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PFC synchronising circuit not working

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Advanced Member level 5
Jun 13, 2021
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The attached is a synchronised PFC controller using UCC28070A.
The Monostable (CD74HC123M) is not working, ie, its not providing narrow pulses to the UCC28070A. Do you know why?

The UCC28070A now seems to have powered up OK, and the UCC28086D is correctly providing a square wave to the monostable.

At first power up, the UCC28070A Vcc pin was stuck at around the turn-on threshold of some 10.4V. ….And it had 0V on its VREF pin indicating it hadnt woken up. I then changed R7 to a signal diode (anode to CD74..) and then the UCC280780A powered up fine (nice 6V on its VREF pin). However, even though a Square wave (~25% duty cycle) is correctly being sent to the CD74, the square wave is not going back down to 0V at the CD74 side of R3… is being held up at around 4.2V. -So the waveform seen at the “1B” pin of the CD74HC123M is just going continuously from 5V to 4.2V.

This must mean that the “1B” pin of the CD74HC123M is broken, because its supposed to be an input, and shouldn’t be able to source voltage.
Do you agree that the CD74HC123M must have somehow gotten damaged, eg, ESD, or perhaps something to do with R7 when it was in the first place, a 100R resistor? ..Also wondering about differences in supply assertion time between the 12V and 5V supplies. UCC28086D and CD74HC123M are supplied by the same 5V rail. UCC28070A is supplied by a 12V rail. (5V rail actually gets derived from the 12V rail so 12V asserts first)

CD74HC123M datasheet

UCC28070A datasheet


  • Synchronised PFC controller.pdf
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..........I just took off R7, and replaced the CD74HC123M, and it is now giving exactly the right output.........
.so the next thing is to connect it back to the UCC28070A..... the first time ipowered up, a voltage
must have coupled through the UCC28070A pin capacitance-to-VCC, and then blew up the , yes, i will use a
1N4148 in there (in place of R7).....must admit i would still like some resistance aswell.....but the
track goes on an inner layer after R7, so not so easy to add an R.......maybe i can do a "wigwam"
type thing with a 1N4148 and a
1k resistor in series between the vias either side of the R7 pads.....

Must admit, i would also now like a signal diode on the CD74 output pin, (just in case it rings below ground) but i'll struggle for room.

I never realised how utterly delicate these CMOS chips are (CD74HC123M). Just because the 12V powered up first...and then capacitively coupled through the vcc-to-pin capacitance of UCC28070A, ignored the R7/input_pin_capacitance time constant of CD74, and then killed the CD74, with what was probably a tiny overvoltage for a tiny amount of time!
--- Updated ---

Great Many thanks to SunnySkyGuy (Tony Stewart) for the diagnosis of the pin capcaitance situation described.
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..........................Now not working again...........................Back to the drawing board........................

Just put 1k and a 1n4148 instead of R7, (anode to CD74), and it now doesnt work......or should i say, when i probe at the mid point between 1k and 1n4148, it pulses for a second or two after power-up, then just goes flat at ~5V.

So it now goes CD74.......1K......1N4148.......UCC28070A

So i am wondering about this now.....maybe i just need to change R7 for 10k?
However, with 10k instead of R7, the CD74 doesnt work at all.......just get a flat 5v5 output from it. ...So in fact now i am wondering if the UCC28070A is damaged.

Any ideas why the CD74HC123M isnt working?
--- Updated ---

Now disconnected R7 from ucc28070a RDM pin, and it (CD74HC123M) still doesnt work.....though every now and again, it does work, for about a second or two after power up....then the CD74 output goes flat at 5V.
--- Updated ---

Now just changed the CD74HC123M timing capacitor to fresh one, same it works but only when not connected to the UCC8070A.
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