Operating frequency in TOP256Y

6 posts / 0 new

I had a suitable PCB design with a TOP224, but I needed to control ON/OFF so I converted to a TOP256 using the same xfmr.
ON/OFF works nicely and I get 50 watts out. But I observe TOP256 operating frequency to be about 35 kHz, regardless of whether F pin is connected to S or C. It's not a stable 35 kHz and varies with load.
When I go back to the TOP224, operating freq is a stable 100 kHz.

Please help soon, as I need to get this design released in a couple days.

I don't see how you could get full 50 watts with frequency so low...

Thanks, but my question is about the operating frequency, not power. Yes, i was surprised also. I changed back to TOP224 and confirmed 100 kHz; back to TOP256, and measured 25-35 kHz.

Send voltage and drain current waveform attachments at 50 watts. Also what is your max power for this design?


Thanks for your note; this is timely as our PCB designer is laying out a new circuit to make a clean layout, using AN43 as a guide.

I made an error in my haste...
Attached are two pix:
7.1 VDC into 1 ohm (50 W) operating at 127 kHz
7.3 VDC into 3 ohm (20 W) operating at 39 kHz

These are at 120 VAC input, 'scope traces at Drain pin; i don't have an easy way to give you current tracs, sorry.

We expect to need 50 W out of this design.

So the question is why does freq change with load?

I found that the 'soft finish' circuit (R100, C100, D100) causes the circuit to stop or have reduced output. THe pictures i attach are without this circuit.

I very much appreciate your help.



I can see that supply is in frequency reduction mode at lower output power which is normal for this device as you can see from datasheeta and app note.

what is max output power of your design?