# MOSFET Selection for DC Controller



## MPaulHolmes (Feb 23, 2008)

You should have a gate resistor for the mosfet. Maybe 10 or 20 Ohms or something. 1/4 watt is fine. You also probably need a mosfet driver to drive teh mosfet on and off hard. I think the 555 timer can't put out the high burst of current necessary to turn the mosfet on and off really fast. If it happens to slowly, it will heat up really bad because if it stays in the "partially off/partially on" state, it dissipates a LOT of power. The gate of the mosfet is like a capacitor sort of. You want to charge the gate to full and discharge it really fast to turn it on and off quickly. Then you also need a resistor from gate to source on the mosfet. Like 4.7kOhm or something so that it won't just float. You want to make sure the gate goes off when there's no obvious default state for the mosfet. You don't want it to hang out in the "sort of on, sort of off" state. It will get toasted really fast.


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## 0110alxcpa (Jan 13, 2012)

hmm, maybe ill try w/ a 556 and increase the frequency...


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## bjfreeman (Dec 7, 2011)

my eyes can't make out the values.
is the Diode a untrafast. you should also have a freewheeling one across the mosfet, for Back EMF.


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## 0110alxcpa (Jan 13, 2012)

So I wired up this schematic today, and it works!!, but I need to change some values because the range of control is VERY narrow, like 60-80%. Any ideas on how to increase the range to 0-100%??

Also I found it strange that the large electrolytic cap C8 sits across the source voltage as opposed to the motor voltage, is this correct??


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## DJBecker (Nov 3, 2010)

Your previous schematic had the big capacitor across the freewheel diode. This is bad.

The capacitor will try to instantly charge when the MOSFET turns on, which causes a large current spike.

The reason for having a capacitor there at all is to compensate for a slow diode, and to reduce EMI. Your freewheeling diode needs to be "ultra-fast" and "soft", not "fast" (which means somewhat slow) or "snappy" (which means harsh and likely to self-destruct with rapid switching).

Generating PWM with a 555 timer is a good beginner learning project, but you wouldn't use it for real. There is no good configuration to generate 0% through 100% pulses.


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## JRoque (Mar 9, 2010)

Hi 011 and welcome to the forum. Once you've done your experiments with motor controllers, decide if you're willing to strap yourself to a 3,000 pound vehicle controlled by a circuit you found on a casual internet search. While a DC controller is fairly straightforward, an EV application requires more than a 555 driver.

Follow Paul Holmes to his website where he has an EV-tested design of a DC controller that you can build yourself. I believe he has the ready made PCBs as well. 

Regards,
JR


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## 0110alxcpa (Jan 13, 2012)

obviously im not trying to drive a car with this circuit... it is for fun


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## Gary B (Jun 2, 2011)

JRoque said:


> Hi 011 and welcome to the forum. Once you've done your experiments with motor controllers, decide if you're willing to strap yourself to a 3,000 pound vehicle controlled by a circuit you found on a casual internet search. While a DC controller is fairly straightforward, an EV application requires more than a 555 driver.
> 
> Follow Paul Holmes to his website where he has an EV-tested design of a DC controller that you can build yourself. I believe he has the ready made PCBs as well.
> 
> ...


I'm interested in checking out Paul's design. Can you give me his website reference? - Gary B.


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## Gary B (Jun 2, 2011)

Gary B said:


> I'm interested in checking out Paul's design. Can you give me his website reference? - Gary B.


Folks. I'm sorry. This looks like the "Paul" of the Paul & Sabrina Open Revolt design (Cougar, et al.) . I've been there and am using some of that info. - (Duh!) - Have also found the following MOSFET that looks pretty good. - APT20M22B2VR - Any comments? - Gary B. - Definite regards to P & S. -


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## DJBecker (Nov 3, 2010)

Gary B said:


> Have also found the following MOSFET that looks pretty good. - APT20M22B2VR - Any comments? - Gary B. - Definite regards to P & S. -


What's the pricing on that part?

Compared to the IXFX230N20T, it has three times the channel resistance while having a higher gate charge. It would have to be very low cost to make up for those drawbacks.


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## Gary B (Jun 2, 2011)

DJBecker said:


> What's the pricing on that part?
> 
> Compared to the IXFX230N20T, it has three times the channel resistance while having a higher gate charge. It would have to be very low cost to make up for those drawbacks.


