Finding the perfect 3.3V power supply for automotive project  - Page 1
Forum section: electronics / projects

author: tru3533
author_rank: Regular Contributor
timestamp: August 02, 2018, 08:23:42 pm
content: I need to put together a 3.3V power supply with low-EMIPowered by automotive Power 13.8VSoC needs 200mA @ 3.3VSOT223?Linear regulatorFor a start I have been looking at AZ1117Ihttps://www.diodes.com/assets/Datasheets/AZ1117.pdfLots of heat to get rid off here!How do you deal with it?

author: iMo
author_rank: Super Contributor
timestamp: August 02, 2018, 10:03:28 pm
content: With 2.1W and sot223 50C/W thermal resistance (with 5cm x 5cm copper under the package) the temperature of the package will be 2.1x50 = 100C above the ambient. With automotive say 100C ambient you are somewhere at 200C.1. TO220 and good heatsink (thermal resistance say 5-10)2. active cooling3. switcher.

author: viperidae
author_rank: Frequent Contributor
timestamp: August 03, 2018, 02:51:02 am
content: AZ1117 is only rated for an absolute max 20V input. I wouldn't connect it directly to an automotive power system. The recommended input voltage is 15V.You can expect high 14's for the charging voltage of a car battery. A 5V spike could kill your regulator. Cars are full of motors and solenoids turning on and off all the time.

author: carljrb
author_rank: Contributor
timestamp: August 03, 2018, 07:20:20 am
content: You'll need more than that for a "proper" automotive regulator. Even if you chose not to support operation while cold cranking, being protected against load dump up to around 125V (not just inductive spikes and ESD which is already quite significant) and jump starts is a necessity, along with reverse polarity protection. Of course, thermal management is also a thing. Typical automotive ambient can reach 85C (185F).Low EMI (both conducted and radiated) is totally doable with a standard buck regulator (switcher). Careful design and layout is necessary of course.When it comes to automotive power supplies and protections, there's no shortage of relevant app notes and reference designs (plus the obvious SAE standards, UNECE Regulation 10 for CE EMI/EMC regulations, etc)You'll often find TVS diodes and/or varistors (along with fuses and/or flameproof resistors) for different over-voltage conditions, inductors and/or common mode chokes and capacitors acting as a filter for conducted emissions (keeping in mind the switching frequency), a diode or an ideal diode controller (MOSFET replacing the diode) for reverse polarity protection, and then wide-Vin bucks to create the voltages you need (again: layout matters a lot)

author: Siwastaja
author_rank: Super Contributor
timestamp: August 03, 2018, 07:50:55 am
content: Using a linear regulator here sounds totally nuts to me. Hard to see any reason why not a switcher.That SoC is going to generate all the ripple and EMI. The switcher is OK. Modern switchers run at very high frequencies, integrate most of the components, and allow using tiny, shielded inductors and almost ESR-less ceramic caps, in a very tight area. Easy to design for low EMI. These 5-component solutions with the datasheet showing the recommended layout in 10x10mm area are hard to do wrong.Using heatsinks is always a complication. Also, dedicating a lot of PCB area to heatsinking copper fills is unproductive, if it can be avoided. Hence, even when the power consumption wouldn't matter, I tend to choose a switcher when the linear P dissipation exceeds around 1W.Use a parametric search with your inputs and sort by price. A lot of options.Despite the forums telling you otherwise by multitude of posters, time after time, you don't necessarilyneedto protect against load dump. It's your choice.Load dumps are about ten orders of magnitude more rare than the frequency of forum posts describing their commonness. Traditionally, there is no protection against them. For example, traditionally a bad load dump kills all the lightbulbs in the car. Yet it's highly unlikely you have ever even heard of anyone experiencing this.Protection againt load dumps became a trendy appnote thing in late 1990's, early 2000's.Of course, load dump immunity is paramount in life-critical systems, or systems where replacements or downtime is very expensive. It makes a lot of sense to protect car's own expensive electronic systems, especially since the integrated protection is not expensive.Even then, it's one of the rare risks, and if you end up thinking it's the only one in its class, you're going to fail elsewhere. Otherwise - you can treat it as an extremely rare occurence that an average Joe will never encounter, but which will kill about half of the external electronics connected anyway; your device doesn't necessarily need to be "better" than all the others, unless you want of course. Depends on what it is. While you are at it, consider protection against moisture, lightnings and theft as well.

