Spikes on gate in Buck Converter - Page 1
Forum section: electronics / general

author: jonathan_nadar
author_rank: Contributor
timestamp: November 03, 2021, 12:31:44 pm
content: I am working on a 60V to 12V synchronous buck converter with the following specs: 12A continuous current, 180kHz switching frequency, feedback provided by a microcontroller.The feedback is stable, however, I am observing spikes(+-5V peaks) when probing the gate of low side MOSFET right when the high side MOSFET turns on. I learnt that high dV/dt at the switching node could cause this issue, and did the following things. The Vgsth of the MOSFET is 1V min and 2.3V max, and I believe the spike would be enough to turn on the MOSFET.1) Increased high side MOSFET turn on time- used up-to 10 ohm gate resistor, didn't produce any significant drop.2) Experimented with snubber values3) Played with the dead time(200ns to 500ns)I have attached the schematic and PCB layout of the buck converter section.Suggestions would be highly appreciated.Thanks in advance

author: Weston
author_rank: Regular Contributor
timestamp: November 03, 2021, 05:07:39 pm
content: You can reduce the gate voltage rise by reducing the impedance of the low side gate drive, or slowing turn on speed of the high side FET. Dead time should not have much of an impact as long as the low and high side gate drive does not overlap, as the high side MOSFET is hard switched.Reducing the low side fet gate drive resistor, adding a turn off diode, reducing the gate drive loop inductance, or using FETs with a higher threshold / lower drain - gate capacitance FET all could help.It looks like the gate drive path to the bottom MOSFET is a bit long / has narrow traces, which could impact the gate drive.However, are you sure the FET is actually turning on? Are you seeing excessive heating / blowing FETs? The switching transition has fast transients that can couple into the scope probe. Are you using a spring ground lead? If you short the probe to the ground plane do you still see a large transient while switching?

author: uer166
author_rank: Super Contributor
timestamp: November 03, 2021, 11:58:50 pm
content: Are you probing with a low inductance spring clip probe? As was said, your gate drive traces are much too thin and long, and seem to go over switch node planes. Move the controller as close as possible to the FET gates, use a Kelvin connection to the FET sources, and increase trace width by maybe 5-10X. But first have to make sure the 5V spike is actually real and not a measurement artifact/noise.

author: jonathan_nadar
author_rank: Contributor
timestamp: November 04, 2021, 11:44:46 am
content: I have tried reducing the low side gate resistance and increasing the high side gate resistance, but they don't seem to make any impact. I'd like to mention that the gate waveform is clean when there's no load, but when I connect a 20W bulb, I get the above waveforms. There are small spikes at the output as well, which align with the spikes at the gate.Quote from: Weston on November 03, 2021, 05:07:39 pmI am not sure whether this is a problem. I found this posthttps://www.eevblog.com/forum/projects/ltc3895-step-down-converter-burns-off/msg2098939/#msg2098939where capt bullshot has attached a layout of a buck converter and the low side FET gate trace is long, thin and passes between the drain and source pads of the high side FET. However in my case, it passes above the switching node, but I don't think it would cause such a significant spike.Quote from: Weston on November 03, 2021, 05:07:39 pmThis could be an issue, I am not using a spring ground lead, it's the conventional clip that is grounded at a point next to the input capacitor. The FETs get warm after a minute of operation with no heatsink attached at 20W load. None of the FETs has blown yet. The high side Vds and low side Vds appear to be clean. I concluded that dV/dt is the culprit and not the probe as it aligned with the high side turn on.Quote from: uer166 on November 03, 2021, 11:58:50 pmI'll get a spring clip and redo the probing. I am planning to probe the input current and see if there's shoot through and will update this post.

author: jonathan_nadar
author_rank: Contributor
timestamp: November 10, 2021, 10:33:34 am
content: The waveform is clean now, thank you for your suggestions. I'll keep this in mind.