# I can' select a N-type mosfet for H-Bridge motor driver

#### Ace Electronics

Joined Oct 24, 2019
4
I'm not very familiar with mosfet motor driver but i selected mosfets depending on these parameters:

Id>10A

Vd>12

Rds<0.02

but the current appears in the n-type at the right side seems too high about -94A

another thing that i'm worried about mosfet's temp. i calculated Power desperate using this formula:

PD=Rds*I^2

then compared using the max power formula:

PDmax=(Tj-Ta)/Rja

and the result was that PD>PDmax

so i decided to use Heat sink

finaly will it be better to use "IRF1405" n-ch mosfet instead of "IRFZ44"

i want to know if this design will work or not

#### ronsimpson

Joined Oct 7, 2019
2,173
You have problems!
Q1=on, Q2=off good
Q3 and Q4 are on. There is 90+ amps going from +14V to ground. You need to fix that.
I get asked this question many time a semester. It must be a home work problem. So, think about it and tell me how to fix it.

#### Ace Electronics

Joined Oct 24, 2019
4
You have problems!
Q1=on, Q2=off good
Q3 and Q4 are on. There is 90+ amps going from +14V to ground. You need to fix that.
I get asked this question many time a semester. It must be a home work problem. So, think about it and tell me how to fix it.
View attachment 189576
I removed R4 and R5 from the circuit and it run successfully without any problem. but i'm not sure how this fixed the problem. please tell me if you see any problems because i don't want to put anything on fire

#### Audioguru again

Joined Oct 21, 2019
4,812
With your R4 and R5 forming voltage dividers with R1 and R2 when a transistor is turned off then what is the gate voltage on both Mosfets on each side? Then the upper and lower Mosfets are both turned on and are shorting the supply with a massive current.

You have the same problem now but for only the moment when a transistor switches off. It will take time for R1 or R2 to charge and discharge the high gate capacitance on its Mosfet then both the upper and lower Mosfets will both be turned on shorting the supply with a massive current.

Drivers for Mosfet H-bridges create a dead time delay when the input switches to avoid the problem.