# SMPS Full-Bridge Converter

• posted

Certainly help yourself to a first project. But our advice is to stick to low power levels at first. This is simply to protect you from burning out all kinds of stuff and becoming discouraged.

If you make a 1.3kW supply, the currents involved will be 20 times more than in a 65-watt supply, all other things equal. Resistive power losses, which create heat, given by P = I^2R, will be 400 times higher, unless you can reduce R by say 50x, so the loss will only be 8x higher. But reducing R by 50x means the gate switching capacitance of the large power FETs will be 50x higher, leading to all kinds of problems, compounded by a need to switch faster.

Recall that every time you switch a current you create an inductive voltage spike, given by V = L dI/dt. When you make a 1.3kW supply its dI term is 20 times more than a 65-watt supply, all other things equal. Furthermore, with high capacitance FETs and large currents being switched, you'll learn about switching losses and the need to reduce switching times in large power supplies. This means dt must be smaller, say 10x faster than you could safely get away with in a 65W supply. Therefore the V = L dI/dt formula tells us the induced inductive spikes will be 200x larger in a hypothetical 1.3kW supply.

For example, a 12V battery powering a 1.3kW supply. In an H-bridge we'll be switching more than 108 amps. We determine that to protect our FETs they have to switch in 30ns. That's 3.56 x 10^9 A/s. Now, if our FETs have 25nH of total source-lead inductance, which is very little and requires effort to achieve, the source-voltage spike will be 89 volts. Which far exceeds the 20V gate-voltage failure limit.

The way you'll experience this is that the converter may work fine at low load levels, but as you turn up the power, it'll suddenly fail. No doubt with substantial explosive accompaniment. When you look to find the part failure, you'll discover there are many failed parts, because as one fails it takes out others, making it hard to determine the exact failure sequence.

Our advice is to start out at low power levels and learn as you go.

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Thanks,
- Win```
• posted

Talking about serious power, a company I worked for had among their products a helicopter jump starter powered by 3 phase 380V. It could supply 1500A@24V

I never bothered to find out how this device exactly worked, I believe it chopped up the high voltage with massive thyristors and rectifying it with big green diode's that looked like stacked plates.

I used to be amazed by the heat the dummy load produced and how hot the wrist thick cables got ;)

Jeroen

• posted

With so much fierce stuff going on, be thankful that noise, in the sense we normally think of it, isn't a factor. Consider a MOSFET gate, with a typical gate capacitance of 2nF. Think of a typical 50MHz equivalent switching bandwidth. The impedance of 2nF at 50MHz is under two ohms. As you know, low impedances like that won't be sensitive to "noise" pickup, especially considering the high voltage levels required to change the on/off state of a power MOSFET.

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Thanks,
- Win```
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• posted

I have seen *If memory is correct* a 75kva full bridge converter switching at 500kHz. It was a research project at UW-Madison 10 years ago. It was about the size of a toaster oven. I am building a 1.5kw pep full bridge modulator that will fit into a small cigar box. regards, Bob

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Well right now I have thought about it and I'm going to try and make some sort of power supply from some solar panels. Like a couple 9V @ 80mA ones! Would be cool to power some of my robotics from it.

• posted

That's impressive. Usually such high-power units are made from many paralleled lower-power modules. For 75kW use fifteen 5kW modules, or ten 7.5kW modules, etc. Whether 5kW, 7.5kW, 15kW or whatever in each module, that's a lot of power to run at 500kHz, so I'm impressed.

I made a 500W 600kHz H-bridge with phase-shift controller, in about half a cigar box. But it was painful to get at stuff, three layers, etc., so when making a second one recently, I doubled the box size.

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Thanks,
- Win```

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