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H-bridges again

E

exxos

Jan 1, 1970
0
Hi all,

I am still plugging away for the 100% perfect H-bridge design. While the
driveing wave is 100% perfect, rock solid, down to just 10mV noise, totally
fantasic, and yet. still H-bridge problems, and that should be the easy bit
right ?

anyway, half bridge mode, can run either side seperate and everything works
fine. but when you plug into full H-bridge mode one half of the bridge is
having a interaction with the other half.

Now I have gone though piles of mosfets, you can have a totally perfect
squarewave or have it totally mashed and the H-bridge output still works but
theres some slight noise ripple there which I can't trace...

Tesiting was as before 50:50 duty cycle and its gets a tad hot, get down to
about 45:45 and works fine(ish), works like any other H-bridge... Run at
50:30 and the noise problem goes away, run at 30:30 and noise comes back...
So deadtime is not having any effect other than stopping cross conduction.

Load on the H-bridge does not matter one bit, you can have it O/C or loaded
with several amps and waveforms etc are solid.

The only thing I can find is when you use a 1K gate resistor to charge the
mosfet gate (read more before you yell), the mosfet gate works perfectly
with no voltage on the mosfets, and as you increase voltage on the mosfet
you start to get what appears to be a switch bounce, and the higher the
voltage is the more bounce you get. While this is to be expected with a poor
gate driver its the only thing I can see which is odd. As you decrease the
gate resistor the bounce gets tighter until you no longer see it, though you
do see dots on the rise so its still there.. but in anycase even though its
a bit odd its not the problem...

If you run some PWM via the bridge, you end up with about 0.5V noise on the
output, this only happens in full bridge, on half bridge everything runs
perfectly... There seems to be some interaction between the 2 half bridges
which is messing up. I've tried placing caps across the mosfet rails and
this makes it worse, the more caps you put in the worse it gets. It appears
to be down to the capacitance of the mosfet itself, the only one which seems
to have effect is the Reverse Transfer Capacitance, I have tried running
diodes in series with the mosfet to block the body diode to see if that had
any effect... and nope...

Voltage on the bridge does not seem to have much effect, it does get (I
think) a little worse on higher voltages though its hard to tell. I can run
at 50:30 ratio and the problem goes away totally, the more you get to 50:50
the more of this noise you seem to get, reducing from 50:30 to 30:30 and the
problem starts to come back again, running half brige works fine... so what
am I over looking here ? some odd interaction between the 2 half bridges ??
Have read over and over about such things, also was using a lot of Texas
stuff and actually been taking to one of thier app guys for months, (nice
that a manufacture actually cares about its customers!!) I sent them all the
results, all the waveforms, explained it all to them for weeks, and they are
clueless, I mean you can't exactly build it wrong, it works but I want to
know why it is not 100%.... and no im not going to be happy with 99% either
;-)

Chris
 
Z

Z80

Jan 1, 1970
0
Hi
When using one half only of an H-Bridge IC - the other half must have it's inputs grounded i.e. at 0v. If they are not they will float at any level, sometimes oscillate and this will cause the type of problems you are describing. It will also get quite hot.
Maybe the circuit you are using is allowing one side to float when the other side is in use.

I know this applies to IC's but it may help in your case.

Alan
 
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