I'm needing an over-voltage protection circuit to put at the output of a switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V. Here's the 3 options I was considering: For all the options the point is to blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output 1. A Zener - I'm thinking these get rather bulky for higher current handling, and they're not very accurate 2. A TVS - Probably better than the Zener option as far as current handling and bulkiness... I think TVS kills Zener for this application 3. An SCR - I could trigger an SCR off of a LM431 to get an accurate trip voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop Anybody have any thoughts or recommendations for OVP circuits like this... I'm likeing the SCR option right now much thanks!
Over Voltage Protection Crowbar Circuit
Started by ●November 30, 2012
Reply by ●November 30, 20122012-11-30
panfilero wrote:> I'm needing an over-voltage protection circuit to put at the output of a > switching regulator that takes in 100V input and drops it to 24V... I'd > like to protect the output from going over 28V. > > Here's the 3 options I was considering: For all the options the point is > to blow my 5A fast acting fuse that's at the input, so I'm trying to > decide on a way to short my output > > 1. A Zener - I'm thinking these get rather bulky for higher current > handling, and they're not very accurate > > 2. A TVS - Probably better than the Zener option as far as current > handling and bulkiness... I think TVS kills Zener for this application >I've seen some Chinese power supplies (under 100 W) that used TVS devices in the intentional fail-to-short mode. That seems to work to protect the load. But, the device needs to be replaced after it is triggered.> 3. An SCR - I could trigger an SCR off of a LM431 to get an accurate trip > voltage and I think I could probably find a decently sized SCR that could > handle the current required to make the fuse popYup, a well-sized SCR is probably the best at producing a low-drop short when called upon, and surviving the event. Maybe that is not critical in your design. Jon
Reply by ●November 30, 20122012-11-30
On Fri, 30 Nov 2012 15:50:55 -0600, Jon Elson <jmelson@wustl.edu> wrote:>panfilero wrote: > >> I'm needing an over-voltage protection circuit to put at the output of a >> switching regulator that takes in 100V input and drops it to 24V... I'd >> like to protect the output from going over 28V. >> >> Here's the 3 options I was considering: For all the options the point is >> to blow my 5A fast acting fuse that's at the input, so I'm trying to >> decide on a way to short my output >> >> 1. A Zener - I'm thinking these get rather bulky for higher current >> handling, and they're not very accurate >> >> 2. A TVS - Probably better than the Zener option as far as current >> handling and bulkiness... I think TVS kills Zener for this application >> >I've seen some Chinese power supplies (under 100 W) that used TVS devices >in the intentional fail-to-short mode. That seems to work to protect the >load. But, the device needs to be replaced after it is triggered. >> 3. An SCR - I could trigger an SCR off of a LM431 to get an accurate trip >> voltage and I think I could probably find a decently sized SCR that could >> handle the current required to make the fuse pop >Yup, a well-sized SCR is probably the best at producing a low-drop short >when called upon, and surviving the event. Maybe that is not critical >in your design. > >JonJust use the SCR to clear a fuse. Just make sure your PCB traces are up to the pulse current. I have measured ~100A while clearing a 1A fuse... makes a teeny little flash when expiring ;-) Very fast. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●November 30, 20122012-11-30
On 30 Nov., 22:17, panfilero <panfil...@gmail.com> wrote:> I'm needing an over-voltage protection circuit to put at the output of a =switching regulator that takes in 100V input and drops it to 24V... I'd lik= e to protect the output from going over 28V.> > Here's the 3 options I was considering: =A0For all the options the point =is to blow my 5A fast acting fuse that's at the input, so I'm trying to dec= ide on a way to short my output> > 1. =A0A Zener - I'm thinking these get rather bulky for higher current ha=ndling, and they're not very accurate> > 2. =A0A TVS - Probably better than the Zener option as far as current han=dling and bulkiness... I think TVS kills Zener for this application> > 3. =A0An SCR - I could trigger an SCR off of a LM431 to get an accurate t=rip voltage and I think I could probably find a decently sized SCR that cou= ld handle the current required to make the fuse pop> > Anybody have any thoughts or recommendations for OVP circuits like this..=. I'm likeing the SCR option right now> > much thanks!why not something like this: http://www.analog-innovations.com/SED/OverAndR= everseVoltageProtection.pdf no need for fuses then -Lasse
Reply by ●November 30, 20122012-11-30
