Class D vs speaker impedance

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u301021

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#1
What determines whether a Class D can withstand playing in 4, 2 or 1 ohm?
The output filter will of course differently frequency rolloff and the coil in the output filter must be able to carry the current.
However, when the FETs "only" switch on and off, they can probably not be compared with, for example, a bipolar transistor in a class A / B amplifier ?

If a class D amplifier uses a pair of IRFB23n20d, does that say anything about how low impedance the amplifier can be loaded with?

The powersupply to the amp are 2 x 44 Vac (2 x 62 Vdc)

http://www.irf.com/product-info/datasheets/data/irfb23n20d.pdf
 
#3
#4
Dissipation in a FET (or filter inductor) goes with the square of current, thus with the inverse square of load impedance, so a 2 ohm load is 4 times as stressful as a 4 ohm load and 16 times as stressful as an 8 ohm load, etc.

Also inductors have a saturation current - exceed that and its just an piece of wire, with negligible impedance to the switching frequency, so you start putting substantial EMI into the environment. And ferrite inductors tend to degrade with heat too, so the effects of overload may become permanent. Or put another way lower load impedance means larger filter components are required.

The IRFB23N20D has 0.1 ohms on-resistance, so you have 0.2 ohms of loss just from the output devices (ignoring switching losses), which is 10% of a 2 ohm load for instance, not including filter and wiring losses. That's about 1dB lower output and lots of heat to dissipate (still better than class B!)

The IRFB23N20D is a pretty poor performer, these days you can get many better MOSFETs, for instance the SQP90142E has 0.015 ohms and yet is the same gate charge rating so just as easy to drive. That would reduce FET resistive losses to 1.5% for a 2 ohm load (switching losses might be the limiting factor though).
 
#5
Thomas C said:
Do you think it will be able to drive 2 ohms with a little extra cooling?
Click to expand...
Given its AliExpress I suspect the specs are very "optimistic". I doubt you'll get the rated power before it smokes and trying to give it 2 ohms is a bad idea (its probably not going to like 4 ohms much). Its likely to have very little in the way of protection circuitry too.

Did you see this line:

the sound field is wide. The stereoscopic effect is strong
Click to expand...
Which is impressive for a mono amp module...
 
#9
Mark Tillotson said:
Dissipation in a FET (or filter inductor) goes with the square of current, thus with the inverse square of load impedance, so a 2 ohm load is 4 times as stressful as a 4 ohm load and 16 times as stressful as an 8 ohm load, etc.

Also inductors have a saturation current - exceed that and its just an piece of wire, with negligible impedance to the switching frequency, so you start putting substantial EMI into the environment. And ferrite inductors tend to degrade with heat too, so the effects of overload may become permanent. Or put another way lower load impedance means larger filter components are required.

The IRFB23N20D has 0.1 ohms on-resistance, so you have 0.2 ohms of loss just from the output devices (ignoring switching losses), which is 10% of a 2 ohm load for instance, not including filter and wiring losses. That's about 1dB lower output and lots of heat to dissipate (still better than class B!)

The IRFB23N20D is a pretty poor performer, these days you can get many better MOSFETs, for instance the SQP90142E has 0.015 ohms and yet is the same gate charge rating so just as easy to drive. That would reduce FET resistive losses to 1.5% for a 2 ohm load (switching losses might be the limiting factor though).
Click to expand...

SQP90142E is a very good one same like IRFB 4227
 
#10
Thomas C said:
Ok, thank you Mark

Can you switch the FETs to the ones you recommend without any other intervention?
Click to expand...

SHORT answer: NO!!!!

Maybe wrong glasses but can´t find on that board the large output inductors that amp needs, can somebody find and point them to me?

If not, that amp is KRRAPP, only suitable (if that much) to be mounted inside a Subwoofer cabinet, with a few inches of speaker wire, and relying on typical Subwoofer high VC inductance.

And even so .... 🙄
 
#12
JMFahey said:
SHORT answer: NO!!!!

Maybe wrong glasses but can´t find on that board the large output inductors that amp needs, can somebody find and point them to me?

If not, that amp is KRRAPP, only suitable (if that much) to be mounted inside a Subwoofer cabinet, with a few inches of speaker wire, and relying on typical Subwoofer high VC inductance.

And even so .... 🙄
Click to expand...

61-TQyl-AG3v-L-AC-SY355.jpg
 
#13
NMOS said:
Dont play with Class D without switching power skills
Click to expand...

I have tried and used
T3 mono TK2050 amp, Zappulse, Pascal S pro2

T3 tk2050 runs 4 ohm without problems
Pascal S Pro2 runs 2 ohm without problems
Zap pulse 2,3 SE runs 1 ohm without any problems.

I just want to know a little about these IRAUD boards

It sounds weird for me, that even a T3 tk2050 can play in 4 ohm, but these Iraud amp can´t
 
#17
Yes they are similar to each other, the capacitors, the output coil and the heat sink, but you do not know if they are the same transistors beyond the 2092
Measurements from a completely different board are also mentioned and shown; IRAUDAMP7D-150
 
#20
Started out with an 8 ohm unit and about 1/3 level on the phone. After about 5 min a little more level for 5 min and then full level for about 10 min.
There is not enough level from the phone for me to get the module to clip.

Same as above but with 2 pcs 8 ohms in parallel, full level from phone to the module in 4 ohms for about 15 min - no clipping

Last attempt today was the same as the first just 3 pcs 8 ohm units in parallel, about 2.7 ohms.
Measured temp on the FETs´ and heat sink to see how it behaved as time went on.

Of course still not enough output from tel, but the oscilloscope said about +/- 50 V maybe +/- 55 V in 2.7 ohms and without clipping.
Temp was 59 degrees directly on the FETs and 42 degrees on the cooling plate when the load was 2.7 ohms and all the level I could give it and that's how I was allowed to play for about an hour.

Well done a module for that kind of money

246730648-1031089114355542-4663289357934908848-n.jpg


247145149-184964057118916-8204736503894108732-n.jpg
 
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