Using SG3525, can't put the feedback section to work - Page 1
Forum section: electronics / projects

author: psydaddy
author_rank: Contributor
timestamp: August 15, 2025, 12:57:44 am
content: I'm building a Half bridge. Made a schematic so you can see what I did so far (There are Snubbers that i didn't put there).The SG3525 works fine but I didn't put all the components there. It works fine at 50Khz and it also has a soft start.Do you have suggestions on how I can make it?(Image attached)

author: moffy
author_rank: Super Contributor
timestamp: August 15, 2025, 05:09:53 am
content: Quote from: psydaddy on August 15, 2025, 12:57:44 amThe schematic provided though well drawn is at best a rough outline, missing important details like MOSFET types, transistor types, electrolytic voltage ratings as well as a whole lot of missing components around the SG3525 etc. It's all in the detail, and what exactly do you mean by "It works fine at 50kHz .....", what works fine? And if it "works fine ..." then why the question "Do you have suggestions on how I can make it?" It is a little confusing.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: August 15, 2025, 06:25:31 am
content: have a look here:https://www.ti.com/lit/ds/symlink/sg2524.pdf, Fig. 10-9hth

author: psydaddy
author_rank: Contributor
timestamp: August 15, 2025, 09:55:29 am
content: Quote from: moffy on August 15, 2025, 05:09:53 amThank you. Yes it is a rough outline.What i meant was that if I force the feedback loop as shown in the picture, it works as intended. I put the potentiometer and I can vary the pwm duty cicle.Quote from: Le_Bassiste on August 15, 2025, 06:25:31 amThanks. Why, on COMP pin there is a cap in series with 20K?

author: moffy
author_rank: Super Contributor
timestamp: August 15, 2025, 11:36:22 am
content: Quote from: psydaddy on August 15, 2025, 09:55:29 amThe 'comp' pin helps set the gain and frequency response of the feedback loop for stability purposes. If you are asking that then you might lack a basic understanding of stability and control loops which is important to understand for PSUs, though you can sometimes get away with copying something close to what you want.For isolated feedback which you might want use a TL431 and optocoupler like:https://www.onsemi.com/pub/Collateral/TND381-D.PDF,if you don't need it isolated from the output use the suggested figure 10-9 with modifications.In your schematic Q4 collector should be connected to the source of Q1, not C1 and C2, there are a few other basic errors that suggest some lack of understanding which around mains can be lethal.Do you know how to design and wind your own transformers? It would be essential to know for what you are proposing.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: August 15, 2025, 12:06:26 pm
content: Quote from: psydaddy on August 15, 2025, 09:55:29 amthe COMP pin is the output of the voltage-regulating op-amp. as opposed to a "standard" opamp that has a "voltage source" output, this one does have a "current source" output. that allows you to "bend", i.e., low-pass the frequency response of its output by simply shunting a portion of the output current to GND by means of an RC network. that's what they have done here.

author: psydaddy
author_rank: Contributor
timestamp: August 15, 2025, 12:36:30 pm
content: Quote from: moffy on August 15, 2025, 11:36:22 amThanks for pointing out the issue with Q4. Yes, I had it wrong in my schematic, but it’s correctly connected in my prototype.For the transformer, I’m using a calculator called “Excellent IT.” It’s running with a fixed PWM and so far no complaints. I’m also testing it with my lab power supply (60 VDC). When connected to the mains, I use a series lamp to prevent damage.I understand your concerns, mate, but I want to learn. This has been my hobby for a long time. From memory, I’ve only had one capacitor discharge in my hands, and nothing more than that. I hope not to increase that count! Besides, this is not a challenge for me to kill myself!At this point, I don’t want to go back to school for this. I’ve already done that for IT and earned two master’s degrees, that’s enough for me. I also love electronics too much to risk ruining it by sitting in classrooms filled with old, tired, almost burnt-out teachers. That’s hardly motivating and could make me start hating electronics because of bad tutors.Could you elaborate on what exactly doesn’t look right? Keep in mind that my schematic doesn’t show the RC snubbers for the diodes and transistors, but they are present in my prototype. I also have clamping diodes connected to the driver transformer.Let me know if you’d like me to draw up the complete design as it is in my prototype.

author: psydaddy
author_rank: Contributor
timestamp: August 15, 2025, 12:45:52 pm
content: Quote from: Le_Bassiste on August 15, 2025, 12:06:26 pmThanks. Oh I treating it as a regular opampThis provided datasheet is for SG2524. Does this applies to SG2535 as well?

