Unusual Gate and Drain Signal Behavior in K2503 MOSFET Normal or Faulty? - Page 1
Forum section: electronics / repair

author: miette_Kruemelchen
author_rank: Contributor
timestamp: August 16, 2025, 08:23:09 am
content: Hello everyone,I have captured oscillograms of a K2503 MOSFET used in the inverter stage of a TV and am trying to better understand some details. In the diagram, the gate drive signal is shown in yellow, and the drain signal is shown in turquoise, both measured against ground.At the beginning of the rising edge of the gate signal, a noticeable drop occurs. The voltage rises to 6 V after approximately 100 ns, but then drops to 1 V within only 80 ns. It then increases again, reaches a small plateau at 6 V, and finally rises to the final 11.5 V after about 200 ns.When the voltage at the gate drops to 3 V, the drain voltage at the drain pin rises briefly to 35 V, then settles at the final 24 V. When the gate voltage increases again, the previously described behavior happens, and I can't fully explain it. Could this indicate a defective MOSFET?The inverter module controlled by this MOSFET is generally working, and I get both the backlight and an image. The actual fault I previously found in this TV was a short in the electrolytic capacitor C3721 (47 µF, 50 V), which required me to replace diode D3710 (1SS250). After replacing these components, I had a picture again. However, during subsequent electrical measurements, I noticed the gate and drain signal behavior described above.I want to understand if this behavior points to a potential issue that might develop into a failure after prolonged use, or if it's simply a normal phenomenon. I'm kind of stuck on this one! :-)I look forward to your insights, explanations, and suggestions!Thanks in advance!Catch you later, Sven

author: m k
author_rank: Super Contributor
timestamp: August 22, 2025, 01:19:51 pm
content: It's a switcher, so if it is switching you get a secondary voltage, end of story, usually.The component is a FET, so field effect something, indicating that junction is less than solid.Some kind of a connection there still must be or function is not possible.Don't think it as an ideal something, it's far from that and has properties that are not obvious.You're also measuring only half of the situation.It's energy that affects, but you're only measuring voltages.So current.There are several capacitances, like probe and switcher IC, and all of them must be loaded first.Maybe switcher's reservoir capacitor is too small.When FET goes off components above it rearrange their energy and at the beginning there the inductor is kicking back.Many components have also a property of negative feedback.When FET goes on a rapid voltage drop between Drain and Source eats Gate charge and Miller Capacitance can start turning the thing off.But as you can see, nothing is really disturbing the drop, so no real problems there.Another thing is how normal or abnormal all of this is.Best way to learn is to get few extra FETs and test how the picture changes with different components.

author: langwadt
author_rank: Super Contributor
timestamp: August 22, 2025, 01:28:48 pm
content: google, miller plateau voltage