LM3478: use it in a flyback converter and the PWM drive waveform oscillate

Howard Zou

Genius 12365 points

Part Number: LM3478

Hi,

my customer designed a schematic as below:

the feedback circuit is:

They find that:

1. At 150kHz frequency, the PWM will oscillate as below: (yellow- DR pin, green- COMP pin). The oscillating frequency is about 760Hz.

2. adjusting RC at the feedback circuit won't help.

Are there anything wrong with this circuit?

over 5 years ago

0 Garrett Roecker over 5 years ago

TI__Mastermind 21720 points

Hi Howard,

Regarding the schematic there are a few things that need to change.

1. FB pin should be grounded.
2. C188 and C130 should be connected to the COMP pin not the FB pin.

Here is a good reference for designing the LM3478 as a flyback.
www.ti.com/.../snva761a.pdf

Please let me know if you have any questions.

Thanks,

Garrett

0 Howard Zou over 5 years ago in reply to Garrett Roecker

TI__Genius 12365 points

Hi Garrett,
By saying C188 do you mean R188?
And " connected to the COMP pin" you mean between COMP pin and GND?

0 Garrett Roecker over 5 years ago in reply to Howard Zou

TI__Mastermind 21720 points

Howard,

Yes I mean R188.

These to components should be connected to the COMP pin not to the FB pin.

Thanks,

Garrett

0 Howard Zou over 5 years ago in reply to Garrett Roecker

TI__Genius 12365 points

Garrett,
as shown in the schematic, FB is already connected to AGND.

0 Garrett Roecker over 5 years ago in reply to Howard Zou

TI__Mastermind 21720 points

Howard,

I see that, but the components that I mentioned above need to be moved. There is nothing connected to the COMP pin other than the opto-coupler, this is not optimal.

Thanks,

Garrett

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LM3478: use it in a flyback converter and the PWM drive waveform oscillate