Strange Bridge Controll issues(again)

[Jon Slaughter]

.......

So I increased the speed of the PWM to 60khz and it worked... at least for a
few seconds. It seems if it was too low of a speed then the IC's(MAX5063)
would burn up because I guess they would try to drive the highside gates
without a good reference voltage.

But now I'm driving it at 60khz and I think the mosfets are burning up(or at
least getting hot... I do not know what is happening to the drivers). Even
when driving a small motor the fets get warm(not much actually but probably
shouldn't even change temp) to the touch.

But mosfets use more power the faster the switch! So how the heck am I
suppose to choose the optimal pwm frequency that provides the MAX5063 with a
stable boost and minimize the fet switching power losses?

Nothing in the datasheet discusses the pwm speed(which is why I initially
did it at about 100hz for testing which only worked for driving led's).

The's damn ic's are driving me nuts ;/ at $4 a pop I think I'm just going to
go with dual drivers and dual powerfets. (would be about the same prive and
much easier to work with giving all the problems I'm having with the
MAX's(they could at least have some overload protection)).

What I was noticing when I ran it at < 1khz was that about a duty cycle of
75% the motor would stutter(like it would stall). I have no idea what's
causing this but I'm thinking one of the max's is bad or possibly the power
fets.


I can't believe this damn thing is causing so many problems ;/

(for those who don't remember or haven't been following...

What I have is a pic24 generating a pwm signal. It goes into some logic that
takes the 2 input's and generates 4 outputs that represent states of the
fets. Basically toggling one side's high and low for the boost generation
and locking the other side properly... one of the input's completely flips
everything so it can be ran "backwards".

Everything works up until this point. I get problems when going into the max
chips(which simply take the output's of the logic as input's).

The circuit is extremely simple

http://i36.tinypic.com/2vblekk.jpg



Now that I think about it, the fets could be getting warm due to
"shoot-through" since I'm not delaying the signals but I imagine the
transitions on the logic are just fine. I don't recall the fets getting warm
when using a small load(led's) so it seems to be load dependent which would
point to a different problem.

Surely 60khz is just fine and shoudln't cause the IC's to screw up? (with
maybe a 0.5A load.. the fets are rated at 40A)

 

[legg]

Everything works up until this point. I get problems when going into the max
chips(which simply take the output's of the logic as input's).

The circuit is extremely simple

http://i36.tinypic.com/2vblekk.jpg



Now that I think about it, the fets could be getting warm due to
"shoot-through" since I'm not delaying the signals but I imagine the
transitions on the logic are just fine. I don't recall the fets getting warm
when using a small load(led's) so it seems to be load dependent which would
point to a different problem.

You can't troubleshoot this thing blindly - You'll need to scope input
signals, corresponding gate drive and actual Vgs waveforms for
integrity. This should be possible, firstly, without fets attached (HS
grounded), effectively reducing scrap at the early stages of
troubleshooting.

Gate waveforms can be modified, after the fact, to slow down turn-on,
while maintaining turn-off speed, if logic can't be trusted to provide
appropriate dead-time intervals.

Shoot-through issue investigation will benefit from fet current
waveforms, after logic and drive integrity is established and fets are
actualy in place.

60KHz switching of the large gate charge mentioned in previous
postings can be expected to produce losses in the driver. It doesn't
take many 100's of milliwatts to raise driver IC body temperatures.
These driver losses can be intentionally distributed into the series
limiters present in typical gate drive turn-on delay networks,
reducing actual driver loss by nearly a factor of two.

If you can do 60KHz, why can't you do 1KHz or 10KHz, as an
intermediate step? This might be easier to troubleshoot, with reduced
high-frequency effects. It was only 100Hz or static-DC control that
was considered unsuitable in your application.

In totem-pole high-frequency power circuits with signifigant
freewheeling current, the internal fet body diode can be very slow to
recover and contribute considerable stored charge to shoot-through
currents, regardless of dead-time precautions.

RL