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Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
I designed this because the idea of paying hundreds of dollars for a commercial model scared me. My completed unit puts out between 2.5v and 25v. I bet some people out there could use this type of equipment, and I figure I can give you a very good start with this design.
My PSU uses an sg3525 for PWM and a dual opamp for OC protection. Output is something like 10a at 24v max. Ive tested it to about 3a at 24v.
Heres a .pcb file. If anyone is interested in my design, Id be happy to answer any questions and post pictures. ]1181684227_51_FT0_smps1-final.zip[/file]
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Sounds good, and very useful. Do you have a conventional schematic? I've been planning a bench smps with a TL494 for a while now, I think I'll start on it soon. I'm thinking regulated 12 - 50V output at 10A+. What kind of over-current protection are you using? Current sense resistor on the secondary side and CT on the primary?
Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
I use a shunt resistor on the return ground path of the output. Its a .01 ohm resistor so there should be .1v across it at 10a. This is then fed into an opamp and multiplied by 10 then put into a comparer. If its above about 1v (for a 10a limit), the output of the opamp goes high and latches an scr on. This holds 5v on the shutdown pin of the sg3525 killing the PWM output. The OC protection circuit is reset simply by power cycling the PSU.
Sorry, I never drew a schematic, Its in my mind.
The design is pretty straight forward though. There are actually 2 switching power supplies in the unit. There is a 12v standby supply good for about 1a(using at TOP202 I think). This is used to power the control circuitry and fan. This standby power supply is always on, but only connected to any load when you turn the unit on. The sg3525 puts out a PWM signal which goes through a GDT (like whats used in an SSTC). This signal gets put across the gates of 2 MOSFETs in a half bridge. The half bridge is fed from a voltage doubler from mains voltage. The half bridge switches about 170vac at about 65khz into the primary of a ferrite cored transformer (primary is something like 30 turns). The transformer steps the voltage down at about a 6.5 to 1 ratio. The output of this transformer is center taped, and goes through a 30a, 60v Schottky rectifier. This is then put through an LCLC filter. This output voltage goes through a voltage divider that includes a pot. on the top of the case. Then comes the OC section as mentioned above. I also included an EMI filter, inrush current limiter, and fuse on the primary side for protection.
Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
I like the sg3525 but I don't think it shines in an off-line application. Too much of a hassle figuring out how to drive it and keep the output fully isolated from the line. I think there is now a uc3825 chip that requires a much smaller operating current than the 3525, around a milliamp or so, which makes building an off-line smps easier. Also, sg3525 is designed for push-pull. Using it to run a half-bridge is pretty unusual.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
SG3525 is a very nice IC for driving mosfets in offline applications.
It is used on low voltage side with GDT instead the mains side like UC3842, so it anyway needs some kind of auxiliary supply to start.
I even played with SG3525 on mains side in flyback mode and I started it up with a simple diac-based charge pump. The charge pump was actually much more efficient than UC3842's UVLO circuit, but at expense of aditional capacitor and a diac.
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Free 240w switch mode PSU design, PCB
