Buck converter - MOSFET turns on before gate drive - Page 1
Forum section: electronics / repair

author: Dobby
author_rank: Newbie
timestamp: October 16, 2024, 02:49:20 pm
content: The Buck converter shown is part of the power supply in the PSU board of an HP 8560 RF Spectrum analyzer.For debugging the input voltage is turned down to 40V and the output voltage into load (not shown) is ~30v. Output voltage on load (not shown) is STEADY.Scope:- Yellow/1 is PWM drive into pulse transformer is 80 kHz 12V.- Blue/2 is MOSFET Gate- Purple/3 is MOSFET Source- Math Red is MOSFET Gate-Source- Green/4 is flywheel diode CR108.Now what could be causing MOSFET to turn on BEFORE the rising edge of pulse drive ??At this point Vgs is still low. Replaced MOSFET but not change.(not sure the function of L104 but bypassing is doesn't alter this behavior).[ Specified attachment is not available ]

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 16, 2024, 04:52:38 pm
content: What's output voltage and load current? -- are you sure that the transistor is turning on when voltage rises again?Tim

author: inse
author_rank: Super Contributor
timestamp: October 16, 2024, 05:05:43 pm
content: Where do you think the MOSFET would be turning on - when the source voltage rises?When the freewheeling is finished and the current in the coil has collapsed, the voltage on both sides of the coil is equal.That means the source voltage is equal to the output voltage.If you had the chance to graph the current as well, things would get easier to understand.The operation mode is called discontinuous conduction mode which happens when the load is low.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 16, 2024, 07:28:29 pm
content: Yes this is discontinuous conduction mode.It is interesting that the current in the coil at some point begins to flow back through the MOSFET's diode.Let us start to track. Please see the Purple-Source voltage trace as well as White-coil current curve that I tried to draw.Points 1-2: MOSFET opened, flywheel diode closed, the current rises.Point 2: Mosfet starts to close, the coil current can't stop and decrease voltage at Sourse until flywheel diode opens at point 3.3-4: coil passes the energy to the load and the current falls until zero - point 4.At point 4 we have at left coil side 0V and at right side +30V - current reverses its direction and flows back until bouth coil sides catch up 30V - point 5.Here the current flow starts to decrease but cann't stop immediately and at point 6 the Source voltage reaches more than 40 V and opens back MOSFET diode.Points 6-7: the current flows back via MOSFET.Point 7: MOSFET opens, current changes direction to forward and starts to increase.You can see that 6-7 is one pixel higher than 7-8 (MOSFET diode drop voltage).PSIt would be interesting to intall the current sense resistor and get the current trace.

author: Dobby
author_rank: Newbie
timestamp: October 16, 2024, 10:34:47 pm
content: Thank you. I don't think its loaded enough. Silly me was assuming because the source is high its in conduction. Thank you.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 17, 2024, 02:49:15 am
content: Many thanks to you for such an interesting case!