I was interested in the schematic just to see if it conforms to some i have seen over the years, and yes it mostly seems the same arrangement, but i wont copy yours, so dont panic. It reminded me of the atx 200-250w types, the tl494 and typical style of drivers. It's a well known style and quite reliable, so yours should be good for a long time :-D. The bits that were very different were the power output transistors would be supplied via the primary of the final transformer from a single capacitor that connects to a capacitor divider across B+, yours replaces the single cap with a link. And your method of making it switch on isnt normally needed on an atx, as it has a 16-18v standby supply to feed the 494 and stages, so it starts straight away. And your current limiting is different. Don't kill me for slight misunderstandings, i don't look at switchmodes that close :-D
Nice job there! I have a question about the driving transistors, though. What is the purpose of the diode in series with the 1.5K resistor that go into the center tap of the driving transformer? Keep up the good work!
Overcurrent protection. Such supplies require so called deadtime, when all power transistors are off. In this circuit, to make sure it is so, two small transistors connected to the drive xfmer are turned on simultaneously. Effectively causing the drive xfmer to behave like a piece of wire shorting bases of power switches
@@mrjohhhnnnyyy5797 Thanks for your answer. I have a similar +24V SMPS with a problem, though. Practically same configuration except for a bit different component values, and an independent winding from main transformer with rectifiers and filter to feed TL494 and pulse transformer circuit. With no load, both the pulses coming from TL494 (pins 8 and 11) and the ones on transistors collectors at drive transformer primary are 2.6us wide, with 200 ohm load, pulse widths are 5.6us, 150 ohms: 6.4us; 100 ohms: 7us; 66 ohms: 8us. Once load is 50 ohms, pulse width at transistors' collectors grows no more, and pulses coming from TL494 go almost as wide as the sawtooth width, not quite because of dead time, I presume. Output voltage drops naturally down to 20V. This is a 150W PS. I was wondering if there is an inherent limit in pulse width or duty cycle for pulse transformers, or maybe something I'm overlooking which is not apparent to me due to "work shop blindness". What is the maximum pulse width at full load at transistors' collectors? By the way, my switching frequency is 25KHz. Yours seems to be twice as much. I do really appreciate any light you may shed on this as it seems to me you know this subject quite well. Thanks, sir.
@@rafaellarios3707 Seems like your power transformer is calculated wrong. Put a 50 Ohm load, scope the voltage right on the secondary (to be safe), if it looks like pure square wave with just a little hint of deadtime, it means your transformer is indeed whack. Are you using ready-made transformer or you made it yourself, if it is the second, give me: AC input voltage, output voltage, and number of turns. I can do a quick check
@@mrjohhhnnnyyy5797 This is a Mean Well PS-150 power supply. I like repairing stuff and this one has proved to be a challenge, so far. But you have a point right there as there is something unusual on output transistors waveform. Instead of being a flat line between half cycles, there is a voltage bump prior to transistors being told to go into conduction. Base pulses are only on when they are supposed to be on, so I don't know whether this voltage bump is caused by transistors, or maybe something to do with main transformer. On the other hand, I wonder if transistors were bad this waveform would be as symmetrical as it is. Send me an email to rafael.larios@gmail.com so I can send you pictures of waveforms. An image is worth more than a thousand words. Thanks, sir.
