PMP8740: LM5023 OVP triggered due to 14V @ Vcc FB

Hello Pooja,

May I ask whether you use the same transformer part number 7508111323?

I agree with you and it's pretty strange you get overvoltage on VCC_FB output (12Vout), also because you have 200mA load on it.

Can you please add a further load on the 12Vout, maybe extra 200mA...500mA? Of course, in reality this is not necessary, because this auxiliary PSU must work also at zero load, it's just to understand if maybe there is an issue in the feedback network (U5, R28, R29 and R35).

Thanks,

Roberto 

Hello Roberto, Thank you for responding,

Yes, we are using the same transformer 7508111323, but the optocoupler we used is a similar alternate 140817143400 from Wurth.

We also observed that for a load of upto 500mA (12V Fan), OVP seems to be triggered with VCC_FB going upto 14V and then gradually reducing.

On further increasing the load upto 800mA, the Vcc FB seems to be oscillating almost similar to the hiccup mode, assuming the overload condition being triggered.

Also before increasing the load, we changed the OVP from 14V to 16V of LM5023 (by increasing R26) to check regulation. But we observed the same response. The Vcc FB reaches peak 16V and OVP gets triggered

Regarding the components, U5 seems to be fine. But R28 & R35 is 5% Tolerance instead of 1%, we will change this to 1% and check.

Are there any hard rules for layout? as our layout is very similar except for a few components.

Hello Pooja,

Can you please double check the value of R28? If this value is too high, the converter would not close the loop to the programmed voltage because the CTR of U5 is maybe too low. In this regard, you can make a test and use 4.7 kOhm on R28 instead of 10 kOhm. This will impact loop compensation, but at least gives you and idea where the issue is. If changing R28 to 4.7 kOhm gives you stable Vout, then there is something not correct with the optocoupler.

Thanks

Roberto

Hello Roberto,
We are facing another issue.
From UCC28180D, there is no gate signal to UCC27517, although the IC is working fine. Due to this, the boosting is not happening and there is only rectified output 340V across the output capacitors.
Here are the parameters that we have observed with no load:
VCC: 12V
ISense: 0V
Icomp: 0.18V
VSense: 4.24V

We checked for shorts across all components and component values. Everything seems to be good. 
Is there anything specific we should check to that is causing the UCC28180 to shutdown and Icomp being <0.2V.

Hello Pooja,

Do you have in your board also the jumper J6 populated? This jumper is intended only for testing, forcing the Boost to supply low output voltage. If you have this jumper populated and have 340V on the output, the controller UCC28180 will switch the gate drive off.

If, instead, J6 is not populated, please check out the value of VCC_Primary, which should supply the PFC controller. If this voltage is >= 12V, then you can activate the PFC controller by applying 3.3V on the net PFC_ON, connected to R21 and the optocoupler U4. This way, Q4 is tying VCC_PFC to VCC_Primary.

Please let me know if that solves the issue.

Thanks,

Roberto 

Hello Roberto,

J6 is populated but left unused on our board. We confirm that Vcc PFC is 12V after supplying 3.3V to the net PFC_ON.

Pooja Singh said:

Here are the parameters that we have observed with no load:
VCC: 12V
ISense: 0V
Icomp: 0.18V
VSense: 4.24V

These are the values that we have measured on the UCC28180

Hello Pooja,

It is weird that even though 12V is present on VCC of UCC28180, it is not switching. Please consider that this controller runs already at Vcc = 11.5 nominal, but the worst case value is 12.1V. Maybe you have a controller that has this higher threshold. In order to understand if this is the case, can you please check whether the 12V on VCC of UCC28180 has exactly this value or is it lower? If it's 11.5V or less, you can do a test. Increase VCC_FB to higher voltage, let's say 12.5V or even 13V. By consequence also the VCC_PFC will increase to a bit higher value. If the PFC stage stars, then I suggest to keep VCC_FB at 13V and verify that all components supplied with this rail supports 13V.

Please provide update when you test the board.

Thanks,

Roberto