UCC3805: DC-DC

Ram yellanki

Part Number: UCC3805
Other Parts Discussed in Thread: UCC2805, UCC2803, UCC2802, UCC2801, UCC2804

Hi,

I'm using the UCC3805 controller in a flyback design, converting 8V to 500V. The existing design has a 12k and 1.1nF on the RC but when measured the frequency is coming out at 36kHz.

Could you please let me know why?

10 months ago

0 Hong Huang 10 months ago

TI__Guru 50675 points

Hi,

I assume you measured frequency from pin 6 (OUT). For UCC3805, frequency on OUT (pin 6) is half of oscillator frequency, i.e., fosc = 72kHz, fsw (OUT) = 36kHz.

If you check Fig 8-4, you will see fosc instead of fsw(OUT).

0 Ram yellanki 10 months ago in reply to Hong Huang

Hi Hong, Thanks for the prompt reply.

which datasheet are you referring to? I have the datasheet UCC280x Low-Power BiCMOS Current-Mode PWM Controllers datasheet (Rev. G) (ti.com)

that information on the fsw is missing in it.

0 Hong Huang 10 months ago in reply to Ram yellanki

TI__Guru 50675 points

Hi,

The same datasheet

https://www.ti.com/lit/ds/symlink/ucc3805.pdf?ts=1692009532292

If you look at Section 8.5, oscillator, and UCC2805 note (2) you will see:

"(2) Oscillator frequency for the UCCx800, UCC2802, and UCC2803 is the output frequency. Oscillator frequency for the UCC2801, UCC2804, and UCC2805 is twice the output frequency"

0 Ram yellanki 10 months ago in reply to Hong Huang

Thanks for that Hong, it was hard to find as this was given in the notes. Would have been ideal if that was written where the frequency formula was!

0 Hong Huang 10 months ago in reply to Ram yellanki

TI__Guru 50675 points

Hi,

Maybe helpful and will consider. But this family of devices has been used for a long time and it has been ok. Anyway I will make suggestions to see how to improve.

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UCC3805: DC-DC