3. For an appropriate MOSFET model with drain-source voltage of 100-V or greater

Question

3. For an appropriate MOSFET model with drain-source voltage of 100-V or
greater (I used the IRF530) use SPICE to simulate the simple MOSFET
driver (like figure 10.1.a) and the emitter follower driver (figure 10.1.b,
using the 2N3904 npn and 2N3906 pnp transistors). Use a switching frequency
of 200 kHz, switch duty factor of 0.5, load resistance of 10-Ohms, a
source voltage, Vs = 80V , and supply a peak gate voltage VG = 15V . Use
a pulse voltage source with a series resistance Rg = 100 Ohms to control the
switch/driver (shown as R1 in figure 10.1).

(a) Plot voltage and current in the switch.

(b) Plot the power dissipated in the switch.

(c) Plot the gate current.

(d) How does the average power lost in the switch change between the
simple driver and emitter follower driver?

(e) How does the average power lost in the switch change as Rg changes
in either case?

Explanation / Answer

Many power MOSFETs require a high gate voltage for high-current loads, to ensure that they are fully turned on. There are some with logic-level inputs, though. The data sheets can be misleading, they often give the gate voltage for 250 mA current on the front page, and you find that they need 12V for 5A, say.

It's a good idea to put a resistor to ground on the gate if a MOSFET is driven by an MCU output. MCU pins are usually inputs on reset, and this could cause the gate to float momentarily, perhaps turning the device on, until the program starts running. You won't damage the MCU output by connecting it directly to a MOSFET gate.

The BS170 and 2N7000 are roughly equivalent to the BJTs you mentioned. The Zetex ZVN4206ASTZ has a maximum drain current of 600 mA. I don't think that you will find a small MOSFET that can be driven from 3.3V, though.

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