Controlling high voltage PWM signal with microcontroller

Thread Starter

killerpiraat

Joined Feb 17, 2020
16
Hi!
I am currently designing a wiper system for my boat. I have 3 200VDC wipermotors (Nominal current is 2 - 3 Amp)
Im controlling them with a 5 VDC PWM output with a frequency of 1kHz, coming from a PIC16F18875. I need to control the speed, so thats why im using PWM.

At the moment i am selecting componentents . But my main question is, what does it take to convert the 5V PWM signal into a 200V PWM signal? I am choosing the right mosfets at the moment
I have a 230VAC voltage in the boat. Can i just rectify and transform this to 200VDC and control this with voltage with a mosfet?
 

MrChips

Joined Oct 2, 2009
26,088
230VAC rectified will have a peak voltage of 325VDC. That is too high for your motor.
You may have to use a step down transformer to reduce the AC voltage.

Make sure you use a MOSFET that is rated well over your requirements, such as 15A @ 600V with low gate voltage thresholds.
 

MaxHeadRoom

Joined Jul 18, 2013
25,202
The high voltage can be used for the motors if you limit the PWM maximum pulse width.
The voltage in and of itself should not damage the motors, only over speed or over current.
This is done all the time in many applications, Treadmills & servo motors for a couple.
 

Ian0

Joined Aug 7, 2020
5,097
PIC - MOSFET driver(low side driver) - MOSFET
is all you need. (But don’t forget the flyback diode across the motor)
There are scores of low side drivers - MCP1402 is an example.
if you use 230V AC rectified to 325VDC, bear in mind that your PIC is effectively connected to mains live.
 

crutschow

Joined Mar 14, 2008
29,779
As Max said, you can just limit the PWM duty-cycle to limit the average voltage to the motor.
Since the rectified 230Vac will give about 320VDC, you would need to limit the duty-cycle to no more than 200/320 = 62%.
For a 1kHz PWM signal, that means it can't be on for more than 620µs.

Of course the MOSFET driver would need to be rated for at least 500V, a current at least twice what the motors take, and a Vgs(th) maximum value of no more than 2V.
Also, as Ian0 noted, you will need a free-wheeling diode across the motor (cathode to plus) to carry the motor current when the MOSFET goes off.
It should be rated for at least 500V and the maximum motor current.
 

Thread Starter

killerpiraat

Joined Feb 17, 2020
16
Not sure how to quote here, so sorry for this

@MrChips, that is correct. That voltage is too high. The plan was to implement a step down transformer in the design
@atferrari It is a large old decomissioned cargo ship (50m), for some reason they have 200VDC wiper motors. They are very hard to remove, since they are tucked away Else i would have replaced them by 24VDC.

@Ian0 Correct. Thats very dangerous and harmful for my PIC. For this, i want to make a seperation between the high voltage and low voltage (microcontroller gets its voltage from the 12V supply on the boat.) That is the reason i want to add a opto-coupler between the PWM output (1kHz) and the gate of the mosfet. That is one of the problems, im having problems with selecting the optocoupler.

@crutschow Thanks, thats a good idea. Altough i was in the understanding that it was the maximum value of the voltage that could hurt the dc motor?But as long as the average voltage doesnt exceed the specs i should be good? (it is a permanent magnet motor by the way)

By the way, is it possible to control 2 mosfets from 1 driver output? The switch on time will be longer, because of the capacitance between Gate and Source, but it should be possible right?
 
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MaxHeadRoom

Joined Jul 18, 2013
25,202
Higher voltage does not damage the motor unless really extreme, a 200vdc motor will have a much higher tolerance to the voltage itself, as long as the voltage is not of a continuous value, which would cause over-speeding.
Using PWM allows the maximum RPM of the motor to be observed by setting a limit to the PWM as explained.
 

crutschow

Joined Mar 14, 2008
29,779
one of the problems, im having problems with selecting the optocoupler.
A digital output type opto will have more than the desired speed.
But you will also need a DC voltage (e.g. 10-12V) to power the opto and drive the MOSFETs.

And you can drive more than one MOSFET from the same signal.
 

MaxHeadRoom

Joined Jul 18, 2013
25,202
There are also logic level (5v gate) Mosfets, but if using higher gate voltage versions, I tend to include a logic to Mosfet driver such as the MIC4427
 

Thread Starter

killerpiraat

Joined Feb 17, 2020
16
Thank you very much once again for the suggestions. I am looking into them as we speak!

Question about the max voltage vs. average voltage on DC motor.
Just to be sure, this doesnt apply on the max voltage of the Drain Source connection? Lets say a mosfet has a max Vds of 400V. And i apply a PWM with 50% d.c. at the gate, and the drain voltage is 800V. (i guess sparks will occur on the drain source connection inside the fet?)

I have a BTW7960 driver module laying around, but thats only rated for max 27VDC...
 

MaxHeadRoom

Joined Jul 18, 2013
25,202
And where is this line on what is possible and what isnt? Lets say i have a 24VDC motor and a 200VDC power supply? Would this be possible with a low enough duty cycle (PWM)?
That would be stretching it a little too fine!
It is RPM that would be the limiting cap.
 
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