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sg3525 gets mosfets heated
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Zero999:
Oh, I remember now. You want to make an inverter on the cheap, so it can be used in poor countries.

This is not the way to make the cheapest inverter possible.

Big, iron cored transformers are relatively expensive. The cheapest way is to use a small, high frequency transformer to build a DC:DC converter to boost the 12V to the peak voltage of the mains, so 340V, assuming 240V out and add the H-bridge to convert the 340VDC to 230VAC on the secondary side. If you really want to make it cheap, then you can skip the isolation and voltage regulation. I accept it may seem more complex, but the it works out cheaper, because the high frequency transformer in the DC:DC converter is a fraction of the size of a mains frequency transformer and as the power level is increased further, the cost saving becomes ever greater.
T3sl4co1l:
Iron core transformers may not be all that unavailable.  Keep in mind 3rd world countries get all our junk.  Which includes tonnes of wall warts that are too heavy for our tastes anymore.

I've built that before, which is also basically a commercial unit (which you might rightly ask, isn't very expensive either so why not just buy one and have it shipped in? -- but therein lies the problem, what if you can't even do that?).



Shame I didn't write down the schematic, I don't remember what all it does.  Bottom-center: TL494 control board (with opto feedback, I think), bottom left: push-pull forward converter (think there's a dual gate driver on the board).  Current mode control.  Also a UC3843 flyback converter for HV side supply; this is a fully isolated design.  Transformer is good for about 200W, and runs at 100-some kHz I think.  FWB output and choke filter.  Uses SiC schottky because I can (they run very cool; you'll have to be mindful of heat dissipation using FR307s or whatever).

Power (170V DC) crosses to the inverter board on the right, a TL494 at ~120Hz and fixed (~50%) duty, and, I forget what I did for current limiting but there is active current limiting at the output.  Chokes on the breadboard are for output filtering, and I later added some film caps to filter things further.

Output: "modified sine" (50% pulsed bipolar) waveform, correct peak and RMS but dV/dt obviously wrong.  Current limiting gives excellent performance into capacitive loads and short circuits (I didn't test shorts, come to think of it, but it should be okay for at least a little while..).

But as you can see, it takes a lot of parts to do this, and a heck of a lot less to use an iron core transformer.

OP: would it not also be attractive to use BJTs instead of MOSFETs?  A discrete circuit could be made, without needing anything more than C1815 and A945 (or any other GP BJTs), some power transistors, and which drives a 12V winding H-bridge style, 100% square wave, without current limiting.  (And could add current limiting for probably less than a dozen additional components.)

Tim
Zero999:

--- Quote from: T3sl4co1l on December 30, 2019, 11:57:29 am ---Iron core transformers may not be all that unavailable.  Keep in mind 3rd world countries get all our junk.
--- End quote ---
That's a good point, although the schematic in the original post shows a 60W (the apparent power will probably be higher) fan which is too big for a wall wart transformer, unless lots are used in series/parallel which would be very bulky and inefficient.

I wonder if it's possible to overclock a wall wart transformer by using it a higher frequency? A 120V unit could be run at double the primary voltage and 120Hz, to give 240V, thus double the power rating. Although it will be difficult to find the correct ratio to get 340V out with 12V in and a wall wart rated to even 30VA is still rare.

How about using salvaged cores from old PC switched mode power supplies? It shouldn't be too difficult to design something which would work with a rewound PC PSU transformer.

Another thing to bear in mind is the load. A modified sine wave is unsuitable for powering refrigerators and cheap LED lamps which use capacitive voltage droppers.
T3sl4co1l:
Just an example, anything will do of course.  Hm, I wonder what offhand would use a lot of transformers that size, it's enough VAs for a medium TV or small desktop PC, but neither of those have used iron transformers, almost forever...  Audio amps of course, but always at higher voltages.

In any case, OP seems to be confident in their supply.  Yes, rewinding is an option, cheap labor and recyclable materials (and a lax demand for safety compliance) would allow lots of mix and match.

Seems like 12-20V 50-300W SMPS (power packs, laptop chargers, etc.) are hella common these days (and have been for a decade or more), for which a solution like I suggested in the other thread would be fine,
https://www.eevblog.com/forum/beginners/mc34063-high-voltage-dc-dc-boost-converter/msg2850352/#msg2850352

Tim
Zero999:
That seems reasonable. I suppose if you really want to cheap out, skip the SG3525 and go for a self-oscillating topology.

I wonder how well a small mains frequency transformer would work for driving the MOSFET h-bridge on the secondary side? Obviously it would need to be rewound to give the desired gate drive voltage and the mains PWM circuit buffered with snubber diodes to work properly.
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