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Subject: Re: H-bridge amplifier design

Date: 11/28/01 at 6:05 PM
Posted by: Brian Snell
E-mail: bsnell@lara.on.ca
Message Posted:

In Reply to: H-bridge amplifier design posted by Mark on 11/20/01 at 6:21 AM:

Hello Mark. Your project plans for an HBridge motor Driver can very quickly get bogged down unless you do your homework. 5volts is not a very good voltage to use as a motor-supply, particularly if you use this same rail to power your control electronics, as there are present in motor circuits all sorts of electrical glitches which can be difficult to get eliminate. Keeping your 5v control rail clean is probably rule #1, so try to use a second power rail to power the motor output stages. Also, if you plan to use silicon transistors in the HBridge you will be losing 1.2v (2x 0.6v per VCE) leaving you with only 3.8v for the motor. If your torque requirements are modest, you may get away with a 3v model motor but you are limiting your options. If you plan variable speed then you will probably need to employ some form of PWM drive, unless, once again, the motor currents are small and the bridge transistors large. My advice would be to get copies of Data Books from suppliers including "SGS-Thomson","Unitrode","Sprague", "Analog Devices", covering Motor Drive products and study well the Application Notes contained therein. If you decide to use PWM speed control then additional requirements for a motor with enough, but not too much, self-inductance, a "Kt" (Motor constant --sort of a formula involving amp-turns and oz-inches) gets into the act, and then managing to run the PWM frequency high enough to keep the losses low, while still minimising internal motor power losses due to ripple current. A full design can be very involved and tricky. If you decide to go this route, use a motor driver chip from one of the above sources (most designs now have current-limiting, common cross-conduction protection, etc), and follow their circuitry recommended for the driver's implementation, as circuit layout can influence performance quite dramatically --particularly if the bridge in the chip is a Hexfet. It would be difficult to create circuitry using discrete components and with the degree of added protection which these chips offer for the price asked.
Lotsa luck.

Brian Snell

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