TL431: TL431

SHREELEKHA KOTEESWARAN

Part Number: TL431
Other Parts Discussed in Thread: LMC7221

Hi am using TL431 and LMC7221 in my feed back loop. which is for isolated flyback three phase ac to dc converter. My only doubt what would be the cause of negative ramped sawtooth ripple output of the flyback converter. The image is attached below.

8 months ago

0 Walter Zollinger 8 months ago

TI__Expert 7430 points

Hello,

Can you send a schematic?

Can you please resend the image? It did not attach

Thanks,

Walter

0 SHREELEKHA KOTEESWARAN 8 months ago in reply to Walter Zollinger

Prodigy 70 points

0 SHREELEKHA KOTEESWARAN 8 months ago in reply to SHREELEKHA KOTEESWARAN

Prodigy 70 points

0 Walter Zollinger 8 months ago in reply to SHREELEKHA KOTEESWARAN

TI__Expert 7430 points

Hi,

The compensation network of R90 and C43 may need to be adjusted to increase stability and reduce the sawtooth output. Is there a load on the output of the converter?

Thanks,

Walter

0 SHREELEKHA KOTEESWARAN 8 months ago in reply to Walter Zollinger

Prodigy 70 points

Hi I am just connecting a DC programmable load to the output. But the sawtooth ripple output seems to observed under no load condition. When i turned on the dc programmable load. Then it is steady and straight.

0 SHREELEKHA KOTEESWARAN 8 months ago in reply to SHREELEKHA KOTEESWARAN

Prodigy 70 points

Also could you please confirm whether the diode D25 is causing this , Which has its place to limit the overvoltage across optocoupler.

0 SHREELEKHA KOTEESWARAN 8 months ago in reply to SHREELEKHA KOTEESWARAN

Prodigy 70 points

Could you please share me the design steps to calculate the compensation network values. Because the designer has just used the values from the application note.

0 Walter Zollinger 8 months ago in reply to SHREELEKHA KOTEESWARAN

TI__Expert 7430 points

The D25 diode should not cause an issue. The switching and crossover frequency is determined by the converter device. The transfer function of the converter and filter determine how much gain and phase margin to add using the TL431 compensation network.

https://www.ti.com/lit/an/slua671/slua671.pdf provides some helpful design guides.

There will also usually be some ripple at no load which goes away when a load is applied to the circuit as the circuit is designed to be under load.

Thanks,

Walter

0 SHREELEKHA KOTEESWARAN 8 months ago in reply to Walter Zollinger

Prodigy 70 points

Hi Walter. Many thanks for the reply. One of the DNP component ( zero ohm resistor ) was surprisingly fitted in our circuit at the SIP32419 current limiter and this has been removed and tested. All I got is start up spikes which was observed before got rid off and now I could see the output is with sawtooth ripple output at no load and when I turn on the load there was a drop to zero voltage and back again to 24V. But one thing i want to mention here is I have damaged the PCB eyelet at the output rectifier D24. Does this would cause an issue.

+1 Walter Zollinger 8 months ago in reply to SHREELEKHA KOTEESWARAN

TI__Expert 7430 points

The PCB damage might cause an issue but it depends on the board layout. Updating the compensation might improve the transient response when the load is applied. There could be another part of the design that is slowing down the response.

0 SHREELEKHA KOTEESWARAN 8 months ago in reply to Walter Zollinger

Prodigy 70 points

Hi many thanks. The psu output is designed to produce 30W with 1.25A, but what I observe is the output voltage drops to 0V. When load is step from 0.1A to 0.9A it is fine. At 0.95A the voltage drops to 0 and back to 24V. This fluctuation also would be the cause of compensation resistor. Do I need to adjust further.

0 Walter Zollinger 8 months ago in reply to SHREELEKHA KOTEESWARAN

TI__Expert 7430 points

Hi,

This voltage drop could be caused by the output filter or somewhere else in the converter circuit. Do you have a waveform of the output current draw? The SIP32419 current limiter might be triggered.

0 SHREELEKHA KOTEESWARAN 8 months ago in reply to Walter Zollinger

Prodigy 70 points

Hi Walter, many thanks. One thing comes to my mind is when I turned on the psu for the first time after manufacturing the assemble. It was stable till 1.1 A and output voltage glitches at 1.12 A. Even though it’s rated to 1.25 A. Next time when I keep on turning on and off the psu the output voltage starts to glitch at .95A. Could you please explain is this behaviour happens still because of output filter or some other part of circuit. Is there any chance it can be because of over temperature or soldering issues. Your guidence is much appreciated.

0 Walter Zollinger 8 months ago in reply to SHREELEKHA KOTEESWARAN

TI__Expert 7430 points

Hello,

The decreasing current capability might be an issue with the regulator device either being damaged somehow or is overheating. Have you contacted support for the the converter device you are using?

Best,

Walter

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TL431: TL431