# Mosfet and bridge question

#### bytraper

Joined Sep 28, 2010
127
I wanted to ask something that's been puzzling me for a while.

I'm interested in dc motor controllers of all types, and I've been studying them over the past year while I learn about electronics. One thing I never unerstood is this:

For a single direction controller, a N-channel mosfet needs just +10v at the gates to switch it on and off, this is the same in a H-Bridge with P and N channel mosfets, but in a circuit with 4x N Channel mosfets in the bridge, the voltage has to be +10v OVER source voltage through the gates!

Why does it have to be +10 over the gates, whereas a normal single speed controller just has to be +10v at the gates?

#### praondevou

Joined Jul 9, 2011
2,942
but in a circuit with 4x N Channel mosfets in the bridge, the voltage has to be +10v OVER source voltage through the gates!
What does that mean???

In a fullbridge with 4 n-channel MOSFETs you need separated High-side drivers, so your high-side MOSFET's source pin will be at a higher voltage than the low-side source pins. Is that what you mean?

#### SgtWookie

Joined Jul 17, 2007
22,230
Because Rds(on) is specified when Vgs is a certain value; usually 10v for standard N-ch MOSFETs, and 4.5v or 5v for logic-level N-ch MOSFETs.

Note that Vgs means "voltage on the gate, as referenced to the source terminal".

A MOSFET "doesn't care" about where the "real" ground is; what matters to the MOSFET is its' three (sometimes four) terminals; gate, source, and drain (sometimes substrate, but this is usually combined with another terminal).

When Vgs is below the threshold value, the MOSFET is considered to be turned off.

So anyway, let's say you have a DC motor rated for 24v, and you have a 24V DC source. In order for the low-side N-ch MOSFETs to turn on, you need to bring the Vgs to 10v.

The high-side MOSFET also needs a Vgs of 10v. As the high-side MOSFET turns on, the voltage on the source terminal rises; voltage is dropped across the load (the motor). Since the voltage on the source terminal is increasing, the voltage on the gate must also be increased to keep Vgs at 10v. If the only power source is 24v, then as the source terminal voltage nears 24v, the Vgs will reduce until the MOSFET threshold is reached. This results in high power dissipation in the MOSFET, as it is then in a partially conductive state.

If you try to reference the high-side gate to ground instead of the source terminal, you can get in trouble quickly.