Increasing the capacity of stepper driver ics

[Dryjoint]

Hi all. I'm looking to increase the output capacity (current) of a stepper motor driver ic. Above about 4A, the only commercial units are complete "systems" on a board. So, in principle, how's this: Connect the existing outputs from the ic to a pair of comparators (with pull up/down) to clean up the signal, then use the comparators' outputs to drive high current fets. So I've added a "power stage". Any good, or am I missing something fundamental here? Steve.

 

[BrianG]

Hi Dryjoint, For my two pence (cents) worth, I don't think you need comparators unless the existing output stages are likely to have poor turn on/off times without the high current load connected. Unless you need level shifting circuits as well, the main requirement is to ensure there is enough voltage swing to make sure the FETs turn really hard on and fully off.

You may also need additional clamp diodes as the original chip driver stages probably has these on board.

 

[richs]

Hi Dryjoint-

I concur with BrianG. A good healthy swing of the voltage
to the power fets should produce the required current drive
without comparators in the circuit. You might need a
small buffer amp in between, but I would suggest looking
at a simple semiconductor switch in the circuit instead.

Cheers,

Rich S.

 

[Dryjoint]

Thanks. I'll give it a go.

 

[BobbyNewmark]

How about instead of a comparator, use a logic buffer with a Schmitt trigger input (which has hysteresis). That will help the circuit switch cleanly even if there's a lot of inductance in the ground lead.

For a given winding, do you drive with one FET or an H-bridge? If an H-bridge, use a FET driver chip that prevents shoot-through.

Be gentle to the gates. Even the big power MOSFETs can usually only handle a gate-source voltage of 20 volts.

 

[Dryjoint]

Yes, I intend to emulate the internal H-bridge structure of the ic. At the end of the day, my comparator was merely a diy schmitt circuit, anyway. So a buffer would be neater, but I wasn't sure if I might have to adjust the switching (threshold) level due to unanticipated output voltage levels. Having said that, it should be rail-to-rail at the ic o/p's. And thanks for the driver tip.

 

[MikeHalloran]

I've seen a 74C14 running on 12VDC used as a FET driver.

Mike Halloran
Pembroke Pines, FL, USA