Switched Capacitor Boost Converter stability issue - Page 1
Forum section: electronics / beginners

author: torndar
author_rank: Newbie
timestamp: January 27, 2022, 12:11:41 am
content: I've been working on a switched capacitor boost converter and it's working pretty well when it works. However, it's very unstable with load transients and seems to wind up in some kind of oscillation with shoot through on the lower two transistors. I upped the current limit on my power supply even more since it had been going into constant current mode and if I let it go for a while it actually stabilizes. It seems something odd is going on with driving the upper gate.Schematic and video of the "oscillation" hereand hereThere is a small bug in the schematic, C2 should not be connected to the left side of D2.I have two smaller issues, the actual power transfer rate is pretty poor (~2-3W) and startup is not reliable. For the latter if I briefly short PHASE and ground that kicks it into operation pretty reliably. It seems that PHASE winds up stuck at 2.5V presumably through leakage currents and that somehow keeps things from switching correctly at first. I'm hoping getting it more stable at higher frequencies will help power transfer along with adding more parallel capacitance (to reduce ESR and ESL).I've built the circuit on perfboard and increased the gate resistors to 50 Ohm as that helped the breadboard version but I'm still having startup and oscillation issues. I think the startup issues are related to the lowest MOSFET being partially on (I measure 2.5V with respect to source/ground on the gate) when it tries to start switching because of leakage currents but I'm still stuck on what the cause is and what to do about the oscillation.

author: torndar
author_rank: Newbie
timestamp: January 31, 2022, 10:47:44 pm
content: While I was doing some more troubleshooting I hooked the scope probe on the gate of the second stage and load transients stopped causing issues. I should probably try to do some simulation again but my best bet is the load transient was dragging the output voltage down enough that the gate was staying too high and the transistor wasn't properly turning off. The gate is raised to at least just under the source voltage by a diode to the source and is driven through a capacitor, but that doesn't give anywhere for "excess" charge to go. I think the recovery over time was from gate and diode leakage currents slowly draining off the excess charge and the scope probe made that recovery quickly enough that I couldn't catch it misbehaving on an analog scope, still might for a couple cycles though.Since a 1MOhm resistor to ground (the scope probe input impedance) was enough I'll probably do something in that range for all the gates. Not sure if I'll tie each gate to the previous stages source voltage or to ground though, at 1MOhm even 20-30V to ground is minimal power loss (a couple milliwatts and I'm looking at 2-20 watts as my target range).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 01, 2022, 12:38:20 am
content: The schematic shows the transistors backwards?Tim

author: torndar
author_rank: Newbie
timestamp: February 02, 2022, 01:40:57 am
content: I've gone back and made another (much more successful this time) LTSpice model of my circuit and replaced the previous schematic with that, also linked here:Based on the model it looks like I was right about the upper gate being high and load transients messing with its ability to turn off, although not entirely on target. Based on the sim with a regular diode instead of the 4.7V zener I'm now using the gate would drift high with respect to source even without a load transient and mess with my output voltage as well.The simulation is about 0.5V higher than my actual measurements with a 30Ohm load, I'm not sure where the additional losses are coming from at this point. I even tweaked the capacitor ESL to take into account lead lengths which got it from 1V high to 0.5V but I can't quite get it closer. I'm running it a 100kHz currently, it's stable from 10,000 (which hurts my ears) all the way up to 1MHz although anything past about 250kHz seems to increase switching losses by notable amounts with only hundredths of a volt of output voltage change i.e. very diminishing returns.I've attached the LTSpice model

