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| opamp transistor follower oscillates to buggery when in CC mode |
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| David Hess:
--- Quote from: nemail2 on July 26, 2019, 11:20:06 pm ---hmm i have always had the footprints for base-emitter resistors in my dual BD139/TIP3055 design (the working circuit) but never populated them as someone in another forum told me not to do so. never had (obvious) issues with that. do you have a link maybe which explains, why they should be used? --- End quote --- The base-emitter shunt resistor does two things: 1. It removes charge allowing faster turn-off and this makes it important in a power supply for good transient response when the load decreases. 2. It sinks the Icbo leakage current which can be considerable in a hot power transistor. If this leakage is large enough, then it can force the transistor on. --- Quote ---@R20, R21: no, as those are balancing resistors and in this (cheap) design I only use one TIP3055, I did not include them. but interesting to know that they can affect stability as well. certainly wasn't aware of that. --- End quote --- A low impedance load at the emitter of the transistor can reflect into the base as a negative resistance causing oscillation. The output capacitor presents a low impedance AC load limited only by its ESR. This is why I suggest removing the 0.1 microfarad output capacitor; if it is necessary for RF decoupling, it should be located at the chassis anyway. This is an advantage of placing the current shunt in series with the emitter; it improves stability. Some linear regulators take a portion of their AC feedback directly from the emitter before the series resistor placing the series resistor in series with the output capacitor. This allows a low ESR output capacitor like a film or ceramic to be used while maintaining good stability. Feedback from the base through C4 is doing the same kind of thing. |
| nemail2:
i guess I should really have that base-emitter shunt resistor then. is there a formula or a rule of thumb what value that resistor should be? guess what, the thing oscillates again. i don't know for sure whether i wasn't measuring correctly or like whether i forgot to measure across the whole voltage range but at 4 Ohms at about 450mA it starts to get ugly. below that, it is good. i'll later (tomorrow) swap the MAX4080T for a MAX4080F as the F version is the one which i used in the working circuit (in an earlier revision, before going to the LTC6102). Maybe that's adding problems as well, as the T version has a much higher gain. otherwise, the differences left between working and non-working are these now: - LM324 vs. OPAx197 - 1 vs 2 BD139/TIP3055 Darlingtons - therefore no current sharing resistors vs. current sharing resistors - LM334 load vs. zener diode/npn load (in the old, working revision with the MAX4080F) oh boy.. i'd love to get away with just some minor parts value changes... |
| David Hess:
--- Quote from: nemail2 on July 27, 2019, 01:28:16 am ---i guess I should really have that base-emitter shunt resistor then. is there a formula or a rule of thumb what value that resistor should be? --- End quote --- 1/20th of the base current is typical for a power transistor (the TIP142 is about that for its output transistor) but I usually figure 1/10th to 1/4 of the base current should flow through the base-emitter shunt resistor depending on the application. The resistance is calculated based on the Vbe voltage which will be between 0.6 and 1.0 volts depending on the current level. Small signal transistors will be 0.6 volts and power transistors will be 1.0 volts or even a little higher. The value is not particularly critical. Note that just because it does not oscillate without the base-emitter shunt resistors, does not mean that it is working correctly. The transistor's slow off-time without the base-emitter shunt resistor masks the problem. |
| nemail2:
i did put it the 0.1R current balancing resistor now like in the working circuit and replaced the MAX4080T with a MAX4080F and oscillation now really seemed to stop. i still don't have the base-emitter shunt resistors in place, though. so at 1A max output current through the darlington, values like 200R and 10K should be right, i guess? do you have any idea where I can learn everything I need to know to be able to calculate or at least guestimate how to compensate control loops like this? are there like youtube videos or entry-level documents on this, you are away of? I WANT to learn it but unfortunately I'm quite lacking a fair amount of the basics and I had absolutely no technical education. I'm only around two years a electronics hobbyist now (besides learning really basic basics from my grandpa when I was a kid and always having some technical understanding of stuff). analog stuff is really evil.. :-) Thanks! |
| David Hess:
--- Quote from: nemail2 on July 27, 2019, 01:25:49 pm ---So at 1A max output current through the darlington, values like 200R and 10K should be right, i guess? --- End quote --- Yes, that will work. Like I said, the values are not critical as long as at a minimum, the resistors can sink the worst case Icbo leakage current. I normally use much lower values sized based on the excess available current from the previous stage which helps remove charge from the emitter-base junction more quickly. --- Quote ---do you have any idea where I can learn everything I need to know to be able to calculate or at least guestimate how to compensate control loops like this? are there like youtube videos or entry-level documents on this, you are away of? --- End quote --- Ultimately this comes down to how negative feedback is used and the relationship between gain and phase. I do not know of any good practical books on specifically this but I am sure one exists and someone can suggest it. I first picked it up in bits and pieces from various places. |
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