design or build a smps from discrete components - Page 1
Forum section: electronics / general

author: jackbob
author_rank: Regular Contributor
timestamp: January 03, 2012, 03:11:30 am
content: Dave has made some five star videos in the past on smps chips and designing linear power supplies but I have yet to find a video from any user on youtube on how to design smps from large discrete components such as smpt's, mosfets and smps driver chips. These power supplies are well known for their use in atx computer supplies and their use is far too common to ignore.

author: hans
author_rank: Super Contributor
timestamp: January 04, 2012, 12:03:58 am
content: They are commonly made and designed, and sure enough there is plenty of stuff about it out there.. in articles and books. I think the main reason, as you see with the EEVblog, the audience watching is not going to build his own ATX power supply. A simple buck converter on a PCB so you can power low voltage stuff @ 2Amps from 24V adapter is fine, many usages there..Because in a way you can also talk about multi-phase SMPS designs, like used for proccesors and graphic proccesors these days. But I don't think it will be very fun to watch and much of interest to most people.Not saying I wouldn't watch it though, I would love to see stuff about it. Following a power electronics course on college which is one of the most interesting courses I had this year.

author: jackbob
author_rank: Regular Contributor
timestamp: January 05, 2012, 05:25:37 am
content: I suppose it may not help some people, like you said most just need a small bench supply and they can buy them or build a simple linear one. But they do play a very important role from cell phone chargers to several kw electric car chargers and I'm sure many would enjoy a video explaining a simple smps design. Sometimes a simple buck or boost converter may not work though as they are not isolated supplies which may be a problem in many cases.

author: Psi
author_rank: Super Contributor
timestamp: January 05, 2012, 05:35:51 am
content: You can design a simple switch-mode power supply from discrete components using a flashing circuit, few transistors/mosfets and a center tapped transformer.It doesn't need to be an inductor based buck or boost design to be switch mode.

author: jackbob
author_rank: Regular Contributor
timestamp: January 05, 2012, 05:48:57 am
content: A flashing circuit? I have a few smpt's that are a graphite type of core if thats what you mean. you cant build a good smps with an iron core transformer. The transformer duty cycle needs to adjust according to the load which i have trouble doing. I have tried making a simple 555 at about 10% duty cycle and about 50kHz driving a mosfet and using the fet to drive the transformer. It works but when there is a significant load the duty cycle needs to change to accommodate for that. when the duty cycle gets higher things get too hot, regardless of the load size.

author: Psi
author_rank: Super Contributor
timestamp: January 05, 2012, 07:38:27 am
content: Quote from: jackbob on January 05, 2012, 05:48:57 amyeah, a simple circuit to flash like an led or something. Obviously it needs to be able to switch fast enough, but that's normally just a matter of changing the cap and resistor.Basically like a 555 timer made from discrete components (it doesn't need all the options a 555 timer has so can be a simpler design)Quote from: jackbob on January 05, 2012, 05:48:57 amDepends what you mean by "good".You can build a 50hz switchmode psu and drive a iron core if you want to. It's just quite large and heavy due to all the iron.So you might as well use 20-50khz and use a ferrite core so you can make the transformer smaller.Modern AC TIG welders normally output 50-300Hz generated through a switchmode system, but that's because the frequency has advantages for cleaning the oxide off metal as you weld.Quote from: jackbob on January 05, 2012, 05:48:57 amI've had a 555 timer driving a toroid at 30khz with some fets.The output of the 555 went through some inverter/buffers to get both a Q and not-Q output.Those outputs each went to the base of a logic level mosfet which grounded each side of a center tap (the middle of the tap was at 12v)The 555 was outputting exactly 50% duty cycle. So the mosfets on state alternated and each was on for the same time.It was fine under no load condition and could handle up to 2A, but since it had no feedback the output voltage wasn't very regulated.

author: jackbob
author_rank: Regular Contributor
timestamp: January 05, 2012, 08:13:21 pm
content: Pretty close to how I set mine up. Did you have any protection diodes or snubber caps? I found it worked better when I put a 220000pF in parallel with the transformer. The big problem is that when I use a smpt the primary is so short and whenever the fet is on its almost shorting out because the coil small this is why things get to hot and wont do high duty cycles without burning up.

author: Psi
author_rank: Super Contributor
timestamp: January 05, 2012, 11:06:58 pm
content: Quote from: jackbob on January 05, 2012, 08:13:21 pmTo reduce ringing/back emf I had an inductor in the power feed to the center tap as well as some diodes to snub high voltage from blowing up the mosfets. There were some big caps in there and I think i also had an RC filter somewhere. It was quite a while ago so i don't really remember the specifics.Quote from: jackbob on January 05, 2012, 08:13:21 pmIt shouldn't be doing that.If its drawing current under no load it's because something isn't optimal.I tried few different numbers of turns before i found one that works the best for me.From memory the primary was a center tap with 16 windings total, 8 per tap and the frequency was 33khz with an input of 12VDC. (i had a pot i could turn to find the most efficient frequency for the core.)Basically, when you have the correct number of turns and the correct input frequency for your core type (and input voltage) it will couple nicely with the transformer core and should draw little to no current under no load.

author: jackbob
author_rank: Regular Contributor
timestamp: January 07, 2012, 08:26:53 am
content: I need to do what you did with the frequency adjust pot. Thanks a lot for the help I appreciate being able to talk to people who understand what I'm talking about. I have a digital electronics teacher at my high school and he is hopeless when it comes to designing electronics, all they do at our school is build circuits off of schematics. Design is much harder.