to smart switch or not to smart switch - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: March 19, 2021, 05:49:27 pm
content: I want to design a cost sensitive device. I need to switch 3 loads and measure the current of those loads. It makes a lot of sense to use at least one smart switch as these are robust and will provide electronic fusing and current feedback. Infineon seems to be the main provider of parts that are reasonably cheap and rugged but they have a nasty habit of making stuff obsolete with no direct or even close replacement and given my low volumes I am left hoping that there are enough other people using them to keep stocks flowing. The BTS7080 is ideal but there are like 10 available around the world right now.My other option is use a BJT with an emitter resistor to limit current in the first instance whilst my microcontroller measures the excessive current draw and turns the power off. it looks like I can get a suitable BJT with low gain for about £0.5 but anything with sensible gain like a darlington or with low collector to emitter saturation to limit power dissipation is well over £1 which is the cost of the BTS7080 for 2 channels.How do others do this? I need to switch 2-5 A.

author: langwadt
author_rank: Super Contributor
timestamp: March 19, 2021, 06:07:51 pm
content: what voltage? has to be high side?

author: Simon
author_rank: Unknown
timestamp: March 19, 2021, 06:17:54 pm
content: it's for 12V which is what most automotive smart switches seem to be aimed at if you want full load dump capability. I don't mind high or low side. i am looking at the VNQ5050AK-E from STM, not sure why RS has it for £2 when everyone else has it at £4 but it's comparable to the BTS7080

author: Benta
author_rank: Super Contributor
timestamp: March 19, 2021, 07:55:40 pm
content: Go for automotive grade parts. These are pretty secure from obsoleting. Infineon only does the automotive grading if there's a major customer behind.

author: Simon
author_rank: Unknown
timestamp: March 19, 2021, 08:05:11 pm
content: Not really. Just had to buy 430 infineon smart switches when we only need 20 because they have just made it obsolete, seems it was so popular that the dealers are now charging half the production price rather than twice like they normally do and we need to make a minimum £250 order. They were automotive, every package version obsolete, data sheet gone, they forgot to remove the app note, no replacement suggested. They had even gone to the effort to put UI in front of it which is the new automotive denomination and it's been ditched.

author: ajb
author_rank: Super Contributor
timestamp: March 19, 2021, 09:08:46 pm
content: A lot of the smart switches I've looked at for various applications had somewhat poorly-controlled current limits, and the range of parts with useful current monitor outputs is fairly limited, and their accuracy is often compromised by using the internal pass transistor as the current sense resistor. So if you need tightly controlled limits or accurate monitoring you may be better off with a somewhat DIY solution. Completely replicating the functionality of a protected switch using jellybean parts would eat a fair bit more board space, but there are intermediate solutions. You could use a basic protected switch to provide short circuit and overtemperature protection with your own current sense resistor if you just need more accurate monitoring but don't particularly care about the accuracy of the current limit. Or use a conventional opamp+transistor+resistor constant current topology and reference input from an IO pin via a resistor divider, which would give you an on/off switch with programmable (based on the divider and sense resistor) current limit, which you can customize to the setpoint and accuracy level you need--and you can tap the sense resistor to an ADC input as well. Or you may be able to leverage an MCU's built in analog comparators to trigger interrupts for reduced response time to overcurrent conditions with fewer external parts. It all depends on what things you need the device to do and how well you need it to do them.

author: Simon
author_rank: Unknown
timestamp: March 19, 2021, 09:26:02 pm
content: Current sensing does not need to be too accurate. So i want to drive a little pump that has a brushed motor. i want the output to be short circuit protected and it would be worth monitoring the current draw to pick up issues so it's like is twice the current being drawn could mean it's jammed and worth turning off.That STM part has about a 15% variation at 2A which is fine for what I am doing.

author: tszaboo
author_rank: Super Contributor
timestamp: March 19, 2021, 09:28:57 pm
content: I would do it like this:https://www.ti.com/power-management/power-switches/efuse-hotswap-controllers/products.html#p2192=Analog%20current%20monitor;Current%20monitoring;Current%20sense/monitor&sort=p1130;ascI dont touch Infineon products with a ten foot pole. Maybe if it is IRF, but even then I disinfect my hand after it.Probably suitable:https://www.ti.com/product/TPS25980

author: langwadt
author_rank: Super Contributor
timestamp: March 19, 2021, 09:39:52 pm
content: Quote from: Benta on March 19, 2021, 07:55:40 pmand if that customer stops using it, poof!! it is gone

author: langwadt
author_rank: Super Contributor
timestamp: March 19, 2021, 09:43:37 pm
content: Quote from: Simon on March 19, 2021, 06:17:54 pmfor low side there is many different ones in DPAK

author: Simon
author_rank: Unknown
timestamp: March 19, 2021, 09:44:22 pm
content: Quote from: langwadt on March 19, 2021, 09:39:52 pmYea, the other one is kingbright. Their high power LED's only exist on paper or when someone places a big order and a few thousand extra go on the shelf for poor suckers to buy then wonder why 4 years later you can't get any.

author: Simon
author_rank: Unknown
timestamp: March 19, 2021, 09:45:58 pm
content: Quote from: langwadt on March 19, 2021, 09:43:37 pmMost manufacturers are concentrating on high side for the auto market. High side would be better but i can live with low side.

author: langwadt
author_rank: Super Contributor
timestamp: March 19, 2021, 09:47:40 pm
content: Quote from: Simon on March 19, 2021, 09:44:22 pmMaxim is famous for seemingly doing the opposite, make a few thousand to give out as samples, then unless someone places a big order it then just disappears

author: Benta
author_rank: Super Contributor
timestamp: March 19, 2021, 10:05:32 pm
content: Quote from: langwadt on March 19, 2021, 09:39:52 pmWrong. Automotive customers need long time support for spare part availability. You're talking consumer goods.

author: langwadt
author_rank: Super Contributor
timestamp: March 19, 2021, 10:13:32 pm
content: Quote from: Benta on March 19, 2021, 10:05:32 pmin what alternative reality does automotive electronics get repaired on component level with spareparts?

author: mikerj
author_rank: Super Contributor
timestamp: March 19, 2021, 10:52:13 pm
content: Quote from: langwadt on March 19, 2021, 10:13:32 pmThey sell you complete new modules instead, and they still have to be manufactured.

author: langwadt
author_rank: Super Contributor
timestamp: March 20, 2021, 12:08:25 am
content: Quote from: Simon on March 19, 2021, 09:45:58 pmmight need to look for "protected mosfet" e.g.https://www.onsemi.com/products/discretes-drivers/protected-mosfets

author: Simon
author_rank: Unknown
timestamp: March 20, 2021, 07:15:28 am
content: Quote from: mikerj on March 19, 2021, 10:52:13 pmYes but as soon as that part falls out of use by a big auto maker those of us using a few dozen loose access.

author: Simon
author_rank: Unknown
timestamp: March 20, 2021, 08:58:35 am
content: Quote from: langwadt on March 20, 2021, 12:08:25 amLooking at a couple of parts they are on an MOQ of 2500 and there is no stock. The STM ones are actually cheaper when you consider the fact that they are multi channel and have the current sense.