Switching several high current loads; how to do fusing? - Page 1
Forum section: electronics / projects

author: fitzgerald
author_rank: Contributor
timestamp: July 04, 2015, 09:09:23 am
content: Hi!I am working on a PCB that will switch on/off six 12V loads. There are two 8A outputs, two 2A outputs and two 1A outputs. These values are maximum values, so total load of 22A maximum which I find a lot.Loads will be controlled via a microcontroller that is connected to a PC via USB. I plan to isolate the microcontroller from the high current side to prevent any return current via the USB in case of a GND disconnect.For the switching I am looking at Infineon ProFET smart high-side switch devices, but it is difficult for me to select the right part for the application. Ideally 'd like to use the current limiting or overcurrent shutdown capability of the ProFET's so that I do not have to individually fuse all outputs and only use a fuse on the input line. However, the current limit of the devices can vary quite a bit. For example the ITS711L1* has a current limit of 4.5A min. to 11A max. at 25C (see table on page 6). If I still have to fuse the outputs I could save a bit of money an use regular MOSFET'sThe loads are commercial-off-the-shelf modules. Most of them behave fairly resistive, but there is some input capacitance that causes a rush-in current. The most complex load is a stepper motor driver that has massive input capacitance (~2000uF) and will cause issues when disconnected during motion (the motor acts as an inductor).About the how/why of fusing:- The main goal of the fusing is safety. In all cases it should prevent a fire hazard.- Secondly it should allow to use thinner wiring to the loads. Input wiring is 12AWG and I would like to use AWG20-26 for the loads (easier routing)- Preferable it should include some form of diagnostics to aid troubleshootingIn case of a fault I prefer to latch-off instead of current limit and then let the microcontroller decide if it wants to give it another try.Possible solutions:- Traditional fuses with p-type MOSFET's- Polyfuses with p-type MOSFET's- 'Smart' high-side switches such as Infineon ProFET's or similar- ...There are no practical budget constraints; but board space is at a premium.How would you design such a system? Any parts in particular that you can recommend? Any things to look out for?Thanks!* ITS711L1:http://www.farnell.com/datasheets/1681363.pdf

author: fcb
author_rank: Super Contributor
timestamp: July 04, 2015, 09:22:35 am
content: ProFET's are great, but not cheap. If your building a few of these units (and care about costs), I'd probably use P-channel MOSFET's (simpler to drive) with sense resistors (and opamps to detect current, fed back to a microcontroller to do current vs. time fusing).There are some high-side current sense IC's that could help (e.g. MAX4172/3) but at a cost.

author: Psi
author_rank: Super Contributor
timestamp: July 04, 2015, 09:35:01 am
content: Does it really matter if the load is faulty and draws the full 11A ?If you put a 4A fuse on the output its not going to blow until at least 8A anyway.Also, that chip has a diagnostic output pin which you can use to detect if the current limit or thermal shutdown kicks in.So your MCU can detect an overload and switch off the output. In effect you have a fuse, it's just done in softwareSome of these style of chips even have an analog output proportional to current however it doesn't look like that one does.

author: fitzgerald
author_rank: Contributor
timestamp: July 04, 2015, 09:46:27 am
content: Thanks for your reply.@fcb:Cost is not a major concern, the cost of the ProFET's are within budget. Board space is a bit limited so I prefer a low component count solution. I have had a look at TI's eFuses; they are nice but they need external protection (load can be inductive) and the high-current ones are a pain to solder@Psi:The cable for the 1A loads is a 2x 20AWG multiconductor cable. If the load is faulty, I'd rather not run 11A continuously through this cable

author: Psi
author_rank: Super Contributor
timestamp: July 04, 2015, 09:50:42 am
content: Quote from: fitzgerald on July 04, 2015, 09:46:27 amI've added some more info to my post above. (The ITS711L1 has diagnostic outputs you can use to detect the 11A overload and shut off the load in software)

author: leblanc
author_rank: Regular Contributor
timestamp: July 04, 2015, 12:24:04 pm
content: I've used this circuit for overcurrent protection. In the scenario where your output shorts to ground, it doesn't just limit current, it will actually latch into a protection mode.This can allow you to have protection for individual channels.If the load is not on your PCB (you have cables going to your load), it may be worth adding protection on the low side as well (in case it gets shorted to unprotected 12V, you don't want to destroy your PCB).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 04, 2015, 04:22:12 pm
content: Quote from: leblanc on July 04, 2015, 12:24:04 pmIf there is a small but positive load, you can never turn it on.A less severe form of this might be a foldback current limiter. This is the usual emitter shunt resistor / Vbe current sense thing, but with a resistor divider to the base, from shunt to the output. Thus the current limit is biased down when the voltage drop is large.See the 2SD1273 here:You'll have to mentally move around the emitter and collector ends, and replace with source and drain.It's latching if the foldback intersects zero current, in which case you need a second transistor to force reset.At this point you're looking at something fancier and easier though, like the ProFET things. Speaking of:Quote from: Psi on July 04, 2015, 09:35:01 amYou're fine with the ProFET things -- they handle that autonomously. Don't make the mistake of doing it with naked MOSFETs, and trusting fuses (especially polyfuses, they're *slow*) to protect the transistor! Only circuitry can protect a MOSFET, so you need the stuff around it, or you need the integrated kind.Tim

author: nctnico
author_rank: Super Contributor
timestamp: July 04, 2015, 04:54:35 pm
content: First of all I wouldn't omit real fuses (and not use polyfuses). A fuse is there to protect against fire and burning traces from a PCB. Now think again why you want current limiting... Do the other devices need to continue working if one develops a short?You can do overcurrent protection using a circuit but it is a major PITA to get right. You'll need to allow a grace period for rush-in currents and overcurrents. A normal fuse solves these problems without needing to think twice about them. All in all I would use normal p channel MOSFETs with a high current handling capability and fuses.

