| 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. |
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| 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 ...
:smash: |
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| 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. |
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| 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. |
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| VEC7OR |
:cannotbe: 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 ? |
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| 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. |
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| Pafi |
| quote: | | 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. |
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| 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. |
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| Gorilla |
Don't get caught up is the maximum r.m.s. power you want your amplifier to be able to output based on its supply rail voltage. Music isn't a square wave.
Also funny is a university friend and I have been thinking for a while about making full-bridge amp with similar specifications to what you have decribed. Its quite funny, as we made exactly that brain fart, and couldn't figure out why we were only getting half the expected power in simulation :cannotbe: for far to long. |
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| Pafi |
| quote: | | For 400W I need 40V rail. |
More then 56 V!
And don't forget about switching loss! |
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| VEC7OR |
| quote: | Originally posted by Pafi
More then 56 V!
And don't forget about switching loss! |
Why ? It is for 4 Ohm speaker.
And whats with switching loss ?
40V roughly gets us to 400W maybe a bit less, also I think I'll have a soft limiter with it (maybe LDR-LED combo) to make sure I dont get over ~95% of modulation.
And yeah, it is for a sub, 0-100Hz range.
Maybe I'll fit it with mixed/higher order feedback.
To reveal more of what I'm planning - it will probably use HIP4080 or 2x of IRS20124S driven by something fast like LT1016 (it's fast and has complementary outputs), it will have classical triangle + signal modulator with feedback added to triangle or to signal. |
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| Ouroboros |
Pafi is correct once again.
400W into 4 Ohms requires 40V rms ((40 x 40)/4 = 400)
40V rms is 113V pk-pk.
Even if there were no resistive losses anywhere in your amp you would still require a 56.5V power rail for a bridged amp. (or +/- 28.25V if usimng symetrical supplies).
You might just manage it with +/- 32V rails, or a 64V single supply. |
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| Pafi |
Thanks, Ouroboros!
| quote: | | And whats with switching loss ? |
You have to add it to conduction losses to get the value of dissipation. |
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| VEC7OR |
| Bah, you are right, I dont see any small packages that can pull this off, maybe DirectFETs, but I dont have hot air at home. IRF6644 maybe, at ~3W per case, BTW how do You calculate switching losses? As much as I googled on that roughly its (Ton + Toff)*I*U*f |
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| Pafi |
I'd rather say tr+tf instead of ton+toff. There is an additional loss due to reverse recovery charge. It's about qrr*U*f, but qrr is usually higher then specified on datasheet, because of higher di/dt and temperature. |
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| VEC7OR |
| Well, that what I meant initially, turn on/off delays are useless here, only rise/fall times count, I just wanted to know if there is another way/formula. |
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| VEC7OR |
So, SO-8 FETs kinda fall out of the picture, but anything that is easily reachable is kinda slow (leads to 10-20W of switching losses per transistor), but with DirectFET it possible to cut the losses to measly ~4.6W.
Another thing that I want to try is to run this Thing on SMPS, around 500-600W (half bridge topology, current mode control, ~50Khz) with some LM3886 dangling around. One thing bothers me, how ugly can all this get if the SMPSU is loaded by class D amp, at work I've seen how 3 phase 2kW supply can sing/chirp/make funny noises without synchronization...
Proper filtering, big caps, all boards two sided, maybe in a shield...
Why ? 500-600W of power needs ~1kVA trafo, and ouch, that one is expensive and heavy. |
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| Pafi |
| quote: | | So, SO-8 FETs kinda fall out of the picture, |
Maybe not, because this will be only an amplifier, and average power of a music signal is very much lower (10-20 dB!) then peak power. |
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| VEC7OR |
I've been playing around with output filters and found out one rather ugly thing.
I've assemled an function generator outputting 200khz squarewave at 50% duty cycle, followed by LC filter with with cutoff @ ~7kHz (100uH, 0.47uF), connected to 4 ohm resistor.
With resistor in place everything is OK, but then I've assembled a dummy load (from ESP site) with loudspeaker impedance (you can see the impedance plot on lower right, I've tricked Bode plotter to measure impedance by using voltage controlled current source on input :D ), things got real ugly, you can see system responce with dummy load at upper left image and the trace on lower left, with output being a full blown sine wave at the frequency of that resonance instead of flat line with small ripple.
The question is, how ugly this gets in reality ? It is obvious that on higher frequency load impedance rises and filter gets unloaded. Sure, you can compensate some of this with feedback but opamps tend to run out of gain at higher f... sure, for sub it OK, nor it will be reproduced, nor it will bother anyone, but having alot of ultrasonic noise in wire is kinda wrong. |
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| VEC7OR |
What, no replies ?
So far I've discussed it with couple of people and we think maybe an impedance compensation will make things right. |
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| BWRX |
| Put a series cap and resistor in parallel with the load and tweak the values to help reduce the resonant peaking. |
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| Gorilla |
Part of the reason why post-filter feedback makes the UCD such a sucess.
| quote: | | Put a series cap and resistor in parallel with the load and tweak the values to help reduce the resonant peaking. |
| quote: | | compensate some of this with feedback but opamps tend to run out of gain at higher f |
Some people call this a Zobel Network, and this is quite often used in linear amps as they tend to have a limited gain-banwidth product.
Also would you mind describing your plots...its hard to interpret them. |
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| BWRX |
| quote: | Originally posted by Gorilla
Some people call this a Zobel Network, and this is quite often used in linear amps as they tend to have a limited gain-banwidth product. |
Technically it isn't a Zobel network as implemented on the output of a class d amp. The purpose of a Zobel network is to level out the rising impedance of an inductive load. The purpose of the series resistor and cap on the class d output is to tame that resonant peak. |
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| Gorilla |
| quote: | | The purpose of the series resistor and cap on the class d output is to tame that resonant peak. |
Which is caused by the rising impedance of a speakers inductive load?
I do know where you are coming from tho, with the names traiditonal reference in Class AB design. |
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| BWRX |
| quote: | Originally posted by Gorilla
Which is caused by the rising impedance of a speakers inductive load? |
It's not just from the speaker's inductance, it comes from the reaction between the speaker's impedance and the amplifier's output filter. |
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