Reduced power with lateral fets ???

Tekko

Banned
2005-01-01 3:33 pm
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 ?
 
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
 
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.
 
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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
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

Banned
2005-01-01 3:33 pm
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.:D