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| TL431 + common base amplifier stability? |
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| Zero999:
The old TL431 is limited to 36V. For higher voltage circuits it needs to be buffered somehow. A common base amplifier is the most obvious solution but is it stable? LTSpice says it is stable but I don't trust the model of the TL431. The extra voltage gain is bound to reduce the phase margin but could this be offset by the fact the gain needs to be high (less negative feedback) and a common base amplifier doesn't have much phase shift? The circuit below has an output voltage of just over 50V, that's a gain of over 20, so it might be stable. I suppose I'll have to build it and see. EDIT: Added .asc file and changed the package to the PDIP/SOP)/TSSOP so the file can be read, without any bespoke symbols. |
| T3sl4co1l:
If you bring in the gain-phase plot for the TL431, you can figure it out. I would suggest one thing, for two reasons. An emitter resistor. First, this regulates the voltage gain of the transistor, so that rather than having transconductance proportional to current (and rather high to begin with, because BJTs are awesome), it's limited by that resistor. Second, this allows some voltage swing to appear at the TL431, so if nothing else, you can compensate it locally with a C or R+C back to REF. (Without the emitter resistor, the emitter voltage will be largely the same as the base voltage, less a fixed drop, so the feedback C or R+C will simply feed back no voltage.) Note that this circuit doesn't have straightforward gain and phase and all that, because there's no independent collector load, it's all shunted back to the same point. Likewise, base voltage is set by collector voltage, so the transistor looks more like a fixed gain noninverting (not really "common base" anymore) stage. On the upside, the limited gain will probably correct the ills of the transistor to a frequency much higher than the TL431 understands (i.e., the roll-off for the gain shown will be in the 10s of MHz), and one should be able to make the hand-waving assumption that, as long as feedback gain is less than transistor gain, it will be stable (because, dividing the two out, the TL431 is effectively still running at or above unity gain, and we know it is unity gain stable, so it should still be stable). That's no guarantee, but it's a start. BTW, SPICE models of the TL431 are notoriously bad, even as simplified macro models go. Info: http://www.audio-perfection.com/voltage-regulators/linear-regulators/realistic-spice-model-for-tl431-stability-noise-impedance-and-performance-simulation-of-tl431-shunt-regulator.html Helmut is very knowledgeable at LTSpice so, if that's the default model used, you're probably not too bad off. I would recommend that model, or the "Eugene" model given in the link. Tim |
| Zero999:
Thanks for the reply. It seems like I have a very basic model for the TL431. I do remember downloading it from the Yahoo group but it was some time ago. I couldn't find a detailed phase/gain plot on the datasheet so it's not something that can be calculated. I think I'm just going to build this and test the step response with an oscilloscope and signal generator. |
| Zero999:
I've tested it and it works. I added a 100R emitter resistor and changed R2 to 47k, in order to reduce the gain and provide more headroom for the voltage drop across the resistor. It seems to be very stable. The gain of the voltage booster is equal to 1+R1/R2 which is 3.1 and it's inside the feedback loop of the TL431 which has a gain of 19.9 which explains why it's stable, especially as the common base configuration has minimal phase shift. I also tried reducing R3 to 510R and cranking up the voltage to 65V for a short period of time. It got hot and would probably meltdown if left continuously but it survived a few seconds and was stable. The output voltage was 50.5V which is within tolerance. EDIT: Schematic changed to one showing the changes described above. |
| schmitt trigger:
An old thread, but I'm glad that I found it. I had experienced first hand a mild instability in a cascoded TL431 (to emulate a 100 volt zener), and I was unsuccessful in taming it. I had attempted the usual cures (a compensation cap between the TL431's cathode and its adjustment pin), and had wondered how come it did not appear to behave. Your following phrase is priceless, and cannot thank you enough. In hindsight it should be obvious! I would suggest one thing, for two reasons. An emitter resistor. First, this regulates the voltage gain of the transistor, so that rather than having transconductance proportional to current (and rather high to begin with, because BJTs are awesome), it's limited by that resistor. Second, this allows some voltage swing to appear at the TL431, so if nothing else, you can compensate it locally with a C or R+C back to REF. |
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