Need Help Please!! Making Multi-Source Power Distribution Circuit - Page 1
Forum section: electronics / projects

author: jwhitten
author_rank: Newbie
timestamp: September 10, 2018, 06:04:12 pm
content: Hello,I have been working on a circuit for a mobile robotics application in which my goal is to have several available power sources. Sources 1 & 2 are onboard battery packs (w/I2C monitoring), source 3 is wall power, and source 4 is "any/whatever" power that might be available in a pinch. Forget source 4 for this discussion.To this end I have selected (and purchased several for testing) the following part: VN7003ALHTR: (http://datasheet.octopart.com/VN7003ALHTR-STMicroelectronics-datasheet-66373282.pdf). As I don't want to let the smoke out right away (we'll save that for later!), I was hoping that some kind person could help me in answering the following:1. My plan is to use this part to route one (each) of the power sources to a common power rail, according to a circuit that I've already worked out for that purpose and tested. It simply needs the final stage for power handling. My question / hesitation is that I mocked up the power part with MOSFETS one day to see how well it would work and promptly blew the top off my (25A!) MOSFET the moment it switched to a different source. In my benchtop test I was not using any diode steering (blocking) to protect against reverse voltage coming into the circuit from the other side of the MOSFET (and would prefer to not have to do it). I'm pretty sure that was why it blew up. So what I want to do is use this commercially-available part which was designed for power distribution / switching in automotive applications and use it instead. I've read through the datasheet multiple times and I cannot find any clear statement that this part would be able to withstand the reverse power input condition that blew the MOSFET, although my strong suspicion is that it can. I have written to the manufacture for clarification but it's been over a year now and haven't heard anything back. So I'm hoping someone here could help me.Can somebody help me answer this question please?Thanks!JWhitten

author: ajb
author_rank: Super Contributor
timestamp: September 10, 2018, 06:29:19 pm
content: If you look at the block diagram on page 5 of the datasheet, you'll see that the switch they're using is a single MOSFET, which, as MOSFETs do, has an integral diode from OUT to VCC. That means that if your common rail is at a higher potential than the power source connected to the switch, that diode will become forward biased and will conduct. Now, if the higher voltage supply is also connected to the rail via one of these switches, it should limit the amount of current, but best case this will cause the lower voltage supply to load down the higher voltage supply, and worse case, the lower voltage supply could be damaged. Also, the current limiting on devices such as you've linked is fairly coarse, so it may not provide adequate protection if you don't have enough headroom between your system's power consumption and the capacity of your various power supplies.If you want to prevent power flowing into *or* out of a given power supply you'll need to use a pair of MOSFETs back to back. This is commonly used in battery protection circuits as well as AC solid-state relays, as illustrated here:https://e2e.ti.com/blogs_/b/industrial_strength/archive/2016/07/26/a-modern-approach-to-solid-state-relay-designI'm not sure if there are integrated power switch ICs that incorporate this sort of switch, but you might look for parts that advertise reverse current protection. Alternatively, a power switch IC that is designed to be used with an external transistor would probably work as well.

author: station240
author_rank: Unknown
timestamp: September 10, 2018, 08:01:42 pm
content: I suspect the reverse battery protection on the VN7003ALHTR VCC input, would also prevent power flowing back into the battery.The datasheet isn't too clear, and shows things like reverse current flow.An example using two mosfets to isolate a battery, is the DW01 circuit below.Note the logic appears odd, as the negative side of the battery is switched.Both mosfets off: no current flow.Discharge mosfet on: Current can flow into the battery negative, and discharge power.Charge mosfet on: Current can flow from the battery negative, and charge the cells.

author: Apollyon25_
author_rank: Regular Contributor
timestamp: November 15, 2018, 03:10:59 am
content: Linear Tech, and others make some ideal diode parts... I've used the LTC4415 with very few issues (other than $)