MOSFET testing - Page 1
Forum section: electronics / general

author: Grandchuck
author_rank: Super Contributor
timestamp: February 19, 2025, 05:19:30 pm
content: I have some suspicious N-channel enhancement mode devices and wanted to make some quick checks. I employed a DP832 and used CH1 to apply fixed 5 volts across source and drain and used CH2 to step from 0 to 5 volts across the gate and source. The results were as expected (0 drain current) until a 4 volt step was reached and then I noticed that reducing Vgs back to 0 did not turn the device back off (there was still a lot of drain current). Aha! It was oscillating. Sure enough, the oscilloscope showed a MHz signal at the gate. Thinking this was an anomaly, I tried various physical lead positions and several different MOSFET types (different part numbers, but all were N type enhancement) and they all started oscillating and did not stop when Vgs was reduced to zero.Just wondering if anybody has ever noticed this.

author: RoGeorge
author_rank: Super Contributor
timestamp: February 20, 2025, 11:39:21 am
content: Did once something similar with a DP832 and a MOSFET, but didn't notice any oscillations:https://www.eevblog.com/forum/testgear/rigol-dp832-power-supply-as-automated-curve-tracer/

author: benseno
author_rank: Regular Contributor
timestamp: February 20, 2025, 12:26:38 pm
content: What is the output impedance of a device used as Vgs voltage source?

author: David Hess
author_rank: Super Contributor
timestamp: February 20, 2025, 01:42:50 pm
content: QuoteTo expand on this, your function generator output is a pretty good 50 ohms, but any wire leads act like inductors and combined with the capacitance of the MOSFET, easily produce an oscillator. Adding high impedance in series with the leads lowers the Q of the circuit preventing oscillation. This is easy at the base where currents is low, so a resistor can be used, but more effective at the source lead where the current is higher. Lossy ferrite beads on the leads would work also, and sometimes circuits will put a lossy ferrite bead around the emitter lead where the current is higher for maximum effect.Curve tracers usually have ferrite beads located in the test fixture as close to the transistor package as possible for every lead.

author: Grandchuck
author_rank: Super Contributor
timestamp: February 20, 2025, 02:50:14 pm
content: Thanks to all. Adding a 3.3 kohm resistor in the gate lead stopped the oscillation.

author: Grandchuck
author_rank: Super Contributor
timestamp: February 20, 2025, 05:47:39 pm
content: OK, with a ferrite bead and no resistor, there is again oscillation but only around a Vgs of 4 volts (from about 3.3 to 4.5).Note the differences in amplitude and frequency.

author: Sassy Taste
author_rank: Contributor
timestamp: February 27, 2025, 11:23:16 pm
content: Quote from: David Hess on February 20, 2025, 01:42:50 pmAcross the gate, to get the mosfets in the active mode, 0.7 volts. Impedance would be low output impedance. Just like how the radio OP AMP is. They have a low output impedance.

author: Xena E
author_rank: Frequent Contributor
timestamp: February 27, 2025, 11:50:10 pm
content: Quote from: Sassy Taste on February 27, 2025, 11:23:16 pmThis is either AI generated garbage text, or the poster has suffered a stroke?X

author: David Hess
author_rank: Super Contributor
timestamp: February 28, 2025, 02:24:51 am
content: Quote from: Sassy Taste on February 27, 2025, 11:23:16 pm

author: temperance
author_rank: Super Contributor
timestamp: February 28, 2025, 02:40:08 am
content: I didn't know you could die twice. But I just did.