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brushed dc motor driver

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Joe G

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I was looking at using this fet "AUIRFS8409-7P" for a 55lbs thrust trolling motor driver. it will be pwm'd and driven from a pic. I had used a BTN8962TA half bridge driven straight from the ccp1 pin, which worked great up to half speed, I was over the current limit on that fet and could only run at top speed for a few seconds until it shut down, now my question is would I need a fet driver for this "beefier" Nfet? and if so, what might be a good match for it.
pic voltage is 5v.
motor voltage is from 12 to 16 volts
pwm range is from approx 4Khz to 20Khz
thx

 

audioguru

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Most Helpful Member
Datasheet Archive (.com) never hear of your Fet. We do not know the gate voltage for it to fully turn on, many Mosfets need 10V. The high gate capacitance might not allow a Pic to drive it at the high frequencies you want.
You forgot to tell us the maximum stalled current of your motor.
 

audioguru

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Your Mosfet turns on fully when its gate is 10V. The gate driver must provide fairly high current to quickly charge and discharge its high gate capacitance. But you do not have a gate driver yet.
 

Joe G

Member
correct, you answered my question, I thought I may need a gate driver but wasn't sure, Now I can start searching for one.
thanks
 

jpanhalt

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At 3V GS, it only conducts 0.1A at 10V DS (RDS=100 Ω )
upload_2016-2-24_1-15-25.png


From what I could find, a 52 # thrust draws about 30A fullbore. That will produce a lot less heat, if you use a gate voltage >5V. You also have to consider the capacitance of the gate. I woud use a driver. One could also build such a driver from components, but I wouldn't bother to do that for a one of.

Why are you using variable frequency for your PWM, or did you simply mean that you tried PWM at different frequencies with the same result of over heating? Of course, slower PWM will decrease the number of transitions per period and reduce heat from that source, but if RDS is high, resistive heating (I^2)R will be your main producer heat. .

John
 

audioguru

Well-Known Member
Most Helpful Member
Where on Earth (or on Mars) can you buy a Mosfet that has "typical" spec's?? Mosfet graphs in datasheets show only typical devices so that an engineer who uses them can calculate how many millions of the products made with them will not work!.
The typical graph posted shows that with a gate voltage of 4V then the Mosfet conducts 100mA but the datasheet says some conduct only 0.25mA. If you heat it with a blowtorch then it conducts 100 times more.
 

jpanhalt

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Joe G
You mentioned using a PIC microcontroller. While that will work, is do don't absolutely need the MCU, there are dedicate PWM+driver chips. I have used the TPIC2101 (TI). It has some nice features, such as builtinsoft start and resistor setting. It can drive 50 mA and run up to 16V.
**broken link removed**

Another option is an NE555/LM555. Either of those can provide the higher voltage and even more current, but they require a little more effort to get the features of the purpose-designed chip.

John
 

Joe G

Member
the pic controls the steering, alarm speaker, leds and "other" items besides the pwm motor, I was running it at less then 10Khz with the smaller fet, In the future I hope to get a larger motor, that is why I'm looking at a bigger fet.
also, the pic is powered by 4.9volts
 

jpanhalt

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In that case, I would simply go with a driver. Two choices: 1) A driver supplied by the 12V system; or 2) A driver with a charge pump that can give a voltage higher than the 5V PIC supply. Note, although not particularly capable of providing high current, one of the operating modes of the TPIC2101 is MCU control. I am not stuck on that particular controller. It is the only one I have experience with, and that experience was 10 years ago.

John
 

Joe G

Member
yes, I will go with a driver, I'm liking the one RONV posted so far...the good thing is, is that I'm still learning:woot:
 

jpanhalt

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One other thing. Remember that the stopped motor/rotor current is much higher than the maximum operating current. Be sure to have some sort of overload protection in case the prop fouls on something. You surely don't want to pop the mosfet in that situation.

John
 

Joe G

Member
yep, got a circuit breaker and alarm system in the pic code, even with flashing leds in case I don't hear the pieza spkr in the wind. sea weed is a killer in the salt water flats;)
 

jpanhalt

Well-Known Member
Most Helpful Member
Please give us an update. I am sure all of us are interested in how it works out for you.

John
 

Joe G

Member
pop brd.png blank brd.png well, thanks all , I got everything on the board working with a smaller 12v>2amp motor, so far so good. I ended up going with a IRFS8407 fet.
It had some better quality's for my end use and did use the MCP1407 driver. I have it set for 9984 hertz for now, ( I programed ccpr1l to ramp up/down every 2 seconds by 10). If it all works good in actual use then I'll redo another board in a high side drive.
 
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