Tracking pre-regulator not linear. - Page 1
Forum section: electronics / projects

author: fc3000
author_rank: Regular Contributor
timestamp: June 30, 2023, 12:30:25 pm
content: Hi all.I am using LTSPICE to simulate the circuit below.It is a PSU with a Tracking Pre-regulator.The circuit exhibit a non-linear tracking voltage.The plots shows two extreme outputs.I want to maintain the input to LM317 to as low as possible, to maintain low power dissipation.But at lower outputs the tracking voltage is about 5V.Any way of correcting this?Is this a characteristic of this type of design?Thank you.Regards

author: Marco
author_rank: Super Contributor
timestamp: June 30, 2023, 01:06:50 pm
content: The switcher voltage is zener + 0.6 above your output voltage, if you want less output voltage from the switcher reduce or remove the zenerTo get rid of the small nonlinearity, remove r12.

author: fc3000
author_rank: Regular Contributor
timestamp: June 30, 2023, 01:44:03 pm
content: Quote from: Marco on June 30, 2023, 01:06:50 pmIf remove R12 the FB voltage will not change. It will be same as the voltage at the pre-regulator.Regards

author: Marco
author_rank: Super Contributor
timestamp: June 30, 2023, 02:31:16 pm
content: It will be ground until Q1 gets triggered ... after which it will be high and the switcher will turn off.PS. the FB voltage when Q1 gets triggered is too high at high output voltage BTW. Regardless of R12, Q1 can raise FB way above 10V, which is absolute maximum rating.

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 30, 2023, 03:43:33 pm
content: +1 to @Marco's solution

author: fc3000
author_rank: Regular Contributor
timestamp: June 30, 2023, 04:31:55 pm
content: Quote from: Marco on June 30, 2023, 02:31:16 pmHi Marco.Thank you for your help.Following your suggestions, I have modified the circuit as shown below.Now I 4V tracking voltage, when Vout=0. And 2.4V tracking voltage when Vout=30V.This is an improvement from before.Regards

author: Siwastaja
author_rank: Super Contributor
timestamp: June 30, 2023, 05:31:48 pm
content: Note that by lowering the voltage drop over the regulator, PSRR at high frequencies probably suffers to worse than what's specified in the datasheet. At relevant switcher frequencies (500kHz?), LM317 is already down to some 20dB, and with less voltage overhead, it's unspecified, possibly just 10dB or so. Are you sure you really want a tracking pre-regulator with linear post-regulation? It's a lot of work with little gain. A switcher forced to work in CCM, designed using high frequency and good low-ESR output capacitors (MLCC bank) is already very low in ripple.The true reason to add linear post regulation stage is to get faster reacting current limit. Do you need this?

author: fc3000
author_rank: Regular Contributor
timestamp: June 30, 2023, 05:37:40 pm
content: Quote from: Siwastaja on June 30, 2023, 05:31:48 pmThank you for that technical insight.It is for a Bench PSU.What do you expect to be the noise level, for this design, at the output?Regards

author: Siwastaja
author_rank: Super Contributor
timestamp: June 30, 2023, 05:42:19 pm
content: You have the SPICE model up. Add parasitics (L for wiring, e.g. 2nH / cm of wire, L & R for capacitors) and simulate it!I don't see an adjustable current limit. That's one of the most important and defining features in a lab bench supply. OTOH, if you just need a voltage source, just use a modern switcher design. What types of load you are going to power it with, that you think are sensitive to noise?

author: fc3000
author_rank: Regular Contributor
timestamp: June 30, 2023, 06:48:17 pm
content: Quote from: Siwastaja on June 30, 2023, 05:42:19 pmThe design is not conclusive. I will add current sensing and adjustment. I prefer high-side kind.Thanks again for your input.Regards

author: Marco
author_rank: Super Contributor
timestamp: July 01, 2023, 11:22:45 am
content: I didn't really want to discourage independent design, but you should know that Analog has a nice series of linear regulators with adjustable current limiting and a reference design for their use in a tracking switching+linear power supply (the current limit isn't very precise, but that could be adjusted with an extra control loop).https://www.eevblog.com/forum/projects/linear-technology-dc2132a-cvcc-adj-bench-power-supply-board/

author: fc3000
author_rank: Regular Contributor
timestamp: July 01, 2023, 04:26:46 pm
content: Quote from: Marco on July 01, 2023, 11:22:45 amHi Marco. Thank you for the info.Yes. By an amazing coincidence I found in my draw an a couple of LT3080.They different from the LT3081 in the fact they do not have current monitor.I will definitely review my design now.The fan is in the learning!Regards.