Selectring MOSFETs for H-Bridge

kubeek

Joined Sep 20, 2005
5,677
Sounds about right for that kind of power. Note that it will not survive the stall current for more than say a minute.

What strikes me is that the NTD20P06 claims 65W dissipation when mounted on a 1 in square pad, with 25°C ambient. Either their definition of ambient temperature is completely different from mine, or they are straight out lying.
On the other hand the IRLR024N has a much more reasonable claim of similar 45W, but when the drain pad of the transistor is held at 25°C, which is a way more believable figure.
 

crutschow

Joined Mar 14, 2008
24,318
Sounds about right for that kind of power. Note that it will not survive the stall current for more than say a minute.

What strikes me is that the NTD20P06 claims 65W dissipation when mounted on a 1 in square pad, with 25°C ambient. Either their definition of ambient temperature is completely different from mine, or they are straight out lying.
On the other hand the IRLR024N has a much more reasonable claim of similar 45W, but when the drain pad of the transistor is held at 25°C, which is a way more believable figure.
I think the first case is the same as the second.
In each case the pad is being held at 25°C.
I agree that otherwise it's not reasonable.
 

Thread Starter

Dritech

Joined Sep 21, 2011
847
Hi again,

I connected the MOSFETs as shown in the diagram below. Am I missing something? Should I use pull-ups resistors of something?
Also, the datasheet states that the gate can be driven at logic level. Why is the circuit not working correctly (not fully switching) when I apply 5V? it is passing the full 12V only when I apply 12V.

 

kubeek

Joined Sep 20, 2005
5,677
You need to apply 12V, otherwise both transistors will be on at the same time. Also you want to turn the lower off before you turn the upper on, otherwise you will short your 12V supply and see magic smoke escape, probably with some visual effects as well.
 

kubeek

Joined Sep 20, 2005
5,677
Because the bottom one needs 2v above ground on the gate to turn on, and the top one need 2V below Vcc to turn on. With 5V at the common gates you have both on, you need to avoid having both gates at the same time in that region 2V to 10V.
 

GopherT

Joined Nov 23, 2012
8,012
Thanks. Do you suggest that I use gate bleeder resistors with all four MOSFETs?
Do you see that there is no way to directly turn OFF the P-Channel MOSFETs with a 5V signal (if the top of the H-bridge is at 12 V).

An additional NPN transistor or other method will be needed to get the P-channel gate up to 12 volts.
 

crutschow

Joined Mar 14, 2008
24,318
Thanks for the reply. Why will the transistors be on at the same time if I apply a logic level (5V) ?
Yes, they are logic level, but that logic-level voltage is measured from gate-to-source (Vgs), not gate-to-ground.
If you calculate the Vgs of all the transistors, you will see why the P-MOSFETs won't shut off with 5V referenced to ground.
 

GopherT

Joined Nov 23, 2012
8,012
Yes, logic level for a p-channel means, if the H-bridge is powering a 5 V motor and you pull the P-channel gate from 5 V (where it is off) to ground, the p-channel will turn on.

If, however, you are running a 12 v motor with that h-bridge, then the P-channel gate must be at 12V to be off and it will be fully on by pulling it 5V lower than 12 V = 7 V.

That means, your 5V logic gates or Microcontroller cannot turn a p-channel off. Because it will either be on (when you apply 5 V to the gate (pulled down 7 volts below Source Pin ), or be very on when the logic gate is at ground (12 volts below Sourcee Pin voltage).
 
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