panfilero wrote:> I'm needing an over-voltage protection circuit to put at the output > of a switching regulator that takes in 100V input and drops it to > 24V... I'd like to protect the output from going over 28V. > > Here's the 3 options I was considering: For all the options the > point is to blow my 5A fast acting fuse that's at the input, so I'm > trying to decide on a way to short my output > > 1. A Zener - I'm thinking these get rather bulky for higher current > handling, and they're not very accurate > > 2. A TVS - Probably better than the Zener option as far as current > handling and bulkiness... I think TVS kills Zener for this > application > > 3. An SCR - I could trigger an SCR off of a LM431 to get an accurate > trip voltage and I think I could probably find a decently sized SCR > that could handle the current required to make the fuse pop > > Anybody have any thoughts or recommendations for OVP circuits like > this... I'm likeing the SCR option right now > > much thanks!#3 is the only option I ever consider. Personally I like to have the SCR right at the circuit side of the fuse. If the fuse were at the input and the SCR were at the output you'd have the upper FET and the inductor in the path. This will slow down the fuse tripping and there is a chance that the FET grenades before the fuse trips, something that is generally not desired. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●November 30, 20122012-11-30
On Friday, November 30, 2012 3:17:59 PM UTC-6, panfilero wrote:> I'm needing an over-voltage protection circuit to put at the output of a switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V. > > > > Here's the 3 options I was considering: For all the options the point is to blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output > > > > 1. A Zener - I'm thinking these get rather bulky for higher current handling, and they're not very accurate > > > > 2. A TVS - Probably better than the Zener option as far as current handling and bulkiness... I think TVS kills Zener for this application > > > > 3. An SCR - I could trigger an SCR off of a LM431 to get an accurate trip voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop > > > > Anybody have any thoughts or recommendations for OVP circuits like this... I'm likeing the SCR option right now > > > > much thanks!Thanks for the input, can anyone tell me why the SCR is a better option than the TVS? I was considering using one of these guys... http://www.littelfuse.com/products/tvs-diodes/surface-mount.aspx
Reply by ●November 30, 20122012-11-30
panfilero wrote:> On Friday, November 30, 2012 3:17:59 PM UTC-6, panfilero wrote: >> I'm needing an over-voltage protection circuit to put at the output of a switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V. >> >> >> >> Here's the 3 options I was considering: For all the options the point is to blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output >> >> >> >> 1. A Zener - I'm thinking these get rather bulky for higher current handling, and they're not very accurate >> >> >> >> 2. A TVS - Probably better than the Zener option as far as current handling and bulkiness... I think TVS kills Zener for this application >> >> >> >> 3. An SCR - I could trigger an SCR off of a LM431 to get an accurate trip voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop >> >> >> >> Anybody have any thoughts or recommendations for OVP circuits like this... I'm likeing the SCR option right now >> >> >> >> much thanks! > > Thanks for the input, can anyone tell me why the SCR is a better option than the TVS? I was considering using one of these guys... > > http://www.littelfuse.com/products/tvs-diodes/surface-mount.aspxThe SCR is much faster. Assume this scenario: The FB resistor comes unglued for some reason, gradually. Now the voltage slowly rises, the TVS begins to conduct, gets hot, gets hotter, starts to smell, smoke arises, a smoke alarm goes off, your lab begins to become drenched in sprinkler water, the bell rings at the local fire station, engine company 27 roars down the road with wailing sirens ... The SCR, in contrast, can be control by a "snap action" circuit. Voltage exceeds a certain threshold ... WHAMMO ... fuse blows, the day is saved. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●November 30, 20122012-11-30
On 11/30/2012 1:17 PM, panfilero wrote:> I'm needing an over-voltage protection circuit to put at the output of a switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V. > > Here's the 3 options I was considering: For all the options the point is to blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output > > 1. A Zener - I'm thinking these get rather bulky for higher current handling, and they're not very accurate > > 2. A TVS - Probably better than the Zener option as far as current handling and bulkiness... I think TVS kills Zener for this application > > 3. An SCR - I could trigger an SCR off of a LM431 to get an accurate trip voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop > > Anybody have any thoughts or recommendations for OVP circuits like this... I'm likeing the SCR option right now > > much thanks!Is this a dedicated supply where you KNOW what it's gonna be hooked to in EVERY case? There are times when it's not a good idea for the output to short. Like when you're charging a battery.
