UC3843: Flyback - unstable dutycycle due noise? - Page 1
Forum section: electronics / beginners

author: GRFViper
author_rank: Newbie
timestamp: July 06, 2025, 05:11:20 pm
content: I am designing a flyback converter, with the following parameters:Input: 50-60VDCOutput: 150VDCI use a simple solution using UC3843, TL431, and PC817. Currently, the circuit is running and has a stable 150V output. However, there is a problem that the transformer is making a loud sound, I found out that the cause is due to the uneven gate pulse, as shown below. I think this noise comes from the Isense part. Can anyone with experience help me with this case?

author: Sorama
author_rank: Frequent Contributor
timestamp: July 06, 2025, 06:03:29 pm
content: Are you sure about that R7 value?Seems a bit low.

author: mikerj
author_rank: Super Contributor
timestamp: July 06, 2025, 07:43:31 pm
content: This is very likely me being blind, but I don't see where the UC3843 gets its VCC supply?

author: D2236
author_rank: Regular Contributor
timestamp: July 06, 2025, 08:01:20 pm
content: Taking feedback after the post-filter LC makes the regulator try to compensate for the filtering. It "sees" a slower, delayed version of the actual voltage and tries to respond faster — often leading to loop instability, overshoot, or oscillation.

author: PGPG
author_rank: Frequent Contributor
timestamp: July 06, 2025, 09:28:53 pm
content: Quote from: mikerj on July 06, 2025, 07:43:31 pmFor me such schematic should not use labels at all to be readable.

author: D2236
author_rank: Regular Contributor
timestamp: July 06, 2025, 10:00:36 pm
content: Operating near or above 50% duty cycle with peak current-mode control (as the UC3843 does), slope compensation is mandatory.

author: petemate
author_rank: Regular Contributor
timestamp: July 08, 2025, 03:25:50 pm
content: Your duty cycle is either very large or very close to zero. It varies rapidly, that is, within two adjacent pulses. That should not be the case.Are you testing with a constant load on the output, so that you would produce a reasonably constant duty cycle? Does it match your calculations? If you don't have a load, you're probably going to see weird behavior, since the UC3843 doesn't have any low-power mode. So it will just send out the shortest possible pulse(which you kinda see) and then, when output voltage has droped, will send a one or a few pulses to "recharge" the output.Why are you using a UC3843 unit? It allows for 100% duty cycle, but a flyback can't handle that, since you'll need part of the duty cycle "reserved" for the 2nd half of the switching cycle, where the inductor current is delivered to the load. The precise max duty is dependent on your particular configuration, but you need to verify that max duty cycle possible will still allow sending the stored energy into the output side. May I suggest you switch to UC3845, just for the sake of having a limit to 50% duty?If you are running the output with a load that should provide you with a reasonable duty cycle, then something is wrong and that is probably the control loop. Consider doing away with the calculated components and just add a large capacitor to form a pole at low frequency. This will give you a really slow response, but it will most likely be stable. From there you can continue your debugging.