author: lewis
author_rank: Frequent Contributor
timestamp: July 04, 2015, 05:05:15 pm
content: I use the VN920 a hell of a lot for this kind of thing, it's so much easier than other methods involving P-FETs and high-side current sensing. It has thermal protection too.http://www.st.com/web/en/resource/technical/document/datasheet/CD00276134.pdfThe diagnostic output is proportional to current (albeit with a significant difference between parts which can be overcome by calibration during production) which is good enough to relatively accurately measure load current.For overcurrent protection where ratiometric measurement is not necessary, using a 1K resistor for example on the diag pin to VSS gives 5V output at between 13-16A load current depending on temperature and varying between device.It's also an automotive grade device, and extremely rugged but you don't want to subject it to ISO7637 pulse 5 load dump without external fettling.

author: rx8pilot
author_rank: Super Contributor
timestamp: July 04, 2015, 07:02:42 pm
content: Quote from: nctnico on July 04, 2015, 04:54:35 pmI have eliminated all fuses on my designs because the electronic fuses work better, are FAR more accurate, they can behave exactly how I need them to, have VERY low resistance, and therefore very low heat is generated. When a fuse blows at the north pole, you cannot go to the store and get another one. There is no doubt that a traditional fuse is simple in design, but in my case they are generally way too slow to prevent damage. My goal is not just to avoid a fire, but to ensure the circuit can be shorted and overloaded 10's of thousands of times without consequence other than a temporary shutdown. This, I have learned, is not all that easy since there are so many details to balance. Traditional fuses force you to make the PCB handle huge transient currents during a short. An e-fuse has allowed me to control the currents during a shorted condition enough to keep the traces on the PCB reasonable size and never worry about overheating. I also have three independent and different protection stages so that if one fails, the others will catch the problem - not sure if your application requires multi-stage protection or not.The first one that comes up is how fast does an overload shutdown the circuit. Whether you build pseudo-discreet op-amp current sense or use a dedicated controller - you have to decide how to react to a small/medium/large/short overload condition that will protect the whole system including the protection circuit itself. I learned a lot about inrush currents and how to manage them without violating the safe operating area of the controlling MOSFET's. I have used linear's LT1910's and LT4356 (which is a 'Surge Stopper') for a while - both needed very delicate balancing to allow normal transient currents to pass while still shutting down when a true fault condition appeared. In the end, they offer easy load switching with very predictable fault detection. The part the OP listed is conveniently packaged in a 4 channel device, but the Rds(on) is rather poor for a full load. I would consider using an external MOSFET design with very low Rds(on) which could be N-Ch or P-ch. On the LT1910 (n-ch driver) I used an RC network (suggested by the data sheet) to control the di/dt reaction time to suit the enormous variety of loads I needed to supply. It took a few rounds of testing and simulating to get a good balance and to understand the needs of the typical loads. You can set these up to auto-reset or latch off. In my case, I use a mix of those options but need to make sure that only a real problem will shut it down. My circuits are high-availability critical and a false shutdown will make enemies fast. A missed fault that damages something is just as bad so I have to be careful to consider everything. My circuits do not have any practical budgetary constraints, which is nice. Not sure how you are budgeted for this project.The variety of controllers available is significant, but they all have the benefit of eliminating many of the challenges with the analog design elements. A current sense amplifier is not rocket science, but there are many details and subtleties in the circuit design and the PCB layout. Also, most of the pre-done IC's offer logic level ENABLE/DISABLE and FAULT STATUS that make uController interfacing super simple without any external components. It is hard to suggest a specific part without fully understanding your application, but at least you can look at the concepts behind the two parts above (which are significantly different).

author: leblanc
author_rank: Regular Contributor
timestamp: July 04, 2015, 08:36:24 pm
content: Quote from: T3sl4co1l on July 04, 2015, 04:22:12 pmRight, I forgot about this issue. You could have a capacitor parallel to the 10k to let the EN signal through just long enough to enable the MOSFET.Your other solution sound interesting though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 04, 2015, 11:08:17 pm
content: Also for those in the thread who haven't seen it,https://youtu.be/cQWnLOCGpXMMike talks about using these kinds of devices. Specifically, for LED lighting installations -- where you'll have techs wiring (and mis-wiring) and hot-plugging things of all sorts. Inrush probably not so much of a problem, but a related issue.I also made these, which are in a similar range for ratings, but which can be expanded for much greater ratings than any monolithic device:http://www.seventransistorlabs.com/FuseTim

author: rx8pilot
author_rank: Super Contributor
timestamp: July 05, 2015, 02:21:33 am
content: What are the loads? Do you have control over them or does your circuit need to deal with anything/everything?Resistive? Capacitive? Inductive? Pulsing?Switching a resistor is much easier than a pulsing capacitive load connected through a long inductive cable.

author: fitzgerald
author_rank: Contributor
timestamp: July 05, 2015, 11:04:05 am
content: I have added more information in the opening post. I will investigate the LT1910 with external n-type FET. That seems a nice alternative

author: nctnico
author_rank: Super Contributor
timestamp: July 05, 2015, 11:13:39 am
content: I see one of your goals is thinner wires. I suggest you look into wires with PPE insulation. This is much thinner than PVC so the end result is less bulky.