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rjon17469 14th July 2005 09:30 PM

Beginning SMPS?
Hey everyone,

I have a good amount of experience with regulated and unregulated power supplies, and want to move into the SMPS arena.

I've done a good amount of research on the subject, and I understand the different topologies, and if you don't know what you're doing, you might not get another chance. :att'n:

So, with this in mind, where would be a good, safe place to start? What should I do if I eventually want to be able to build say a 400-500 watt SMPS off 120VAC?

Any and all help appreciated, including articles, schematics, parts, etc.


carvinguy 14th July 2005 11:44 PM

I've seen someone in here tweek out a K6 power supply. It is a kit you can purchase. It was designed for audio amps, although there are some improvements you can do to make it more robust. They are located in your neck of the woods!

You could start with the kit and study the wave forms, etc. You can download the schematics at the website first, if you wish. There's a legthy thread on it in the Class D forum here.

rjon17469 15th July 2005 01:54 AM

Great link! Thanks for passing that my way. Looks like a very nice PSU. I'm going to try and study the schematic, and especially the layout of the PCB to get a feel of how things should be.

Many thanks!


thomas997 23rd July 2005 01:29 AM

Yeah I am in a similar situation..

I had thought direct off line smps discussion was not allowed, or maybe the design of them (there were a few locked posts).

Anyway I can find a lot of 12V smps circuits and designs, but not a whole lot of 120V.

I would like to learn some board design as well as how to wind the transformer.

btw that k6 psu schematic, 2200uF seems like an insane amount for 200V input caps. Although they dont look too big in the pics.

Stocker 23rd July 2005 01:45 AM


Originally posted by thomas997
Yeah I am in a similar situation..

I had thought direct off line smps discussion was not allowed, or maybe the design of them

Who holds the keys to Texas?

Eva 23rd July 2005 04:55 AM

I've seen some closed threads where people suggested directly connecting amplifiers to mains line, and I can't complain because that is quite an insane idea. However, I think I haven't seen any serious thread about SMPS closed or censored.

Anyway, there are some safety guidelines that anybody willing to experiment with off-line SMPS should follow:

- Never power your SMPS prototypes directly from mains line, use isolation transformers instead. A variac is also recommended in order to test circuits at different voltages. In case of money shortages, several transformers scrapped from audio amplifiers may be used with their secondaries connected in series to get the desired voltage. For example I routinely use a 500VA unit with two 40V secondaries and a 750VA unit with two 60V secondaries, this metod is quite smart because I can test my circuits at 40V, 60V, 80V, 100V, 120V, 140V, 160V, 200V and even 20V AC, depending on how I connect the windings. Lower voltages are safer and render less catastrophic results in case of failure.

- Keep 5mm or more clearance between 'mains-side' and secondary side PCB tracks. Note that mixing both sides in the same breadboard is not recomended at all.

- When winding transformers, pay great attention to output isolation. There should be three layers of isolating tape between primary side and secondary side windings. Magnet wires from different sides should never be in direct contact, there must be either isolating tape, proper plastic tubing or 3mm of air inbetween. There should be 5mm or more clearance between solder pins of different sides.

- Use fuses rated at the smallest possible current placed before and after the isolation transformers. Use light bulbs or even halogen lamps placed in series with isolation transformer output, they work fine as current limiters. Use primary-side peak-current limiting or shutdown. All these tips are very useful to prevent catastrophic failures.

And finally, remember that there are some mistakes that will hurt you, others will put your house on fire, and some others will kill you, so always act at your own risk.

rjon17469 23rd July 2005 05:23 AM

Thanks for the tips!

My ultimate goal is a well-regulated, efficient supply (I guess that's nothing too unique;)). So however I can get that, assuming it's safe, reliable, and won't break the bank, is fine.

So, idea (I don't know if this has been said else where or not, but what you said just inspired me): get a standard toroidal transformer, say 30V (of good power rating), hook it up to the mains line and rectify/filter its output to DC, probably giving me around 40VDC. This will give me mains isolation and lower voltages to work with. Then feed this to a SMPS, which will regulate the voltage for the final output.

Good? Bad? Different? Too much work/cost for too little gain?

Eva 23rd July 2005 06:40 AM

Yes, this is how I tried my first half-bridges and full-bridges some years ago. I was using IRFP460 MOSFETs with transformers for isolated gate drive, and a SG3525 PWM IC with a potentiometer to adjust duty cycle and powered with an auxiliary supply. At first I didn't even have got power transformer cores nor ultra-fast rectifiers to experiment with, so I used lightbulbs as a load for the switches. It was very frustrating until I got an oscilloscope...

Also, I tested my first PFC prototype with 40V AC input, and with the circuit temporarily patched for producing only 200V DC output. When it worked fine I removed the patches and tested it with 120V AC input and nominal 440V output. New problems appeared... :)

rjon17469 23rd July 2005 05:23 PM

Hmmm, very cool. The parts you were using sound a lot like some of the parts I'm planning on. :)

So would that suit as a permenant design, having a step-down transformer before the actual SMPS?

Eva 23rd July 2005 09:17 PM

It depends on what you want. If you just want to learn and experiment with switching circuits then this kind of setup will be fine.

However, if you are more ambitious and have put your aims in efficient off-line SMPS design, then it will be just a begginer's setup for learning, training and testing. As you gain experience, your designs may start to be reliable with high (isolated) voltages like 120V or 230V AC and you may learn how to avoid risks and catastrophic failures.

After uncountable hours of testing, you might consider that your circuits are mature enough and met reasonable realiability, EMI and isolation requirements in order to be actually operated off-line.

Note that off-line operation is a potentially harmful practice where mistakes are not allowed. Shall you decide to do it, you will be following your own criteria and acting at your own risk.

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