High powered SMPS design

[tstitans]

I've been perusing these forums for a long time and finally registered Excited to get involved.

In reading, I've found a recurring theme and wanted to see if I could get consolidated info on it. A lot of users want to design high powered (kw range) converters and typically get responses like "start lower and move up" or "that's really hard for beginners" in between the advice and replies. Another one is that beginners (like me ) need a lot of guidance and time. I've built hobby low-powered SMPSs and understand the theory, but have no idea what problems start popping up at high powers.

In looking around, I found alot of info spread out across lots of threads, some of them many pages long.

I'm looking for somewhere that has basically a list of things to take into account as you increase power like:
-parasitic behavior to take into account
-physical design of PCB and case to isolate components
-peak currents that can cause avalanche breakdown in FETs
-ZVS for dummies (I've read a dozen articles on this and still don't understand how it differs from normal function - maybe I'm just not smart enough don't even understand what manages it, the PWM?)

and other things I've seen but don't fully understand. I'm in the process of getting what's been called the "bible" of SMPS by someone here (Chryssis, Pressman, and Brown), but I just finished college and am a bit burnt out on text books, so going through page-by-page of texts might take me a while. Is there a "complete guide to high power design considerations"? If not, maybe the pros here can post some replies and make this thread the goto guide!

[luka]

Well lets say what others and yourself already did, start at low voltages and make that work, trust me, I've been there, and I got nowhere until I undertood what is going on.

Also If you have low power to work, ist only steping up in components rating for higher power. Its nothing to it, to get 1kw+, but make it work under all and every condition it something else [like feedback, unregulated supply is easy, anyone could do in then sleep ]

After you get your answers on your qestions, make some schematic, so you can show, what you'll go with, and so that we change something if needed

Good luck

PS: I think I got those books you are looking for

[infinia]

Hi
What do you want to do?
Its really hard to write a list of things to watch out for with no specific topology or requirements. I think the advice for lower power is generally fair advice for someone with little to no experience. Also things start getting expensive with higher power esp if you start blowing stuff up and have to redo your PWB/magnetics a few times. What test equipment do you have or think you need for 1KW+ SMPS development?
The general knowledge you need covers a lot of ground .... what guides or experience you have with the list below can let you advance more quickly than most newbies.

Safety requirements
control theory
gate drive
magnetics design
snubbers
EMI/ PFC
V/I regulation/slow start/overcurrent/overvoltage

[star882]

What's the largest power supply you have built so far and what are its specifications? If that one works, the next one you should build should be just a bit bigger and more complex.

Then again, high power is not that difficult if the circuit is simple. Here's a buck converter rated for 72kw for an EV: http://ecomodder.com/forum/showthrea...ller-6404.html
Note that the builder has had very little previous electronics experience. But the circuit is simple as the windings in the motor act as the inductor.

Another good example of a high power but very simple power supply is the electronic controller for an electric tankless water heater. They usually just have two SCRs acting like a very large light dimmer. (It is considered a switching power supply since the power devices are practically completely on or off at any given instant.)

[tstitans]

Originally posted by luka
Also If you have low power to work, ist only steping up in components rating for higher power
...
PS: I think I got those books you are looking for

I have a half-bridge topology on a pretty small supply. I don't fully understand the magnetics involved and therefor the actual power limits based on the transformer, but I'm purposely current limiting it with a couple power resistors. I was under the impression that I'd have to change the topology for higher power, although I don't know why - the full bridge topologies still use FETS with similar currents, and you can always parallel the FETS if you need more power. Any insight on the need for different topologies?

Originally posted by infinia
Hi
What do you want to do?
Its really hard to write a list of things to watch out for with no specific topology or requirements. I think the advice for lower power is generally fair advice for someone with little to no experience. Also things start getting expensive with higher power esp if you start blowing stuff up and have to redo your PWB/magnetics a few times. What test equipment do you have or think you need for 1KW+ SMPS development?
The general knowledge you need covers a lot of ground .... what guides or experience you have with the list below can let you advance more quickly than most newbies.

Safety requirements
control theory
gate drive
magnetics design
snubbers
EMI/ PFC
V/I regulation/slow start/overcurrent/overvoltage

I'm not sure what I want to do I have a bunch of hobbies that will probably require specific power supplies, so I'm trying to learn about them (I used a flyback from an old CRT to build a little arc speaker, which makes potentials in the kV range! crazy, and I have no idea how it works). Mainly, I want to be able to convert Mains AC to DC (simple boost circuit or PFC to get 340-400Vdc or so) and then a DC-DC converter which will drop it down to some useful voltages, like 12 or 24. As far as test equipment, I have an o-scope, a multimeter, a few current sensing transformers I'm learning to use, and a variable power supply for my bench that can do up to 24V.

