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'[EE]: Mosfet N-Chanel switching'
2001\10\14@163838 by Andrew Hooper

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I'm trying to build an H-Bridge for inductive loads using n-channel fets.

The fet that has its source connected to ground and the load connected between
the 18V+ rail and drain I get full power. (around 18 volts appears between negative and drain)

But the fet that has its drain connected to the vcc and the load connected between
the negative rail and source I'm getting a considerable amount less power.
(only about 15.5 volts appears between negative and the load)

Am I barking up the wrong tree or should this work?

The reason for using n-channel is that they are easier to get and less expensive.

Thanks in advance for the assistance.

Andrew Hooper

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2001\10\14@173724 by Wade Carpenter

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Hi,

I'm no expert in CMOS technologies, but I know that in typical CMOS circuits
your P-channel devices are attached to Vdd and your N-channel devices are
attached to Vss.

For the case of passing a signal using the transistors for switching, if you
try to pass Vdd with an N-Channel its value will be degraded, but passing
Vss will be fine.  The opposite holds true for P-Channel, ie passing Vss
will be degraded but Vdd is fine.

The reason for this is as follows:
N-Channel is turned on with Vdd.  If you are passing Vdd, ie you want Vdd on
drain and source, then your output Vs will actually be Vdd-Vgs because of
the material properties of the device.

P-channel is turned on with Vss. Passing Vss, you'll have Vss at gate, drain
and, presumably source, but the same holds. you need Vgs.  so your output
will be |Vgs|.

Hope that helps.

Wade

{Original Message removed}

2001\10\14@175349 by Spehro Pefhany

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At 09:36 AM 10/15/01 +1300, you wrote:
>I'm trying to build an H-Bridge for inductive loads using n-channel fets.
>
>The fet that has its source connected to ground and the load connected
between
>the 18V+ rail and drain I get full power. (around 18 volts appears between
negative and drain)
>
>But the fet that has its drain connected to the vcc and the load connected
between
>the negative rail and source I'm getting a considerable amount less power.
>(only about 15.5 volts appears between negative and the load)
>
>Am I barking up the wrong tree or should this work?

The gate drive for the high side driver needs to go to something like 28V
(10V
*above* the +Ve rail) in order to use N-channel MOSFETs (5V if you happen
to have
logic level MOSFETs). If you use half N-channel and half P-channel devices
you can
drive each half-bridge with a 0/18V signal (but that's getting close to the
punchthrough voltage rating - typically 20V - of the gate, so zeners may be
in order,
particularly with inductive loads that can cause multiple-volt transients
on the
supply lines with only small parasitic circuit inductances.

>The reason for using n-channel is that they are easier to get and less
expensive.

If it makes your driver more expensive, it may not be a good trade off. It's
quite common at high voltages where P-channel devices are truly rare to
non-existent,
but maybe not such a good approach at such low voltages.

If you still want to do it, one way is to make a driver that goes 0-10V and
level
shift it with a capacitor and diode, but practical driver circuits also often
include undervoltage lockout to remove drive when the voltage drops close to
the linear region. This prevents destruction of the high-side driver. You also
have to maintain a minimum "low" time and maximum "high" time to keep the
level
shifter working properly.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2001\10\14@204043 by Lawrence Glaister

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Hi Andrew,
I have been dabbling in H bridges as well for a cnc project. I have a
web page done up with most of my notes on various h bridge circuits
found around the web. Feel free to check it out.
http://members.shaw.ca/cncstuff/bridges.html
You dont mention if you are using logic drive fets of regular n channel
fest, but it sounds like you are talking about the upper fet in the
bridge not turning on fully. In a bridge with n channel fets, you will
need a charge pump or a higher value voltage to drive the upper fets
gate. A normal fet likes at least 10v between gate and drain.

cheers

On Sun, 2001-10-14 at 13:36, Andrew Hooper wrote:
{Quote hidden}

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2001\10\15@002527 by Bob Blick

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At 09:36 AM 10/15/2001 +1300, you wrote:
>I'm trying to build an H-Bridge for inductive loads using n-channel fets.

The IR2112 is a favorite of mine for doing this. Almost no parts needed.
Don't know how available they are where you are.

Cheers,

Bob Blick

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