29/08/2025 5:20 am
Topic starter
Notifications
Clear all
Theoretical questions
1
Posts
2
Users
0
Reactions
888
Views
0
I’m trying to understand the voltage differences across a MOSFET in my circuit. I’m noticing that the source, drain, and gate don’t behave exactly as I expected, and I’m getting confused about why the voltages don’t line up the way the theory suggests.
Could someone explain how to correctly interpret the voltage differences in a MOSFET (Vgs, Vds, etc.) during operation? Also, what are some common mistakes or misconceptions that could cause unexpected readings when probing a MOSFET in a real circuit?
1 Answer
0
30/08/2025 4:42 am
When analyzing a MOSFET in a real circuit, it’s important to focus on the voltage differences rather than just absolute values measured to ground.
The key parameters are Vgs (gate-to-source voltage), which determines if the MOSFET is on or off, and Vds (drain-to-source voltage), which affects whether the device is operating in the saturation region or behaving more like a resistor.
A common mistake is measuring all voltages with respect to ground without considering that the source may not be at ground potential, especially in high-side configurations. This can make the gate voltage look incorrect when in fact Vgs is fine.
Other factors that cause confusion include conduction through the MOSFET’s body diode when it is “off,” a floating gate if it’s not properly driven, and unexpected results when using a multimeter in a switching circuit since it only shows average values rather than the real dynamic waveforms.
The best approach is to measure Vgs and Vds directly across the MOSFET’s terminals, which provides readings that align with how the device is actually supposed to operate.