DJ - If i remember the cost was about $17 each (small quantity). - What really attracted me to it was its nice "Safe Area of Operation." - I will also look at the device you referenced. Does the higher gate charge make the driver design more difficult (costly)? - Thanks for the input. It strikes me that the higher channel resistance would not be a problem. - Gary B.


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## Gary B (Jun 2, 2011)

DJBecker said:


> What's the pricing on that part?
> 
> Compared to the IXFX230N20T, it has three times the channel resistance while having a higher gate charge. It would have to be very low cost to make up for those drawbacks.


O.K. DJB - I checked out that part and the Safe Operating Area looks like it will do my needs. - Your other points are quite legitimate. Also, i believe the price is down to about $9 (or something). - Greatly appreciate your input. - You got me before i made my purchase (for example). - I'm roughly following the P & S controller design. THANKS ! - Gary B.


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## DJBecker (Nov 3, 2010)

What is your target voltage?

If you are experimenting with under 100V, there are a bunch of MOSFETs that have a lower gate charge, lower threshold and lower on resistance. And are much less expensive to blow up.

IRFB4310ZPBF 100V 120A 250W TO-220 4.9mOhm [email protected] $1.32ea Avnet
IRLS4030-7PPBF MOSFET 100V 190A 7-pin D2PAK $3.62
IRFP4110PBF MOSFET 100V 180A TO-247 $1.94


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## Gary B (Jun 2, 2011)

DJBecker said:


> What is your target voltage?
> 
> If you are experimenting with under 100V, there are a bunch of MOSFETs that have a lower gate charge, lower threshold and lower on resistance. And are much less expensive to blow up.
> 
> ...


 Target voltage 144 v. (500 amps) - Intermediate development, 72 V. - Initial tests, 12 v. - (130 amps - motor runs). - ME1002 motor. - I needed two of the MFets i mentioned. I can just use just 1 of yours for this level of test and development. - I have copied your other MFet info for reference. THANK YOU. - Gary B. -


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## Arlo (Dec 27, 2009)

Im about to do some testing with some irfp4468 irfp4568 and irfp4668. They are all TO-247 and can shed a lot of heat. The 4468 is 100v and uber low rdson the 4568 is 150v and the 4668 is 200v but has a higher rdson. I am building a brushless controller so I will see how it goes but I am playing with different winding counts and differnt voltages at the same time all for an end rpm 8000-9000 rpm on a bldc hence the reason for all the testing.

Anyways those three fets all seem great for $4-8 depending on who you get them from.


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## Gary B (Jun 2, 2011)

Arlo said:


> Im about to do some testing with some irfp4468 irfp4568 and irfp4668. They are all TO-247 and can shed a lot of heat. The 4468 is 100v and uber low rdson the 4568 is 150v and the 4668 is 200v but has a higher rdson. I am building a brushless controller so I will see how it goes but I am playing with different winding counts and differnt voltages at the same time all for an end rpm 8000-9000 rpm on a bldc hence the reason for all the testing.
> 
> Anyways those three fets all seem great for $4-8 depending on who you get them from.


Arlo - Thanks for the info. - I will check out the 4668 because i like the voltage headroom. - Good luck on your BLDC. (definitely more complicated than mine). - Gary B.


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## Gary B (Jun 2, 2011)

Arlo said:


> Im about to do some testing with some irfp4468 irfp4568 and irfp4668. They are all TO-247 and can shed a lot of heat. The 4468 is 100v and uber low rdson the 4568 is 150v and the 4668 is 200v but has a higher rdson. I am building a brushless controller so I will see how it goes but I am playing with different winding counts and differnt voltages at the same time all for an end rpm 8000-9000 rpm on a bldc hence the reason for all the testing.
> 
> Anyways those three fets all seem great for $4-8 depending on who you get them from.


