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Old 7th December 2012, 02:17 AM   #11
AKSA is offline AKSA  Australia
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I have used FQA36P15 and FQA40N25.
These are not well suitable to audio, but their 300W Pd rates and their cheap make them attractive.
Both have high gm, around 30S.
It is mandatory that 47R and 220pF series snubber between gate and drain to prevent cross conduction.
Powerful gate drive is required, but they perform well.


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Old 7th December 2012, 03:43 AM   #12
DUG is offline DUG  Canada
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Junm: "You mean Crss, Coss and Ciss..."

Yes, I grouped them together as I usually work with these parameters in a switching mode where Ciss and Crss interact (as Miller capacitance) to create a simpler model of "Gate Charge".

For linear operation Ciss and Crss still interact but not quite in the same way.

I would match Ciss as a first priority. Crss second and I don't use too much calculating time on Coss.

You will find that if you get P and N channel closely matched for the C's then the max current can be as much as 60% different.

That is why I think that:

"Max V, max I, SOA, Pdiss, don't need to match, just make sure they exceed requirements."

There will be variations in the gain curve of each but I feel that the feedback circuit should take care of that.

Doug We are all learning...we can all help
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Old 7th December 2012, 04:08 AM   #13
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Yes, it seems the only choice from Fairchild for the P-channel device is the FQA36P15. Other N-channel that look like they may play well with the noted P-channel device are the FQA28N15 and FQA32N20C.

Ok, so I agree that voltage, current, dissipation are not important parameters to match so long as the weakest link is strong enough so to speak. I have been looking at input capacitance, transconductance and threshold voltage as my primary parameters to match as close as possible. I figure the closer the devices match in the most critical parameters, the more the NFB goes to keeping the amplifier linear and less to making up for grossly different devices in the first place.

So, the IRFP240/IRFP9240 appear to be in common use. What else are folks using? I'm looking to see what is being used in the DIY community for MOSFETs that are relatively inexpensive and readily available. If you have a favourite set I'd like to hear from you!
---Jason Kuetemann---
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Old 7th December 2012, 04:12 AM   #14
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So, an equivalent p-channel mosfet will always have a larger die than an equivalent n-channel device. Maybe try matching an N-channel part with a p-channel one die size larger. 9540 vs. 530, for example.
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Old 7th December 2012, 04:20 AM   #15
CBS240 is offline CBS240  United States
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It depends on the topology as to which capacitance is more effective to the circuit. If you intend to use them as complementary source follower, Cgs is for the most part bootstrapped. You have the change in Vgs vs Id that will be effected by Cgs directly. If the load is driven by the drain then Cgs is directly driven by the previous stage. Cgd is really the problem when dealing with complementary follower. When Vds becomes small at clipping, Cgd increases exponentially and can reak havoc. (This is where a lot of the distortion mosfets create come from.) For this reason a strong driver stage is needed to reduce distorton, prevent rail sticking, cross conduction leading to failed components. Also Cgd, being a variable dependent on Vds, can cause the mosfet to resonate at a frequency dependent on that and the lead and PCB trace inductances. This usually happens under load when the signal peaks and Vds is reduced. There are simple ways to remedy this.

Originally Posted by wrenchone View Post
So, an equivalent p-channel mosfet will always have a larger die than an equivalent n-channel device. Maybe try matching an N-channel part with a p-channel one die size larger. 9540 vs. 530, for example.
Due to this fact, the P-ch of equal Gm tends to be more robust. Also the input capactitance is higher. This is good because the N-channel is usually much cheaper so if you design the circuit so that a burnt ouput device results in deactivation of the amplifier output stage(s) then the fault of overdriving the amp would be on the cheaper N-ch device. This way if you overdrive and blow the OPS, you simply replace a transistor that costs maybe a dollar.
All the trouble I've ever been in started out as fun......

Last edited by CBS240; 7th December 2012 at 04:28 AM.
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