# H Bridge motor control clarification

I am trying to drive a 12V motor that has max amps of 15amps. I am using MOSFET RFP30N06LE. Can someone explain to me why these are used for motor control when they control the current? When I use my circuit below, turning the gates on using TTL 5v from an Arduino my voltage on the 12v circuit drops dramatically and the motor barely turns. What am I doing wrong? These are TTL MOSFETS from my understanding.

simulate this circuit – Schematic created using CircuitLab

• Have you done the math for M1 and M3? Jun 21, 2018 at 16:14

M1 and M3 are wired as source followers. This means that the source voltage will be at best 1 to 3 volts below the gate voltage. Given that the gate voltage is 5 volts, the source voltage from M1 and M3 will be at best 4 volts and, under heavy load probably only 1 or 2 volts.

Quite often P channel MOSFETs are used in positions M1 and M3 and you would pull the gate to 0 volts to activate them: -

If you are using N channels MOSFETs in M1 and M3 positions, a bootstrap circuit is normally used that raises significantly the gate driving voltage. Here's a half bridge that uses a bootstrap driver IC: -

Or sometimes, isolated techniques are used like this with 4x N channel devices: -

• So on an N channel mosfet, the source Gate Threshold voltage has to be above that of the source pin? I'm still learning to read the datasheets. What part of the datasheet should I be looking at to calculate what that voltage would be from source to drain? Jun 21, 2018 at 16:54
• Look at figure 7 in the data sheet (saturation characteristics). It tells you what voltage you need on the gate (relative to the source) to obtain a certain drain current at a particular drain-source volt drop. You are aiming for high drain current (15 amps) with low DS volt drop. Jun 21, 2018 at 17:01
• @mScientist - Yes. For an n-type, the gate must be higher than the source, and for a p-type the gate must be less than the source. When reading the data sheet, look for Vgs(th), the threshold gate voltage. Note that this is specified to produce a very low drain current, typically 250 uA or 1 mA. Take the maximum value given and (for high-current loads) multiply this by 2.5 or 3. Jun 21, 2018 at 17:26
• Or good P channel mosfets might be an easier find. Any suggestions for P and N for the above circuit? The motors max amps is about 15 amps. I have read that starting and stall amps can jump double the max. Jun 21, 2018 at 18:01
• @Andyaka The PWM is going straight into the gates of the two N mosfets? Jun 21, 2018 at 18:02