OP Amp PWM generator for Buck/Boost - Page 1
Forum section: electronics / beginners

author: TheDood
author_rank: Regular Contributor
timestamp: December 16, 2019, 06:30:18 pm
content: Im trying to use a couple OP Amp PWM generator ccts to manage the load power.There's a small section labeled "Buck Boost Chooser" which turns on the MOSFET controlling the output to the corresponding PWM generator. When Feedback is below Reference I want to turn on the boost PWM output, and when Feedback is greater than the Reference, I want to switch the Buck PWM output on.The "OP Amp power rail" is good and consistent, as well as the "Reference Voltage" though aside from that, I'm having difficulty even simulating. Maybe my computer is just slow?I can use PWM Voltage sources but they aren't dynamic like I'm trying to make this simulate as. I'm sure there are other chips that can create PWM better (SG3525a ect), but I don't know if LTSpice can simulate them ect. I'd also just like to learn a little bit and wanted to see if I could make the OP Amp circuits viable.I'm looking for advice on how to go about something like this given the attached schematic. It doesn't seem like the duty cycle is adjusting dynamically like I'm wanting. Without the "Buck/Boost Chooser," both PWM generator output operate simulataneously and the boost FET flows outrageous current. So I'm trying to make it an either-or type of situation rather than operating at the same time.Also, the feedback V signal is not the same for each input of the PWM generator, I've added blocking diodes to stop the chatter but it seems that its still a bit different for each tie in.Maybe my OPamp sensing signals need to be reversed, I've switched them back and forth several times but I'm sure I'm missing something. I'd just like to see a current through the load that isn't 3A+, I've been tweaking it for quite sometime trying to gain a conceptual understanding of "when I do this, that happens" but I can't seem to find consistency. I'm not sure if my understanding of the "Buck/Boost" principles are correct, ie like inductor placement and switch placement ect. If you see something glaring please let me know, or if you see an obvious fix tell me that too, thanks!EDIT:The end goal is to be able to adjust the POT to create greater or lesser Reference V which the PWM generators/Choosers manage the load based off of. I'd like to learn how to use smaller parts while achieving the same end result, I've been simulating 100mH coils to level current in capacitive dropper ccts, but I'd like to see how to create a steady current without using giant capacitors ect..

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 16, 2019, 09:06:30 pm
content: I have not analyzed your circuit yet, but here are a few comments on the simulation.1) I tried to simulate it and after only some number of ms, it failed to converge.2) Divide and conquer. Simulate the AC+regulator. I did that and got 52 volts for the opamp power rail and about 33.5 volts for the reference. If this is what you wanted, just substitute dc voltage sources for these.3) Instead of using the actual opamp model, use something simpler CCCS with soft limiter and moderate gain--this might help the simulator4) Can you separate the two pwm loops and simulate by themselves...at least at first until you get the parts working?There may be some problems with your design...I did not try to figure that out.

author: TheDood
author_rank: Regular Contributor
timestamp: December 17, 2019, 08:45:30 am
content: Thanks Wimberleytech, Ill keep at it. I realize now (per your comment) that the OP Amp power rail will be effected by the increased resistance when messing with the POT, so I'll have to isolate them from each other. I've added a second point for feedback V amoung other little changes and it's simulating faster now.If you notice the "Buck/Boost Chooser" section you'll see diode D24, I can't figure out how to stop the current flow from the Refrence V line through that Chooser section. If I don't put that diode there (D24) then my nice steady voltage becomes erratic? The Refrence V is different on either side of that diode by a fair margin and I can't figure out where or how?