Bridged class D on small mfets

[VEC7OR]

Question mainly for class D gurus.

Is it possible to pull off something like 400W in 4 Ohms in full bridge configuration with something like IRF7468 by using 2 per corner on 200kHz

As I see it:

IRF7468 is an 40V 15.5mOhm device with fast swithing times and small gate charge.
Assuming 30V rail and 0.95 max modulation index, we get ~40V swing across the load, this equates to 10A rms current through 4ohm load ant that is in turn ~400W. By using 2 devices we get ~7.5mOhm, and we drop ~0.4W per FET on Rdson, adding switching losses that this grows to roughly ~1.1W per FET with 5A. And you can dissipate all of that by copper, yeah I need to do thermal calcs but I think it wont get that ugly.

So is it doable or my calculation is fundamentally flawed somewhere ?

P.S. Another thing I considered is DirectFET, but those are rather tricky to solder without stencils.

[FastEddy]

... looking at the NX-150 at http://aussieamplifiers.com ...

[VEC7OR]

Originally posted by FastEddy
... looking at the NX-150 at http://aussieamplifiers.com ...

Um why ?

[FastEddy]

... Because you wanted an example of a Class-D full bridge of 400 watts > 4 ohms ...

" ... Is it possible to pull off something like 400W in 4 Ohms in full bridge configuration ..."

I consider the author at http://aussieamplifiers.com to be a decent "guru" ... you might drop him a line directly for a more definitive answer.

I realise that you are trying to use your own parts for this, but maybe considering alternate power MOSFETs and configurations will produce a good or better answer. Granted the NX-150 runs at a considerably higher speed ... but, truth is one, paths are many ...

[VEC7OR]

But isn't NX-150 a linear amplifier ? Maybe I missed a point but I think It IS, and it isn't full bridge either.

[BWRX]

If you're using a 30V rail with a brdiged configuration then the max voltage swing across the load will be 30V and the max power into 4 ohms will be (30^2)/4 = 225W. That's assuming the output stage will be able to source and sink ample current for the 4 ohm load.

With a 40V rail (definitely not a good idea with FETs rated at 40V) the max power would be 400W. RMS power will obviously be lower.

[VEC7OR]

I'm tripping or something?

In full bridge config we can get 30V across the speaker one way (+/-) and another (-/+), this results in full 60V p-p swing, converting this to RMS is ~40V, and that gives 10A of current, and 40x10 is 400W, isnt it ?

[BWRX]

Think of a bridged amp this way:

You have each side of the load connected to the middle of a half bridge. The middle of each half bridge is sitting at half the supply voltage with no input signal. When the input signal has enough amplitude to drive the output signal to the rails, one half bridge will be pulling one side of the load to the rail voltage while the other half bridge will be pulling the other side of the load to ground. Thus, it will only see 30Vpp with a 30V rail.

If you had a brdiged output stage with +/-30V supply then you could get 60Vpp across the load. That would give you a maximum of 450Wrms into a 4 ohm load.

[Pafi]

60V p-p swing, converting this to RMS is ~40V,

No, 60 V peak-to-peak is 30 V peak (to zero), ~21V eff!

It's obvious that a bridge cannot put out more then power supply voltage! Peak-to-peak is not a real voltage, just a calculation.

[VEC7OR]

O_o OK, that was a major brain fart, what I thought was correct for amps with +/- supplies and this is not the case.

Back on topic.
For 400W I need 40V rail. That means we need voltage rating atleast 55V. I think maybe IRF7855 could do it ?
9.4mOhm, 26nC.
I estimate ~1.25W per device (more when temp rises) with 8 total devices.

Why SO-8 ? - at where I work (SMPS production, roughly PSU/UPS for telecomms), I saw an attempt to do a 300W 48>220V inverter and they had problems with magnetics, so it is shelved for some time, but the fun part that it was made with SMD FETS.