# The 4 N channel mosfet H Bridge trig voltage

Joined Nov 21, 2018
814
I made a all N channel Mosfet H Bridge, as picture attached, few questions:
1. the H Bridge can be driven by the voltage of 6 - 24 V, not like said need a voltage of higher than output V, that is 24 V here?
2. The gate voltage can't go to 0 V when the switch is off?
3. Can I use this Bridge to drive a 400W motor manually or need few Mosfet in parallel？

Thanks

sorry about the picture does not show, I posted at https://imgur.com/gallery/XFwHX0z and I don't know why?

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#### bertus

Joined Apr 5, 2008
21,945
Hello,

The pictures don't show.

Bertus

Joined Nov 21, 2018
814
@bertus

#### crutschow

Joined Mar 14, 2008
30,131
the H Bridge can be driven by the voltage of 6 - 24 V, not like said need a voltage of higher than output V, that is 24 V here?
I don't really understand what that means.

The gate voltage for those top MOSFETs needs to be at least 10V higher then their source voltage to full turn-on with minimum ON-resistance.

This is usually done with a bootstrap, high-side MOSFET gate driver circuit.

Joined Nov 21, 2018
814
I don't really understand what that means.

The gate voltage for those top MOSFETs needs to be at least 10V higher then their source voltage to full turn-on with minimum ON-resistance.

View attachment 170932

This is usually done with a bootstrap, high-side MOSFET gate driver circuit.
Because I got message that the N channel MOSFET can be driven by the voltage higher than the output voltage, say if the output V=12v, the drive V = ~20v, maybe I am wrong.
Do you have idea about the questions 2 and 3?
Thanks

#### Alec_t

Joined Sep 17, 2013
12,906
If the high-side FETs (Q1,Q3) drain voltage is 12V then you need 24V gate voltage to turn those FETs on fully. But you are applying the same voltage to the gates of the low-side FETs (Q2,Q4). The absolute maximum gate-source voltage for the specified FETS is only ±20V (per the datasheet), so the lo-side FETs would die.
2) Yes it can.
3) 400W at assumed 12V implies a running current of ~33A. That is within the 80A continuous rating of the FETs. However, motor stall current could be well over 150A, which the FET can sustain for only a few mS to keep within the safe operating area, so you would probably need to parallel-connect FETs. That is not a trivial thing to do if the FETS are to share the load equally.

Joined Nov 21, 2018
814
If the high-side FETs (Q1,Q3) drain voltage is 12V then you need 24V gate voltage to turn those FETs on fully. But you are applying the same voltage to the gates of the low-side FETs (Q2,Q4). The absolute maximum gate-source voltage for the specified FETS is only ±20V (per the datasheet), so the lo-side FETs would die.
2) Yes it can.
3) 400W at assumed 12V implies a running current of ~33A. That is within the 80A continuous rating of the FETs. However, motor stall current could be well over 150A, which the FET can sustain for only a few mS to keep within the safe operating area, so you would probably need to parallel-connect FETs. That is not a trivial thing to do if the FETS are to share the load equally.
Thank you Alec_t,

1. What can I do if I need turn on two MOSFETs one at high side and one at low side? say Q1 and Q4? do I need to put different voltage to different MOS ? if I employ a high side driver, just driver the high side MOS only?

2. I measured the Vg_GND = ~0.4v, how to make it 0 when turn off the switch?

3. I’ll parallel them 2 or 3. BTW, the SOA of MOSFET shown different time area, say 10ms / 100us? Is it mean I can use the 100us area ampere only by the PWM cycle less than 100us?