Best high current linear regulator configuration - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: April 18, 2020, 11:37:08 pm
content: Hi,What is the best design/solution for a regulator for a PCB with the following characteristics:- linear (because of stringent noise requirements)- 2A out (regardless of Vin and Vout)- Vin from 12V to 26V supplied by an external wall adaptor.- Vout is variable (through an ADC connected to a uC or other means) from 2V to (Vin-2V).- the PCB with the regulator (not the wall adaptor obviously) is to be placed inside an enclosure without any air vents (and possibly even potted).That means that with respect to heat dissipation the worst case scenario is when Vin is 26V and Vout is 2V. At 2A load that is 48W dissipated just by the regulator.1) What is the best solution using linear regulators? The classics are two regulators (but then have to add balancing resistors) or bypass transistor. Which is best and why? Any parts/designs suggestions?2) Any other topologies/solutions?3) Any other concerns? For example, I'm thinking the 48W dissipated inside an enclosure without ventilation? The elements that heat up (regulator, bypass transistor etc) would have heat sinks but how safe is that?4) Would any potting (of the entire PCB) improve or worsen the thermal situation and general safety?Many thanks

author: Ian.M
author_rank: Super Contributor
timestamp: April 19, 2020, 12:07:41 am
content: 3) With 48W dissipation, it will cook itself to death unless its in a large metal enclosure, with enough surface area to dissipate the heat and with the high dissipation parts on heat spreaders thermally coupled to the enclosure, or alternatively if you fit a large enough exterior heatsink with all high dissipation parts mounted to its inner face.

author: Whales
author_rank: Super Contributor
timestamp: April 19, 2020, 12:39:29 am
content: Common alternative topologies:A. Multi-tap transformer followed by linear reg. Relays select which tap gets used. Dramatically lowers P loss in linear reg. Requires an array of comparators or a micro.B. Switch-mode followed by linear reg. Special attention may be needed for common-mode noise, depending on your requirements.If this is a one off: computer heatsinks + fans make cheap and easy coolers. Edit: yes for 48W you will need forced airflow; or a crazy amount of thick-whole-case-side-multi-kg heatsinking if you want it passive.FETs may be easier to parallel than BJTs - can someone comment here regarding runaway? I believe (but I may be wrong) that FETs have better thermal coeffs?Make sure the check the SOA of your linear transistors. If the datasheet doesn't provide a DC SOA then don't trust it. I can't recall exactly, but I think the higher-voltage capable FETs tend to have better SOA. Requires lots of looking around, the only obvious search term hits (like the Linear L2 series) cost a lot per FET even thought other manufacturers make similarly specced items for less.

author: Whales
author_rank: Super Contributor
timestamp: April 19, 2020, 12:47:56 am
content: > 4) Would any potting (of the entire PCB) improve or worsen the thermal situation and general safety?You have to think of the individual thermal and safety problems.Potting should have nil effect on your main pass transistors, because theyhaveto already be secured to a big heatsink anyway. As long as you don't pot the fins of the heatsink, that is"Safety" is not a one-word topic. You need to look at what parts of your supply can fail and what bad stuff could happen because of this. I would avoid potting anyway unless this is for production or you have a specifically harsh environment (eg wet), it makes it very hard to repair/modify.

author: Circlotron
author_rank: Super Contributor
timestamp: April 19, 2020, 01:12:17 am
content: Quote from: Whales on April 19, 2020, 12:39:29 amMOSFETS parallel great when they are fully switched on. The hotter one increases it’s drain-source resistance and so conduction backs off.BUT...A MOSFET partially switched on as per a linear regulator, the hotter on will have a lower gate threshold voltage causing it to turn on even more than the increased drain-source resistance tries to back it off and so hogging the current. Not what you want. A resistor in each mosfet source lead may help somewhat.

author: ricko_uk
author_rank: Super Contributor
timestamp: April 19, 2020, 02:15:23 am
content: Thank you all for your detailed replies!Not familiar with the thermal calculations to choose a heatsink, could someone please point me in the right direction or even better provide me some equations or examples?Many thanks again!

author: David Hess
author_rank: Super Contributor
timestamp: April 20, 2020, 02:56:55 am
content: Quote from: ricko_uk on April 18, 2020, 11:37:08 pmCurrent sharing between multiple regulators can also be enforced with current shunts on their inputs and operational amplifiers to control the additional regulators. This has the advantage of perfect current sharing and not compromising the load regulation with ballast resistors.However some regulators however have an output voltage tolerance good enough to use low values of ballast resistors and this would be the simplest way.QuoteA discrete solution with transistor, operational amplifier, and reference is eminently feasible but gives up the inherent protection features of an integrated regulator. Or an integrated regulator can be used in place of the power transistor for its built in protection features; see below about lower noise.Some low dropout integrated regulators work down to 0.6 volts so could drive an external power transistor with a total dropout below 2 volts. Most low dropout regulators use a Sziklai pair for higher efficiency but these have a dropout of 1.2 volts which would be marginal when driving an external power transistor.You mentioned the importance of noise. An external reference and operational amplifier can control a regulator to produce much lower noise than the regulator alone. This also makes remote sense practical.QuotePower is a real problem in a sealed enclosure. I would mount the heat sinks on the outside of the case and mount the power devices to them through the enclosure.Integrated regulators have limited power dissipation and that is the criteria which will require multiple regulators in parallel, which is just as well because it spreads the heat out making it easier to dissipate. However operating the regulators at a fraction of their individual current limits means the total current limit will be much higher than the design current which may be undesirable. So some consideration should be given to implementing an external current limit.Integrated regulators also have secondary breakdown protection which requires them to be used in parallel at high input to output voltages.QuotePotting could improve the situation however mounting the power packages to the case would be better and sufficient.

author: ricko_uk
author_rank: Super Contributor
timestamp: April 22, 2020, 03:09:27 pm
content: Thank you all for your replies and suggestions!!

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 22, 2020, 03:33:18 pm
content: The LM350 is the higher powered big brother to the LM317.It can provide up to 3 amps if proper thermal management is followed. Which as others have mentioned, this will be your biggest technical challenge.In addition to fans and a large heatsink, I also vouch for the transformer tapping method to dramatically decrease dissipation at the lower output settings.To perform your thermal calculations, start with this web page. There are many others on the web, Google is your best friend here:https://www.designworldonline.com/How-to-Select-a-Suitable-Heat-Sink/

author: Kleinstein
author_rank: Super Contributor
timestamp: April 22, 2020, 05:34:35 pm
content: Simple linear regulator all have essentially the same power loss and heat. So if possible one should reduce the loss by something like tap switching / using multiple raw voltages. One does not need relays, but could also electronic switching.With only 2 A there is no real need to parallel transistors or MOSFETs.There are higher power regulator available, like the LT1083-5 series.

author: Vovk_Z
author_rank: Super Contributor
timestamp: April 22, 2020, 06:35:31 pm
content: 26V max input voltage is low enough to use almost all powerful integral regulators: LM338, LM350, LM1083/LD1083/1084/1085 etc. We can even use several parallel LM317.I'm only not sure about number of parts - use only one or two or even three to safely dissipate all that power. It depends.Of cause nowadays it is better to use well-designeg (low-noise) dc-dc or tracking dc-dc and linear after-regulator.