MOSFET Motor control - PWM

[Pheetuz]

Hi folks, new to this forum so dont know if i am posting this in the right place.

Am currently doing a project at uni where i am building a fighting robot, all was going fairly well and i had my control circuit all drawn out untill a student in the year above me told me that when he had built a similar circuit last year he couldnt drive his MOSFET from the PIC that he was using.

Basically i am implementing a H-Bridge circuit using DPDT relays with the negative line from the relays going through the high powered MOSFET and down to ground.
My intention was then to use pulse width modulation to turn the MOSFET on and off to control the amount of power being delievered to the motor and thus its speed.

As i said the concern that i have is that the output of the PIC wont be powerful enough to drive the gate of the MOSFET and was wondering if anyone had any suggestions as to some circuitry to use ???

The MOSFET i am using is 75nf75 MOSFET and am using a 12V drill battery as a power supply.

Many thanks.

 

[IanP]

Keep in mind that the 75NF75 part requires up to 10V Ugs ..

Power MOSFETs can be driven with so called Complimentary Emitter Follower ..
See attached picture ..
More details on this concept can be obtained from:
**broken link removed**

Here are some other options:
https://www.picotech.com/applications/pwm_drivers/


Rgds,
IanP

 

[Tahmid]

Hi,
You could use a logic level driver such as TC427 / TC426 / TC428.
Datasheet:

ww1.microchip.com/downloads/en/DeviceDoc/21415C.pdf

Hope this helps.
Tahmid.

 

[Pheetuz]

Thanks Ian, i tried using the emmiter follower totem pole circuit but with no luck.

I have just noticed in the datasheet for the 75nf75 that it says the gate-source voltage is + or - 20V, does this mean that i need to apply 20V to the gate for the channel resistance to switch completely??

Cheers aswell Tahmid, dont know why i didnt think of looking for a chip to solve my problems

Just wondering now if 12V is going to be enough to drive this particular MOSFET, would be strange if it didnt seen as my lecturer gave me the MOSFET???

Many thanks folks

/Pete

 

[Tahmid]

Hi Pheetuz,

The 20v stated is the maximum which means if you apply more than 20v, the MOSFET gate will "break down" and you'll have a non-functional MOSFET.
If you check the graphs in the datasheet, there is one that shows required VGS vs Required Current. You can drive you MOSFET from 4v, but you won't be able to take much output from there.

If you apply 8v to 12v, you will get most efficient MOSFET drive.

There are logic level MOSFETs (IRL series) which can be directly driven from 5v logic, that is, directly from the PIC.

Hope this helps.
Tahmid.

 

[Pheetuz]

Thanks again Tahmid.

I am using a 12V battery so that should be fine then. Am opting to use the MOSFET Driver 1.5A Dual High-Speed PDIP8 as it seems suited to what i need, the only concern i have with using this chip is that if i connect its VCC to the same line that the motors are going to be drawing current from then it may not function properly due to noise being emposed on the line by the motors starting to turn and stopping turning????

I am using a diode in parrallel with the motors and will use de-coupling capacitors on the line itself and the chip.

All the other IC's that i am using are going to be on a seperate board with opto isolators inbetween them and the motor drive circuit.

Thanks

/Pete

 

[Tahmid]

Hi,
Place bulk capacitor (220uF) across the decoupling cap from the chip VCC to ground and you won't face that problem, as when the motor draws, the bulk capacitor will supply the current. And at other times the cap will be charging.

Hope this helps.
Tahmid.

 

[Pheetuz]

So put the bulk capacitor in parrallel with the usual de-coupling capacitor for the chip then ???

Many many thanks

 

[Tahmid]

Yes, and that should be ok.
The value for bulk capacitor isn't very critical, 100uF, 220uF, 470uF, all are fine.
Hope this helps.
Tahmid.