Auto-Adjusting a linear regulator and switching regulator - Page 1
Forum section: electronics / projects

author: LoveLaika
author_rank: Frequent Contributor
timestamp: April 06, 2023, 03:33:49 pm
content: I was thinking about this circuit as a mental exercise, and I wanted to get some feedback about it and see whether or not my idea could be feasible. For power supplies, using a DC/DC switching regulator feels efficient, but because it's a switching regulator, there's ripple noise. You could filter it out, but that might not be enough. So, you take the output and pass it to a linear regulator which would help reduce the noise even further for a clean voltage output. Question is, is there a way to "auto-adjust" the switching regulator given the final output that you want? I thought that by doing this, you adjust the input to the linear regulator so as to not waste power by keeping it within the dropout voltage. Not sure whether or not the idea is a good one, but this is just a mental exercise after all. I'm not really looking to build this, so please excuse the lack of capacitors and such.So, I kind of picked some example parts to get started: the LM2596 and the LT3081. The LM2596 is my adjustable switch mode DC/DC regulator (feedback pin should be at 1.23 volts ideally), and the LT3081 is my linear regulator (dropout voltage of 1.5 volts max, min load 5 mA). So, the LM2596 is like the datasheet with the output going to a resistive divider and to the LT3081. The LT3081's output voltage is controlled by RSET (I picked the value to set it to limit the max output to 15 volts). From the circuit, the idea is that OA1 takes the difference between the input and output voltages of the LT3081. That goes to a comparator which compares the difference in voltage to 1.5 volts. That way, the LT3081 should stay within the dropout voltage. If it exceeds the voltage, then the op-amp output will go high and send a voltage through D3 to the feedback pin, which should adjust the LM2596 output until the difference in voltage is met. (Looking at it now, I got the input pins wrong for OA2; sorry, just flip them around; perhaps get rid of R2 as well as there's no need to check the LM2596 output if you're looking at the LT3081 output)Fundamentally, does the idea make sense? Would there be a reason that this would be a bad idea?

author: jonpaul
author_rank: Super Contributor
timestamp: April 06, 2023, 03:50:43 pm
content: Ancient idea, used since 1970s, bravo for your rediscovery!many hifi power amps did exactly this.in 1985, we used similaires in first sound cards for PC ISA busNowadays the MHz soft switching SMPS have much less noise on outputsJon

author: LoveLaika
author_rank: Frequent Contributor
timestamp: April 06, 2023, 03:53:35 pm
content: I was just thinking about what I've done before and what I can do differently in the future. I don't have a book of common circuits to draw upon for reference.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 06, 2023, 04:02:46 pm
content: The idea is old / stanard for using SMPS preregulator. However instead of the 2 OP amps this usually is more like a single transistors and a few resistors and maybe a diode to simplify things.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: April 06, 2023, 04:11:30 pm
content: Quote from: Kleinstein on April 06, 2023, 04:02:46 pmThanks for your reply. I went with op-amps because they were easier for me to understand than transistors at a high level. I guess they felt more intuitive to me, though I don't doubt that you could do it using BJTs. It's just that thinking of the voltage drops and such for correct BJT operation doesn't come to me intuitively as does an op-amp with its basic rules.If I may, going back to the circuit, how would one ensure that the FB voltage pin remains at 1.25 volts for correct operation? With the way I have it now when there's a voltage difference, OA2 would go to +VCC (the op-amp power source, assuming that's more than 1.25 volts), pass through the diode, and go to FB. With the way it is now, would the switching regulator be constantly adjusting its output because it won't be fixed at 1.25 volts? What would be a way to fix that?

