PNP mosfet as a current limiter

Thread Starter

stanislaw

Joined May 29, 2020
3
Hey Guys,

I have build H bridge for a stepper motor and I would like to add current limiter on PNP side of bridge:
1593807718243.png
Voltagesource is from 12V to 48V and I would like to limit current. Best option would be to have a dip switch or potentiometer to set current limit between 1-6A.
Could you point me to good direction. I have basic understanding of electronics.
 

dendad

Joined Feb 20, 2016
3,595
It will be better to have a comparator to switch the FET on and off, not just run analog as it looks like you are doing. At least I think that is what your circuit may be. I've not tried to hard to figure it out.
On my boards, the current is monitored via a resistor in the 0V line, and feeds to a comparator to gate the drive on and off.
 

Thread Starter

stanislaw

Joined May 29, 2020
3
Signal IN is 5V DC coming from a micro controller.
Dendad by comparator you mean to use analog input from a for example micro controller or to use some IC which does it automatically?
 

dendad

Joined Feb 20, 2016
3,595
Signal IN is 5V DC coming from a micro controller.
Dendad by comparator you mean to use analog input from a for example micro controller or to use some IC which does it automatically?
No. Have a comparator that reads the load current and if it is above a set point, it turns the gate drive off. Then the gate drive will come on again after a delay that can be set with an R/C network.
This way, the FET is either on or off, not running in an analog way that wastes power and generates heat.
There will be a bit of loss during the FET switching time, but that will be a lot less than running it as a variable resistor to limit the current.

A way to try it is to have a series gate resistor to limit the gate drive current so a open collector comparator output can pull the gate to 0V to turn it off. This may not be the ideal purist way to do it but in practice, it works well, increasing the FET on and off switching time, but that in itself helps to reduce RF interference generated. It will make the FET run a bit warmer but I have used this for years quite ok.
 

Irving

Joined Jan 30, 2016
951
Have you actually built this or is it just 'in design'

As far as I can see you have a very odd arrangement of a npn/pnp pair driving the gate of a P-channel MOSFET that goes to OUT. I don't see an H-bridge driving the stepper which presumably is connected to OUT and ???

I don't understand the statement "on the PNP side of the bridge" since your PNP isn"t actually the bridge, its the gate drive. Did you mean on the "P-channel side of the bridge"?

Current limiting via a sense resistor would normally be in the ground end of the bottom pair of the H-bridge, since sensing on the high side has all sorts of issues with common mode voltages, etc., especially at 48v. Why do you want it on the high-side?

Please publish all of the H-bridge circuit showing the stepper motor connections and we might be able to be more helpful.
 

Thread Starter

stanislaw

Joined May 29, 2020
3
Have you actually built this or is it just 'in design'
Yes, I have build this driver, and it works well, there is almost no heat on PNP transistor.

As far as I can see you have a very odd arrangement of a npn/pnp pair driving the gate of a P-channel MOSFET that goes to OUT. I don't see an H-bridge driving the stepper which presumably is connected to OUT and ???

I don't understand the statement "on the PNP side of the bridge" since your PNP isn"t actually the bridge, its the gate drive. Did you mean on the "P-channel side of the bridge"?
Well I'm not an electronic engineer thus I don't know proper nomenclature, but you understand me right. I meant P side of H- bridge.

I will get home in few hours and post full bridge.


@crutschow Thanks for idea - LM339/393. I'm only afraid that I will blow it up - it work only up to 36V. Do you know any comparator which works to at least 48V? I have been looking in electronic shops, but max I have found is 38V
 

Irving

Joined Jan 30, 2016
951
If you do low-side current sensing you don't need to work at such a high voltage. When you post your full schematic we'll be able to advise better.
 

Irving

Joined Jan 30, 2016
951
No.
Do you not have a lower supply voltage available?
If not, you could use a resistor and zener to reduce the voltage (such as to 12V).
Not if he's doing high-side sensing, he needs the common-mode range too. I suppose you could move the comparator up to the 48v rail by lifting its v- supply but then you'd still need to level-shift it all back down for the control loop, or opto-isolate. There's many reasons why low-side sensing is done generally!
 

schmitt trigger

Joined Jul 12, 2010
312
@stanislaw:
It seems that you are eager to learn....good for you! :):)

-As such you should use the proper terminology to convey information accurately. NPN and PNP labels are for bipolar transistors exclusively. N-channel and P-channel is what is used for FETs.

-If you want to learn everything you want about current sensing and then more, much more, then there is absolutely no better guide than Linear Tech's (now part of Analog devices) Application Note 105, Current Sense Circuit Collection.

If you require a current sensing circuit, its there, all 118 pages of it. And has a short but very descriptive technical explanation to allow you to understand how the proposed solution works.

https://www.analog.com/media/en/technical-documentation/application-notes/an105fa.pdf
 

sparky 1

Joined Nov 3, 2018
301
He takes the High side you take the low side never get to Scottland.
Two p-channel and two n-channel : forward, backward. Which brings me to next point, What does the application need to do? Also why not use pulse width to limit current ? These things have been around awhile.
 
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