Electronics > Projects, Designs, and Technical Stuff
DC-DC Converter Stability and performance help
FotatoPotato:
Hey guys,
So after some more experimentation, I finally got my LTSpice simulation to work out perfectly. The schematic and sim results will be linked below. I plan on making a 4 channel bench supply using the exact schematic that I made in LTSpice. I decided to first make a perf-board prototype of the DC-DC regulator, just to make sure the design would work in the real world and I ran into some weird problems.
The switching frequency is only 85Hz :wtf: when it should be around 600Khz. With no load attached the LTC3864 reaches around 65C and I'm able to adjust the voltage using a potentiometer but the output is not stable at all and there is a lot of voltage ripple. The second I attach a load, no matter how small, the voltage drops significantly, the IC gets to around 80C and the frequency drops to 80Hz. I already double checked all of my connections based on the schematic and I'm really not sure what is wrong. Also once a slightly bigger load of over 100mA is connected the output drops to 0v and the LTC3864 almost instantly goes to 110C.
Any suggestions would be greatly appreciated!
Thanks :)
mvs:
--- Quote from: FotatoPotato on February 24, 2019, 05:51:04 pm ---The switching frequency is only 85Hz :wtf: when it should be around 600Khz.
--- End quote ---
It is not switching frequency what you see on scope, but parasitic oscillation.
Put C13 from DC2132A eval board schematic back in circuit. It is important component, that closes local feedback loop at AC.
FotatoPotato:
Will that have the same effect, even if I'm not using the same controller?
mvs:
--- Quote from: FotatoPotato on February 25, 2019, 01:37:32 am ---Will that have the same effect, even if I'm not using the same controller?
--- End quote ---
Yes, but you may need to adjust its capacity.
FotatoPotato:
So I was working on trying to fix the regulator last night and I think there was more than just parasitic oscillation. The whole board had problems and even after adding decoupling caps all over the board It still didnt work. I re-built it on a breadboard but this time kept all the component leads short and made the connections as short as physically possible. This seemed to fix most of the issues but there are still some confusing things.
First is that the switching frequency is still super low (300 - 600Hz) but as the output load increases the frequency goes up with it, so with a 250mA load the frequency will go up to arround 3kHz but that’s still much lower than the 700ish KHz that I want. I’m assuming that this is part of the burst mode operation but I don’t really know.
The other problem is that the with any loads over 100mA the LTC3864 gets super hot, the output dies and it wont re-enable switching unless I turn off the input power and turn it back on. It seems as though the chip is thermal throttling at high loads because when I cool it down with freeze spray it works perfectly fine, even with large 500mA loads. The thermal throttling doesn’t make much sense to me though becuase the IC isn’t handling any of the power so it shouldn’t be getting so hot. I tried adding a 50 ohm resistor to the vin pin on the LTC3864 to limit its current and that seemed to do the trick but its still weird to me.
At this point I’m going to make a test PCB so that I can eliminate the parasitic resistance, capacitance and inductance of the breadboard and to add some proper heatsinking to the LT chip. Hopefully that will fix most of the problems but if you have any suggestions I’d be happy to hear them!
Thanks :)
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