# MOSFET in series

#### rreddy426

Joined Jan 24, 2019
18
Hello All,

wondering whether we can connect MOSFETS ( which are rated for 500V) in series to increase voltage output. I tried simulation in LTSPICE. I connected 3 MOSFETS in series and I am observing decrease in output voltage. I have attached configuration picture.

Does over all output voltage increases when FETS are connected in series ??

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#### wayneh

Joined Sep 9, 2010
16,285
In theory you can get a higher voltage rating by stacking the MOSFETs ... IF you could ensure that the voltages distribute evenly. In reality it's impractical because you would need all sorts of protection schemes to ensure the voltage does not, even for an instant, exceed the specs of any individual MOSFET.

The scheme in your drawing won't work - a MOSFET cannot turn on unless the gate-source voltage is taken up to ~10V or more. If there was, for instance, 200V across the lowest MOSFET because it is turned off by the 0V on its gate and not conducting, your 12V applied to the gate of the second MOSFET would be -188V from the point of reference of the source pin of that second MOSFET, which is at +200V.

What are you really trying to do?

#### rreddy426

Joined Jan 24, 2019
18
In theory you can get a higher voltage rating by stacking the MOSFETs ... IF you could ensure that the voltages distribute evenly. In reality it's impractical because you would need all sorts of protection schemes to ensure the voltage does not, even for an instant, exceed the specs of any individual MOSFET.

The scheme in your drawing won't work - a MOSFET cannot turn on unless the gate-source voltage is taken up to ~10V or more. If there was, for instance, 200V across the lowest MOSFET because it is turned off by the 0V on its gate and not conducting, your 12V applied to the gate of the second MOSFET would be -188V from the point of reference of the source pin of that second MOSFET, which is at +200V.

What are you really trying to do?

Hello Wayne,

Thanks for the reply. I am using a h bridge driver to drive motor. Single FET is not able to supply required voltage. So I googled abt it. Came to know that connecting FETS in series would increase voltage and connecting FETS in parallel would increase current. Bottom mosfet will act as a controlling switch ( when it turned ON, most of the current goes through ground).

From your words, if MOSFET 4 is turned OFF( see attached pic) and MOSFET 1,2,3 are turned ON , if voltage across "source" of FET 3 is around 200 V, then possible potential difference between FET 3 gate and FET 3 source shall be -188 v, which would in turn turn OFF FET 3. Is my understanding is current. Let me know.

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#### wayneh

Joined Sep 9, 2010
16,285
Bottom mosfet will act as a controlling switch ( when it turned ON, most of the current goes through ground).
If it is rated for and capable of that duty, then you don't need the other ones. Let's talk about the actual voltage and the specs of the devices you want to use. I would MUCH rather spend a little more to get a device rated to allow using a single MOSFET.
From your words, if MOSFET 4 is turned OFF( see attached pic) and MOSFET 1,2,3 are turned ON , if voltage across "source" of FET 3 is around 200 V, then possible potential difference between FET 3 gate and FET 3 source shall be -188 v, which would in turn turn OFF FET 3. Is my understanding is current. Let me know.
Yes I think you've got it. I pulled 200V out of the blue sky as an example. If your diagram is correct and those upper three MOSFETs are somehow conducting (they will not be, with only +12V on their gates, but let's assume they are), then the lowest MOSFET's Source pin will be at +500V. For the MOSFET to be conducting with its S pin at +500V, the Gate pin would have to be at 510V. If you apply +12V, that's 498V too low to cause the MOSFET to conduct.

#### rreddy426

Joined Jan 24, 2019
18
If it is rated for and capable of that duty, then you don't need the other ones. Let's talk about the actual voltage and the specs of the devices you want to use. I would MUCH rather spend a little more to get a device rated to allow using a single MOSFET.
Yes I think you've got it. I pulled 200V out of the blue sky as an example. If your diagram is correct and those upper three MOSFETs are somehow conducting (they will not be, with only +12V on their gates, but let's assume they are), then the lowest MOSFET's Source pin will be at +500V. For the MOSFET to be conducting with its S pin at +500V, the Gate pin would have to be at 510V. If you apply +12V, that's 498V too low to cause the MOSFET to conduct.
Thanks Wayne,

Those voltage numbers i just gave as example.

