Implementing resettable overcurrent protection in a commercial device - Page 1
Forum section: electronics / projects

author: Corporate666
author_rank: Unknown
timestamp: August 28, 2014, 10:26:11 pm
content: I think I've asked about this before, but the particular project in question just got put on the front burner.Let's say you have a device like a CNC machine and you want the controller to be able to run various other devices (for example, a light, a motor that pumps coolant, a camera, etc). Let's say the system sources power and you want each output to be able to supply 10A of juice. You want to implement overcurrent protection so that the unit can't be harmed by external events, such as a shorted load, or a load that draws too much power.You would want to drive the outputs with FET's, but a FET trying to drive a dead short could fail in very short order given the amps available from a CNC control power supply. There are protected FET's available which claim to be impervious to this. One could then use a microcontroller measuring voltage over a resistor inline with the output to calculate current draw and turn off the output if that specific channel exceeded a pre-set limit.Is that how it's done in commercial devices that need to be highly reliable? I've also seen some FET's that use a resistor divider to program an output current and have an "error" pin that might be high when OCP occurs... but these are usually more expensive and complex and physically larger, and putting 10+ of them on a board would get expensive and make for a tight fit.So I'm just curious how "commercial grade" OCP is achieved?

author: SuzyC
author_rank: Frequent Contributor
timestamp: August 28, 2014, 11:16:10 pm
content: Ummm...circuit breaker..they are cheap, reliable, resettable, used in any home fuse box!

author: max_torque
author_rank: Super Contributor
timestamp: August 28, 2014, 11:33:01 pm
content: Plenty of "smart" high side semiconductor switches available now, which incorporate logic level inputs, and so overall are not much more expensive than the, necessarily complex, descrete implementation of the same capability!Infineon ProFets are a good place to start for most low voltage DC outputs!http://www.infineon.com/cms/en/product/power/smart-low-side-and-high-side-switches/smart-high-side-switch-profet-tm/channel.html?channel=ff80808112ab681d0112ab69e2d40357

author: DanielS
author_rank: Frequent Contributor
timestamp: August 29, 2014, 12:31:24 am
content: Quote from: SuzyC on August 28, 2014, 11:16:10 pmBreakers are fine for preventing your 15-20A in-wall wiring from melting down and causing an electrical fire since copper wiring takes a few seconds to warm up enough for this to happen and the breaker takes only a few milliseconds to trip but will not do you any good with saving a MOSFET from getting destroyed by a shorted output in microseconds.In off-line switching power supplies, the primary-side current can be sensed with a small resistor and the PWM controller can end the current switching cycle once a predetermined maximum current is reached, preventing an output short from destroying the primary-side switches and by extension, the output rectifiers as well if they are sized accordingly. This over-current circuitry can react as quickly as 100ns depending on PWM chip and circuit design.For a switching designs relying on synchronous rectification, the output can be split between multiple channels with individual current limits on their output so they can shut down independently. The more common implementation is to simply shut down the primary side when over-current occurs on any of the outputs.

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2014, 04:46:44 am
content: For an off-line AC system, I would use inductive current sensing which is cheap and allows galvanic isolation to be maintained between the control circuits and triacs or MOSFET switches.If I wanted to implement a soft start controller though that would be more interesting.

author: Corporate666
author_rank: Unknown
timestamp: August 29, 2014, 04:55:36 am
content: Just as an aside, the CNC machine was just the first example that came to mind.The particular project at hand will be DC, powered by a 24V lead acid battery, so there is a lot of current available for frying things like FET'sThanks for the replies so far. I'll look at those Infineon FET's... anyone have any specific parts they've used and would recommend in this arena?

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2014, 05:05:06 am
content: DC is a different matter.The simplest design I would try would use low side switching with an N-channel MOSFET and high side current sensing. During turn on the MOSFET gate may be charged relatively slowly to limit surge current. I would use the high side current sense to charge a shunt RC network which triggers an SCR through a zener diode. That allows for a delay and current profile. When the SCR fires, it pulls the MOSFET gate low, signals a fault, and latches on.The only tricky part is the high side current sense and that just takes a current sense resistor, operational amplifier, and transistor although it can be done using just the current sense resistor and two or four transistors depending on the desired level of accuracy.

author: Corporate666
author_rank: Unknown
timestamp: August 29, 2014, 05:34:48 am
content: Quote from: David Hess on August 29, 2014, 05:05:06 amCan't do low side switching. The application doesn't allow it... the device must source power only and won't have control over sinking.Do you suggest the discrete component approach over a "protected" P-Fet?

