Re: A little h-bridge help?

Eriswerks wrote:
Thanks for all the help. I've played around with various different
versions of this circuit and never did get it to work properly. What's
strange is that if you search for h-bridge circuits on google, almost
everything out there is some version of this guy's circuit. Maybe
everyone else is just using different mosfets than I am, or maybe they
just put it online without trying to build it.

Probably the latter...

I have some mosfet driver chips from Microchip on their way right now,
which hopefully will make life easier.

In building simpler test circuits to figure out what exactly was wrong
with this, I came across a couple of quirks. One is that the mosfets
like to stay on once I've given them a gate voltage, even once I take
that voltage away.

Not a quirk but natural, since the conductivity of the drain source channel is controlled by the gate to source voltage, i.e. the voltage across the gate capacitance.

See pretty metal gate + oxide + semiconductor picture at
http://en.wikipedia.org/wiki/MOSFET#MOSFET_structure

Bipolars eat base current to stay conducting (imagine a small resistance between base and emitter), whereas mosfets just need to have the gate charged up once and that's it (small capacitance between gate and source).

The gate capacitance will remain charged if you entirely disconnect the gate voltage. If the n-channel mosfet gate remains positively charged => mosfet remains conducting, until you discharge the gate.

I can get rid of this gate capacitance by running a
resistor from the gate to ground, but then there's an undesirable
voltage drop. What's the preferred method of dealing with gate
capacitance?

Dealing? Errm. Switching mosfets is pretty much identical to driving (charge, discharge) a capacitor.

A bad choice to handle the discharging (turn off) part is to use the resistor you mention. Actually, your reference schematic
http://www.armory.com/~rstevew/Public/Motors/H-Bridges/Blanchard/h-bridge.htm
has such resistors, namely R3, R4, R6, R8, and relies on these for the mosfet to switch off. Not good. Also because of the voltage drop, as you noticed.

Usually you "get rid of" the gate charge with a driver setup that can actively both charge and discharge the gate. Like, complementary emitter
follower, TTL logic buffer, mosfet driver chip.

(Btw, D1..D4 are unnecessary since MOSFETs already contain internal parasitic diodes that are at least as fast as 1N4001's, often even significantly faster).

The other odd thing is that my n-channel mosfets are giving me a huge
voltage drop even when I test them singly, not in a larger circuit.
(Not so for the p-channels, they work as expected.) It'll drop from a
12v supply to a 1.8v output, and that's with 12v at the gate.

Connected how, and output being what pin? If as source follower: the voltage at the source is typically at least by V_threshold (see mosfet data***, Vth, may be e.g. around 4V) lower than the voltage at the gate.

Do I need
to use a gate voltage that's higher than Vcc in all cases for n-channel
mosfets?

Depends on the circuit. Not necessarily Vcc, but the gate voltage Vg has to be at least 8V higher than the voltage Vs at the source, so you get gate->source voltage Vgs of at least 8V.

Btw if the source is not connected to ground, don't make the mistake of measuring the gate->ground voltage instead of gate->source. :)

Or is that an issue that only arises when you try to
incorporate them in an h-bridge?

In the upper n-channel mosfets of a h-bridge, certainly an issue. There you want the h-bridge output to be alternately 0V and Vcc, so to get the Vcc output, the upper mosfet's source will have to give out same as on its drain i.e. Vcc. So, the source is at Vcc, hence the gate to ground voltage must be at least Vcc+8V for that mosfet to have gate-to-source of 8V and be fully conducting.

- Jan
.

Quantcast