H-Bridge Not Working As Expected

Thread Starter

NatGazer

Joined Dec 23, 2020
14
First situation (Q1 and Q3 get 5V):
ScreenShot_20210414141841.png

Second situation (Q2 and Q7 get 5V):
ScreenShot_20210414141851.png

As you see R9 just gets 0.95V or -0.95V, but the bridge is powered with 12V.
And I know why this happens, the saturation of a MOSFET is defined by the voltage between the gate and the source (Vgs) and since the voltage at the source of Q1 and Q2 depends directly on the transistors below, this does not work as it should. I'm probably wrong and that's why I'm doing this thread, any information is very useful to me!
 

bertus

Joined Apr 5, 2008
21,323
Hello,

The input voltages are to low to drive the fets.
Try to drive them with 12 volts and see the results.

Bertus
 

DickCappels

Joined Aug 21, 2008
7,284
Since Q1 and Q2 are N-channel devices they operate as voltage followers and will drop a lot of voltage, depending upon the actual transistors.

If the top two transistors (Q1 and Q2) are changed to P-channel and driven properly with 12V P-P you can get a higher voltage across the load, and also get the bonus of lower DC voltage across the load.
 

crutschow

Joined Mar 14, 2008
27,010
The top P-MOSFETs are upside down.
Their sources must be connected to the positive voltage.

To determine if a MOSFET is connected properly, note the direction of the substrate diode arrow.
They must always be reversed biased in normal operation.
 

Thread Starter

NatGazer

Joined Dec 23, 2020
14
Since Q1 and Q2 are N-channel devices they operate as voltage followers and will drop a lot of voltage, depending upon the actual transistors.

If the top two transistors (Q1 and Q2) are changed to P-channel and driven properly with 12V P-P you can get a higher voltage across the load, and also get the bonus of lower DC voltage across the load.
Yeah, I just wanted to drive them with 5V but now I'm realising that's not possible, at least in this way
 

Thread Starter

NatGazer

Joined Dec 23, 2020
14
The top P-MOSFETs are upside down.
Their sources must be connected to the positive voltage.

To determine if a MOSFET is connected properly, note the direction of the substrate diode arrow.
They must always be reversed biased in normal operation.
ScreenShot_20210414214902.png

Yeah! That way the bridge works better, thanks!
 

crutschow

Joined Mar 14, 2008
27,010
Below is a simple way to drive an H-bridge from a 5V signal (assuming the bottom N-MOSFETs are logic-level type) with a 12V supply.

1618433830659.png.
 
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