AVR-Chat

At 02:00 PM 2/19/05 +0000, velo1_4mb wrote:


>--- In AVR-Chat@yahoogroups.com, Robert Adsett <subscriptions@a...>
>wrote:
> > At 09:11 PM 2/18/05 -0500, Ben Matthews wrote:
> > >I am a 15 year old high school student using a tiny 26 on an
>odyssey
> > >of the mind project (we have a competition next week :) ).
>
>This is Ben's father.  A little more info.
>
>Ben's coach is the person in charge of the program and is about to
>lose it.  She is coaching 3 teams (balsa, technical, and vehicle)
>and all of them are having major problems.  I am trying to help the
>vehicle and technical teams.  I used to do analog hardware (Nike
>Herc missle) but have been a software guy for over 20 years now.
>
>The two types of MOSFETs are in use because I made a Digikey order
>for the first batch.  Between Ben's team and another with an
>impulsive teenager the supply went down in a hurry.  The local
>electronics surplus store has BUZ11s in stock and they looked
>adequate.  Thus the mix of types.


OK, but be aware that may effect the switching.  The two types may need 
different gate shaping circuits.


>The buffer outputs are connected directly to the MOSFET gates.


Um, how are you getting the 10V to drive the gates then?  I don't believe 
either of these devices are logic level devices and while they will sort of 
work at 5V it is asking for trouble.  More importantly the high sides need 
to run at 10V ABOVE the input voltage to turn on,  If they are only running 
at 5V you will definitely have problems.  The gate drive issue, especially 
the high sides, is what worries me the most about what I've heard of this 
setup so far.  Is their a possibility the original circuit called for P 
type FETs on the high side?  That might explain the odd drive 
circuitry.  Even in that case though driving a high side gate off of more 
than 5 or 6 Volts is asking for trouble.  Truthfully I'm rather surprised 
the high sides turn on for any appreciable amount of time.

>The diodes are there for spike supprssion as called for in the book.

Sounds like someone designed the circuit who knew a little bit but not 
enough.  The diodes at best are unnecessary for a MOSFET H-bridge.  The 
MOSFET integral body diode will work just fine.  In some case adding diodes 
will cause problems (including blown power sections).

>He has access to a 65MHz dual trace scope and knows the basics of
>using it.  I can get him through the rest of it.

Definitely needed.  With a low enough load not to cause problems you should 
measure
         - the low side gate
         - the high side gate
         - The gate drive power supply -what voltage is the gate drive 
power?, if it's less than 10V I'd be worried although it might work at 
these low currents.
         - The logic power supply

Both should get 10V with respect to the source of the FET they are driving 
to turn on. You can probably get away with 7V although I'd rather see 12-15 
V on a non-logic level gate drive.  On the off portion of the PWM they 
should be less than 1V.

Check the low and high side turn on and turn off relationships to make sure 
you are not getting shoot through.


>Logic power is supplied by a 4 cell nicad pack.  Drill power is the
>15 cell pack that came with the unit.  Negative is common to both.
>
>The drill is the Harbor Freight 18v unit.  About $17 US complete
>(cheap junk).  There are no noise supprssion caps on the motor.

Noise suppression caps really shouldn't matter very much

>He had blown a transistor earlier in the day from a backwards
>diode.  That one was found with low volts testing.
>
>The 7.2 v pack was a nicad "9v" battery, about 120 mah.  I handed
>him that because it can't source much over a couple of amps.
>
>Oscillation is an interesting guess.  I hadn't thought of that one.
>The only way that I could see getting enough gate leakage to wipe
>the buffer was tranistor failure and since they were not getting
>warm it did not make a lot of sense to me.  I am guessing that when
>the failure occurs he is dumping one battery pack into the other.


Remember the high side reference point.


>Plan for today:
>Check the transistors.
>Put a 300-500 ohm resistor between the buffer and the MOSFET gate to
>limit current when the failure occurs.

Careful, that might make things worse by slowing down the edges and 
increasing shoot through.  Also note that the reference point for the high 
side drive is the high side FET source not ground.  That means its 
reference will vary from 0V (or slightly below) to slightly above B+.  That 
means the output drive on the high side must float with it (and yes I think 
I've said this multiple times, it's important).  The off on the high side 
must float between 0V and perhaps slightly above B+. It should never be 
more than approximately a volt below the high side source voltage.

>Put an auto light bulb in place of the motor in the circuit (dummy
>load).  That will get rid of the motor electrical noise while
>testing.

Good idea if the bulb will draw less current than the motor.

>Fuse it all at 5 amps
>Test with 12 volts (or less).
>I am going to look through the junk box for a low ohm resistor to
>toss in series with the battery as a limiter for testing.

Also a good idea.

Another troubleshooting suggestion.  Hookup only one 1/2H rather than the 
full H.  The other side of the load you are using for testing can go to 
either B+ or B- depending on the test.  For example to test the high side 
turn on hook up the load (say 1K to start to B+ and one of the 1/2 
H's.  Then scope the high side gate with respect to the High side source 
(which will be close to B+) and see how it behaves.  In fact this should 
probably be one of the first tests you do.  This will allow you to verify 
the high side operation at low currents (particularly if you use a current 
limiting resistor on the B+.


One more question (for now), do you have a large capacitor across B+ close 
to the H-bridge (physically close with short trace lengths)?

>Odyessey is a great program.  The vehicle and technical problems are
>really difficult this year.  We are in a small public school (100
>graduates/year) in upstate NY.  Lots of community involement for
>coaching and skills training.


I just wish I had something like that available at that age.


Good luck,

I'll try to help you along.

Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,
be they legal, genetic, or physical.  If you don't believe me, try to
chew a radio signal. "

                         Kelvin Throop, III