LM5002: unexpected pulse-skipping

Sheng Yu

Part Number: LM5002

Hi TI:

I designed a flyback circuit using LM5002. Input 24V, output 200V, transformer 1:5 80uH, switching frequency 250k, output load 0.4W.

Because the C123 is the ceramic capacitor, I just use type I compensation network.

I used the oscilloscope to probe the #3pin of LM5002, I found the unexpected switching pulse-skipping which cause a big output noise。

When I decrease the input to 20V or change the switching frequency to 130k, the pulse-skipping disappeared.

I don't know how to analyse this issue theoretically. Could you do me a favor?

Thanks!

unexpected pulse-skipping

input voltage 20V

frequency 130k

over 6 years ago

0 Jasper Li over 6 years ago

TI__Mastermind 46905 points

Hi Sheng,

Could you share the layout? to check if the issue is caused by the noise.
what is part number of the D1? is it a schottky diode?

0 Sheng Yu over 6 years ago in reply to Jasper Li

Prodigy 40 points

The diode is Panasonic DA2JF8100L.

And here is the layout.

0 Jasper Li over 6 years ago in reply to Sheng Yu

TI__Mastermind 46905 points

Hi Sheng,

it seem the current through the snubber and the diode reverse recovery current lead to very small low side on time in some switching cycles.
maybe you can try to increase the resistor of the snubber RC circuit.

0 Jasper Li over 6 years ago in reply to Jasper Li

TI__Mastermind 46905 points

Hi Sheng,
I haven’t heard back from you, I’m assuming you were able to resolve your issue. If not, just post a reply below

0 Sheng Yu over 6 years ago in reply to Jasper Li

Prodigy 40 points

Hi Jasper:
I increase the resistor of the snubber RC circuit to 150 ohm, there is no improvement.

0 Zack Liu over 6 years ago in reply to Sheng Yu

TI__Mastermind 34201 points

Hi Sheng,

The PCB wire between transformer pin 3 and LM5002 SW pin is not wide enough, and you can solder a wire in parallel with it.
What's the value of R176 and C127 you are using now? Could you decrease the value of R176 and increase C127 and do a test again.

0 Zack Liu over 6 years ago in reply to Sheng Yu

TI__Mastermind 34201 points

Hi Sheng,

What's the latest news about this issue? Have you found the root cause? I'll close this thread first and if you still have any doubts, please send a new thread or repond here. Thanks.

Because of the Thanksgiving holiday in the U.S., TI E2E™ design support forum responses may be delayed from November 25 through December 2. Thank you for your patience.

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LM5002: unexpected pulse-skipping