'Missing pulse' behaviour from LTC4446 driving a custom buck converter - Page 1
Forum section: electronics / projects

author: tom66
author_rank: Super Contributor
timestamp: July 31, 2022, 09:28:52 pm
content: I'm using an LTC4446 dual H bridge driver to drive a pair of MOSFETs to create a high-bandwidth class-D audio modulator, for a Tesla coil project. The control signals from this converter are derived using an FPGA because, why not. The circuit operates from up to a 60V supply, but for now I'm testing at just 12V.As far as I can tell the control signals are all OK going into the converter, I've got plenty of dead time. The switching frequency is high - about 2.5MHz - but this should be within the range for this device (the sum of propagation delays suggest the Fmax is around 12MHz). I'm not expecting this to be pushing the gate driver. It might be pushing the FETs at full power, but I've yet to get to that stage, so I haven't found out yet.I'm encountering a strange problem where the low-side transistor fails to switch sometimes. The result is the output goes high because only the high side is switched on (just about, by whatever charge is left in the flyback cap.) This is definitely down to the gate driver: the low side channel goes 'dead' while the high side continues on.At first, I thought it was the IC, so I changed that. No, issue persists. Then, I thought the 3V3 logic signals - while within range for the device - were a bit too close to the limits. So I modded the circuit so that the logic IC that generates them runs at 5V instead. No luck. Issue is identical. I then thought that this might be caused by the layout of the circuit which admittedly was poor. (The 'flyback cap' trace runs right adjacent to the low side control signal, I reasoned that they could be cancelling each other out somehow.) I made a mod to the board, effectively re-routing this trace. No difference noted.The only issue I can see so far is the logic signal for either side looks distorted, I'm not really sure what's causing this.But the low side one looks good even though that channel is what appears to be faulty.A scope trace of this distortion is attached - the signal still meets the Vil and Vih specifications. My best guess is the logic IC is struggling to drive the input impedance of the LTC chip but this is odd given it is not specified as a low impedance device. Perhaps it's a clue - but I don't know to what.Something interesting is that it does only seem to occur at certain duty cycles, and there's a kind of pattern to it, in that it will consistently appear at those frequencies. It's nonlinear, I would say occuring most at lower duty cycles and least at higher ones, for the low-side channel. And it's sensitive to supply voltage. As far as I'm aware, the LTC device has no crossover protection or timing (certainly the schematic diagram doesn't show any) and thereforeI'm getting to the point of despair and considering making a mod board to replace this fancy chip with something a lot less complex - I selected it to make my life easier and it's done the exact opposite. Has anyone had an experience like this before?Attached is the relevant schematic, PCB layout and some scope traces.Edited to clarify.

author: tom66
author_rank: Super Contributor
timestamp: July 31, 2022, 09:41:12 pm
content: More traces.First shot - non overlapping waveforms from FPGA with plenty of dead time.Second shot - low side control (blue) vs low side FET switching (yellow) - I do in fact see the sagging gate drive waveform for both channels (have edited original post), so maybe this is a clue.I'm a little confused about the 'kick' before the low side FET turns off for good, maybe a Miller phenomenon?N.B. the strange waveforms seem to go away if I disable the power stage, which suggests some interaction between the two parts of the circuit... I thought that one of my traces may be too close to another but I have since modified the PCB to move them. My grounding of my scope is inadequate which is causing some additional ringing here but I'm not convinced this is changing the behaviour seen on the scope in a material fashion.This is my first foray into mid-scale power electronics and I've clearly bitten off a bit more than I can chew, but hoping I can figure this out.