Power Supply circuit design. Need a check over / advice please

eagletalontim · 2013-05-19 16:54

I am working on designing a "plug and play" power supply for a project and would like to have someone inspect my design and and see if there is anything that I should change or if everything looks good.

Here is the parts list from DigiKey with each component labeled as on the board :

SR1-5 : http://www.digikey.com/scripts/DKSearch/dksus.dll?Detail&itemSeq=130650588
C1 : http://www.digikey.com/scripts/DKSearch/dksus.dll?Detail&itemSeq=130650589
C2 : http://www.digikey.com/scripts/DKSearch/dksus.dll?Detail&itemSeq=130650590
LR1-3.3 : http://www.digikey.com/scripts/DKSearch/dksus.dll?Detail&itemSeq=130650592
C3 : http://www.digikey.com/scripts/DKSearch/dksus.dll?Detail&itemSeq=130650593
C4 : http://www.digikey.com/scripts/DKSearch/dksus.dll?Detail&itemSeq=130650594

The power supply will be used in an automotive environment with a few ADC chips, and other "sensitive" components along with the Prop. I need the power supply to be small yet effective. In this design, I am hoping to connect it directly to the vehicle's ignition power and run a few extra things on top of the Prop's normal circuit. Expected current is roughly 300ma on the 3.3V side and up to 800ma on the 5V side. I need to keep heat down as this is in a sealed enclosure. Any input would be great!

tonyp12 · 2013-05-19 18:53

In automotive recommend a power inductor and then a 16v zener.
And large copper pour for heat transfer, use via's to connect heat.to both sides.

eagletalontim · 2013-05-19 19:15

Would the power inductor just be on the 5V side before the 3.3 regulator? I have never used an inductor before so I am not quite sure what I would need or where it would be used. Thanks for the tip on the via's for heat transfer. I am hoping the circuit will not product much heat since another circuit I am using is a simple 7805 that produces very minimal or no heat.

tonyp12 · 2013-05-19 20:15

I through this together.
The inductor will filter power spikes (a high uH with a 1 ohm is a good value)
The zener will redirect power above 16v to ground, the 1ohm from inductor will help to not overload it.
Get the both in 1Amp ratings.

eagletalontim · 2013-05-19 20:42

I will see what I can find for the inductor on Digikey. The height in the box is limited so I need to be careful on what I put on this board since it stands off from the main board. I noticed you added a cap in your schematic. Is there any specific spec that I should use for the C1 in your design?

EDIT : Also... what should I look for in a zener diode other than the voltage of 16v? I see on digikey there are many results with several specs.

eagletalontim · 2013-05-19 20:53

Also searching on Digikey for an inductor and am kind of lost on what I am looking at. Here is the search I have currently :

http://www.digikey.com/product-search/en?pv314=3205&FV=fff40003%2Cfff80013&k=inductor&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=100

frank freedman · 2013-05-19 21:25

You may also want to relieve the connections to components soldered into the ground plane. Will help with making good solder joints on construction and you won't need a blow torch wattage soldering iron if you need to do any repair / rework on the boards.

FF

tonyp12 · 2013-05-19 21:26

Inductor:
http://www.mouser.com/Search/ProductDetail.aspx?R=BRL3225T6R8Mvirtualkey57660000virtualkey963-BRL3225T6R8M
Or a little beefier one
http://www.mouser.com/ProductDetail/Bourns/SRR1210-561M/?qs=sGAEpiMZZMsg%252by3WlYCkUxKwhsadNScO0TvL0TIBf24%3d

Zener: 16v 1Amp any should do, but this one looks good:
http://www.mouser.com/ProductDetail/ON-Semiconductor/P6SMB16AT3G/?qs=sGAEpiMZZMuNo3spt1BaV6ZZhAoz%2fLLcD7ileiny2wE%3d

LoopyByteloose · 2013-05-20 07:51

I can't seem to identify the 3.3v regulator chip, but not all provide enhancements for automotive use. The Digikey link seems broken.

If you are going to this much trouble, you should pick and IC that has particularly improvements for automotive use. These include reverse voltage protection, low voltage dropout, and higher spike protection.

The original linear regulators, such as the LM7805 didn't have these features and suffered for it. The newer chips seem to do very well even when installed without additional protection. But it wouldn't hurt to go both with the external protection and a chip that has internal enhancements for an automotive environment.

An LM2940ct is a good choice for +5 Volt, take a look at the added features

http://www.futurlec.com/Linear/LM2940CT-5.shtml

Sorry, but I am not sure I have a good choice for 3.3V which is specifically enhanced for automotive

Duane C. Johnson · 2013-05-20 07:53

Hi tonyp;

tonyp12 wrote: »

I through this together.
The inductor will filter power spikes (a high uH with a 1 ohm is a good value)
The zener will redirect power above 16v to ground, the 1ohm from inductor will help to not overload it.
Get the both in 1Amp ratings.

