Pre regulator inspired by Linear AN30 Fig 46 - Page 1
Forum section: electronics / projects

author: jaycee
author_rank: Regular Contributor
timestamp: October 27, 2017, 06:23:42 am
content: Since as I might be giving up on winding my own tapped transformer for my bench PSU project, I figured switching pre-regulators were worth a look. Linear's App Note 30 and 32 are full of goodness on this particular subject!One thing I absolutely wanted was for the switch element to be an N channel MOSFET... the fastest and lowest RDSon devices are available in this, but I had the IRFP240 in mind as it's cheap and easy to get. Requiring a higher VGS voltage isn't too much of a problem as a simple charge pump can be used to get it from the transformer. So I came up with this.Thoughts and comments ? I suspect using a high-side driver chip for the MOSFET would be a good idea. The LT1083 would be replaced with a discrete linear regulator and is only being used here to test the pre-regulator. The optocoupler is used because I want to keep the output of the supply floating like most bench power supplies are. I'm also not a great expert on driving MOSFETs so maybe I've missed something obvious and the thing would actually blow up in my face if I built it!

author: capt bullshot
author_rank: Super Contributor
timestamp: October 27, 2017, 06:50:59 am
content: After staring some time at the schematic, I came to the conclusion: it might work indeed.The way it is made is unusual today, but LT had the most interesting application notes back then with many unconventional ideas. A great source of ideas and knowledge.Anyway, I wouldn't build the pre-regulator this way today, as we have today easy to use switch mode regulators.Otherwise, this design still is kind of a beautiful and simple thing and worth giving it a try.So for my thoughts about your variant:I'd replace the 1k (R4) resistor by a bootstrap circuitry for the gate drive, no special gate driver IC since this would destroy this feeling of nostalgia and simplicity. So you need a higher value pull-up from the 35V to initially turn on the MOSFET, when the oscillating starts, the bootstrap will take over the gate drive with lower impedance and reduced losses.I don't think you need that darlington configuration to pull down the gate, ultra fast switching won't be useful here. Anyway, check if the voltage drop across all the B-E junctions pulls the gate low enough to turn off the MOSFET. I'd probably go for a P-Channel MOSFET.Check if the 100uH inductor is suitable here, the original circuit uses a 1mH and I suspect it to have a rather low switching frequency. The switching frequency is limited by the frequency response of the optocoupler and I'd better not go above 10kHz or maybe 20kHz.

author: jaycee
author_rank: Regular Contributor
timestamp: October 27, 2017, 06:41:16 pm
content: I did think about just using some modern switching regulator, but most of them are focussed on overall efficiency at high switching frequencies, and not so much on noise. I also didnt want to base it around a part with an internal switch element, and most of the switch mode regulators i found that drive external MOSFETs are in tiny packages.I dont know about "nostalgia" but simplicity definitely has its benefits. I like the idea that 20 years down the line, this circuit could be repaired should anything go bang - whereas some dedicated switchmode regulator IC will probably be obsolete and unobtainable.The main reason for the darlington was that the D45H11 on its own resulted in poor waveforms on the gate, so the darlington helped. That said the D45H11 probably isnt the right device for the job, a BD140 would do, or as you say some MOSFET. I will have to see whats in my parts storesThe 100uH inductor is a Murata type rated for 7.45A with a resistance of 0.04 ohms. Higher switching speed would actually be undesirable - I'd want to keep the noise down into the lower frequency ranges where filtering is easy

author: David Hess
author_rank: Super Contributor
timestamp: October 27, 2017, 09:03:24 pm
content: The Linear Technology design operates at a much lower frequency with a much larger inductance because switching feedback through the optocoupler is slow. The low operating frequency allows a slow darlington transistor to be used and limits high frequency noise at the input of the linear regulator. A higher frequency design should have a separate LC filter between the switching preregulator and linear regulator.Higher switching frequency to take better advantage of a power MOSFET and lower inductance requires either removal of the slow optocoupler or separating the error amplifier and switching functions.

author: cdev
author_rank: Super Contributor
timestamp: October 27, 2017, 09:15:27 pm
content: You could send them an email and ask them, then repost back here with their answer.Why not? It's a small world and getting smaller.

author: jaycee
author_rank: Regular Contributor
timestamp: October 27, 2017, 09:54:53 pm
content: Quote from: cdev on October 27, 2017, 09:15:27 pmNot sure what you mean with this reply... the original idea was by Jim Williams, so one would have to have a connection with the afterlife to ask him, sadly :/