Reply by ●November 30, 20122012-11-30
On Fri, 30 Nov 2012 14:37:29 -0800, Joerg <invalid@invalid.invalid> wrote:>panfilero wrote: >> On Friday, November 30, 2012 3:17:59 PM UTC-6, panfilero wrote: >>> I'm needing an over-voltage protection circuit to put at the output of a switching regulator that takes in 100V input and drops it to 24V... I'd like to protect the output from going over 28V. >>> >>> >>> >>> Here's the 3 options I was considering: For all the options the point is to blow my 5A fast acting fuse that's at the input, so I'm trying to decide on a way to short my output >>> >>> >>> >>> 1. A Zener - I'm thinking these get rather bulky for higher current handling, and they're not very accurate >>> >>> >>> >>> 2. A TVS - Probably better than the Zener option as far as current handling and bulkiness... I think TVS kills Zener for this application >>> >>> >>> >>> 3. An SCR - I could trigger an SCR off of a LM431 to get an accurate trip voltage and I think I could probably find a decently sized SCR that could handle the current required to make the fuse pop >>> >>> >>> >>> Anybody have any thoughts or recommendations for OVP circuits like this... I'm likeing the SCR option right now >>> >>> >>> >>> much thanks! >> >> Thanks for the input, can anyone tell me why the SCR is a better option than the TVS? I was considering using one of these guys... >> >> http://www.littelfuse.com/products/tvs-diodes/surface-mount.aspx > > >The SCR is much faster. Assume this scenario: The FB resistor comes >unglued for some reason, gradually. Now the voltage slowly rises, the >TVS begins to conduct, gets hot, gets hotter, starts to smell, smoke >arises, a smoke alarm goes off, your lab begins to become drenched in >sprinkler water, the bell rings at the local fire station, engine >company 27 roars down the road with wailing sirens ...We had an incident here. The carpet cleaner guy wanted to close the garage door, saw a red box with PULL DOWN on it, so he pulled it down. It was the fire alarm, of course. In a couple of minutes we got two fire trucks and a supervisor in a car. They were very nice, helped us reset everything, no charge.> >The SCR, in contrast, can be control by a "snap action" circuit. Voltage >exceeds a certain threshold ... WHAMMO ... fuse blows, the day is saved.The SCR gate should indeed be driven hard by some schmitt-trigger sort of thing. A wimpy gate drive (say, a zener from anode to gate) can result in a slow-spreading current concentration somewhere in the SCR die, and damage the SCR. Moto used to make a specific crowbar driver chip for this application... can't recall the part number. OnSemi may still make it. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
Reply by ●November 30, 20122012-11-30
On Fri, 30 Nov 2012 14:22:56 -0800, panfilero wrote:> On Friday, November 30, 2012 3:17:59 PM UTC-6, panfilero wrote: >> I'm needing an over-voltage protection circuit to put at the output of >> a switching regulator that takes in 100V input and drops it to 24V... >> I'd like to protect the output from going over 28V. >> >> >> >> Here's the 3 options I was considering: For all the options the point >> is to blow my 5A fast acting fuse that's at the input, so I'm trying to >> decide on a way to short my output >> >> >> >> 1. A Zener - I'm thinking these get rather bulky for higher current >> handling, and they're not very accurate >> >> >> >> 2. A TVS - Probably better than the Zener option as far as current >> handling and bulkiness... I think TVS kills Zener for this application >> >> >> >> 3. An SCR - I could trigger an SCR off of a LM431 to get an accurate >> trip voltage and I think I could probably find a decently sized SCR >> that could handle the current required to make the fuse pop >> >> >> >> Anybody have any thoughts or recommendations for OVP circuits like >> this... I'm likeing the SCR option right now >> >> >> >> much thanks! > > Thanks for the input, can anyone tell me why the SCR is a better option > than the TVS? I was considering using one of these guys... > > http://www.littelfuse.com/products/tvs-diodes/surface-mount.aspxWhat Joerg said. And what Joerg said: you can't blow the fuse on the input side by putting a TVS on the output side unless you go through the FET and whatnot. With an SCR you can sense overvoltage on the output and blow a fuse on the input. And what Mike said about a crowbar on the output. And finally -- if I'm remembering correctly, the TVS's that you cite _are_ zener diodes -- they're just optimized for TVS service. If you're dead set on using them, remember that they're _transient_ voltage suppressors -- Joerg's example of things slowly going to hell and the output rising isn't a transient any more. Isn't it possible to make a switching supply such as this with a topology and a transformer with a turns ratio that limits the output voltage to some specific fraction of the input? It's probably not economical unless it's a honkin' big supply, but it would give you a supply that "naturally" can't go above your 28V with a 100V input. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