16V is not high enough voltage in an automotive application.
The alternator's voltage regulator often can go to 18V or greater in very cold weather.
It is temperature compensated.
I would suggest something more like 20V.

Duane J

LoopyByteloose · 2013-05-20 08:12

With regular transistors, automotive applications often select power transistors in the 60 to 90 max. voltage range, just because inductive and switching spikes can hit that high. (Think about the coil on the starter solenoid) So I suspect coils and capacitors need to consider similar high end voltages to live long and prosper.

The 12 volt system is a very deceptive rating. Charging is regularly in excess of 14 volts. And Duane J has mentioned something that is new information to me, cold weather alternator output. The lead acid battery suffers greatly in cold weather, so it certainly does make sense that the alternator regulator would compensate with an even more aggressive output to recover charge.

So I would guess that trapping spikes above 20 volts and providing components that will survive up to 90 volts is optimal.

http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXxyxt.pdf

eagletalontim · 2013-05-20 18:33

Thanks for the help so far! Just to clarify things, I am using this ( http://www.digikey.com/scripts/DKSearch/dksus.dll?keywords=811-2196-5-ND ) switching regulator instead of a linear regulator due to power dissipation and heat. If I want to add on to my circuit at a later time, the power supply circuit will be able to handle it without changing anything. The 7805 is just not quite strong enough to handle my current circuit and builds up quite a bit of heat after 10 minutes of running. Lowering the voltage with an adjustable linear regulator seems to help keep the heat down a little, but it is too complex of a circuit for a power supply that will fit into my already chosen enclosure. Switching regulators don't have as much of a heat issue and the one I have chosen has already been recommended to me in another post quite a few months ago. The input range is great enough to handle any automotive voltage, but I want to protect the regulator circuit and still be able to have a clean output. Since switching regulators have more of a choppy signal coming from them, I need to clean up the signal for use on 5V ADC's and also use it for the input of the 3.3V regulator which will run the Prop and EEprom and maybe a few other things like an LED display.

I don't think I listed the diode in the part sheet, but I do plan on running a zener to protect the switching regulator. If I can protect the switching regulator, then filter the noise from it, I think the circuit will be perfect for my application. Is there a need for a second diode as listed in the above schematic? Would the zener not catch reverse voltage and pop the fuse the power supply is running from? The fuse is normally a 10A fuse that this device connects to so having just a 1A zener may not be enough and a second diode would be better....(answering my own questions). My biggest concern is catching "flyback" voltage from relays or solenoids that may be tied in with the wiring of my device. This has been an issue in the past where a relay would click off and my entire circuit would reset.

eagletalontim · 2013-05-20 21:41

Upon doing more research, I have found that most linear regulators for automotive use are rated to "Up to 16V" which to me sets the zener diode voltage to 18V for my 5V regulator. With more research, I have also decided not to use the switching regulator I was currently designing for and have decided to go with an automotive linear regulator. What I am hoping for is a cleaner 5V power rail to use with ADC chips and a switching regulator requires too much additional circuitry to properly filter the noise or they are just too expensive.

The linear regulator I am hoping to now use is the LM2940 found here : http://www.digikey.com/product-detail/en/LM2940SX-5.0/LM2940SX-5.0CT-ND/3701580 According to the datasheet, it can handle up to 26V, but 18V I still think should be the shut off point which is where the zener diode will come into play.

Now, I am still confused on the inductor specs. I am not sure what amperage rating, what micro Henry to use, nor what self resonant frequency I need. Most of the information I have found online about inductors is rocket science to me so I would like a little help with this part please

LoopyByteloose · 2013-05-21 02:52

Well, if you alternator regulator puts out a steady 18 or more volts, your 18 volt zener is going to burn up. It is better to have a 20 volt zener that isn't exposed to anything except noise and spikes.

The 26 volt maximum is okay as the LM2940c provides an array of automotive protective features specifically intended to survive the noise and spikes that might get through.

But it is a 5VDC output, not a 3.3VDC output. I am wondering if you are going to have a second regulator step-down to 3.3VDC. That one wouldn't need as much protection from the automotive noise and spikes, as the 5VDC would have blocked all that.

