MCU controlled power switches - Page 1
Forum section: electronics / projects

author: sirhaggis
author_rank: Regular Contributor
timestamp: December 08, 2016, 06:49:15 am
content: Hi all,I have a board which supplies power to multiple plugin modules at 3.3V, 5V, +12V and -12V. I want to be able to switch these power rails on and off using the MCU, which pulls signals high/low at 3.3V when it wants to turn the power on/off. I'm looking for a low cost, compact, SMD solution that can achieve this at a couple of amps per rail. Perhaps some sort of transistor IC? I have very little experience with power circuits, more of a digital guy so I'm not quite sure what to look for here. Any part suggestions would be appreciated!ThanksLuke

author: Jeroen3
author_rank: Super Contributor
timestamp: December 08, 2016, 02:55:29 pm
content: Infineon has high side load switches. For off board stuff.NXP has some wlcsp load switches.You will find something you like in Digikeys catalog:PMIC - Power Distribution Switches, Load Drivers.Probaby even hot swap controllers if you have modules.

author: technix
author_rank: Super Contributor
timestamp: December 08, 2016, 05:08:22 pm
content: Maybe not SMD, but you can try relays, which gives you isolated control and good current carrying ability. However you should not switch those contacts too frequently.

author: D-Jack
author_rank: Contributor
timestamp: December 10, 2016, 11:48:33 am
content: For the lowest cost solution, I would recommend using discrete transistors or mosfets. You would use a pnp (or p-channel) as the load switch because of the -12V rail. It wouldn't be the most compact solution though. If your microcontroller has open-drain output, use them when working with transistors

author: sirhaggis
author_rank: Regular Contributor
timestamp: December 17, 2016, 07:05:17 am
content: I have found some products which seem appropriate for my requirements such as this one:http://www.st.com/content/ccc/resource/technical/document/datasheet/d8/20/12/4c/e4/65/49/ff/CD00002209.pdf/files/CD00002209.pdf/jcr:content/translations/en.CD00002209.pdfThe diagnostic feedback is nice as I can detect if the MOSFET has gone into protection mode while in operation.My only issue is dealing with the -12V rail, as I need a p-channel mosfet... there doesn't appear to be a p-channel equivalent to the above IC. I need a p-channel MOSFET that has thermal and short protection, preferably with diagnostic feedback like the one above. I can find plenty of plain old p-channel mosfets, but none with auto protection features.Any ideas?

author: technix
author_rank: Super Contributor
timestamp: December 17, 2016, 08:59:58 am
content: Quote from: sirhaggis on December 17, 2016, 07:05:17 amYou may have to implement it yourself.

author: D-Jack
author_rank: Contributor
timestamp: December 18, 2016, 06:27:21 pm
content: You can add current sensing capabilities using a current sense resistor and an OpAmp depending on the output current. But the real question is: Is that level of complexity really necessary? I mean, what type of output is it going to be? Is the load subjective to variations outside of your control? How much current output current is expected?

author: sirhaggis
author_rank: Regular Contributor
timestamp: December 20, 2016, 04:20:49 am
content: Hmmm current detection with opamp sounds like an option...It will be driving a set of module connectors, which can have unknown modules plugged into them. Although I will specify a current limit on the modules, there will be a wide variation in operating conditions that are outside my control. Better safe than sorry if a faulty module shorts out the rails or draws too much current.

author: D-Jack
author_rank: Contributor
timestamp: December 21, 2016, 12:44:12 am
content: Ohh ok having unknown load characteristics is definitely a concern. Temperature monitoring sounds like an overkill to me. You could simply sense the output current and act upon it. Choose a Mosfet which can safely operate across your current range and add a sense resistor (beware of the voltage drop across the resistor due to current!) and use a current sense opamp like ST TSC888 to measure the current

author: Pack34
author_rank: Frequent Contributor
timestamp: December 21, 2016, 01:01:13 am
content: Is there a reason a simple relay wouldn't work?

author: Wilksey
author_rank: Super Contributor
timestamp: December 21, 2016, 12:51:29 pm
content: Tranny or relay

author: BFX
author_rank: Frequent Contributor
timestamp: December 21, 2016, 08:46:45 pm
content: Here is example how I did it.

author: technix
author_rank: Super Contributor
timestamp: December 22, 2016, 02:27:33 am
content: Quote from: BFX on December 21, 2016, 08:46:45 pmYou may want to add antiparallel diodes across relay windings to prevent frying your MOSFETs.