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Author Topic: Mosfets building heat too fast  (Read 681 times)

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Offline TrickyNekroTopic starter

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Mosfets building heat too fast
« on: May 24, 2010, 06:03:08 AM »
Hello guys,

I got some trouble with my homemade motor controller. The motor controller works just fine, there is no short of anything, that's positive....

But when running the motor at low speed heat is building up at the P-channel mosfet too quickly.
That's no good... Especially for a motor controller design that is supposed to run cool and efficient in low speeds...
The ridicules point is that at no power, but also at full power the mosfets are perfectly cool, they start to build heat
when the PWM is lower than about 30%... But the vehicle the motor is on... is supposed to be able to carry out low speed maneuvers... so that's bad... The vehicle start moving at a PWM of about 10% and the wheels start spinning at a PWM lower than 1% PWM... That's pretty good of course but that's the point for the motor controller being efficient....

I know the problem mostly comes from the currents that are generated between the mosfets cause of the difference of the switching times, but I need a way NOT to drastically change the design  :-\
This boards are two face and are homemade... you can imagine how much work they involve.... :-/

Don't really think it's important but the PWM is frequency locked at ~15kHz and only Duty is varied...

Best Regards, Lefteris
Greece
For whom the interrupts toll...


P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

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Offline Cristi_Neagu

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Re: Mosfets building heat too fast
« Reply #1 on: May 24, 2010, 06:13:25 AM »
I see that the MOSFETs you are using are in TO-220 packages. And are rated for 12A. Which beg the following questions:

1. How much power does the motor draw? If it's anything above 4-5A, it's natural that the MOSFETs are overheating
2. Did you mount them on any kind of heatsink? Because you really should mount them on heatsinks.
3. When you say that they get hot, do you mean that they get hot to the touch or 80-90 degrees C hot?

Offline TrickyNekroTopic starter

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Re: Mosfets building heat too fast
« Reply #2 on: May 24, 2010, 06:29:15 AM »
The P channel mosfets are the IRF9540 which are rated at 23A
The N channel mosfets are the IRF540 which are rated at 33A

Heatshinks are usually needed for more than a minute operation....generally for big loads.... these are building heat way too fast...
Although there is both passive and active heatshinks the copper layer should have been more than enough for such small currents...

At 1% PWM the motor runs pretty clean... It shouldn't need more than 200 - 300mA.... Still the mosfets are having
a tequila party... :-\
I wrote it... At full PWM duty, the Mosfets are actually cooling down!!!!  :o
That's why I seriously suspect the switching times...  

I even stall the motor and heat isn't build so fast... It's a design issue... I'm almost sure that it's
caused by the switching times of the MOSFETs but I'll run without a motor to test this assumption...
It should be for one or two resistor values to change per channel nothing more... (cause of the capacitive behavior of the mosfet gate)...

Also for me even 60C is good to touch... I never tested limits... ( I don't really want to tell the truth )
I should find my temp prop to test also this out...
« Last Edit: May 24, 2010, 06:31:33 AM by TrickyNekro »
For whom the interrupts toll...


P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

Offline Soeren

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Re: Mosfets building heat too fast
« Reply #3 on: May 24, 2010, 07:31:20 AM »
Hi,

That's why I seriously suspect the switching times...  

I even stall the motor and heat isn't build so fast... It's a design issue... I'm almost sure that it's
caused by the switching times of the MOSFETs but I'll run without a motor to test this assumption...
It should be for one or two resistor values to change per channel nothing more... (cause of the capacitive behavior of the mosfet gate)...
Break out a 'scope and measure the gates of Q5 and Q3 (or Q4 and Q2) to see if there's a short flash over (that's my primary guess). The P-ch devices are slower than the N-ch. devices, so they really need a bit of dead band. A higher drive current would help as well.

Reduce R10 and R11 to 120..150 Ohm (assuming 12V motor voltage).
You cannot get them anywhere near the datasheet values for switching speed, when feeding them only 2.55mA for the N-Ch. (again assuming a motor supply of 12V). The N-ch. will need close to 5Ás to reach the Miller plateau, but changing to 120 Ohm will get it down to around 120ns (around 1ns/Ohm).

 The P-ch. device won't work reliably from a 5V supply, it needs at least 6V to even consider doing the job and a good deal more to behave. The N-ch. is somewhat better in that respect, but it's not a logic level device either.
Regards,
S°ren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline TrickyNekroTopic starter

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Re: Mosfets building heat too fast
« Reply #4 on: May 24, 2010, 07:47:07 AM »
The assumption about the voltage is correct.... (10V now and 12V with the lipo I plan to use)

The values of the resistors you told are 1K... 4.7K have failed tremendously....
I'm installing the new resistors now...

I'll start with 220 ohm though... and see current consumption when switching on the scope and judge alike.

Thanks for the reply, I'll keep you updated!!!
For whom the interrupts toll...


P.S. I've been inactive for almost a year... Don't give promises but I'll try to complete my tutorials. I'll let you know when..

Cheers!

 


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