UCC28610: excessive ripple at low amplitude AC input

Naoto Fujita

Part Number: UCC28610

We are evaluating a prototype based on the AC/DC part (12V output) of the reference design "PMP5411".
The PMP5411 has a universal input (85VAC to 265VAC), but at full load (3.34A output) and AC input voltage less than about 90VAC, the output ripple will be excessive.

The output ripple of concern has the same period as the rectified pulsating wave in the primary circuit and an amplitude of about 1.5 Vpp.

The circuit is basically the same as PMP5411 (p1/2 of SLUR421.PDF), but the circuit board is our original design.

I would appreciate it if you could give me some advice.

over 1 year ago

0 Mike O' over 1 year ago

TI__Guru 73275 points

Hello,

I am reviewing your inquiry and will get back to you shortly.

Regards,

0 Mike O' over 1 year ago in reply to Mike O'

TI__Guru 73275 points

Hello,

Do you have waveforms that you can share?

Please include VDD, CS, Vbulk and Vout in these waveforms all on the same scope plot.

Regards

0 Naoto Fujita over 1 year ago in reply to Mike O'

Prodigy 10 points

Thank you for your reply.

I can't understand what the CS(Current Sense?) waveform is.
Please tell me what it is.

0 Naoto Fujita over 1 year ago in reply to Mike O'

Prodigy 10 points

In Figure 29 of UCC28610 datasheet, I_DRV current is sensed by a internal comparator.

I don't know how to observe CS signal from outside the IC.

Can you help me?

0 Mike O' over 1 year ago in reply to Naoto Fujita

TI__Guru 73275 points

Hello,

You would have to get a current probe to measure the current into the drain of the FET.

If you don't have a current probe you could a small resistor in-between the drain of the FET and the bulk capacitor. You when then need a differential probe to measure the voltage across it. I = V/R.

Regards,

0 Naoto Fujita over 1 year ago in reply to Mike O'

Prodigy 10 points

Thank you for your help.
Our current probe is now calibrating.
I 'll try at a later date.

0 Mike O' over 1 year ago in reply to Naoto Fujita

TI__Guru 73275 points

Hello,

I was glad that I could help.

Regards,

0 Naoto Fujita over 1 year ago in reply to Mike O'

Prodigy 10 points

Hello,
I share waveforms that you advised.

Measurement conditions:
Input voltage is 85VAC.
12VDC Output current is 3.34A.

CH1(yellow): VDD
CH2(green): Vbulk
CH3(purple): Vout
CH4(pink): CS(FET Drain current)

How is it?
Any advice would be greatly appreciated.

0 Mike O' over 1 year ago in reply to Naoto Fujita

TI__Guru 73275 points

Hello,

I am reviewing your inquiry and will get back to you shortly.

Regards,

+1 Mike O' over 1 year ago in reply to Naoto Fujita

TI__Guru 73275 points

Hello,

Your output ripple voltage is dipping at the valley of the bulk voltage ripple at 90V. Chances are that your transformer turns ratio cannot handle this low input voltage. Try increasing the input capacitance and this should help remove this issue.

The high frequency output ripple is due to your ESR in your output capacitor. If you want to reduce this select a capacitor with a lower ESR.

Regards,

0 Naoto Fujita over 1 year ago in reply to Mike O'

Prodigy 10 points

Hello,
Thank you for your advices.

Output voltage dipping issue was solved by increasing input capacitance 100uF to 220uF (added parallel 120uF to original circuit).

blue: original output
yellow: improved output

Regarding The high frequency output ripple problem, We had already choosed a low ESR input capacitor, so adding a 1uF ceramic capacitor in parallel improved it somewhat.
I will continue to consider it based on your advice.

I really appreciate your help.

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UCC28610: excessive ripple at low amplitude AC input