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I'm looking to build a motor controller where I want to have consistent motor performance. Normally you have a 6v DC motor and you can plugin a 7.2v battery pack directly and be fine, but the motors start out running really hard and slowly fade as the battery level drops. So I was thinking of building/buying a DC-DC power supply so the motors would get a consistent 6v but still be able to draw a lot of current. Digikey has a 10A 6V Murata OKR-T/10-W12-C DC-DC converter for $10. While I can buy one, I always like learning to build my own and would like the ability to have a final product have both the motor controller and DC converter in one. The motors I'm working with have a stall current of 6A each.

My questions:

1) Is a DC-DC converter a good solution for my situation?

2) Would that Digikey DC-DC converter work in this situation?

3) Are there better alternative solutions?

Any pointers in the right direction would be much appreciated.

Here are some TI components I'm going to experiment with:

LM3150 - SIMPLE SWITCHER® CONTROLLER, 42V Synchronous Step-Down

PTN78020W - 6-A Wide-Input Voltage Adjustable Switching Regulator

LM2587 - 5A Flyback Regulator

TPS56921 - 4.5V to 17V Input, 9A Synchronous Step-Down SWIFT? Converter with VID Control

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  • \$\begingroup\$ I've added some TI components that I'm going to experiment with \$\endgroup\$ – getSurreal Mar 20 '13 at 15:33
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    \$\begingroup\$ Its worth mentioning that this is probably for an RC-type application (the 7.2V screams NiCd battery pack). Those motors can draw several dozens of amps in little bursts, such that your DC-DC converter will just laugh and then promptly die. \$\endgroup\$ – Bryan Boettcher Mar 20 '13 at 16:21
  • \$\begingroup\$ It's a robotics application using geared motors. 7.2v NiMh in current testing with potentially moving up to 9v and possibly 12v lead acid. \$\endgroup\$ – getSurreal Mar 20 '13 at 16:47
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You don't need a separate DC/DC converter. A motor controller that does Pulse Width Modulation (PWM) into an inductive (motor) load is essentially a buck-type non-isolated DC/DC converter. At an intermediate duty cycle, the voltage across the motor will be less than the battery voltage - try measuring it. Power is conserved, so

Vbatt * Ibatt = Vmotor * Imotor + [switching losses]

Note that if you limit the maximum duty cycle, you can safely use a higher battery voltage than the motor can withstand. This may allow you to add more energy storage (more batteries) to your project without being constrained by motor voltage.

Whether you decide to close the control loop with voltage (like a DC/DC converter), current (torque control), or go open loop (most small electronic speed controls) is up to you.

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  • \$\begingroup\$ Without the DC-DC converter wouldn't there still be the motor performance fade as the battery drains unless you continually adjust the duty cycle with the battery voltage? \$\endgroup\$ – getSurreal Mar 20 '13 at 15:15
  • \$\begingroup\$ Yes, but this will happen with a DC/DC converter also. The net effect of having a separate DC/DC is that your maximum motor voltage will be limited by the DC/DC, which may delay the point where the battery is depleted enough to cause performance degradation. Note that limiting the maximum PWM duty cycle on your motor controller will accomplish exactly the same goal. \$\endgroup\$ – HikeOnPast Mar 20 '13 at 16:40
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1) Yes

2) Yes but be aware that on continuous use it has a MTBF of only 273 days. There are better devices i would have thought

3) Yes if you want it cheaper and you want to learn

On point (3) you might want to consider the plethora of circuits and chips from Linear technology (amongst others) for a step-down (or buck) switching regulator.

You might also consider designing your own circuit based on an op-amp, a comparator a switching FET, inductor and diode.

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  • \$\begingroup\$ Thanks for pointing out that MTBF. I'll scratch that one off the list of options. \$\endgroup\$ – getSurreal Mar 20 '13 at 15:30

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