PWM and Chopper Drivers

Pulse width modulation and chopper drivers for stepper motors

If there’s anything that can be said about modern electronics it’s going to include MOSFET technology, reliability and the application notes brought out by the manufacturers.

LMD18200 - 3A 55V H-Bridge
LMD18200 – 3A 55V H-Bridge

Calling back the past

Going back a few years we had the dreaded GCS (Gate Controlled Switch) which Sony used in abundance in their first Trinitrons, we had the SCR and commutation circuits, conventional inverters which would have been a couple of beefy BJTs driving a 50 or 60Hz transformer.  All this stuff was, to put it mildly, utterly unreliable. Your standard antiquated auto vibrator power supply was more reliable, ditto motor alternator converters.

I worked on radars and television receivers.  Both shared similar semiconductor topologies, both unreliable compared to post 1990s circuitry using MOSFETs.  The problem was that I think engineers over-engineered their designs. Think of the old Triad, Aim and KIC television receivers which ran on and on and on and cost R800.00 from the store.  Entry level but more reliable than the juice eaters of the previous decade.

Arduino and RPi for additive or subtractive printing

Moving up a gear we move into the world of Arduino, RPi and embedded systems.  These can be used to plot the movement of a 5kg spindle motor in a CNC machine with absolute precision through G-code, steppers and drivers.  Ditto robotic arms and robotics in general.  And they are dirt cheap by yesterday’s standards. Stepper motors have been around for eons and are extremely reliable. Costs have come down substantially, thanks to Asian imports.

Stepper motors can be driven at manufacturer spec or through a chopped DC voltage sometimes up to 80V and higher (using Pulse Width Modulation).  CNC manufacturers prefer this method because a) it can provide constant current and b) very high torque because of the instantaneous current at low RPM.  (Electric motors deliver most torque at stall, like steam engines)

Why the brick wall for the home designer?

The DIY and Maker brotherhood try in vain to build their own motor drivers but are frequently against a brick wall.  CNC experts will discourage this practice. Is this a real problem though?  Looking through the masses of information on CNC builds one rarely comes across a professional tool where expensive parts are integrated into “home-baked” stepper or servo drivers. Why is this so though?

Motors, whether DC, three phase, poly-phase, stepper and servo all play an integral role in most automation systems where accurate mechanical positioning is required.  All motors vary in size,  common sizes not always reflecting the torque spec and current rating.  This must always be obtained from the manufacturer.

Stepper motors are poly-phase synchronous induction devices, the coils being activated by a low DC voltage and in a sequence which allows for C.W. and C.C.W rotation.

See https://www.sunfounder.com for a 4 phase stepper motor explanation.

Building Bridges

H-Bridges are commonly used to drive stepper motors. Although common in class-D audio, inverters and power supplies the careful design and operation of this topology is not for the faint hearted.  More often than not, and I have built a few PWM supplies, they fail without prior notice, usually in a spectacular fashion.  Gate drive control is usually the biggest reason.  There must be a dead time and there must be adequate drive current.  Of course one mustn’t tinker with the load whilst powered either.

Manufacturers of reliable driver technology have a thick wallet and electronic engineers at their disposal.  Cutting an expensive piece of aluminium when thermal runaway or self-oscillation in a driver stage takes place won’t save your work-piece.  As most critics warn, it’s going to fail when you least expect it. And don’t believe that thermal runaway doesn’t happen with MOS devices.

Paralleling devices

This is a practice not always fully understood and in most cases designers of MOSFET circuits don’t advise this.  It is imperative that no current hogging takes place when sharing devices.  It is also imperative that they are equally matched, for this reason of course.  (current through both devices should be matched).  Thermal transfer must be equal. Drive must be equal. TI (see below) advise that their products can be shared to increase current handling, but with caveats. Read application notes carefully.

The LMD18200 is known for it’s reliability. App note below.

Texas Instruments to our rescue:

Being a big fan of T.I. one would be surprised if there wasn’t some sort of solution offered on their website.

Main:

http://www.ti.com/motor-drivers/stepper-driver/fets/products.html

 

 

 

 

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