Wondering , is it possible to use power MOSFETS for power amplification. For example, I need to get 400 V total. I am using FETS which are rated at 100V.

What is the best simulation tool for circuits. Right now I am using LTSPICE. Some times it does not yeild correct results. I tried PSPICE, but I could not find proper libraries to work. ( Example IR 2110). Apart from these are there any other better software tools that will simulate circuits correctly.

Thanks

#### wayneh

Joined Sep 9, 2010
16,285
Thanks Wayne,

Those voltage numbers i just gave as example.

Wondering , is it possible to use power MOSFETS for power amplification. For example, I need to get 400 V total. I am using FETS which are rated at 100V.

What is the best simulation tool for circuits. Right now I am using LTSPICE. Some times it does not yeild correct results. I tried PSPICE, but I could not find proper libraries to work. ( Example IR 2110). Apart from these are there any other better software tools that will simulate circuits correctly.

Thanks
I use LTspice.

I can’t imagine a way to reliably accomplish what you want the way you are proposing. That doesn’t mean it isn’t done, only that I haven’t seen it. Have you looked for a 400V MOSFET? What sort of current level are we talking about and what are you using for a power supply?

#### bertus

Joined Apr 5, 2008
20,333
Hello,

It will not work when the gates of the upper fets are connected to together.
When you want to make such construction, use seperate (isolated) drivers for the fets.

Bertus

#### BR-549

Joined Sep 22, 2013
4,938
It can be done on silicon. But I would forget it with discreet components.

The devices would need to be closely matched......and each gate would need a common reference to be proportioned with. Much worse than any hair trigger.

Think of balancing four long stacked poles in the air. It's hard to keep stabilized from the bottom, because of the time needed for correction.

But that balance can be had on a silicon die.

#### rreddy426

Joined Jan 24, 2019
18
I use LTspice.

I can’t imagine a way to reliably accomplish what you want the way you are proposing. That doesn’t mean it isn’t done, only that I haven’t seen it. Have you looked for a 400V MOSFET? What sort of current level are we talking about and what are you using for a power supply?
Hello ,

I tried to simulate a MOSFET in LTSPICE.If VD is around 250 V, at the source I observing 88V. Not sure where the loss. See the attached screen shot. Not sure where the loss is. Mosfet I used it irfp460p. Max gate cutt of voltage is 30 V.But I am observing voltage of around 18V at the source.

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#### wayneh

Joined Sep 9, 2010
16,285
Hello ,

I tried to simulate a MOSFET in LTSPICE.If VD is around 250 V, at the source I observing 88V. Not sure where the loss. See the attached screen shot. Not sure where the loss is. Mosfet I used it irfp460p. Max gate cutt of voltage is 30 V.But I am observing voltage of around 18V at the source.
More or less as predicted. Your image is hard to see but it looks like your gate voltage is 10V? With R1 in place, a 1A current would bring the source up to 10V, the gate-source voltage would be zero and the MOSFET would cease to conduct. There’s probably a steady state solution in between the extremes but the simulation may not be able to reach it - I have no idea.

#### shortbus

Joined Sep 30, 2009
7,467
Wondering , is it possible to use power MOSFETS for power amplification. For example, I need to get 400 V total. I am using FETS which are rated at 100V.
Then why not use mosfets of that rating? Or switch from mosfets to IGBTs? both of them use the same type of gate drivers.

Also don't be fooled by the data sheet voltage and amperage ratings, the "package" of the device is the real limiting factor, no matter what LTspice will tell you. The common TO- 220 package is only good to around 75W.

#### rreddy426

Joined Jan 24, 2019
18
More or less as predicted. Your image is hard to see but it looks like your gate voltage is 10V? With R1 in place, a 1A current would bring the source up to 10V, the gate-source voltage would be zero and the MOSFET would cease to conduct. There’s probably a steady state solution in between the extremes but the simulation may not be able to reach it - I have no idea.

Hello Wayneh,

Thanks for explanation. gate voltage is at 10V. By your analysis, transistor would not conduct at all. Any ideas on how to limit the current at source terminal.

#### ian field

Joined Oct 27, 2012
6,539
Hello All,

wondering whether we can connect MOSFETS ( which are rated for 500V) in series to increase voltage output. I tried simulation in LTSPICE. I connected 3 MOSFETS in series and I am observing decrease in output voltage. I have attached configuration picture.