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2022, 02:50:02 am
content: What's the driver circuit? Is it powered from the supply as well? Do the timings match what's in LTSpice? Is SI5515 actually what you're using, or what?Tim

author: torndar
author_rank: Newbie
timestamp: February 02, 2022, 03:30:10 am
content: The driver circuit is based on a TI MOSFET driver IC "The TPS28225 is a high-speed driver for N-channel complimentary driven power MOSFETs with adaptive dead-time control."https://www.ti.com/lit/ds/symlink/tps28225.pdfThe IC is supplied from the same 5V supply and is currently getting its PWM signal from a PWM pin on a Raspberry Pi. It does have a decoupling capacitor between VDD and GND. The phase pin is connected between the 1st and 2nd MOSFETs of the first/base stage i.e. the source of the upper MOSFET per the datasheet. The actual MOSFETs I'm using are IRLB8721PBF. I updated an old simulation from before I built the actual prototype and haven't looked for a spice model for it yet. I'd say the dead time is probably longer in LTSpice than the actual circuit timings. The old analog scope I have almost can't see the edges things are switching so fast. I do have 50ohm gate resistors in circuit currently. I updated the spice model to give the PULSE sources a 50ohm Rser to match but that had essentially no effect on the output voltage.The implemented circuit has 2 10uF capacitors in parallel for each of Vin, C2, and C3 so it actually has higher capacitance than the model which should help with ESR and power transfer but I'm only getting ~2.5W into the circuit according to the power supply. The nice thing is that something like 95% or more of that is actually making it to the output from what I can tell so the actual power transfer efficiency seems to be pretty good.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2022, 04:34:45 am
content: What does the breadboard look like? (Layout matters!)The TPS28225 is a very fast driver, and has very little deadtime: 14ns typical. Your simulation shows a whopping 2us, if I'm not mistaken? Also doesn't show any gate resistance, and a much smaller device (so, less Qg, more Rds(on); Qg being the important figure here, giving faster gate rise/fall time). It's not obvious if this is the problem, but it's a definite possibility that the transistors are mildly on at the same time, shorting out the supply.An IR2109 would be a much better fit for breadboarding, having a ~0.5us dead time:https://www.digikey.com/en/products/detail/infineon-technologies/IR2109PBF/812234Tim

author: torndar
author_rank: Newbie
timestamp: February 02, 2022, 11:18:35 pm
content: I started on breadboard but moved to perfboard once I got it semi-working because I've seen so many "layout matters" in datasheets and across the internet. I definitely see less ringing on things now that I don't have 3-4 inch flying wires all over the place like on the breadboard. I hope the next version I build I can spread things out a bit more as they current layout is super cramped.I definitely had issues with transistors not turning off enough and partially shorting the supply before I replaced the diode with a zener at the uppermost gate-source connection but I don't see any evidence of that happening now. It drifted high in the LTSpice sim as well causing power loss (the output voltage in the sim would actually peak and then start dropping) and attaching the scope probe definitely had an effect in the physical circuit so tried a resistor in the sim. Since I needed a resistor and the diode but a zener has the same end result I just went with a zener for now since I have some 4.7V zeners on hand. The power transfer efficiency seems to be really good based out output voltage*current versus input voltage*current (90% or better). At this point I'd just like the output to droop less under load. My understanding is that there are two ways to help that, increasing the frequency so charge is transferred more often and increasing the flying capacitance. I seem to have reached the limits in terms of frequency at this point so I suspect I need to add more capacitance but I'm not sure of that and would rather be able to see/show that the issue is a lack of capacitance before adding more. It could be too much inductance or too much ESR and I'm not entirely sure how to interpret the scope traces to tell, if that even can. I'm also debating have the switching converter feed into a final inductor based boost stage since output regulation for a switcher can only happen by making it more lossy and a small ratio inductor is relatively efficient compared to trying to do my 6x goal in a single inductor based stage.As context for the goal of the projectEventually I want to extend the circuit with additional stages to get more efficient voltage boosting from 5V to 25-30V for a series battery pack/USB port power bus that feeds individual QC3/USB PD buck converters. I've tried using inductor based converters but the losses were significant especially when my goal was to be able to go as low power as 500mA at 5V for a basic USB port. I realize it's not the most practical thing at that point and higher power USB is easy to come by these days (the concept goes back many years now at this point, it's been a learning journey) but I'd still like to get it working for the learning experience.