author: moffy
author_rank: Super Contributor
timestamp: August 15, 2025, 11:41:21 pm
content: Quote from: psydaddy on August 15, 2025, 12:36:30 pmThanks for the reply, my concerns have been nicely addressed and I am relieved that you have some experience and the use of the light bulb tester is a lovely old school safety measure.Please show the completed drawing, the more complete the better the advice. Put dots on the transformer windings to show phase, and there is a question as to where the SG3525 gets its startup supply and its run supply from. Normally the SG3525 would be on the HOT side with a winding of the output Tx providing its running power.P.S. Your supply is reminiscient of the schematics for those generic but decent Mains to DC supplies from companies like Meanwell etc. What is your expected power output?P.P.S. Keep your documentation updated with succesful changes, as well as a record of what those changes were e.g. history, it helps especially if there are breaks in the project and you need to remember where and why you were up to. We all hate paper work but it is a necessary evil.

author: psydaddy
author_rank: Contributor
timestamp: September 10, 2025, 09:46:44 am
content: Thank you, I'm rebuilding everything, then I post it hereI got it working with an external opamp, which is fine because I could control other stuff like maximum current.I haven't put the right values yet. Do you see something that doesn't look right (PDF attached)?

author: moffy
author_rank: Super Contributor
timestamp: September 10, 2025, 11:07:47 am
content: Very neat drawing well laid out, just a few questions:1. The LM393 is a comparator that needs a pullup, I assume you are using it because it has the same pinout as the opamp you are using?2. Could you explain what all the CY capacitors around the transformer(CY7, CY8, CY9 etc) are for?3. The +12V is supplied externally, the PSU needs an external supply to work, not the best solution.4. The 1N4007 is too slow to clamp effectively but you might have meant the UF4007 which is much faster.

author: psydaddy
author_rank: Contributor
timestamp: September 10, 2025, 02:32:27 pm
content: Quote from: moffy on September 10, 2025, 11:07:47 amIt's really nice to see how much help people provide in this forum. I have to thank you for being one of those people.1) Oh yes, same pinout but a different IC. I'm using a TL072. But out of curiosity, where exactly would I need the pull-up resistor with the LM393?2) I believe those capacitors are there to keep the switching frequency balanced between the primary and secondary, so anything that spikes out of the switching frequency gets attenuated. This way, the switching stays in sync on both sides (hot and cold). That's how I understand it, though I might be wrong.3) Yes, i got you. But my plan is to use an auxiliary power supply just for startup. The relay that bypasses the NTC will first power the auxiliary supply. Then, after about 1 second, the bypass occurs and the auxiliary supply is switched off. From that point, the pwm controller will be powered by the secondary side. Is this a bad plan?4)You're right! Thanks. Are the MOSFET gates enough to place them, or should I also use a UF4007 (instead of 1N4007) across Drain and Source as well?

author: moffy
author_rank: Super Contributor
timestamp: September 10, 2025, 03:06:49 pm
content: Quote from: psydaddy on September 10, 2025, 02:32:27 pm1. The pullup resistor for the LM393 is on the output as it is open collector, but not necessary for the TL072. Might be better to use a single supply opamp that has full input and output range.3. It's awkward because you need two supplies instead of just one.4. You probably mean the body diodes, they will do in a pinch, but a couple of fast diodes can be helpfull in clamping ringing.

author: psydaddy
author_rank: Contributor
timestamp: September 10, 2025, 04:23:08 pm
content: Yes I got you, A power supply that needs a power supply is stupid. Although I have seen some ATX that has those that also work as standby voltage.Thank you for the help.About this " I believe those capacitors are there to keep the switching frequency balanced between the primary and secondary, so anything that spikes out of the switching frequency gets attenuated. This way, the switching stays in sync on both sides (hot and cold). That's how I understand it, though I might be wrong." is this right?

author: moffy
author_rank: Super Contributor
timestamp: September 10, 2025, 10:47:32 pm
content: Quote from: psydaddy on September 10, 2025, 04:23:08 pmI am a bit unsure, normally you try to keep capacitance between primary and secondary low, I have seen a small high voltage cap between primary and secondary grounds, but not the arrangement as shown, the cap is normally on just for connecting grounds together.

author: psydaddy
author_rank: Contributor
timestamp: September 10, 2025, 11:14:36 pm
content: Quote from: moffy on September 10, 2025, 10:47:32 pmI'll investigate. I'm not sure where I saw this. Probably some random guy decided to put it online and got me confused.Thank you for your time