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[DPRG] Voltage Conversion

Subject: [DPRG] Voltage Conversion
From: Quinones, Jose jquinones at ti.com
Date: Mon Nov 30 12:37:13 CST 2009

I am going to agree with Paul here!

The DC/DC converter needed to take down 48V to 12V and at 20A is going to be ridiculously bulky and expensive. I am going to assume a Buck converter here. If you have 20A DC at the load, the peak current has to be way larger than that. Hence, the FET Switch will need to deal with this. Not to mention the feed forward diode. But my biggest worry is the inductor. It will have to be huge! I just can not imagine such hassle just to drive 12V motors when an H Bridge is such a nice step down converter by itself, when properly PWM'd. Of course, there can be other DC/DC converter topologies I am not aware of, so I am going to plead ignorance for the time being.

Now, if you decide to go with the 48V H Bridge to drive your 12V motor, there are still some challenges that need to be overcome. First, look into an OSMC (from Open Source Motor Controller) H Bridge. Start on www.robotpower.com, although I believe theirs are only good up to 50V. However, the OSMC was designed around the HIP4081A from Intersil and if I am not mistaken, they will do up to 80V on the high side. Datasheet can be found here http://www.intersil.com/data/fn/fn3659.pdf. I did a design on this chip a few years ago, but was not successful in making it work. I was able to generate funny noises as they detonated into obliteration, though...

My biggest fear is what happens if the motor gets the full 48V for some unknown reason. I don't know, maybe the microcontroller driving the H Bridge goes crazy and PWM goes to 100% Duty Cycle. Clearly in order to safely use a 48V power source with a 12V DC motor, you would need to limit the PWM to 25% for full speed. Of course DC motors are not going to get killed if you go beyond their rated voltage a little bit; they will just spin faster. But at the same time, I would not run a 12V motor at 48V (heck, I am not certain I would at 24V either!) as this is way beyond their limit. So there are two deadly scenarios I can already sense:

1. The voltage is too high and the motor spins in such a fashion that the mechanical load is severely affected, or

2. The voltage is too high and the motor burns out.

Hence, making sure this voltage (or PWM) does not go above something like 18V or 20V at an ABS MAX must be ensured. I can see a small controller measuring the voltage at the motor (or the Duty Cycle at the PWM input) and safely shutting down the H Bridge if something goes haywire. Redundancy may not be a bad idea.

Seems like a painful endeavor, but it will certainly be way less painful than a 48V at 20A Buck converter.

Best regards,
 
JIQ
Texas Instruments - Applications Engineer
13560 North Central Expressway
MS3744
Dallas TX, 75243
Office: 1-214-567-7275
Mobile: 1-214-240-3179
-----Original Message-----
From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org] On Behalf Of Paul Bouchier
Sent: Saturday, November 28, 2009 6:41 PM
To: kip at kdream.com; 'Charles Thurston'
Cc: 'dprglist'
Subject: RE: [DPRG] Voltage Conversion

Kip - you responded with the right answer to the wrong question. Clearly,
wanting that amount of power means Charles is driving motors, and it makes
no sense to regulate 48V @ 5A down to a nice smooth 12V @ 20A only to PWM it
to motors. 20A is harder to deal with than 5A. Better to use a motor PWM
module that accepts 48V and designed to drive 5A average. The only thing
that might make that problematic is if he was hoping to use an
already-installed motor controller that runs off 12V; but really, if you
need to run off 48V, ditch the 12V module & get a high-voltage module. Jose
might be able to steer you in some good directions.

A separate, professionally-built regulator to power the electronics makes
total sense - all kinds of protection built in.

Paul

> -----Original Message-----
> From: dprglist-bounces at dprg.org 
> [mailto:dprglist-bounces at dprg.org] On Behalf Of Kipton Moravec
> Sent: Friday, November 27, 2009 7:14 AM
> To: Charles Thurston
> Cc: dprglist
> Subject: Re: [DPRG] Voltage Conversion
> 
> 
> 
> 
> On Thu, 2009-11-26 at 00:41 -0500, Charles Thurston wrote:
> > Hello DPRG,
> > 
> >   I am looking to convert 48 VDC into 12VDC at about 20 to 25 amps. 
> > The supply is Wet cell batteries on a golf cart (I need 
> something to 
> > run steering motors, brake motors and my electronics with). 
> > Suggestions on tapping 1 or 2 batteries are out, These are 8 volt 
> > batteries. Low power consumption at idle is important.
> > 
> 
> For the steering motors, brake motors use high speed PWM with 
> MOSFETs. Just never go above 25% duty cycle.
> 
> I am not sure what you mean by break motors, but the easiest 
> way to break is to short the motor terminals (after removing 
> power of course). 
> 
> Use an off-the-shelf power supply like Dale recommended for 
> the electronics. You will probably be under 0.5A for electronics.
> 
> 
> Kip
> -- 
> Kipton Moravec AE5IB .- . ..... .. -...
> 
> "Always do right; this will gratify some people and astonish 
> the rest." --Mark Twain
> 
> 
> _______________________________________________
> DPRGlist mailing list
> DPRGlist at dprg.org http://list.dprg.org/mailman/listinfo/dprglist


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