Any ideas for DCpower supplly ORing and power source reverse polarity protection - Page 1
Forum section: electronics / beginners

author: osvaldo
author_rank: Newbie
timestamp: August 21, 2013, 05:06:22 pm
content: Hi all,For a current project of mine, I have to switch between two voltage sources (wall power supply or battery pack). When an external power source is present it should be used to power the device (with higher than the battery priority). There can be one case, where the wall adapter voltage is 9V and the battery voltage is 12V, but also vice versa. My goal is a device powered from as many power sources as possible (laptop power adapters, etc., for the external power, and different battery power pack configurations). It will be good if there is some protection against connecting the external power source (or the battery) with reverse to the device circuity polarity.The wall adapter or the battery should power a step-up regulator, where the peak power consumption of the device is around 2W (not all the time, usually the consumption should be less than 1W).First I tough about using a bridge rectifier on both power source terminals (that would give to my device a protection against reverse polarity, and if it is present a protection to the other power source as well). Unfortunately there is voltage drop across th? diode bridge rectifiers, which I want to avoid for the cases, when the device is running only on battery. Of course I could use Schottky diodes for the diode bridge rectifier, but in that case there is still some voltage drop and there will be small current leakage as well. Also with that simple implementation I cannot guarantee, that with higher priority will be used the external power source and not the battery power pack (attached on the battery terminals).Please, can anyone suggest a solution, that can cover my requirements.Thanks in advance for your ideas and thoughts on that problem!

author: TerminalJack505
author_rank: Super Contributor
timestamp: August 21, 2013, 05:44:41 pm
content: They make ICs that will do what you're looking for. They're called OR-ing controllers.You'd have to fish through all the specs to find one that meets your needs. Based on the banner specs, theLinear Technology 4417might work.

author: osvaldo
author_rank: Newbie
timestamp: August 23, 2013, 02:33:08 pm
content: Thank you very much for the suggestion.Yeah in my case LTC4417 definitely will fit for this task, it has all the bells and whistles (all kind of protections, a high input voltage range, power path priority selection, etc.). Unfortunately in my country I could get that chip for a relatively high price ~11$ (the combined price of almost all my device components is relatively the same).However I come to a simpler device by linear - LTC4412. There was an example in its datasheet (http://cds.linear.com/docs/en/datasheet/4412fa.pdf) that almost covers my needs (On page 10 underIdeal Diode Control with a Microcontroller- see the attachedFig 4.pngwith snapshot of the schematic). There is a suggestion to control the power source selection via MCU on the CTL pin of LTC4412.Here what is written in the data sheet:QuoteAs I only need a priority set on the external power source, I decided to provide a high voltage to that CTL pin, when external source is present via strong pull-up/weak pull-down resistor configuration and diodes for reverse polarity protection of that pin. I also added LED to light up when the battery is connected in reverse polarity.Please see the attachedPower Selector.pngfor a schematic with my current idea.However I'm not completely sure if that is the correct solution, I have certain concerns if there will be some issues like battery drainage by that UC, when the device is powered off. Also I have no idea how to implement the over-voltage and over-current protection of the external power source connection and it's positive and negative terminals.I'll be glad on any opinion. I'm open to other schematic suggestions and UC's as well (If they are relatively cheap and does not involve a plethora of hard to find components).Thanks in advance for your response

author: TerminalJack505
author_rank: Super Contributor
timestamp: August 23, 2013, 03:22:47 pm
content: I've used the LTC4412 before and it works fine. I use it to switch between a plug pack and battery backup.Your schematic should work, however, I'm pretty sure it has the requirement that it powers up from the primary. Only then will it provide the switching. This has to do with the fact that the IC actually has two power pins: Vin and Sense. However, the Sense pin in this case won't see power at power-up because of the back-to-back MOSFETs on the auxiliary input.

author: osvaldo
author_rank: Newbie
timestamp: August 23, 2013, 06:29:21 pm
content: I see... OK, so in that case I can compromise with using a diode on the aux power supply line and the resulting voltage drop there.In that case I can even reduce the P-Mosfets count to 1.I reworked my schematic using that idea (see the attachedPower Selector v3.png).Am I right by saying, that the circuity will be protected against reverse polarity accidental connections on both power sources?For the over current protection I'm considering to put some kind of fuse before the load. I'll welcome other suggestions as well.Also I still don't know, what to put for over voltage protection (something that will strip any voltage above 20V).EDIT: Updated the schematic

author: marshallh
author_rank: Unknown
timestamp: August 23, 2013, 06:42:14 pm
content: On one design where I had to switch between two 5v sources I used a low Vf schottky diode on each DC input. The drop was 220mV so the effect was 4.8V ORd. Since this was regulated down further it was not a problem.