author: 2N3055
author_rank: Super Contributor
timestamp: August 03, 2018, 08:58:33 am
content: I agree with Siwastaja on switcher. Look around, there are are even ready made switcher modules that look like 7805. For 200 mA it is not hard to find something already made. I'm not talking about no name E-bay little boards.As for load dump, I disagree with all or nothing approach. I'we seen elaborate over voltage protection designed to keep everything running normal during transients. Or people decide the do nothing and occasionally have burned boards. I would go with an over voltage protection that will disconnect or burn a fuse if one of those very rare incidents occur. Just to minimize damage. It doesn't have to seamlessly work trough transient.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 03, 2018, 09:37:55 am
content: No all-or-nothing attitude here. My point was actually just the opposite.Not saying you "should" design your thing to blow up on load dump. Just saying it's a viable option. A lot more viable than people may think! Not all things are irreplaceable; you can actually analyze the situation and reach the optimal goal for the purpose, which is case-by-case.For example, having a $2 commodity thing (like a phone charger) with expected usage time of 100 hours during 5 year lifetime, fail at a rate of one device per 10000 during its lifetime makes much more economical - and ecological! - sense than making it a bigger $3 thing with bigger environmental impact, which would still fail at maybe one device per 11000 due to other reasons.OTOH, it would be horribly poor design not to include load dump protection on a permanently installed high-tech automotive navigation, communication or scientific system costing $1000. In the extreme case, surviving the dump is not enough; it needs to stay operationalduringit. In this example, all the gazillion other reliability-relevant questions would be closely considered as well.In these discussions, people often give an impression that:A) load dump is very common,B) it's the #1 reliability issue in automotive environment, so all the others can be ignored for the first round of "optimization".Neither is true.Fuse is always required for fire protection, regardless of the style (or lack) of protection. Even the protection can fail, in which case fuse will protect from fire, if things short out. A 20A fuse somewhere upstream may not be enough to protect from fire at the small device.

author: tru3533
author_rank: Regular Contributor
timestamp: August 03, 2018, 10:16:10 am
content: OK switcher it will beYou can read about the SoC here:https://www.nordicsemi.com/eng/Products/Bluetooth-low-energy/nRF52832To clarify SoC will be made in 2 (3) IoT versions. (low cost)Version 1 who is coin cell operated, so no power problems here.Version 2 Cigarette lighter powered, I estimate 50mA total with the added IO, 1 input, 1 output, (BC817)I think my 200mA version should be on hold for now.So Siwastaja your suggestions is where I want to go.With this in mind and 50mA what would be your favorite chip?

author: 2N3055
author_rank: Super Contributor
timestamp: August 03, 2018, 10:18:09 am
content: Quote from: Siwastaja on August 03, 2018, 09:37:55 amAgree on all counts...

author: tru3533
author_rank: Regular Contributor
timestamp: August 03, 2018, 10:40:54 am
content: 2N3055Nice tip, where you thinking about something like this?https://www.cui.com/product/resource/vxo78-1000.pdfOr is it even better solutions out there?

author: Siwastaja
author_rank: Super Contributor
timestamp: August 03, 2018, 11:36:18 am
content: Quote from: tru3533 on August 03, 2018, 10:16:10 amFilters:In stock, normally stocking, fixed output at 3.3V, max input voltage at least 40V (to give some margin here; you'll find more options at lower max input voltage), switching frequency at least 500kHz (to get rid of ancient chips and large solutions requiring massive inductors), sort by price:https://www.digikey.no/products/en/integrated-circuits-ics/pmic-voltage-regulators-dc-dc-switching-regulators/739?k=&pkeyword=&pv384=6&pv384=38&pv384=22&pv384=94&FV=a4027c%2C8f40010%2C8f400a7%2C8f400aa%2C8f400db%2C8f40021%2C8f40034%2C8f40043%2C8f4005a%2C1bcc0032%2Cffe002e3&quantity=0&ColumnSort=1000011&page=1&stock=1&nstock=1&pageSize=25

