# zener Floating Voltage to Bias a P-Channel Mosfet without Exceeding Source-Gate Max Voltage

Posted on Feb 6, 2014

The premise of the circuit is this: I want to power L1 coil with square pulses. You can see I`ve got a freewheeling diode to handle the back emf field collapse to protect the circuitry. From previous experiments and talking to others on here, I learned that using an N-channel mosfet on the high side of the coil like this doesn`t work because the large negative voltage during the back emf coil collapse puts negative voltage

on the mosfet source which causes the gate (at zero volts) to turn on while the field is collapsing. I experimented with BJTs and they make it easy to fix the problem, but they also get hot. I really want to use mosfets for the ability to pass a lot of current with minimal resistance. So I`ve switched to a P-channel mosfet in the circuit shown. This seems pretty simple but I`m adding a level of complexity. I want to have arbitrarily high voltage in the main circuit - 80volts shown here but it could be more or less. Since the voltage is prone to change in use conditions, the circuit design must be agnostic to the actual main power voltage. In the 80 volt example shown, since the p-channel mosfet requires 80volts at the gate to turn it off, I have added a small resistor network to supply the required voltage from the power src. You can see I`m using a 555 and some BJT`s to pull down the gate voltage to turn it on. Now the tricky part. From my understanding, I can`t drop the mosfet gate all the way to 0V because that would make an 80volt difference between the Source and the Gate which would destroy it. I need to make sure the gate drops to about 12 volts less than the Source but drops no further. THe solution I`ve come up with is to add a 12 volt zener diode and adjust the resistances accordingly so once the gate starts to become more than 12 volts less than the source, the diode will reverse bias and bring the gate back up....

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