Re: H-Bridge Problem
- From: Winfield Hill <Winfield_member@xxxxxxxxxxx>
- Date: 8 Feb 2006 04:08:21 -0800
Walter Harley wrote...
James wrote ...
I am having a problem controlling an H-Bridge Circuit that I put
together, it is a variant of Eugene Blanchard's design. Anyway
the schematic can be found here:
http://www.geocities.com/jj_legend_83/circuit.jpg
I two power supplies, one of the driving voltage, and one for the power
to the gates of the MOSFETs. I have a function generator as a logical
input to the circuitry. The voltages that are involved are on the
circuit diagram. I have the grounds of the two bottom MOSFETs attached
to the negative terminal of the driving voltage DC power supply, and
the rest of the grounds are attached to the negative terminal of the
control voltage power supply. The floating terminal of the function
generator is also attached to the negative terminal of the control
voltage power supply.
Anyway, the upper-left BJT and MOSFET are always "on", and I am having
trouble figuring out why. If anybody could give me some pointers here
that would be great. Thanks!
I don't think there's any way that circuit can work. Something is
missing, e.g., it's been copied incorrectly.
Or badly designed. :-) E.g., how can one expect a 1V square
wave to switch an NPN at ground *and* a PNP at up to +18V???
If the signal in is between 0v and 1v, then you're right, the upper-
left BJT will always be turned on. Its base would have to get close
to Vcp for it to turn off.
It also suffers from basic architectural problems. The "signal"
should first be squared up and prepared as a valid logic (on/off)
signal, with one circuit. Then it can be inverted, level-shifted,
buffered, etc., and prepared for driving the MOSFETs. But this
needs to be done without delays in the wrong places, to prevent
high rail-to-rail shoot-through currents through the FETs.
Next the MOSFET gates need high drive currents to turn them on
and off. James' IRF3205 has of 3247pF gate capacitance, forming
a disastrous 32us time constant with a 10k pulldown resistor.
This big MOSFET hungers for amps of gate drive.
The easiest way to solve all these tough problems is to use an
inexpensive H-bridge driver. I like to use the Intersil HIP4080A
and 4081A for high-frequency bridges (e.g. 1MHz). There are many
other good choices for use below say 200kHz.
--
Thanks,
- Win
.
- References:
- H-Bridge Problem
- From: James
- Re: H-Bridge Problem
- From: Walter Harley
- H-Bridge Problem
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