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2014, 02:33:11 pm
content: Quote from: Corporate666 on August 29, 2014, 05:34:48 amI would still use the same design then with high side switches. I am guessing that you cannot use low side current sense but that is not a problem either.QuoteI might but the protected switches still require external circuitry to latch off. Some of them produce a current sense output in which case no external current sense will be required. There are also parts at an intermediate level which do not include active protection but do provide the current sense output.Disadvantages include Infineon's web site and their EULA for merely looking at their datasheets. I would try to avoid their products.

author: salbayeng
author_rank: Frequent Contributor
timestamp: August 29, 2014, 03:02:41 pm
content: Hmm, I use PTC's to supply all the auxiliary loads on my CNC controllers, seems the guys in the plant like mashing up the proximity cables, and expect everything to work normally during the fault and after they fix the cable. Ratings up to 5A at 30v are common, the 10A to 12A ones get a bit big.Working with big batteries is pretty scary, best to put a fuselink right at the battery terminal, for that one time you drop something metallic in the wrong place.

author: Seekonk
author_rank: Super Contributor
timestamp: August 29, 2014, 03:49:55 pm
content: I second PTC's. I use them everywhere. They are ideal for protecting source from morons. You should remember that the power generally needs to be completely removed for them to reset. Once they heat up they need only about a quarter watt to remain hot. So a momentary short usually requires some human action to get them to reset. That can be a real hassle in many applications. Since they are basically resistors they can be paralleled with success in many applications for higher current allowing stocking a single item. Specs are like nailing jelly to a tree. Amount of mass they are soldered to, air currents, ambient temperature, all can change trip current. Epoxy a resistor to one and it makes a good temperature oven at about 100C to better than .5C.

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2014, 05:10:22 pm
content: I like the idea of PTCs but tend to use them in addition to other safety systems.I agree that low impedance sources like batteries should always be fused as close to the source as possible with one possible exception; some systems require fail-safe operation even in the event of catastrophic overload where it is better that branch wiring burn than loss of power occurs.

author: salbayeng
author_rank: Frequent Contributor
timestamp: August 30, 2014, 01:47:26 am
content: The other neat trick with PTC's is using them to protect other components from overheating, eg an overvoltage zener next to a PTC with a teflon heatshrink sleeve over both , this gets overvoltage and overcurrent protection.And with a (surface mount) buck or boost SMPS , use the SMD PTC to connect the MOSFET drain to the SMD inductor, so if things start to overheat the PTC goes "open" .

author: poorchava
author_rank: Super Contributor
timestamp: August 30, 2014, 02:00:16 am
content: MAX4374 is a very nice chip for doing that. Downside is that I think only Digikey has stock, and but this shouldn't be a problem for a commercial product.Sent from my Samsung i9100 by Tapatalk

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 30, 2014, 06:42:53 am
content: I finished one of these today:Unidirectional8A current carrying capacity (on resistance < 70mohm)40V open circuit capacityFault current: opens in under 10usModerate overload: opens up to 1ms+ later (adjustable)Current limit: adjustableThermally protected (~100C at R4)Manual set/reset (pushbuttons)Battery operated (180uA current draw; was supposed to be under 50uA, I need to look into that -- but still, several months on a typical 9V battery)With some mods, it could also be tweaked for continuous current limiting (thermally protected), foldback or otherwise; automatic retry (tick-tick-tick-tick...), and various interfaces (electronic set/reset, logic level indication, battery eliminator -- isolated supply), etc. And, needless to say, scalable to whatever voltage/current rating needed.Tim

author: AndyC_772
author_rank: Super Contributor
timestamp: August 30, 2014, 07:59:08 am
content: Have a look at 'hot swap' controllers for backplanes. They measure current through a small resistor in series with the load, and can directly drive a FET to switch off the supply in the event of a fault. Linear Tech do a good range.I'm not sure I'd necessarily trust one to be "fire safe", if it's approvals you're looking for rather than functional protection, but as a way to prevent damage in 99% of common fault cases, they can work really well.