author: jwet
author_rank: Frequent Contributor
timestamp: April 06, 2023, 04:35:14 pm
content: As stated, The concept is solid and has been used. With modern swtichers at higher frequencies and better topologies (contant frequency PWM), it might not be necessary unless you're powering a really sensitive load like a clock PLL, etc. There are also some potential implementation issues- these are range, startup and stability. Making a widely varying buck converter takes a bit of care- you have a fixed frequency and fixed inductance and you're changing Vout- you're going to go over a range of duty cycles and maybe traverse through 50% duty cycle- a tough spot. I would stick this in a spreadsheet and look at all the corner cases and find an inductor value that was a good comprimise. On startup with the linear output at 0V and the chip not really operating, the switcher is going to maintain 1.5 volts across it, you might get stuck in a startup loop where you can't get started- something to watch for. You have the FB for the 1.5v drop shown as a comparator basically (open loop). This needs some thought. Since you're trying to reduce power dissipation and have a bit of time, I would slow things way down. The servo action of the switcher doesn't have to be instant- slow down the loop (add integrators)- this might include the diff amp. You can also maintain the drop across the linear with a few Vbe's or a floating voltage reference like a Low Voltage TL431. Have fun.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: April 06, 2023, 05:00:11 pm
content: Thanks for the vote of confidence. Honestly, at first glance, that sounds like a lot to try and balance...almost way too much. I don't have a lot of power/analog electronics experience as a lot of others here seem to have, and that makes me really question myself and my abilities...but I strive to do my best here, because exercises like this may be useful in the future.Makes me wonder how other people went about this in the past.

author: Ian.M
author_rank: Super Contributor
timestamp: April 06, 2023, 05:01:37 pm
content: The LM2596 and LT3081 are poor choices for this - see theLT3081 data sheet, page 8, graphs 'Ripple Rejection' - which show you that the LT3081's PSRR at the operating frequency of the LM2596 is little better than 20dB, approximately three orders of magnitude worse than its performance when fed from a traditional line frequency transformer, full wave rectifier and adequately sized reservoir capacitor.If active filtering is required, consider using a capacitance multiplier (seehttps://www.eevblog.com/forum/blog/eevblog-1116-the-capacitance-multiplier/), as the HF performance of a single or Darlington transistor is typically much better than that of most linear regulators. If using an adjustable switching regulator, it can be placed within the regulator's DC feedback loop to auto-compensate for voltage drop across the pass transistor, but AC feedback needs to be taken from before the capacitance multiplier, to avoid instability on load transients, and even oscillation.

author: donlisms
author_rank: Frequent Contributor
timestamp: April 06, 2023, 08:27:38 pm
content: I was thinking the original poster might enjoy the LT3045-1 datasheet, just as an example of what's possible with these ideas.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 06, 2023, 08:52:37 pm
content: Commonly called a pre-tracking regulator. Nothing new as you have found out.One TI note:https://e2e.ti.com/blogs_/b/powerhouse/posts/design-a-pre-tracking-regulator-part-1-for-a-positive-output-ldo

author: jwet
author_rank: Frequent Contributor
timestamp: April 07, 2023, 02:53:32 am
content: That TI app note uses a nice clean solution. I would test and make sure it starts and is stable.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: April 07, 2023, 02:41:53 pm
content: had similar thoughts a few months back and found this page:https://sound-au.com/articles/preregulators.htm#s5(Figure 5 - Switchmode Buck Converter Pre-Regulator)cheers,rob :-)

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 08, 2023, 03:06:23 pm
content: Quote from: SiliconWizard on April 06, 2023, 08:52:37 pmI've used a similar solution in my DIY laboratory power supply but with a MOSFET. Found it in some datasheet. Works fine.

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 08, 2023, 03:17:08 pm
content: Indeed, tracking pre-regulators predate the widespread use of monolithic switching regulators.A POPULAR ELECTRONICS article in the late 1970s showed a pre-regulator utilizing SCRs.Anyways, congratulations on your efforts! I don’t want to detract from your interest in electronics, so by all means, build a circuit and learn from the experience.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: April 12, 2023, 10:13:50 pm
content: Thanks for your message. I picked those two parts because I was familiar with them. Certainly, I could go with better parts, but they were the first things that came to my mind.If I may ask, what do you think about the last part of the feedback circuit, going to FB of the switching IC? In order for the output voltage of the IC to be consistent, it has to have a voltage of 1.23 volts as per the datasheet. But, with the way I have it now, the voltage coming from the op-amp is the one providing it. Is that a good way to go about it to complete the loop, or would it be better to have the op-amp control a BJT instead and have that provide the voltage to the feedback pin?