Control two regulators with one variable resistor - Page 1
Forum section: electronics / beginners

author: sverzijl
author_rank: Contributor
timestamp: May 05, 2020, 12:22:53 pm
content: Hi all,I've been trying to design myself a lab power supply via experimentation in LTSpice.One thought I had, justified or not, was to have a switching regulator in series with a linear regulator via a capacitance multiplier. What I wanted to achieve was an increase in the overall efficiency by first stepping the voltage down with the switching regulator.My problem is: how can I set it up so that by adjusting a single variable resistor I'm adjusting the voltage of both regulators simultaneously?One naive thought I had was that I could take the voltage output from the second regulator and somehow use a jfet on the voltage divider of the first regulator - though I'm still working out how to correctly use jfets. I figure the slow feedback due to the capacitance multiplier isn't a big deal since the first regulator is only suppose to perform a crude step down - the linear regulator being what gets the true target voltage.In terms of which voltage regulators, the only adjustable ones I've seen are controlled by a voltage divider. I note that they all seem to have different reference voltages and current requirements for the voltage regulators which further complicates matters.Is anyone able to point me in the right direction please? My preference would be hints not answers as working it out myself will help me with the learning process...

author: xavier60
author_rank: Super Contributor
timestamp: May 05, 2020, 01:34:49 pm
content: I have often seen examples for doing that using an LM2675 feeding a linear regulator IC. Circuitry consisting mainly of a PNP transistor monitors the voltage drop across the linear regulator and controls the voltage on the FB pin of the LM2675. The idea is to maintain some small voltage across the linear regulator.I cant find any example schematics at the moment.

author: xavier60
author_rank: Super Contributor
timestamp: May 05, 2020, 01:52:10 pm
content: Dave explains it in this video but using a Boost rather than Buck SMPS IC. Also a P-ch MOSFET instead of a PNP BJT.

author: sverzijl
author_rank: Contributor
timestamp: May 06, 2020, 04:12:41 am
content: Ah! I was frustratingly close!Thank you for pointing me to that.

author: xavier60
author_rank: Super Contributor
timestamp: May 06, 2020, 05:38:18 am
content: Expect to have some regulation stability problems that should be easily fixable when the time comes.If you don't already have a small MOSFET, I'm certain that a PNP transistor will work if applied correctly.

author: sverzijl
author_rank: Contributor
timestamp: May 06, 2020, 12:09:05 pm
content: So my brain triggered on your statement about maintaining a voltage difference between the input and output of the linear regulator.I tested that idea by using an op amp to subtract the input and the output of the linear regulator and used that as the input to the switching regulator (where I had the voltage divider set to maintain 3V as an example).Is there any reason why I wouldn't achieve the outcome the way I did using an op amp?

author: xavier60
author_rank: Super Contributor
timestamp: May 06, 2020, 12:17:56 pm
content: An op-amp configured as a balanced amplifier could work. Because it will have very low gain, it shouldn't cause regulation instability.

author: xavier60
author_rank: Super Contributor
timestamp: May 06, 2020, 12:51:48 pm
content: I'm curious to see if something as simple as this will work.

author: sverzijl
author_rank: Contributor
timestamp: May 06, 2020, 01:34:42 pm
content: Funny you should mention that because I just noticed there was a part 2 to that video you posted and I saw this:https://www.eevblog.com/forum/blog/eevblog-329-tracking-pre-regulator-ltspice-simulation-part-2/Which seems to be the same idea.I'm simulating it now but I'm having trouble getting my head around what is going on? Am I right in my understanding that the voltage of the emitter is 0.7 V greater than the output voltage of the second regulator. The collector is 1.25V because of the Vref in the first regulator. The current going through R2 is the same as R1 (which is 0.125 mA) which therefore gives the voltage drop of R2?

author: sverzijl
author_rank: Contributor
timestamp: May 06, 2020, 02:17:07 pm
content: Interestingly enough it works - but only if I remove the capacitance multiplier. With the capacitance multiplier in between the two regulators the slow response of the second regulator seem to cause it to converge to the steady state. Something like 0.001 V/ms.The op amp appears to be able to converge much much faster.That said - looking at the output of the linear regulator I'm not sure why I'm bothering with the capacitance multiplier... (Not that it hasn't provided a good learning experience).

author: xavier60
author_rank: Super Contributor
timestamp: May 06, 2020, 02:22:54 pm
content: Quote from: sverzijl on May 06, 2020, 01:34:42 pmYes, that explains it. The configuration of Q1 is Common Base. The Base is tied to a point that's considered as fixed. So the Emitter voltage is fixed also. The current caused by the voltage drop across R2 is output from the Collector causing about half the voltage drop across R1.I missed the 2nd part.

author: xavier60
author_rank: Super Contributor
timestamp: May 06, 2020, 02:40:06 pm
content: With no Base resistor, the only concern is that if a negative voltage is applied to the output, it will be forcibly applied to the FB pin of the SMPS IC, I don't know if this will cause any extra damage. a diode could be placed in series wit the Collector of Q1.

author: sverzijl
author_rank: Contributor
timestamp: May 06, 2020, 10:23:44 pm
content: Great!Thank you for helping me with this.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 06, 2020, 10:27:24 pm
content: Quote from: xavier60 on May 06, 2020, 12:51:48 pm- yes, but use MOSFET instead of bipolar. You'll have just fine voltage drop about 4 V. For example, look for LT3080 datasheet, Typical Applications, page 21 "Adjustable High Efficiency Regulator".(Or LT3083 Typical Applications, p.21-22, the same).I used XL4016 with Mosfet and LM350/LM338/LD1083 with success.

author: xavier60
author_rank: Super Contributor
timestamp: May 07, 2020, 12:14:42 am
content: Quote from: Vovk_Z on May 06, 2020, 10:27:24 pmWith the PNP transistor method, the voltage difference across the linear regulator can be altered by the value of R2 or R1.Using the LT3080 makes it much easier to be able to adjust down to 0V output.More: The ability to adjust down to 0V opens the possibility of having CC mode.

author: sverzijl
author_rank: Contributor
timestamp: May 07, 2020, 06:24:42 am
content: Quote from: xavier60 on May 07, 2020, 12:14:42 amWhat does CC mode mean?I did find it interesting that different methods are faster at converging to the desired voltage than others. Your concept did work in LTSpice.

author: xavier60
author_rank: Super Contributor
timestamp: May 07, 2020, 07:09:29 am
content: Quote from: sverzijl on May 07, 2020, 06:24:42 am"Constant Current" mode. It can be used as current limiting for minimizing possible damage when powering loads with unknown faults,or for occasional battery charging.If CC mode works precisely enough, it can be used for measuring very low resistances.I see that the LT3080 is rather expensive compared to other regulator ICs. Oddly, element14 have stopped stocking it.