Automotive DC Step Down Power Supply: Switchmode or Linear? - Page 1
Forum section: electronics / projects

author: ONETEN
author_rank: Contributor
timestamp: September 15, 2015, 03:54:50 pm
content: Hi Guys,I have an automotive PIC32MX primarily logic based project I have been working on for a couple of years, I have been teaching myself the works along the way (C, Eagle, Comms protocols etc) so it has been slow going at times! I've reached a bit of a logger head in terms of power supply design and could really do with some experienced input.The basics of my project is a PIC32MX communicating with vehicles via CAN and or UART, and transmitting some data via Bluetooth. I have just gotten a copy of Eagle and am working on my Schematics now, knowing the time was near to design the power supply for the unit. Values for supply are Vin +12v (but ranging from 8v to 14v) and Vout of +5v.Some of my immediate concerns when thinking about the supply are:- Nature of Automotive supplies being full of RF- Rapid & considerable changes in voltage, especially during crank and if alternator regulator fails and an overcharging condition occurs- While brown outs\drop outs are not an issue (BOREN to the rescue!), it would be nice to avoid them if possible (during engine crank etc).Initially I only considered using a LDO Linear regulator along with supporting passive components to get the job done, and as I am yet to draw a schematic I was thinking along the lines of:- LM2490 or similar- Generous amounts of cap on bot Vin and Vout- 1N4001 on Vin for reverse polarity protection- Reverse biased Zener with 18v breakdown? for over voltage protectionTHEN I saw Dave's video:http://www.eevblog.com/2010/09/10/eevblog-110-lets-design-a-dc-to-dc-switchmode-converter/while researching switchmode supplies and it got me thinking "would this be a more stable/better/efficient design?". I always believed Switchmode to be generally accepted as a more stable for power supply and regulation.I have looked at a few of the DIY Engine control units and other schematics around the place and all of them seem to use Linear regulators... Not sure what the reason for this is.So I guess my questions I am struggling to find answers for are:Switchmode has advantage of not dissipating heat, but is it suitable to be used in close proximity with logic devices like the PIC? Can oscillation RF cause an issue?In this application, Is Linear advantageous at all over Switchmode other than being more simple to design? My unit will be lucky to draw 200ma so heat is somewhat negligible.Any other suggestions/advice greatly appreciated. I have a nasty habit of over-engineering everything I do which is why I am questioning the good ole linear reg in the first placeThanks for taking the time to read & assist!Cheers,Hayden

author: Kleinstein
author_rank: Super Contributor
timestamp: September 15, 2015, 05:10:42 pm
content: The choice wer much depends on the current drawn. 200 mA at (14 V - 5 V) is 1,8 watts. Thats about the value where you change over from a linear regulator to a switched mode. It also depends on the size of the case and how long the unit runs. A simple switcher (with or with out trade mark) is not that complicated.

author: DanielS
author_rank: Frequent Contributor
timestamp: September 15, 2015, 08:31:45 pm
content: Switching power supplies are especially well suited for logic since logic is far more tolerant of moderate amounts of noise on their supplies compared to analog stuff where remotely significant noise on the supply can readily ruin your day. PCs, tablets, smartphones, etc. have mutiple switching DC-DC converters.Properly designed switching regulators will produce output much easier to clean than the power and signals coming from the rest of the vehicle. If you do not know how to cope with noise from the power source, how do you expect to deal with noise from your other inputs?Here's a reference design from TI for inspiration:http://www.ti.com/tool/TIDA-IPAD-AUTOMOTIVE-CHARGER-REFERENCE-DESIGN

author: ONETEN
author_rank: Contributor
timestamp: September 16, 2015, 06:37:32 am
content: Thanks guys for the replies. Appreciated!Gonna do some reading on the switch mode and see if I can wrap my head around a prototype. It is a winner from both a cost perspective as well as design it seems, and if I bite the bullet and get the hang of switchmode now, it will probably pay off on future projects.Will update with a schematic when I reach that point.This video probably sums up my position pretty accuratelyCheers,Hayden

author: Andreas
author_rank: Super Contributor
timestamp: October 14, 2015, 06:14:16 am
content: Quote from: ONETEN on September 15, 2015, 03:54:50 pmHello,You should first read the automotive standards regarding the requirements for your cirquit.A 1N4001 will never be sufficient for the (up to) -200V transients within a car.(and is probably not fast enough).A 18V zener will not survive the "jump start" test. (24-27V for 1 minute)(and may also be at the limit when having a bad battery with voltages up to 18V)From 1 OEM the requirements are available here:(other standards may vary especially with radiated emissions).http://www.fordemc.com/docs/requirements.htmWith best regardsAndreas

author: prasimix
author_rank: Unknown
timestamp: October 14, 2015, 07:39:02 am
content: I can recommend both Simpleswitcher LM(2)5574 (up to 500mA) and LM(2)5575 (up to 1.5A). They are pin compatible and can survive high input transient (up to 75V in case of 5574 and 5575). Tested design you can findhere(ignore upper part which is soft-start for main transformer). I'm also using LM5574 with continuous Vin from rectified 48VAC (~68V in peak) now for many months.TI also has handful on-line Webench calculator/simulator and simple spreadsheet calculator.

author: Scrts
author_rank: Frequent Contributor
timestamp: October 14, 2015, 03:04:37 pm
content: We mostly use DC/DCs for automotive apps. E.g. sometimes we need PMICs for FPGA or DSP designs, but sometimes it's just TPS54140 to get into 5V and then bunch of other converters with low input for the lower rails.You also have to consider cold start, where the input voltage might drop to as low as 2.7V, but CAN bus of 5V still have to operate. We got this requirement from one OEM. We also had 3.6V as cold start before. In this case, either buck-boost or sepic is the way to go.For the jump start you basically have to withstand 32V continuous DC and have more buffer for the load dump.One critical point is not to go into AM band. Your switching frequency has to be really low or above 2MHz. I usually select something like 2.2MHz if possible.Do proper filters with inductors and caps on the input, so no noise will come in and no current noise will go out.

author: ONETEN
author_rank: Contributor
timestamp: October 15, 2015, 02:12:05 pm
content: All great info, thank you very much for taking the time to reply.I will have a good read through the fordemc site, did not know such resources existed online.I am almost certainly going to go with a switchmode unit, I have a few things drawn up on paper but will design in eagle eventually and post up here for those interested when I get around to it. I have reverse engineered some of the circuit in some automotive electronics I have here and quite a few use variants of the MC34063 from various manufacturers. Its a good place to start I think. LT had some really cool switchmode regs but they are very expensive...Cheers!

author: Scrts
author_rank: Frequent Contributor
timestamp: October 15, 2015, 03:31:31 pm
content: Quote from: ONETEN on October 15, 2015, 02:12:05 pmI'd say go for TI, they're not expensive and work fine.

author: batteksystem
author_rank: Regular Contributor
timestamp: January 20, 2017, 03:56:20 am
content: MC34063 is a bit tough for a new beginner with no prior experience in switch mode power supply. Agree that simple switcher is properly easier with fixed switching frequency, built in MOSFET and most of the parameter already fixed.