author: salbayeng
author_rank: Frequent Contributor
timestamp: September 05, 2014, 09:15:24 am
content: @ Tim, Nice one.Are you planning to put mounting holes on the next one?I think 180uA is pretty good, compared to , say, the average 78L05 at 400uA.Getting the Vb for the NMOS transistor at low current is the tricky thing, looks like you have a charge pump/rectifier with D5-D8, how much current goes into the boost arrangement?, compared to say using a PMOS.I did a discrete H Bridge PCB a while back, using a charge pump and transistor level shifters for the top NMOS , but even with 5mA consumption, the gate drive was a bit soft at 5kHz , had to reconfigure it so PWM only on lower transistors. If I did it again I would put in complementary emitter followers for gate drive (like your Q3/Q4).BobT

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 05, 2014, 03:54:29 pm
content: No charge pump, only protection (most of those diodes are either clamping the current sense voltage, or the gate drive output, for when-not-if the transistor goes nuclear).I blew one already (avalanche failure; the MOV doesn't clamp nearly as well as I need), and the rest of the circuit is fine. A sample of one is hardly representative, of course. With a proper TVS in there, it's already survived thousands of cycles into various loads (at rated voltage: no external impedance, inductive, resistive).Tim

author: salbayeng
author_rank: Frequent Contributor
timestamp: September 08, 2014, 01:08:21 am
content: @Tim ,Yep I think MOV's are only useful across the mains, even then they need some associated inductance , the lower voltage ones are too soft .TVS's are pretty good, and the clamping voltage can be selected to match your application. Plus they are are available in SMT , I like to place a series 1810 PTC thermally connected to a shunt connected SMB TVS , so with sustained overvoltage, the current drops after a while to relieve the TVS.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 08, 2014, 03:29:39 am
content: Aaand, my first video starring it, heh. So if you're curious about the turn-off waveform,.I have some SMDJ48As on order, in the mean time I put some 1.5KE15s in series. Works like a charm, I've twanged it into a wide range of current-unlimited, variously inductive or not loads.Tim

author: calexanian
author_rank: Super Contributor
timestamp: September 08, 2014, 04:06:54 am
content: Agreed about the panel mount type circuit breakers. Even well above trip current they take a few seconds to trip. We use them on out products with success, but 10 seconds or so is acceptable in a marginal over current condition. The best Trimax breakers (which we use) can take a second or two at full short. I recommend an active fault protection. Even one that is latching relay based,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 08, 2014, 05:40:03 am
content: Polyfuses tend to go slowly, too (they don't dissipate all that much heat, in order to rise to a fairly modest temperature = long time constant). Even crowbarring into one could be a challenge (you need an SCR or something else beefy enough to handle fault current for a really long time, 100s ms).Tim

author: Corporate666
author_rank: Unknown
timestamp: September 08, 2014, 05:54:34 am
content: Quote from: T3sl4co1l on September 08, 2014, 05:40:03 amLove your circuit Tim, thanks for posting it!Regarding some of the other ideas posted... in the application I am thinking of, PTC's and breakers would be much too slow to protect a high or low side FET conducting an instantaneous current an order of magnitude above it's rating, killing it. If there were a PSU in play that had some sort of limiting, then fine - but I don't want to rely on up-steam protection (including fusible links). Of course they should be in place, but if the device in question dies, it still ends up coming back to our shop for repair at our cost, so it's better to make it fully protected (at a reasonable cost).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 08, 2014, 07:26:42 am
content: Absolutely, first step should be current mode or current limited control. Next step after that, perhaps an active protection line an electronic fuse, current source, etc. Next up I'd place a polyfuse + crowbar, since it could fail in the process; finally, last resort is a fuse fuse.Coming back to the topic at hand, stepper drives are generally high voltage driving low voltage coils (e.g., 36V into 3-5V motors), with current mode PWM intrinsic in the driver. Should be pretty robust, but I guess they can still fail?Tim

author: David Hess
author_rank: Super Contributor
timestamp: September 08, 2014, 01:15:26 pm
content: Quote from: T3sl4co1l on September 08, 2014, 07:26:42 amUsually they fail because of improper derating or overheating.The reason for using a high voltage and current limiting is to reduce the effects of the stepper winding inductance which would otherwise limit current transition time lowering torque at high step rates. The same thing may be accomplished with a high voltage and series resistor. It is not as efficient but the design is simpler and more reliable.