5V and 3.3V Automotive Power Supply with Battery Backup - Page 1
Forum section: electronics / projects

author: Izzni
author_rank: Newbie
timestamp: May 30, 2014, 02:55:58 am
content: So I'm working on a power supply that runs off automotive power, which means the voltage can spike to 150V, but usually hangs around 14.1V. If the automotive power is available, I want to use it, if not, I want to use a battery backup that uses 4 AA batteries.The automotive input (through the barrel jack), needs overvoltage, spike voltage, and reverse polarity protection. The automotive input doesn't need to be efficient.The battery backup needs only reverse polarity protection, but needs to be as efficient as possible.The circuit uses a polyswitch fuse with zener diode for the reverse polarity and over voltage protection, and a TVS diode for spike voltage protection. The battery backup uses one P channel mosfet for a low voltage drop reverse polarity protection.The second P Channel mosfet turns the battery backup on when the automotive supply is disconnected.I have that 1000 ohm resistor in there to pull the gate voltage of the second p channel mosfet to zero when the supply is disconnected, but it has the problem of always having power flow through the circuit, even when it is off.I'm looking for feedback on whether this is a workable design, and ways to get rid of the resistor so that when the switch is turned off the circuit isn't drawing so much current when the automotive supply is still active.

author: Clear as mud
author_rank: Regular Contributor
timestamp: May 30, 2014, 04:05:10 am
content: First time using Kicad? You don't have to put junctions every place a wire attaches to a component. I suppose you can if you want to, but it takes extra time to do, and it looks funny to me that way. You really only need the junctions where a wire attaches to another wire, and the program usually puts them in for you.The circuit has some errors. I'm not going to try to identify all of them, because I am pretty tired, but one of the most obvious to me is that the 7805 can have voltage at its output when there is no voltage at its input. So, power can back-feed through it. Unless the data sheet for the regulator you're using specifically says it can be used this way, you should try not to do so. Some regulators will be damaged by that. Linear regulators that specifically block reverse current are few and far between. I think many of Linear Technology's regulators do, and some ON Semiconductor products do. If you want to use a cheaper regulator that doesn't have that feature, you have to figure out a way not to apply a higher voltage to the output than to the input. Even the 100 uF capacitor will potentially hold enough charge to damage some regulators when power is removed from the input. I know I have seen data sheets that recommend 10 uF maximum on the output for this reason.What is the purpose of Q1? It looks like it does nothing, with the gate tied to ground, but I'm thinking maybe it is for reverse voltage protection? I haven't see one used that way before, and I'm too tired to think all the way through that and figure out if it will work.By addressing your problem of possible back-feed of voltage through the regulator, you can also address the problem of power losses in the resistor that you were concerned about. If you could be sure that no current could come backwards through that 7805, then a much larger value resistor could be used. 100k or larger would still make a strong enough pull-down for a MOSFET gate.

author: Clear as mud
author_rank: Regular Contributor
timestamp: May 30, 2014, 04:23:28 am
content: I just realized something: since you are not using the 5V for anything except to feed a 3.3V regulator, you can put a diode after the 5V regulator to block reverse current into the output. You'll have some voltage drop from the diode, but it will still have enough voltage to run the 3.3V regulator if you get a low dropout one.

author: rdl
author_rank: Super Contributor
timestamp: May 30, 2014, 04:33:36 am
content: There are 3 terminal regulators made specifically for automotive use. You might look at one of those instead of the 7805. The only one I can think of right off the bat is the LM2940, but I'm sure there are others.Quote

author: Izzni
author_rank: Newbie
timestamp: May 30, 2014, 12:40:49 pm
content: First of all, thank you both! I appreciate the fast feedback and good suggestions.I'm not completely new to Kicad, but I ran into some issue the first time I was using it where I thought adding junctions at the components fixed some ERC errors. It is good to know they are not needed. I eventually will clean up the schematic so that it isn't cluttered by those.The most power hungry module in the system can run off of 3.3V to 6V, so I have that one running directly off the batteries to reduce power consumption. The 3.3V LDO would waste more than I want.I replaced the LDO with the LM2940, dumped the zener diode since it isn't required anymore, and increased the resistor to 47k so that it draws only about 20uA. I can bump it up to 100k if you really think that I don't need that strong of pull down.Oh, and Q1 is there for reverse polarity protection on the batteries.Let me know if you can see any other issues, and thanks again!

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 30, 2014, 01:23:48 pm
content: You'll want to add a resistor and zener to protect Q7 from voltage spikes. For reverse protecting the backup battery connection, it would probably be more economical to use a fuse and inverse diode.

author: Izzni
author_rank: Newbie
timestamp: May 30, 2014, 06:13:09 pm
content: Thanks again for the feedback. I've added a zener and a resistor to protect the gate of the mosfet, hopefully it is what you were thinking of.I also converted the reverse protection of the battery input with the fuse and diode like you suggested. I'll be using a polyswitch there since I don't want users to have to change a fuse (I'm sure they will call before they realize it was simply a fuse, and not mention they installed the batteries backwards either...).Let me know if anyone can think of any other issues.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 30, 2014, 09:27:31 pm
content: That looks pretty good. It might be a good idea to add a diode on the output of the regulator (with another diode in series with the ground pin of the regulator to compensate for voltage drop) if the regulator needs backflow protection or at least give you the option of using another regulator that does. Also, if there's enough spare space on the PCB, you might want to add a space for a diode from the gate of the MOSFET to the drain to have the option of trickle charging the battery. (Aim for at most a 1/20C rate, 1/100C is more reasonable if the batteries are only rarely used.)

author: DutchGert
author_rank: Frequent Contributor
timestamp: June 12, 2014, 12:20:28 pm
content: If u really want to make a decent Automotive proof supply us something like a LT4363 surge stopper. They can do loaddump protection, UV, OV, reverse polarity, current limiting and all kinds of weird and wonderful things.Have a look at:http://www.linear.com/product/LT4363and/orhttp://www.linear.com/solutions/4385I used something like this to make a fully protected and stable 8-30V rail to power a complex FPGA based automotive system from. Works a threat.

author: Niklas
author_rank: Frequent Contributor
timestamp: June 12, 2014, 06:34:16 pm
content: What are the specs on the TVS diode at the input? The pulse power rating must be sufficient and the selection of breakdown and clamping voltages are also important. There can be a large difference between the breakdown voltage, typically measured at 1 mA, and the actual clamping voltage. Also be careful about if the TVS diode is bidirectional or unidirectional. If it is bidirectional, the rest of the circuit must be able to handle the complete input voltage range of -Vclampin -> + Vclamping. A wrong polarity diode, with a reverse voltage rating above the Vclamp of the TVS, would solve it. The schematic symbol is bidirectional, but check before you order the components.I would suggest that you add some inductive filtering between the input and the first regulator. A common mode filter and some more capacitance, both an electrolytic buffer and ceramic noise filter, would be good. 1 uF is not very efficient at filtering noise, somewhere around 1-10 nF would be better.Perhaps also think about something "lossy" inductive, like Murata's BLM-series, between the switch and the 3V regulator. Add more capacitance, both electrolytic buffer and ceramic noise filter, directly to the input of the 3 V regulator.