UCC2897A: No Vout, IC doesn't start up.

Maxim Shatrov

Part Number: UCC2897A
Other Parts Discussed in Thread: TPS2378, TPS2378EVM-602,

I'm using a POE reference design which uses the TI part UCC2897A.

The part with TPS2378 works propertly (ithink), but PWM doesn't start. The LIneUV is 1.7V, LIneOV is 0.9V.
There are schematic and some waveforms

Vdd waveform

AUX waveform

SS waveform

Vin waveform

Anyone with more experience can help me or give advice?

over 5 years ago

0 John Griffin1 over 5 years ago

TI__Mastermind 34465 points

Hello Maxim,

You application is using the AUX signal to drive a synchronous rectifier on the secondary in a flyback application.

Please disconnect AUX from D6 and C15.

AUX is a switching waveform and you are not seeing this.

It may be that AUX is not properly loaded.

Regards

John

0 Maxim Shatrov over 5 years ago

Prodigy 70 points

Sorry for the confusion about [Vout]. In my cricut Vout is output voltage 12V / 5A in the secondary circuit. In question i meant PWM signal (pin15[OUT] of UCC2897).

update:

I've disconnect SS circuit [removed R42] and then IC starts generate out PWM signal (don't know yet why). But when I load the output with a 15 ohms load Output voltage becomes unstable:

Dual PD section schematic.

0 Maxim Shatrov over 5 years ago in reply to John Griffin1

Prodigy 70 points

Hi, John, thank you.
I'll try it, but can you explain why it needed to disconnect AUX when in reference design (TPS2378EVM-602 Evaluation Module) AUX connected the same way?
Difference between these designes only in POE transformer (19V to 12V) and values of some components.

Upd: No. It did not help

0 John Griffin1 over 5 years ago in reply to Maxim Shatrov

TI__Mastermind 34465 points

Hello Maxim,

Your signal for AUX is not right .
Did you attach the wrong scope plot ?
I suggested disconnecting AUX because this simplifies troubleshooting and it is possible that you have a short somewhere on AUX.

If now the system is unstable then you should run the power supply open loop.
By putting a variable resistor on SS you should be able to adjust the output voltage so that the output voltage is correct with the load you are using.
If this works then you can see about changing the loop feedback.

Also can you please attache the schematic as a pdf.
If you embed the document in the post then it makes it very difficult to switch between schematic and post.

Regards

John

0 Maxim Shatrov over 5 years ago in reply to John Griffin1

Prodigy 70 points

Thanks for the explanation.

I increased resistor on SS up to 180 kOhm and now the circuit starts up normally.

Next problem is - PWM signal seems not correct when i connect load (15R) but output 12V is correct. Duty cycle not stable and bouncing.
When i connect higher load output voltage may drops up to 2 Volts.

Attach the schematic as a pdf: https://drive.google.com/open?id=1bLbFsQVwWy-EH8LT4eZTJUjr-ZIJvd5h