High power AMP design

[ecotack]

I need to design a high power voltage follower and most of my Googling keeps bringing me here, so although its not an audio circuit, I hope you will forgive me and offer some suggestions.

I need an output range of +/-50V and 50A. A 1 ohm load will be expected so it must be capable of 50A at 50V. The other main requirement is reasonable linearity through 0V. Our first effort using several high powered op-amps just went up in smoke.

[firechief]

What is the maximum frequency? Talk a little bit about your earlier efforts , please.

[batteryman]

Originally Posted by ecotack

I need to design a high power voltage follower and most of my Googling keeps bringing me here, so although its not an audio circuit, I hope you will forgive me and offer some suggestions.

I need an output range of +/-50V and 50A. A 1 ohm load will be expected so it must be capable of 50A at 50V. The other main requirement is reasonable linearity through 0V. Our first effort using several high powered op-amps just went up in smoke.

Did you not try some paralled power transistors driven from one of your high powered op amps?
How are you varying the voltage? I.e does it have to swing from - 50 to + 50 at 50amps, or is it dual 50v 50a per rail?
Ian

[a.wayne]

I like this guy ....

[firechief]

Tall order, to say the least.

[alizarraga]

Hello your project sound like a programable waveform linear power supply, you can do it with a ixys linear power mosfets have a great SOA.

[Conrad Hoffman]

IMO, the first thing you need to do is look at some SOA diagrams for various output devices. Those will give you an idea of what's necessary to do 50A @ 50V. You need a detailed and intimate understanding of what those diagrams mean. Amplifiers are often designed backwards, so from that decide how many output transistors it will take to do the job, then what it will take to drive the outputs, working your way back to the input. Get a copy of Doug Self's amplifier book and Bob Cordell's amplifier book. All will be revealed!

[simon7000]

With a really good heatsink and fan you can get about 80 watts of power from a well mounted "150 watt" power transistor. So if you are actually going to want to do 50 v at 50 a you might think you require 35 power transistors on each rail! However if you set your rail voltage to 55 volts then when the output is 50 volts there is only five volts across the output transistors. So the power requirement would drop to 4 transistors. But as you should understand at 0 volts out into a resistor there would be 0 current so no power dissipation would be required. At 27 volts out the power requirement would be 10 transistors but only if the load is purely resistive!

So if you are using an AC waveform half of the transistors would be on at a time so it would be five per rail.

If you want a general purpose power supply that will not fail under any conditions It should really be about 40 power transistors per rail.

One amplifier manufacturer starts out with the minimum number and adds more output devices if they get warranty failures.

For example using ON Semi MJL3281A (NPN) MJL1302A (PNP) A safe minimum gain for the output stage would be 30 so a drive current of 1.67 a would be required. Also use emitter resistors of around .22 ohm on each transistor.

[a.wayne]

Originally Posted by Conrad Hoffman

IMO, the first thing you need to do is look at some SOA diagrams for various output devices. Those will give you an idea of what's necessary to do 50A @ 50V. You need a detailed and intimate understanding of what those diagrams mean. Amplifiers are often designed backwards, so from that decide how many output transistors it will take to do the job, then what it will take to drive the outputs, working your way back to the input. Get a copy of Doug Self's amplifier book and Bob Cordell's amplifier book. All will be revealed!

He possibly could buy that much power for the same cost ... i kid, i kid ..

[batteryman]

Originally Posted by simon7000

With a really good heatsink and fan you can get about 80 watts of power from a well mounted "150 watt" power transistor. So if you are actually going to want to do 50 v at 50 a you might think you require 35 power transistors on each rail! However if you set your rail voltage to 55 volts then when the output is 50 volts there is only five volts across the output transistors. So the power requirement would drop to 4 transistors. But as you should understand at 0 volts out into a resistor there would be 0 current so no power dissipation would be required. At 27 volts out the power requirement would be 10 transistors but only if the load is purely resistive!

So if you are using an AC waveform half of the transistors would be on at a time so it would be five per rail.

If you want a general purpose power supply that will not fail under any conditions It should really be about 40 power transistors per rail.

One amplifier manufacturer starts out with the minimum number and adds more output devices if they get warranty failures.

For example using ON Semi MJL3281A (NPN) MJL1302A (PNP) A safe minimum gain for the output stage would be 30 so a drive current of 1.67 a would be required. Also use emitter resistors of around .22 ohm on each transistor.

Another option would be a multi tapped mains transformer that keeps the dc input closer to the output thus reducing the dissipation at low output volts at high current.