author: tru3533
author_rank: Regular Contributor
timestamp: August 03, 2018, 12:18:30 pm
content: Thank you so much Siwastaja for sharing your strategy for search with comments.Learned something here: "switching frequency at least 500kHz (to get rid of ancient chips and large solutions requiring massive inductors)"Smart!and direct to my prefered distributorhttps://www.digikey.no/This for sure got me on the right track!Tom

author: JPortici
author_rank: Super Contributor
timestamp: August 03, 2018, 01:23:09 pm
content: I suggest you also look for system basis chips (SBC) from ST, NXP, Infineon, TI.If you open up a number of ECUs you'll find the aforementioned input filtering / TVS / reverse polarity protection, then a SBC.Otherwise, TI released a short while ago this nice switcher + LDO combo:http://www.ti.com/lit/ds/symlink/tps65320c-q1.pdf

author: iMo
author_rank: Super Contributor
timestamp: August 03, 2018, 01:39:15 pm
content: I've got a board here with a TPS82130 on it. The switcher's inductor and diode are integrated in the chip package. The package is 3x3mm. 3A from up to 17V input.

author: 2N3055
author_rank: Super Contributor
timestamp: August 03, 2018, 02:01:11 pm
content: Quote from: tru3533 on August 03, 2018, 10:40:54 amYes. There are similar one from ROHM etc... You don't even need to so small, you can use SIP modules too...

author: tru3533
author_rank: Regular Contributor
timestamp: August 03, 2018, 02:31:59 pm
content: JPortici, interesting. I will keep my eyes open next time I look around an ECUTPS65320C addedimo, TPS82130 Didn't know they existed in such a small size with built in inductorabsolutely interesting2N3055, will check out ROHM, thanks

author: tru3533
author_rank: Regular Contributor
timestamp: August 03, 2018, 03:53:43 pm
content: Very nice feature from TIhttp://www.ti.com/design-tools/overview.htmlBy using the free WEBENCH® Power Designer, specifying your parameters, selecting the chip,you get a complete design with all the bells and whistles. Download a complete PCB design package with BOM to Altium or your favorite designer

author: Siwastaja
author_rank: Super Contributor
timestamp: August 04, 2018, 07:40:29 am
content: TI Webench is quite nice, but limiting yourself to TI switchers is quite limiting - their parts are not very innovative, and not very integrated (with a few exceptions). It feels like TI's stuck in the 1990's.When I choose a TI part anyway, I often use the WEBENCH to sanity check my design.

author: iMo
author_rank: Super Contributor
timestamp: August 04, 2018, 08:52:55 am
content: Quote from: Siwastaja on August 04, 2018, 07:40:29 amCould you point us to some more innovative vendors, I've been looking for some switchers with integrated inductor and diode, variable outp voltage, >=3A output, at least 2MHz switching, <20uA quiescent, 3x3mm package max..

author: tru3533
author_rank: Regular Contributor
timestamp: August 04, 2018, 10:07:33 am
content: imoIs this useful?https://eu.mouser.com/applications/low-power-dc-converter/hmm... maybe to old

author: iMo
author_rank: Super Contributor
timestamp: August 04, 2018, 11:16:07 am
content: Those 82671 3x3mm are 4.5V input only. The TPS82130 fulfills all my needs at this stage

author: tru3533
author_rank: Regular Contributor
timestamp: August 04, 2018, 04:41:41 pm
content: imoNice! Looks like you have followed design spec. I will order some for test.Have you tried to hand solder one? If yes how did you do it?

author: Siwastaja
author_rank: Super Contributor
timestamp: August 04, 2018, 04:55:39 pm
content: By "more innovative", in this context, I mean someone who makes switch mode DC/DC supplies commonplace. They shouldn't be treated as rocket science anymore. Recently, I have ended up choosing Semtech, A&O semi parts among others...I look for:- Lack of overcomplexity- Lack of "traps"- Simple and readable datasheet with a practical example circuit to compare against- Low price- Low price of external components- Minimum area of both the chip itself and the external components- Level of integrationHand soldering is rarely an issue. You can do a lot by tinning the pads with an iron, then preheating the board from the underside, and using hot air to reflow the chip. No paste/stencil necessary.

author: tru3533
author_rank: Regular Contributor
timestamp: August 07, 2018, 10:02:13 pm
content: Thank you Siwastaja,For me the question is about using built in inductor or not.The price and space leans against built in.Unless I find an inductor serie who is reasonably priced. So far no luck on that.