documentation, can you provide some information about this issue and the conditions when this can take place?
If it is possible, i prefer a phone call or a E-Mail contact to exchange more details if needed.
Thank you for your effort in advance and best regards,
John Kock