author: TerminalJack505
author_rank: Super Contributor
timestamp: August 23, 2013, 07:01:08 pm
content: Quote from: osvaldo on August 23, 2013, 06:29:21 pmThe problem that I see with that approach is that if you turn on the MOSFET (to enable the primary) and the primary voltage is less than the auxiliary voltage then you'll be driving current into the primary from the secondary. And to boot, the auxiliary will still be powering Vcc since it is the higher of the two voltages.I was trying to think of how to support power-up from either source earlier and it made my head hurt. There's probably a way to do it but I can't think of it. You might need two sets of MOSFETs. One set for the LTC4412's power (Vin and Sense pins) which would be hooked-up with the body diode shown as in your latest schematic and another set of MOSFETs for Vcc, which would be arranged back-to-back, as in your original schematic.A fuse is a good idea for over current protection. If you go that route then you can use a crowbar circuit for over voltage protection. It protects the load by blowing the fuse when it detects an over voltage condition.

author: MasterOfNone
author_rank: Regular Contributor
timestamp: August 23, 2013, 09:37:33 pm
content: Sorry but it’s back to the 20th century again.Why not try something like using a relay to switch the input when the external power is connected. When the external power isn’t connect the relay uses virtually no power. But unfortunately there is a single diode in the power path when the battery is being used. Actually a Schottky should be used for battery polarity protection diode (D3). The 7085 means the circuit should work on a wide range of voltages.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 24, 2013, 01:06:39 am
content: Hi group,This is a message for the original poster.In this schematic:There is a small problem, the body diode in the MOSFET.Consider the situation where the battery voltage is higher than the wall adapter. The battery will be used to power the load because the body diode in the MOSFET is forward biased. This the reason why in some of the LT4412 and LT4417 circuits you see two MOSFET in series to prevent the body diode from conducting.You really need to assign priority to the wall adapter independent of the voltages.Jay_Diddy_B

author: osvaldo
author_rank: Newbie
timestamp: August 25, 2013, 10:04:37 am
content: Quote from: Jay_Diddy_B on August 24, 2013, 01:06:39 amThank youJay_Diddy_Bfor pointing out that. Then I'll definitely have to use two MOSFETs there.There is also an useful functionality of the CTL (Pin 3). Here what is written in the LTC4412's data sheet:QuoteI reworked my schematic according to these ideas (please see the attachedPower Selector v4.png)Quote from: MasterOfNone on August 23, 2013, 09:37:33 pmThank you for the idea,MasterOfNone. Yeah this seems far more simple. The only concern that I have is the linear voltage regulator 7805 in front of the relay (probably because its coil is rated for 5V). My AUX power supply probably is going to vary in this range: 10V-19V. This means that even for a 30ma coil current the 7805 will dissipate significant amount of power. I was thinking about a 12V rated relay (The one I have in it's specs is given that the coil will withstand 30V - however I'm not sure for how long, and the coil release is around 8V). I think with such relay, I can skip the linear regulator and the additional power losses. What do you think ? I also use a bridge rectifier on the AUX input so I don't have to bother with the reverse polarity issues from the AUX input (also if there is a primary battery present - which will be connected with the AUX trough the common ground, an external positive voltage to its negative terminal could be bad I think). I also placed a P-MOSFET instead of a Schottky diode for the reverse polarity protection.Please see the attachedRelay Power Path Selector.png) for the schematic.

author: MasterOfNone
author_rank: Regular Contributor
timestamp: August 25, 2013, 12:02:16 pm
content: Quote from: osvaldo on August 25, 2013, 10:04:37 amSeems like some great modifications to the relay idea. Now if you could only try the relay with a variable PSU to ensure it will operate over the range you require before you commit to building the circuit. If the relay doesn’t switch at the lowest voltage you could try going back to a single Diode on Aux to see if it helps.I’m not sure if I’ll be able to post again for a few days, but I’m sure others here will offer suggestions as usual.

author: TerminalJack505
author_rank: Super Contributor
timestamp: August 25, 2013, 09:26:18 pm
content: Here's something I've been playing around with. It uses just a quad comparator, a voltage reference and a handful of passive components.It uses OR-ing diodes to power the comparators and a 2V5 voltage reference. The comparators on the left act as a window comparator and switch the supplies based on whether the primary is between 5V and 10V (these are arbitrary values.) There's some hysteresis controlled by R4.The comparators on the right switch the MOSFETs on or off as appropriate. Open collector comparators were used for the simulation but comparators with push/pull outputs would be better since they would turn off the MOSFETs faster. (You'll notice the 'lazy' switch-over in the output.)There's likely to be some problems with it. I didn't test what happens when the secondary supply varies, for example. In the end, I'd leave the task to a dedicated IC. I just thought it would be an interesting challenge.

author: TerminalJack505
author_rank: Super Contributor
timestamp: August 25, 2013, 09:50:07 pm
content: Here's the circuit using a comparator with a push-pull output. You'll notice the switch-over is much cleaner. The problem with comparators with push-pull outputs is that they don't seem to come in versions that work at 36V, like the open collector variety.