Arlo. - I checked out the 4668 but i'm not comfortable with the Safe Operating Area of that device since i am using a 144 v supply (in the final application). - I want more linear (DC) room in the SOA than what is indicated there. - Your application is quite different from mine. - Again, good on your progress. (Mine's slow. Got a lot to learn.) - Gary B


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## Arlo (Dec 27, 2009)

Gary B said:


> Arlo. - I checked out the 4668 but i'm not comfortable with the Safe Operating Area of that device since i am using a 144 v supply (in the final application). - I want more linear (DC) room in the SOA than what is indicated there. - Your application is quite different from mine. - Again, good on your progress. (Mine's slow. Got a lot to learn.) - Gary B


 We have found with the right caps you can usaly push a IR fet to its max voltage eg. I have a brushless controller running irfp4110 fets and I charge my 24s lipo to 100.8 volts and rip on it right after! So 144 on a 150v fet might not be bad.... Espicialy once you get a bit of sag. 150 volt rated caps or 160 will be cheeper from what I have seen too.


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## Arlo (Dec 27, 2009)

I realy recomend a TO-247 package over a TO-220 though it will cool a lot better! I am excited for your project because I might need to build a brushed contoller as well...


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## bjfreeman (Dec 7, 2011)

Gary B said:


> Target voltage 144 v. (500 amps) - Intermediate development, 72 V. - Initial tests, 12 v. - (130 amps - motor runs). - ME1002 motor. -


I suggest you find the device that will work with you target.
there is nothing you will garner from using lesser parts, that will help you with the target.
if you parallel mosfets, you will have a RDSon matching to manually do. what happens is manufactured gives you a RDson for a device, but if the die does better and they need to fill an order you may get some that are less RDson than the rating.
Next select a device for the worst case, heat, cold, voltage spikes, current overloads.
with a motor you are dealing with a inductive load that will kick back EMF. This depends on the Q of the inductive load.
you will have to come up with a Diode across the Motor that will act fast enough to keep the Back EMF below the parameters of you devise from the get go.
114 X 500 = 57KW I doubt you will find any mosfets that can handle that power, I suggest a IGBT


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## Gary B (Jun 2, 2011)

bjfreeman said:


> I suggest you find the device that will work with you target.
> there is nothing you will garner from using lesser parts, that will help you with the target.
> if you parallel mosfets, you will have a RDSon matching to manually do. what happens is manufactured gives you a RDson for a device, but if the die does better and they need to fill an order you may get some that are less RDson than the rating.
> Next select a device for the worst case, heat, cold, voltage spikes, current overloads.
> ...


 BJ - I definitely agree with your guidance on orienting to my final application supply voltage, which is what I’m trying to do. - Actually, its 72 KW (144v * 500a) - Still, a lot. - The IXFX230N20T MSFET you mentioned looks like it may do the job. (Five in parallel). I definitely like it better than the IRFP4668 (which I got from the P&S (Cougar) controller which I am using as a guideline). - (Thank you P&S !) - (and others) - Actually, my approach to the power end is pretty hard tied to their circuit. When I looked at their MOSFET I tried to find something with a Safe Operating Area I was more comfortable with. I did that (and you helped). Unfortunately, I do not know enough about what I am doing to verify their choices for the freewheeling diodes (which you mentioned) and the supply capacitive filters (which they also define). - So I will probably go with their selections. - This thread was focused on the MOSFETs (as I was trying to do). - My approach is quite different from theirs in other regards (on the front end). - I am focusing now on their power output approach. - The inductive load and back EMF I am aware of but not to the level of detail or design knowledge (experience) to make specific component selections (as has been done by P&S). - As far as I know, the Cougar22 (P&S) was designed for 72 KW (144v and 500 a). - As far as the Rds(on) values go, I think I may just take a chance on them matching up sufficiently. (replace burnouts ?) - Has anyone had any special trouble with the P&S power output design (other than MOSFET failures)? - I’m not planning on going with IGBTs at this point because other problems seem to raise their heads there. - Curtis and others (P&S) have gone many years with MOSFETs, I’m just trying to get the latest generation. I am NOT working on the super power controllers that more reasonably qualify for an IGBT approach. - Believe me, I am open to correction and guidance here. - Apologize for the length here. Just need the context. - Gary B.


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## piotrsko (Dec 9, 2007)

you may want to search for Tessaract's diatribe on building and designing controllers: why he went to IGBT instead of mosfets. I believe he was aware what 'Zilla was doing with mosfets in parallel, but he explains the whys and where fores of mosfets selection along with the cures. Great reading, but a wee bit long winded..


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