555 switch mode power supply - Page 1
Forum section: electronics / projects

author: jackbob
author_rank: Regular Contributor
timestamp: December 07, 2011, 08:25:12 am
content: Over the past few days I have been building some janky 555 switch mode power supplies and it worked better than I thought it would but not as good as I want it too. I would like to make a 12vdc power supply that runs on 120vac rectified and filtered of course. I made a 555 oscillator at about 16khz with 10-20% duty cycle. I used that pulse to drive a MOSFET and the MOSFET drove the primary of a ferrite core toroidal transformer I wound. The transormer isolates the power which is useful. It works good when the fet drives the transormer at 12v but 120v things blow up and the fet is rated for 200v. Obviously it's not designed well an I know that, so what is it that a smps driver ic does? I think it adjusts the duty cycle to depending on how big the load is. So is there a way to accomplish a smps with a 555? If not has anyone played with smps driver ic's and could recommend one?

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: December 07, 2011, 10:32:10 am
content: You are getting quite alot of nasty spikes on your fet. If you want to leave this flyback switching topology then use a 1200 or better mosfet and adequate snubber.But go to a gate drive IC and use a halfbridge.

author: Psi
author_rank: Super Contributor
timestamp: December 07, 2011, 11:16:51 am
content: Unfortunately switching voltage from mains down to 12v is more tricky than the other way around.As said above, spikes produced from the switching are huge when your trying to switch mains voltages7 years or so ago I built a 555 timer based SMPS that took 12V input produced 200V for a plasma display.From memory it had a 555 timer producing around 30khz @ 50% duty cycle.The 555 output split in two and went into a hex inverter IC. One when through one inverter the other went through two.So that produced two 30khz signals opposite to each other.The signals connected to two fets and each pulled one side of a transformer center tap to ground.The center tap itself was connected to +12v through an inductor.It wasted a bit of energy as the push/pull switching on each mostfet happened at exactly the same time, so there was a tiny amount of overlap.It wasn't ideal but it did work quite well for just a 555 timer controlling it all.I was able to get around 20W @ 200V, just enough to light a 25W 230V lamp.Later on i ripped it apart and use the transformer core to make a new powersupply using a TL494 controller.It worked much better and was able to produce 12V @ 10A from 10-15VThe TL494 is very common and there's lots of info and example circuits for it but i'm pretty sure its not intended for mains voltages directly.Switching mains voltage down to low voltage can be a little annoying, that's why most designs are flyback type.But i have seen some ICs with built in fets that will switch mains into a transformer directly.They're probably a little specialized though. I think maxim do some.

author: BravoV
author_rank: Super Contributor
timestamp: December 07, 2011, 12:25:57 pm
content: Quote from: AcHmed99 on December 07, 2011, 12:13:48 pmYep, although its not as popular as the common 494 or uc384x families, if you just want a simple and low component counts switcher that regulate high voltage from powerline to low dc volt on low powered device, Power integration's product especially the TinySwitch families are very easy to use and build, with quite minimal component counts compared to the 494 or uc384x controller.To give you an idea how low the component counts, this is the actual circuit example, and pretty simple imo.

author: jackbob
author_rank: Regular Contributor
timestamp: December 07, 2011, 10:36:37 pm
content: Thank you all for the help yes those spikes killed a few fets on the 120v. Those ic's with fets in them sound pretty good but they probably can't do too much power. It doesn't matter though beacuse I don't have a specific use for it, I just want to build a smps that works for something like a small bench supply. I love power electronics and have never built a good smps on mains power. I built a good boost converter smps that ran on a 12v and it's worked damn well after shoking me. I will look into the parts you mentioned and choose what looks best. Also I have been taking lots of computer smps apart and find that some are very simple! I have a supply that puts out 36 amps at 12v and it only has one 8 pin ic driver that drives a couple to220 fets and bingo. I just can't seem to get the same results.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 07, 2011, 11:02:28 pm
content: It takes experience to build a good switching power supply. Nowadays, it makes more sense to find an off the shelf unit or modify one unless you have special requirements.

author: jackbob
author_rank: Regular Contributor
timestamp: December 08, 2011, 05:31:30 am
content: The tl494 appears to be a good choice but looks to me like it is sort of similar to the 555 pwm I made. Does the 494 adjust the duty cycle itself? I had a fixed duty cycle on my 555 supply. I looked at my 36amp spms and it only has one ic running the whole supply it's a td3842, china knows how to make these things right. Although for such a simple supple it is very reliable I have ran 30 amps continuous with no problems. The snubber cap should go between the gate and source right? And should I use a diode in reverse bias in parralel with the transformer primary ( this has helped me cut spikes on the fets but can get hot sometimes).

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: December 08, 2011, 08:57:33 am
content: TL494 is an old beast, and if even widely used still today there is much better replacements

author: jackbob
author_rank: Regular Contributor
timestamp: December 09, 2011, 02:14:30 am
content: I still think I'm missing something, I drew a quick general schematic of the output side of the smps I built. It's only the output section not the ic's and other control stuff. However smpt's have rather low windings and whenever the fet switches on its almost shorting it out and creates a lot of heat even without a load on the transformer. Is there something I'm doing wrong in my schematic? Also the transformers and cores I placed on the schematic are the type I am using. I also put pictures of my 36 amp smps, not much to see but it still impresses me how simple it is. Yesterday I popped open a cheap Wal Mart dc car adapter that takes 12v in and has a switch to select output voltages of 3, 4.5, 6, 7.5, 9, and 12v I was surprised to see it uses a tl494! I believe it is a buck converter, that's how it looks to be configured and looking at it on the oscilloscope it changes the duty cycle to adjust the voltage.