Both transistors and MOSFETS have advantages and weaknesses, and I have discovered multiple uses of MOSFETS that make it a handy device.
Few parameters to keep in mind.
1. Max drain current
2. VTH (Threshold voltage)
3. Max VGS (Voltage across source-Drain)
4. Max Power dissipation
Two types of common MOSFETs - enhanced mode and depletion mode
The voltage difference between source and gate acts like a water faucet
- Enhanced Mode: with VGS = 0, the Source-Drain channel is pinched off, no current flow. Current flows as VGS voltage approaches VTH, current starts to flow between source and drain.
- Depletion Mode: The source - drain channel is fully open with VGS = 0. As VGS increases, the channel starts to pinch off until VGS = VTH.
Posted on: November 21, 2013, 01:04:50 01:04 - Automerged
Example of a depletion mode MOSFET is current limiters. Especially for mixed voltage systems using current input analog channels, and digital optically isolated inputs.
Let's take an optical isolated digital input, a series resistor is used to limit current input from a digital signal source. The series resistor will be one value for 5 VDC input, and another for 15 VDC input. A depletion mode MOSFET in series with the optical isolator can eliminate the need to change component values and allow input voltage to be from 3.3 to 32 VDC without resistor value change.
Place the depletion mode MOSFET in series with the series resistor, then the optical isolator. The gate of the MOSFET is connected to the junction with the resistor and optical isolator. At 3.3 VDC, the voltage developed across the resistor may not be sufficient to cause the MOSFET to pinch off, but the resistor itself is the current limiter. But when you appli 24 VDC to the input, the current through the series resistor will cause the MOSFET to pinch off, regulated by the voltage across the series resistor. The max current through the resistor, and the optical isolator is roughly VTH/R. if VTH is -1.5 VDC, and nominal optical isolator current is 10 mA (> 5 mA), then the resistor value of 150 ohms would be sufficient. With 3.3 VDC, 150 ohm resistor, and forward voltage of the optical isolator to be apoprox 2.1, the current is calculated to be about 8 mA, which is greater than 5 mA.