Experience...
safety requirements, I feel comfortable limiting power, but not sure what safety needs to be taken at higher voltages/currents. I know high voltages can arc, but I don't imagine I'll ever get that high unless I start building tesla coils
Control theory - I've build a PWM circuit that pulses at 100kHz and I can control the deadtime - beyond that, not sure.
Gate drive - some experience using MOSFET drivers, driver transformers (although I can't seem to get a clean square wave off of these). Things get much more difficult when the source voltage fluctuates like in these topologies, but I feel comfortable with it.
Magnetics design - can do some design calculations, but haven't built them myself. Do most people here build there own transformers? :|
Snubbers - used them on previous design, but it was mostly experimental, not calculation based. I have a few guides on how to do these, but don't know the best implementations. Currently, I've only ever used RC snubbers across Source-Drain on FETS.
EMI/PFC - Never played with EMI (I'm assuming you mean on input stages to avoid blowing breakers or affecting my neighbors?), and PFC - never built one, but I've used IR's design guide to make the schematic for one. Do these get crazy at high powers? I assumed it was a simple controller circuit with a switch FET, boost inductor, and output cap...
V/I regulation/slow start/overcurrent/overvoltage - mostly I've done this at lower powers with the Application Notes and Datasheets from my PWM controller with its built in safeties...does that work?

Originally posted by star882
What's the largest power supply you have built so far and what are its specifications?...

Another good example of a high power but very simple power supply is the electronic controller for an electric tankless water heater. They usually just have two SCRs acting like a very large light dimmer. (It is considered a switching power supply since the power devices are practically completely on or off at any given instant.)

I made a simple half-bridge topology SMPS limited to 50W (24V, limited to 2amps with a couple power resistors) or so because I have no idea what the transformer can handle. It works decently in the few setups I've used so far. I also use it with a 12V and 5V linear regulator (just ICs I bought) for powering a PIC controller and some fans. I've found a lot of info that suggests moving to full-bridge and ZVS topologies, with possibly a phase shift on the output (wouldn't a PFC input stage take care of that?) for 1kw+ range of power. Mainly, I've read that a lot of things (parasitic behavior?) creep up as you increase power, but I don't know what they are, what to watch for, etc. Are these conductive or can I avoid them by putting each stage in its own faraday cage? I dunno..

Do you have an example design for the SCR design? I've never used them before, but I imagine they can handle a lot more power than FETS...I just don't know enough theory or have enough experience with them to reliably turn them on and off...I vaguely remember that they have a strange issue with turning off. I'll start looking that up.

[luka]

well half bridge seems to be easyest, fullB just after it and can offer tons of power, like halfB and then some, then flayback is very cool, coz you can use small number of components, and very wide range of input voltage and then you also have buck and boost, this 2 are really great too

[tstitans]

I guess my biggest concern in going to higher power is that I don't know the non-ideal behavior of devices. Even with the low powered SMPS, my "square wave" isn't really square and I get little spikes in voltage on turn-on for the FETS.

The Vgs on the FETS isn't square at all, but like a steep curve up and then a less-steep curve down. I don't want the FETs to turn on at the wrong time and short the 340Vdc input through the FETs, which is the problem that I can guess at. I'm more worried about the problems I can't guess at

[star882]

Originally posted by tstitans
Do you have an example design for the SCR design? I've never used them before, but I imagine they can handle a lot more power than FETS...I just don't know enough theory or have enough experience with them to reliably turn them on and off...I vaguely remember that they have a strange issue with turning off. I'll start looking that up.

Look up "phase angle control". It is commonly used with resistive loads on AC circuits and certain kinds of small AC motors. Basically, a SCR, once turned on, stays on until the current through it drops below a certain level. AC does that for us 60 times every second for each polarity. So you have a circuit to detect zero crossings of the AC, then turn on the SCR after a variable delay. When the AC goes into another zero crossing, the SCR turns off. Since a SCR is only able to turn on in one direction, two are usually used to ensure full power is available to the load, each one switching on opposite polarities. (There are also triacs that can conduct in both directions, but they cannot handle as much power as SCRs.)

[infinia]

Hi
Test Equipment besides an O-scope, DVMs and lab DC supplies youll need at least a line AC variac, isolation transformer, Tektronics current probe, DC electronic loads. I like to add big analog meters (V and I) on the variac so I can see it out of the corner of my eyes.

Safety - http://focus.ti.com/lit/ml/slup227/slup227.pdf this is a good read on safety for supply designers. Gives most of the language and terms involved. Even DIY folks should heed the rules/ guidelines.

[infinia]

Control Theory - stability using feedback ie control systems. This is how you can design your SMPS error amplifier compensation scheme R and C values to ensure stability under variable conditions of line and load. You should understand why SMPS don't like no loads on the main loop.

Control Loop Cookbook - http://focus.ti.com/lit/ml/slup113a/slup113a.pdf
Bode plots - http://focus.ti.com/lit/ml/slup070/slup070.pdf