Bridge With IRF 540 & IRF 9540

A single pair will be just about OK with 8 ohm load on +/- 50V supply, 4 ohms will be dicing with fire. Basically you should expect no more than about 75 watts RMS from a single pair. You can then use this basis to determine voltage rails (and bridge specs) and fuse ratings for your chosen load impedance.

One pair = 75W so however many pairs you use you multiply by that number.

Assuming 50V rails with 5V rail loss and 4 ohm load, a single-ended (un-bridged) amp would be:

Ppeak = (V * V) / R = (45 * 45) / 4 = 506 W

Prms = Ppeak / 2 = 506 / 2 = 253 W

253 / 75 = 3.37

Therefore to run this you would need 4 pairs of output devices.

If you were to build two of these single-ended amps you would have a 2x 253 W into 4 ohm amp and use a grand total of 8 pairs of output devices for the stereo pair. If you were to bridge them, you would be limited to no lower than 8 ohm load (as each amp sees half the load impedance) so you would have a 1x 506 W into 8 ohm amp.

Note also that you must use IRF540 and IRF9540 not the newer IRF540N and IRF9540N devices as these newer types will not cope well with linear amplifier application and may go up in smoke.

With one pair of these devices you can get without problems 100 W into four Ohms...

Hello friends

Some years ago i had build a power amp with this mosfets.
Bridge amp with +- 43 volts supply,2 pair of IRF540 and 2 pairs of 9540 each amp(total 8 mos x channel)with low amount of NFB.
Some difficulties were taken place.
At the first the dumping factor of 25 / 8 Ohms,for me too much low.
Another problem of this mosfets is the difference between us , like result in a great number of refuse:40 mosfets in order to use 8.Neglect this selection,the current would be different and distortion increase.
I advice against to construct an amp whit this topology,i think that a bipolar is much better for a bridge power amp.

It would appeal to me that someone can reply to this

Yes there can be a problem with current sharing because HEXFETs are not as tightly matched, this can be made less of a problem by using 0.33 ohm source resistors.

Without seeing a schematic it's impossible for me to comment further.

That amp will blow up after time because it has no thermal compensation.

Are you going to try and lay that board out yourself? Because it will be quite sensitive to board layout, a simpler design would be advised.

You would add additional parallel output devices by replicating the gate resistor (220R), MOSFET and source resistor (0R22).

I suggest you just build a proven design that is right for your needs and skill level, or you will simply waste time and parts and get frustrated.

You need to measure from the middle of the sine wave to the peak, not peak to peak.

Testing with under-rated fuses will not protect anything, you need a current limited supply and test with lower voltage progressively stepping up.

I have already told you my thoughts on this circuit, you are experiencing exactly what I predicted. The power of the MOSFET is not the issue, the flawed circuit is.

It won't matter if you bridge or not, the circuit has inherent problems as I said.

richie00boy is right, you're wasting your time and money on that circuit.

If you want a bridge amp to run on ±50V, I suggest you use two pair of the MJ15003/04 per half bridge (a total of eight outputs per channel). Crown does this in their Macrotech 1200, a good design.

Depending on how 'stiff' your supply is, you will get about 500W~600W at 4 ohms.

http://www.crownaudio.com/gen_htm/legacy/legacamp.htm

I can show you a design idea for a IRF 640/9640 amp to put out 600W into 4 ohms. It would need ±95V and have eight pairs per channel (16 total). For bias, each transistor could be configured as a constant current source with its own opamp driver. Four quad opamps would be required, plus one more to drive the whole thing.

A conceptual schematic:

http://www.diyaudio.com/forums/attachment.php?postid=182299

Any power amp can be bridged provided the power supplies are sharing the same ground. Be aware though that you must use 2x impedance for bridge, i.e. for 2x 100W into 4 ohm amp, you must use 8 ohm when bridging.