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Old 24th January 2014, 04:58 PM   #1
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Default Matching/pairing MOSFETs

I am planning a power amp with two pairs of IRFP240/9240 driven by a LME49830. Do the MOSFETs absolutely need to be Vgs matched? If yes in which way: all four or just by N-ch/P-ch pairs? Thanks.

Jacques
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Old 24th January 2014, 05:11 PM   #2
h_a is offline h_a  Europe
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Hi,

typically matching of the paralleled mosfets (that would be N-N and P-P) is done to ensure equal turn on. For current sharing source degeneration is used.

N-P matching is rarely done and improves only even harmonic distortion cancellation a bit. However, the IRFP240 and IRFP9240 are much too different to benefit from that signficantly, so I would stick with N-N and P-P.

EDIT: I don't know your LME part, but does it include thermal compensation of the output stage? Otherwise you would need a vbe-multiplier or so to avoid thermal runaway of the output fets.

Last edited by h_a; 24th January 2014 at 05:14 PM.
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Old 24th January 2014, 05:26 PM   #3
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Quote:
Originally Posted by h_a View Post

N-P matching is rarely done and improves only even harmonic distortion cancellation a bit. However, the IRFP240 and IRFP9240 are much too different to benefit from that signficantly, so I would stick with N-N and P-P.

EDIT: I don't know your LME part, but does it include thermal compensation of the output stage? Otherwise you would need a vbe-multiplier or so to avoid thermal runaway of the output fets.
Thanks for the tip H_a. Are you still selling matched pairs?
And yes, a vbe multiplier (BD139 sharing the MOSFET heatsink) is there as well.
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Old 24th January 2014, 08:18 PM   #4
tvrgeek is offline tvrgeek  United States
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Huh? The tempco of a V-MOSFET is the other way. They don't go into thermal runaway to the best of my understanding. That is one of their big advantages. Many designs just use resistors for the bias spreader because of this.
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Old 24th January 2014, 09:52 PM   #5
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Quote:
Originally Posted by tvrgeek View Post
Huh? The tempco of a V-MOSFET is the other way. They don't go into thermal runaway to the best of my understanding. That is one of their big advantages. Many designs just use resistors for the bias spreader because of this.
I am not an expert but I always thought that vertical MOSFETs such as these IRPFs had a positive temperature coefficient (conduct more with temp) whereas lateral MOSFETs have a negative coefficient. Laterals usually don't require thermal compensation whereas verticals do. Please correct me if I am wrong.
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Old 24th January 2014, 10:10 PM   #6
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Just read a datasheet, Vgs-Id graph.
Temperature goes, Id goes up for a vertical MOSFET up till 5-10 amps drain current : +/+ means positive.

Laterals have a positive tempco up to 50-100mA, negative temperature coefficient from there up.
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Old 24th January 2014, 10:12 PM   #7
CBS240 is offline CBS240  United States
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You are correct. Mosfets like hexfets or plainer stripe fets require thermal compensation to operate with class AB bias. The N-type and P-type have different Gm figures. Nothing can be done about that aside from transistor choice but matching the like type pairs and appropriately sized source ballast resistors will help with current sharing.
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Old 25th January 2014, 12:31 PM   #8
AndrewT is offline AndrewT  Scotland
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a pair of irfp9140/240 make a better complementary pair, IF the supply rails are less than +-50Vdc.
for >+-50Vdc you have to use the 9240 for the p.

The parallel Ns must be matched for Vgs at their operating bias current.
Similarly the parallel Ps must be matched.

It would be nice to match at the operational Vds and operation temperature. This needs a lot of resources not generally available to us amateurs.

The Vfets MUST have a temperature compensating bias voltage. Very similar to BJTs.

Lateral mosFETs are the only device I know of that have a negative tempco at normal quiescent currents. i.e >100mA of output bias per pair.

The Source resistor does help in reducing the out of balance current. It does NOT eliminate it !!!!!!!!!! A higher value Source resistor also reduces the out of balance current.
Parallel outputs have in effect paralleled Source resistors. You can use a higher value Source resistor in parallel arrangements without reducing audio performance. Instead of 0r15 for a single pair, you could try 0r22 for 2pr and 0r33 for 3pr.
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Old 25th January 2014, 12:55 PM   #9
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Quote:
Originally Posted by AndrewT View Post
a pair of irfp9140/240 make a better complementary pair, IF the supply rails are less than +-50Vdc.
for >+-50Vdc you have to use the 9240 for the p.

The parallel Ns must be matched for Vgs at their operating bias current.
Similarly the parallel Ps must be matched.

It would be nice to match at the operational Vds and operation temperature. This needs a lot of resources not generally available to us amateurs.

The Vfets MUST have a temperature compensating bias voltage. Very similar to BJTs.

Lateral mosFETs are the only device I know of that have a negative tempco at normal quiescent currents. i.e >100mA of output bias per pair.

The Source resistor does help in reducing the out of balance current. It does NOT eliminate it !!!!!!!!!! A higher value Source resistor also reduces the out of balance current.
Parallel outputs have in effect paralleled Source resistors. You can use a higher value Source resistor in parallel arrangements without reducing audio performance. Instead of 0r15 for a single pair, you could try 0r22 for 2pr and 0r33 for 3pr.
Hi AndrewT,
But why we still use IRFP240-9240 in class A amplifier such as F5 or
F5 turbo with parallel outputs...
can we use 9140 instead?
or better not changed because it is class A

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Old 25th January 2014, 01:00 PM   #10
AndrewT is offline AndrewT  Scotland
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ClassA generally run at lower rail voltages.
9140/240 would make an excellent pair for F5, F5t etc.
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