Problem: I'm using a "zener diode voltage reference" to put 16 V across the HI/LO pins of an optocoupler/MOSFET driver. The configuration constantly draws current, thereby dissipating a lot of power (~watts).
Question: Can I use a MOSFET switch to "turn on" the Zener voltage reference in a window around when I need it, and have it off most of the time?
Details: My application is a negative high-voltage pulse generator (see attached circuit). The output will be connected to a wire (and probe), and nothing else.
Pulse specs:
period = 50 ms
duty cycle = 0.2%
This pulser uses an N-channel Enhancement Mode MOSFET (1500 V, STFW3N150) with the drain connected to a load resistor plus ground, and the source connected to a negative high-voltage supply.
The MOSFET is driven by a FOD3120 optocoupler, whose output floats on the negative high-V rail. For this optocoupler to work, the HI/LO difference must be >12 V. Here I've set it to 16 V.
Originally, I set HI to 16 V above the negative rail with a "Zener voltage reference/divider". The resistor in series with the Zener diode is set to draw enough current (~5 mA) to operate the Zener diode, and have it's voltage drop be 16 V. The Zener diode is bypassed with some caps.
The power dissipation will be an issue in the Zener's resistor. Say the negative rail is at -1000 V. With a current through the Zener of 5 mA, the power will be P = I*V = 5 W!
Important Note: I'm testing this circuit now with lower voltages (~-100 V). Ultimately I'll want to use higher voltages (~-1000 V), and thus some resistors will change. For example, 30k resistor will be more like a 100k resistor.
Solution: Use a P-Channel Enhancement Mode MOSFET (100 V, ZVP2110A) to switch the current through the Zener diode from low to high in a window around the main MOSFET trigger.
Should this work? I've tried wiring it up, but so far it acts as if the P-channel MOSFET isn't even there! Turning the trigger to it on or off doesn't affect the output.... (or the current being drawn, and hence the power).
Question: Can I use a MOSFET switch to "turn on" the Zener voltage reference in a window around when I need it, and have it off most of the time?
Details: My application is a negative high-voltage pulse generator (see attached circuit). The output will be connected to a wire (and probe), and nothing else.
Pulse specs:
period = 50 ms
duty cycle = 0.2%
This pulser uses an N-channel Enhancement Mode MOSFET (1500 V, STFW3N150) with the drain connected to a load resistor plus ground, and the source connected to a negative high-voltage supply.
The MOSFET is driven by a FOD3120 optocoupler, whose output floats on the negative high-V rail. For this optocoupler to work, the HI/LO difference must be >12 V. Here I've set it to 16 V.
Originally, I set HI to 16 V above the negative rail with a "Zener voltage reference/divider". The resistor in series with the Zener diode is set to draw enough current (~5 mA) to operate the Zener diode, and have it's voltage drop be 16 V. The Zener diode is bypassed with some caps.
The power dissipation will be an issue in the Zener's resistor. Say the negative rail is at -1000 V. With a current through the Zener of 5 mA, the power will be P = I*V = 5 W!
Important Note: I'm testing this circuit now with lower voltages (~-100 V). Ultimately I'll want to use higher voltages (~-1000 V), and thus some resistors will change. For example, 30k resistor will be more like a 100k resistor.
Solution: Use a P-Channel Enhancement Mode MOSFET (100 V, ZVP2110A) to switch the current through the Zener diode from low to high in a window around the main MOSFET trigger.
Should this work? I've tried wiring it up, but so far it acts as if the P-channel MOSFET isn't even there! Turning the trigger to it on or off doesn't affect the output.... (or the current being drawn, and hence the power).
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