Try a LM1117 for 3.3v

http://www.futurlec.com/Linear/LM1117DTX-3.shtml

Here is a 5 amp 3.3v linear you might have missed, a LM1084IT-3

http://www.futurlec.com/Linear/LM1084IT-3.shtml

eagletalontim · 2013-05-21 04:23

I am also using a 3.3 regulator as well, but I am not so much worried about it since most of the automotive noise would have been filtered before entering the 3.3V regulator. If I can get the right inductor for my application, I think I would be heading in the right direction. I still don't quite understand what the specs are on an inductor like I posted in the previous post.

eagletalontim · 2013-05-21 18:09

How can I calculate the wattage needed for the zener diode? I also noticed the TVS diode posted above is unidirectional. Not fully understanding how that can help with flyback voltages if it is unidirectional. Should I go with a TVS diode or a Zener? I am searching for 20V rating, but that is about as far as I get.... there are thousands to chose from...just not sure how to calculate which one. I would prefer to stick with using DigiKey as the supplier right now since everything else in my current order is through them.

tonyp12 · 2013-05-21 18:37

With so many to choose from, I always start with: in stock and smt, 'new product' always get first dip.
It takes time to learn how to be a good ‘sourcer’, I like Mouser better when it comes to zero in.

These should be OK for your project.
http://www.digikey.com/product-detail/en/BRL3225T6R8M/587-3032-1-ND/2763403
http://www.digikey.com/product-detail/en/SMAJ18A-E3%2F61/SMAJ18A-E3%2F61GICT-ND/1091631

eagletalontim · 2013-05-21 19:21

Ok, thanks for the help on that.... I have been stressing over these specs for hours

Since I don't have absolute current and voltage readings, I can't plug in numbers into these online calculators which most don't make any sense to me anyways. Still learning the whole math part of electronics and how every spec plays a part in determining what is needed.

Will an additional diode be needed when using the TVS diode you listed? From further research on the LM2940, I have read that it can shutdown in the result of a reverse voltage which would be the whole "flyback" issue.

When reading more on inductors, I have read that they can help with filtering some voltage spikes and helps regulate the voltage a bit better. If I am not mistaken, "flyback voltage" is usually reversed from the input polarity, meaning the flyback voltage would be positive on the negative input. Would this not create an unusable input and cause the regulator to switch off for a split second? Would I need to add an electrolytic cap after the TVS diode and inductor to help prevent my circuit from restarting in the event of flyback voltage?

tonyp12 · 2013-05-21 20:09

>Will an additional diode be needed when using the TVS diode

You should use regular rectifying diode too, as your board may not handle accidental reversed polarity and any electrolytic capacitors definitely can not.

eagletalontim · 2013-05-21 20:11

Ok, thanks, I will look for a rectifying diode and implement that into the new design. Once I have it finished, I will repost it for review.

eagletalontim · 2013-05-21 22:11

Here is the updated design using the LM2940 5V regulator. I will get the parts list posted probably tomorrow. It's 12AM and I need to get to bed. Please let me know if there is anything visibly wrong with the design.

eagletalontim · 2013-05-22 16:19

Here is the current parts list for the above schematic. Please critique this and see if there is something I should change or add. My goal is to have as LITTLE heat as possible, protection from possible flyback voltage, and a decent clean signal to use with 5V ADC chips, then last but not least, a good power source to run the Prop and eeprom on along with an LED display.

DigiKey Parts :
LR5V - LM2940SX-5.0CT-ND	IC REG LDO 5V 1A DDPAK	5V REGULATOR
C2     - 587-1242-1-ND	CAP CER 1UF 10V 10% X7R 0603	FILTER CAP 1
C5     - PCE4384CT-ND	CAP ALUM 22UF 10V 20% SMD	FILTER CAP 2
LR3.3 - MCP1825S-3302E/EB-ND	IC REG LDO 3.3V 0.5A 3DDPAK	3.3V REGULATOR
C3     - 1276-1024-1-ND	CAP CER 1UF 6.3V 10% X7R 0603	FILTER CAP 3
C4     - PCE4166CT-ND	CAP ALUM 47UF 6.3V 20% SMD	FILTER CAP 4
HEADER - 609-3319-ND	CONN HEADER 4POS .100 R/A 15AU	BOARD PINS
TVS   - SMAJ18A-E3/61GICT-ND	TVS UNIDIRECT 400W 18V 5% SMA	TVS DIODE
C1     - PCE3951CT-ND	CAP ALUM 100UF 35V 20% SMD	INPUT BUFFER CAP	1
IND1  - 587-3032-1-ND	INDUCTOR 6.8UH 20% 1210 SMD	INDUCTOR
D1     - 641-1311-1-ND	DIODE GEN PURPOSE 400V 1A DO41	RECTIFIER DIODE

eagletalontim · 2013-05-24 04:19

I guess everything is good to go?

Power Supply circuit design. Need a check over / advice please

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