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MOSFET half-bridge

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audioguru

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Your P-channel Mosfet is connected upside down, or it should be an N-channel Mosfet.
The source of a P-channel Mosfet should be connected to the positive supply voltage and the load connected to its drain.
The source of an N-channel Mosfet should be connected to ground like you show.
 
I got it to work, so I simulated the full H-bridge and became surprised about the voltage to th eload (the 2R resistor). The volatage were only around 4V and I apply 20V. The more strange is that it's only when applying 0.68V to the transistor other wise it'll go low again.
 

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audioguru

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You are not supposed to barely turn on the transistors by applying only 0.68V to their base. You are supposed to apply a big voltage to the 1k series base resistor so that the transistor has enough base current to turn on completely. If +5V is applied to the 1k series base resistor then the base current is (5V-0.7V)/1k= 4.3mA and the transistor is turned on completely.

The Mosfets are too small for more than only 2.7A, which is only 5.4V across the 2 ohms load resistor. If you actually build the circuit then the tiny Mosfets would fry.
 
I am going to build the bridge with IRF9Z34N and IRFZ34N and then use small signal transistors like the BC547B. But is the simulation just wrong while showing that the bridge wil only conduct when applying 0.68V to the base of the FET?
 

audioguru

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Electronics4you said:
I am going to build the bridge with IRF9Z34N and IRFZ34N and then use small signal transistors like the BC547B. But is the simulation just wrong while showing that the bridge wil only conduct when applying 0.68V to the base of the FET?
The bridge will conduct properly when the transistors have enough base current, not base voltage, and when the Mosfets can conduct enough current.

Transistors are turned on with base current.
Mosfets are turned on with gate voltage.
 
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