Implement OCP (overcurrent protection) in digital circuit - best solution? - Page 1
Forum section: electronics / projects

author: Corporate666
author_rank: Unknown
timestamp: January 15, 2013, 10:44:38 am
content: Just curious how others would implement this, or if there is a standard way.Let's say you are using a MOSFET to drive the high side of a device that a user connects. Let's say you want to protect your widget from either failure of that connected device, or open shorts, or user silliness, etc, etc.Almost like a power supply.Is there a standard method for this?Forget PTC fuses and regular fuses - I'm talking OCP like a power supply, where you short the output and it instantly terminates power... and immediately upon removing the short, power is restored.There are OCP chips out there, but they tend to do other stuff and use a fair amount of board space for, as an example, a device that has 10 output lines that all need OCP.

author: poorchava
author_rank: Super Contributor
timestamp: January 15, 2013, 03:45:24 pm
content: Google for 'high side current shunt monitor' .Connect that to a garden variety single supply opamp (LM358 comes to mind) and that in turn to series P-MOSFET. Current shunt monitor output exceeds the threshold, opamp output goes high, p-mos turns off.You could probably replace current shunt monitor with another opamp-based differential amplifier, but that would be less sensitive/accurate.

author: tld
author_rank: Regular Contributor
timestamp: January 15, 2013, 05:18:32 pm
content: Quote from: poorchava on January 15, 2013, 03:45:24 pm... current shunt monitor drops below threshold, opamp output goes low, p-mos turns on, and you have oscillations.Basically the right way to go about things though. You could expand and use something similar to a latching soft switch, perhaps periodically reset by a 555-timer or some such thing.It all depends on the rest of the design really. If you've got an MCU in there, you might want to use a soft latching for turning off the load, but let the MCU reset the soft-switch periodically to test the load, possibly take other actions like light an over-load LED or some such.Depending on the accuracy you need, there are dedicated current-sense amplifiers, down to sot-23 size. You could mix it up any way you'd like, using an ADC for monitoring current, and so on.tld

author: ConKbot
author_rank: Super Contributor
timestamp: January 15, 2013, 05:22:06 pm
content: Quote from: poorchava on January 15, 2013, 03:45:24 pmYou'd have to have some other smarts in there as well, otherwise it'd turn into a linear regulator above the setpoint (very bad, lots of dissipation in the FET) or oscilate rapidly (still not shutting down the load, but not as bad as going linear) You could use an RC delay with diode steering for a fast trip and slow reset, between the current shunt monitor and your opamp/comparitorThis would slow the oscillating down so that way it only tries to reset a few times per second or however often you want it to.

author: free_electron
author_rank: Super Contributor
timestamp: January 15, 2013, 05:40:16 pm
content: There are chips that do that. Siemens has that . St has that.these look like mosfets but they have overcurrent detection , fault output and overtemp protection. typically in a 4 pin or 5 pin TO220 package.Power in , power out , fault output a groudn pin and a pin where you set the current limit using a resistor.On semi NVC8460Siemens BTS100 BTS432STmicro :http://www.st.com/internet/automotive/subclass/1037.jsp. There's 152 different ones ...These devices are specifically made for such applications. they are cheap and readily available.

author: lewis
author_rank: Frequent Contributor
timestamp: January 15, 2013, 07:52:40 pm
content: Quote from: free_electron on January 15, 2013, 05:40:16 pm^^ What he said, definitely the best way to implement a protected high side switch. We use VN920 from ST a lot, it comes in a variety of packages (Google VN920B5, VN920SP, VN920PEP) and it's very rugged if you treat it right. It has an analogue output which is proportional (loosely) to load current. The high side switches tend to be rather slow though, and the switching losses can mount up if your application demands more than about 2kHz.Edit: Oh, and they have a logic level input!

author: Corporate666
author_rank: Unknown
timestamp: January 16, 2013, 08:47:45 pm
content: Quote from: free_electron on January 15, 2013, 05:40:16 pmThanks!I had seen some parts before, I think it was National? I can't remember... but they topped out at a couple of amps on p-channel parts, the Siemens and ST parts look interesting. Thanks!