Does over all output voltage increases when FETS are connected in series ??

As long as you have overall negative feedback - you can Zener clamp the gate of each MOSFET to its drain so its Vds is never exceeded.

MOSFETs in the chain will top out one by one. Apart from dropping their quota of the total voltage - they'll make no further contribution to the control function. This means the total transfer characteristic will change and nfb will be needed to compensate for that.

#### ian field

Joined Oct 27, 2012
6,539
I use LTspice.

I can’t imagine a way to reliably accomplish what you want the way you are proposing. That doesn’t mean it isn’t done, only that I haven’t seen it. Have you looked for a 400V MOSFET? What sort of current level are we talking about and what are you using for a power supply?
It looks to me like the top MOSFET in the sketch might be approximately a current source and the 2 under it presenting RDSon.

#### wayneh

Joined Sep 9, 2010
16,285
It looks to me like the top MOSFET in the sketch might be approximately a current source and the 2 under it presenting RDSon.
?? I don't follow. As far as I can tell, the TS just wants to use MOSFETs in series to increase the voltage "rating", rather than just buying a single MOSFET with the proper rating. Clamping the gates to the drains with zeners may be a clever part of accomplishing that.

#### rreddy426

Joined Jan 24, 2019
18
As long as you have overall negative feedback - you can Zener clamp the gate of each MOSFET to its drain so its Vds is never exceeded.

MOSFETs in the chain will top out one by one. Apart from dropping their quota of the total voltage - they'll make no further contribution to the control function. This means the total transfer characteristic will change and nfb will be needed to compensate for that.
Hello Ian/ Wayne,

Thanks for explanation. Up on reading this and chiming it, there are few things that are buzzing in my mind.

(1) If I have to use power MOSFETs and IGBT I need to make sure Vgs> what ever voltage going to through the source resistor. This shows that Power MOSFET cannot be used for amplification ?. sounds to me like this statement is contradictory. Any more ideas on how to use MOSFETS for power amplification and still turn ON MOSFETS(Vgs~=20 V) and voltage going through source terminal be around 100V?.

#### wayneh

Joined Sep 9, 2010
16,285
Hello Ian/ Wayne,

Thanks for explanation. Up on reading this and chiming it, there are few things that are buzzing in my mind.

(1) If I have to use power MOSFETs and IGBT I need to make sure Vgs> what ever voltage going to through the source resistor. This shows that Power MOSFET cannot be used for amplification ?. sounds to me like this statement is contradictory. Any more ideas on how to use MOSFETS for power amplification and still turn ON MOSFETS(Vgs~=20 V) and voltage going through source terminal be around 100V?.
I'm having trouble guessing your question, what you really want to do. If you want to use an N-channel MOSFET as a low-side switch, the source pin is typically connected to ground and the drain provides a path to ground for the load. In this arrangement, Vgs is also just Vg and needs to be only ~10V, or ~5V for a logic-level MOSFET. If you want a high-side switch, the typical solution is to use a P-channel MOSFET. In that scenario, you drop the gate voltage ~10V below the power rail voltage to turn on the MOSFET and cause the power rail voltage to appear on top the load.

There are other options. For instance you can still use an N-channel MOSFET to switch the high side as long as you have a voltage source ~10V above the power rail to apply to the gate. That allows the source pin to rise as high as the rail voltage with the MOSFET still fully conducting.

#### ian field

Joined Oct 27, 2012
6,539
Hello Ian/ Wayne,

Thanks for explanation. Up on reading this and chiming it, there are few things that are buzzing in my mind.

(1) If I have to use power MOSFETs and IGBT I need to make sure Vgs> what ever voltage going to through the source resistor. This shows that Power MOSFET cannot be used for amplification ?. sounds to me like this statement is contradictory. Any more ideas on how to use MOSFETS for power amplification and still turn ON MOSFETS(Vgs~=20 V) and voltage going through source terminal be around 100V?.
MOSFETs can be used for amplification - but automatic bias arrangements are a PITA.

A version of the cheapo single resistor transistor self bias sort of works, but you need 2 resistors to feed the gate with a potential divider from the drain. This bias arrangement has as much AC nfb as DC nfb and pretty much defeats the purpose of using a MOSFET. You can split the drain to gate resistor and decouple it with a capacitor to short out the AC nfb.