Reduced power with lateral fets ???

[Tekko]

Since i installed lateral mosfets in my mosfet citation 12, i am only getting half the power out on the onset of clipping.

With two IRFP450's i get 120W into 1.95 ohms and 100W into 3.9 ohms.

With lateral mosfets i only get 32 watts into 1.95 ohms and 50 watts into 3.9 ohms for the same supply voltage.

Why is this ? Does laterals need a higher drive voltage for a given output voltage ?

 

[jan.didden]

Since i installed lateral mosfets in my mosfet citation 12, i am only getting half the power out on the onset of clipping.

With two IRFP450's i get 120W into 1.95 ohms and 100W into 3.9 ohms.

With lateral mosfets i only get 32 watts into 1.95 ohms and 50 watts into 3.9 ohms for the same supply voltage.

Why is this ? Does laterals need a higher drive voltage for a given output voltage ?

There's probably two effects:
- the laterals need higher gate-to-surce voltage which means the drivers hit the rails with the output still apprciably below it;
- the laterals have significant internal resistance meaning that with higher output current (especially in low impedances like your 2 and 4 ohms) you lose significant voltage across the device.

Not sure which effect is dominant here.
You could check the difference in Vgs in the data sheets, that should make it clear.

jan didden

 

[wahab]

Since i installed lateral mosfets in my mosfet citation 12, i am only getting half the power out on the onset of clipping.

With two IRFP450's i get 120W into 1.95 ohms and 100W into 3.9 ohms.

With lateral mosfets i only get 32 watts into 1.95 ohms and 50 watts into 3.9 ohms for the same supply voltage.

Why is this ? Does laterals need a higher drive voltage for a given output voltage ?

Their transconductance is much lower,about 1 to 2S, meaning that you
have to paralel two or better, three, to have limited losses with low loads.
Otherwise, they have lower threshold voltage than the verticals IRFPs.

 

[Mooly]

Laterals (a single pair) are not suitable for driving such low impedance loads. The on resistance is far higher than a HEXFET.

Are they "double die" laterals you are using ?

 

[AndrewT]

With two IRFP450's i get 120W into 1.95 ohms and 100W into 3.9 ohms.

is this a quasi complementary that has 1pr of Nchannel devices?

1pair of laterals is probably good for 60W to 90W into 8ohms.
2pair of laterals for 120W to 180W into 8ohms.
4pair of laterals for 240 to 360W into 4ohms.

 

[Tekko]

They are the ACD100NSD model, the same ones used in the NX200 200W into 4 ohms amp from Class D

I do think about adding the second pair, i bought four of them.

 

[Mooly]

Why did you want to fit Laterals ?

Just looked at the price of those ACD100NSD... outrageous

 

[Mooly]

I take it this is the amp,
http://cygnus.ipal.org/mirror/www.passlabs.com/images/citation.pdf

 

[Tekko]

Yup!

I wanned to try out laterals since they are made for audio, nut my ears cannot detect any difference in sound quality.

Btw i've heard these laterals were cheap compared to other laterals.

Heres a video: YouTube - HK pass modified citation 12 Lateral mosfet

 

[wahab]

Why did you want to fit Laterals ?

Just looked at the price of those ACD100NSD... outrageous

Not that much, although he better had opted for 2SJ162/2SK1058,
even this mean more pairs..

Class D - ACD100NSD Lateral Mosfets

 

[Mooly]

Laterals are great for audio, but they are not really suitable for those low impedance loads unless the amp is designed to use them, and certainly not as a drop in replacement for HEXFET's. The characteristics are totally different. If you have the bias generator to sense temperatue (as with bjt's and HEXFET's) then doing he same with laterals will probably give an unstable current value as the lateral has a negative tempco. You might find the current drops too much giving crossover distortion.

Some laterals here,
Magnatec. ALFET Lateral MOSFETs

 

[Tekko]

I had to change the bias pot from 5k to 15k to be able to get the bias down to 100mA, with the same value as 100mA with hexfets i wasd getting like 500mA when i installed the laterals.

 

[Mooly]

Turn on voltage is much lower for laterals as you have discovered.

Prices,
ECW20N20-Z

 

[Tekko]

So i think i'll go back to hexfets in this amp and dream up something else for the laterals.

 

[Mooly]

So i think i'll go back to hexfets in this amp and dream up something else for the laterals.

Why dream when there is this
http://www.diyaudio.com/forums/solid-state/119151-my-mosfet-amplifier-designed-music.html

 

[Tekko]

I dident get any P channel devices. Generally i dislike p channel devices and secondly i read somewhere that quasicomplementary performs better than complementary.

Either way this amp packs some real punch.

 

[Mooly]

As long as you are happy with the "12" that's all that matters. I'd refit the HEXFET's anyway.

 

[Tekko]

Now i have gone back to IRFP450s as output devices and this time i got 132W into 3.9 ohms at the onset of clipping!!

Tho i dunno if this was before or after the dummyload got so hot that a solder joint melted and disconnected.

And squarewave response is beautiful!! No overshoot or ringing.