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 27, 2017, 10:15:40 pm
content: Hi,This circuit is interesting. It first appeared in an29. There is a description and some waveforms in AN29. But this circuit is old. A lot has happened since AN29 was written 28 years ago. The original switching frequency was 2kHz.Although the circuit works, there are much better ways to do this today.Here is one idea:The PNP transistor level shifts the voltage across the LDO to ground. As shown the circuit will maintain about 1.8V across the LDO.The LT8614 was chosen because it from the silent switcher family.I have attached the model.Regards,Jay_Diddy_B

author: jaycee
author_rank: Regular Contributor
timestamp: October 27, 2017, 10:31:18 pm
content: Quote from: Jay_Diddy_B on October 27, 2017, 10:15:40 pmYep, however in this case that QFN package rules it right out - I'm making this at home with home-etched PCB's. Also disaster should something happen and kill the pre-reg chip.Also not sure how well these chips would work with a regulator that has it's opamp rails floated around the output of the main supply, as typical for a bench supply

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 27, 2017, 11:07:15 pm
content: O.K.Here is one that uses the LT1074. Should be easier to build. The older technology means a lower switching frequency. This means bigger capacitors and bigger inductor.I have attached the model.Jay_Diddy_B

author: jaycee
author_rank: Regular Contributor
timestamp: October 27, 2017, 11:16:41 pm
content: Think I did look at the LT1074 once actually.. nice and simple... might have to get a cheeky sample and try it out

author: jaycee
author_rank: Regular Contributor
timestamp: October 27, 2017, 11:58:14 pm
content: Other comments on preregs for bench supplies are welcome.. i know theres a few good bench psu projects on here already, mine probably isnt anywhere near as good as those but maybe the "built from recycled junk" goal might appeal to some

author: timb
author_rank: Super Contributor
timestamp: October 28, 2017, 12:48:47 am
content: If you need low noise, LT makes really nice ultra low noise converter chips that can be setup in boost/SEPIC/flyback/inverting/Cuk configurations, plus converters for push-pull setups.If a 2A switch limit is enough, check out the LT1534. If you want to use an external switch, look at the LT1738. For push-pull with external switches, the LT1683 is nice (it can also be setup in various other topologies such as SEPIC, flyback, etc).I’ve successfully implemented a pre-regulator using the LT1534. I used one of their ultra low noise LDOs as the post regulator and it worked really well. I ended up with unmeasurable noise on the output. I actually used two LT1534s, one in a Cuk topology and one in a SEPIC topology, for positive and negative rails. (I first tried using a single converter in a flyback setup, with a center tapped transformer on the output, but I couldn’t get the power I needed out of the 2A switch limit. (Ultimately it would have been a lot cheaper to go with the external switch version (LT1738) and flyback, but I was pressed for time and the customer didn’t mind the added cost.)Anyway, these converters work on the principle of reducing the slew rate of the switch, to lower noise. With a single stage LC filter on the output you can get to sub-500uV levels of noise on the output of a boost or flyback converter. SEPIC and Cuk topologies put out practically unmeasurable levels of noise (as they’re already low noise topologies to begin with).I can post the circuit I used for pre-regulation feedback if you want. (I need to cut some parts of the non-power supply, customer related parts of the circuit out before I post it.)http://www.linear.com/parametric/Ultralow_Noise_Regulators

author: jaycee
author_rank: Regular Contributor
timestamp: October 28, 2017, 04:19:14 pm
content: Quote from: timb on October 28, 2017, 12:48:47 amThat would be nice if you dont mindI've sampled the LT1074 and a LT1680 to play with. I also have a few LM2576-ADJ in my parts drawer that I might try out!I will probably still use a discrete linear regulator rather than an IC, but who knows...

author: flowib
author_rank: Contributor
timestamp: October 29, 2017, 01:55:17 pm
content: I've actually got a LT1076 prototype supply on my bench right now. I plan on using the 1076 for the axillary supplies (+-15V), from a 24V bus.The 1076 works fine, and seems to be about 85-88 percent efficient at 40Vin 20Vout, I used SB560 (60V 5A) because i have a complete reel of these.an optional filter of 10µH 200µF gets rid of most of the noise, however the high frequency switching residue, (About 400mV PP peaks) passes through both the output filter and the LM317 nearly unattenuated.I was thinking of a similar circuit to yours, the PNP transistor is similar to what i came up with, however i chose to keep the loop of the switcher closed, so the input voltage of the LDO post regulator is not violated if the feedback circuit is too slow or fails.There are SMD D-pack LDO's available that are pin compatible with the LM317s these might be what you are looking for, most if not all of them can be controlled by an opamp to provide CC and CV behaviour.The first 10uH filter is to avoid noise getting out of the supply and influencing other circuits fed from the same bus, I was thinking of adding a common mode toroid. and some SMD filter caps instead.See AN101F it is one of the most valuable pieces of information concerning switching noise.This post on a Dutch forum is about a development of a insanely low noise bench power supply (5uV PP max output noise and 0.1% stability) ,https://www.circuitsonline.net/forum/view/110029he tested different types of preregulators for the power supply.