Hi Sean, the disadvantages are that reversing the motor would still require
a H bridge, response time would be slower and you won't be able to drive
the motor at very slow speeds. (You could still pulse the SMPS output like
a H bridge but response will be slower)

Other then that, u'd have the best of 3 worlds, variable voltage, PWM motor
controller and efficient use of batteries! Should be great for RC and other
portable products.

The easiest way to implement this would be to use a digital pot on the
voltage sense/feedback divider available in some switch mode controllers.
Either that or a bipolar/fet across the voltage set resistor.

Normal digital pots won't go higher then 5V so a discretes solution is
required. I guess rolling your own 8 bit, 256 step resistor array is the
only way to go for better precision...

Terry


At 04:20 PM 7/25/99 -0400, you wrote:
>Hi Terry,
>
>From the experimenting with PWM that I have done recently, I think your
>idea is excellent. The only problem is complexity.
>
>Here is how I understand PWM (someone please correct me if I am wrong):
>
>When you feed PWM to a DC motor, you might think that you are saving power
>as you turn down the motor speed. It is true that less mechanical work is
>being done,but more power is being wasted in the winding resistance. This
>is because the same average current needs to flow (if the frictional torque
>remains the same). If we have a motor that generates 10v of back emf at
>full speed and has 1 ohm of winding resistance, then with 12v PWM at 100%
>duty cycle, we have a continuous 2 amps flowing, we are using 24 watts of
>electrical power,and doing 20 watts of mechanical work (approximately,
>assuming that the transfer of power due to back EMF is purely mechanical).
>The efficiency is 20/24 = 83%
>
>If we now drop to 50% duty cycle, we still need an average current of 2
>amps to oppose the friction,so we have 4 amps flowing during the on time.
>That's 48 watts of power for 50% of the time,or,again,24 watts of average
>electrical power. However, the back emf is now only 8v ( 12-8=4, 4/1ohm = 4
>amps),so we are only doing 8*4=32 watts for half the time, or 16 watts
>mechanical work. The efficiency is now only 67%,and it reaches 0% when the
>duty cycle gets so low that the resistance doesn't allow enough current to
>flow to oppose the friction (in this case, we couldn't operate the motor
>below 16% duty cycle). As we decrease the duty cycle, the speed vs. duty
>cycle curve is nonlinear and drops expecially fast as we near 16%.
>
>With a switcher,however,we could forget PWM and just supply a continuous
>variable voltage to the motor. The current would stay the same and the RPM
>would linearly follow the voltage. The efficiency would not change much
>over the whole RPM range.
>
>Sean
>
>
>At 03:39 AM 7/26/99 +0800, you wrote:
>>Hi, i was just thinking, instaed of using a semiconductor switch to control
>>the motors, why not use a switch mode power supply to directly drive the
>>motors?
>>
>>If your circuit doesn't require reverse motor drive, wouldn't it be ideal
>>to cut out the switches and just use the SMPS's drivers?
>>
>>Battery powered controllers would benefit even more from the SMPS's ability
>>to step up the battery voltage and squeeze out every last drop from the
>>batteries, (don't over squeeze to the point of battery breakdown tho..)
>>
>>Current sense in the SMPS could double as overload protection too. If
>>response time is somewhat more critical, use a smaller filter cap for the
>>outputs.
>>
>>What say you folks? Viable?
>>
>>Terry
>>
>>
>>
>>
>>At 10:24 PM 7/24/99 -0700, you wrote:
>>>At 01:55 AM 7/24/99 -0400, you wrote:
>>>>I am a bit confused here: why would the FETs require more precautions? The
>>>>only thing I can think of would be that the power dissipation for a FET
>>>>goes as the second power of current,and for a darlington/BJT, as the first
>>>>power of current. Granted,this makes a big difference as current goes to
>>>>infinity, but in certain cases, you might burn up the BJTs way before the
>>>>FETs if the RDSon of the FETs is really low.
>>>
>>>It's actually even worse than that. The R of the MOSFET increases with
>>>temperature and current, so unless you go overkill or have protection you
>>>have a big problem. With comparable devices, the bipolar will usually take
>>>a lot more abuse(assuming it has enough drive current). Generally in a
>>>small application it's easy enough to use a larger MOSFET, but I've
>>>designed 1200 amp motor controllers and it makes sense to put in good
>>>protection so you can use fewer MOSFETs.
>>>
>>>Cheers,
>>>Bob
>>>
>>
>|
>| Sean Breheny
>| Amateur Radio Callsign: KA3YXM
>| Electrical Engineering Student
>\--------------=----------------
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