Stacking Mosfets for ClassAB Amplifier

ecool811 · 2nd April 2010, 05:40 PM

Guys,

My first post and thanks for any input.

I guess I wanted to hear some thoughts on things I need to think about when stacking P or N Mosfets (to get to higher voltages) for a ClassAB amplifier. Is a resistor divider from the rails good enough? What about charging the Gates for the top and bottom Fets.

Circuit of Interest:

Also pros/cons on using a complementary output stage vs. a cascode output stage (just using N-Types), especially if I wanted to get an output around 500V.

Yes, I know there is no need to stack the Fets in the attached JPEG since they can easily do more than 80V.

This is just for my personal education.

Regards,
Eric

star882 · 2nd April 2010, 07:57 PM

IGBTs are commonly available up to 1.2kV. It is better to use single devices than it is to build a cascode if the objective is high voltage.
But if you use a low voltage MOSFET to emitter/source switch a high voltage MOSFET or bipolar, you can accelerate switching speeds by counteracting Miller capacitance.

woody · 2nd April 2010, 08:14 PM

Were you talking about 500v p to p if so there are some 600w P chanel mosfets
like the IZTR16P60P available at Digikey.

ecool811 · 2nd April 2010, 10:19 PM

Woody, I have looked at those and they seem like a good choice for my design.

My real question pertains to the correct way to stack the Fets (both the two P's and the 2 N's), if I wanted to go greater than 600V, say 1000V.

1. What are good values for the resistors going from the rails and why?

2. What sort of compensation should I add for this circuit, specifically for the top and bottom Fets?

Thanks again,
Eric

star882 · 3rd April 2010, 11:38 PM

The correct answer is to special order high voltage IGBTs. It is not very easy to ensure voltage sharing with series semiconductors.

1.7kV, 20A IGBTs are just over $5 each, although it will very likely get quite expensive as you'll probably go through a lot of them before you have a working design. IXGH10N170 Ixys IGBT Transistors

ecool811 · 5th April 2010, 04:45 PM

Thanks Star. I will look into these.

But I guess I still don't have an answer on to properly stack mosfets for higher voltages...purely for my own education and not necessarily for this circuit.

-Eric

2nd April 2010, 05:40 PM	#1
ecool811 diyAudio Member Join Date: Mar 2010 Location: United States	Stacking Mosfets for ClassAB Amplifier Guys, My first post and thanks for any input. I guess I wanted to hear some thoughts on things I need to think about when stacking P or N Mosfets (to get to higher voltages) for a ClassAB amplifier. Is a resistor divider from the rails good enough? What about charging the Gates for the top and bottom Fets. Circuit of Interest: Also pros/cons on using a complementary output stage vs. a cascode output stage (just using N-Types), especially if I wanted to get an output around 500V. Yes, I know there is no need to stack the Fets in the attached JPEG since they can easily do more than 80V. This is just for my personal education. Regards, Eric

2nd April 2010, 07:57 PM	#2
star882 diyAudio Member Join Date: Mar 2007	IGBTs are commonly available up to 1.2kV. It is better to use single devices than it is to build a cascode if the objective is high voltage. But if you use a low voltage MOSFET to emitter/source switch a high voltage MOSFET or bipolar, you can accelerate switching speeds by counteracting Miller capacitance. __________________ "Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!

3rd April 2010, 11:38 PM	#5
star882 diyAudio Member Join Date: Mar 2007	The correct answer is to special order high voltage IGBTs. It is not very easy to ensure voltage sharing with series semiconductors. 1.7kV, 20A IGBTs are just over $5 each, although it will very likely get quite expensive as you'll probably go through a lot of them before you have a working design. IXGH10N170 Ixys IGBT Transistors __________________ "Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!

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2nd April 2010, 08:14 PM	#3
woody diyAudio Member Join Date: Jan 2002 Location: Tyrone Ga. U.S.A.	Were you talking about 500v p to p if so there are some 600w P chanel mosfets like the IZTR16P60P available at Digikey.

2nd April 2010, 10:19 PM	#4
ecool811 diyAudio Member Join Date: Mar 2010 Location: United States	Woody, I have looked at those and they seem like a good choice for my design. My real question pertains to the correct way to stack the Fets (both the two P's and the 2 N's), if I wanted to go greater than 600V, say 1000V. 1. What are good values for the resistors going from the rails and why? 2. What sort of compensation should I add for this circuit, specifically for the top and bottom Fets? Thanks again, Eric

5th April 2010, 04:45 PM	#6
ecool811 diyAudio Member Join Date: Mar 2010 Location: United States	Thanks Star. I will look into these. But I guess I still don't have an answer on to properly stack mosfets for higher voltages...purely for my own education and not necessarily for this circuit. -Eric

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