On Fri, 30 Nov 2012 13:54:33 -0800 (PST), "langwadt@fonz.dk" <langwadt@fonz.dk> wrote:>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 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! > >why not something like this: http://www.analog-innovations.com/SED/OverAndReverseVoltageProtection.pdf >no need for fuses then > >-LasseYep. I always like my own circuits >:-} The fuse blower design was done before MOSFET's were common. ...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.
Over Voltage Protection Crowbar Circuit
Started by ●November 30, 2012
Reply by ●November 30, 20122012-11-30
Reply by ●November 30, 20122012-11-30
Tim Wescott wrote:> 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.aspx > > What 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. >Sounds like the OP is using a buck. A sync buck would give at least some level of protection. A white-knuckle event is when the upper FET welds through. Eventually, depending on its control architecture, the lower FET will come on and then one can only hope that the fuse opens. A non-synchronous buck would not protect at all, the output would head straight up to Vin and most likely leave a path of destruction in anything connected. Belts-and-suspenders would be to sense the output and trip a beefy SCR right at the fuse. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●November 30, 20122012-11-30
On Fri, 30 Nov 2012 16:21:16 -0800, Joerg <invalid@invalid.invalid> wrote: [snip]> >Belts-and-suspenders would be to sense the output and trip a beefy SCR >right at the fuse.That's always been my philosophy. ...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 ●December 1, 20122012-12-01
On Fri, 30 Nov 2012 15:50:55 -0600, Jon Elson <jmelson@wustl.edu> wrote:>> 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.Please remember to put some ferrites in series with the SCR gate drive. I once made a power supply for a 100 W RF transmitter. When used with an indoor antenna, the RF signal could get into the protection circuit and trigger the SCR and blow the fuse.
Reply by ●December 1, 20122012-12-01
On a sunny day (Fri, 30 Nov 2012 13:17:59 -0800 (PST)) it happened panfilero <panfilero@gmail.com> wrote in <d5ff67fc-1014-4400-a9ca-ed103cad1805@googlegroups.com>:>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 popYes, I vote for SCR.
Reply by ●December 1, 20122012-12-01
On a sunny day (Sat, 01 Dec 2012 10:23:11 +0200) it happened upsidedown@downunder.com wrote in <1cfjb891c6qafs1hnce5ngtbu8q2t81lel@4ax.com>:>I once made a power supply for a 100 W RF transmitter. When used with >an indoor antenna, the RF signal could get into the protection circuit >and trigger the SCR and blow the fuse.LOL very long time ago, my first job, we were making big controlled rectifiers for the army, used to test their transmitters. We got a call that the voltage was not stable, went way up, and killed their transmission equipment. We went there, and sure enough, voltage went way up, measured some things, case was not grounded, electronics got the RF, bad things happened. After grounding the cases things worked normally.
Reply by ●December 1, 20122012-12-01
On Fri, 30 Nov 2012 14:43:08 -0800, mike <ham789@netzero.net> wrote:>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.With the battery attached, the overvoltage condition would be difficult to detect, until the battery was toast. A current regulator failure in this condition would likely blow the fuse without external aid. If it's a back-up battery, it should be connected into the circuit in such a manner that source failure (including the shorted condition) doesn't interfere with the intended back-up function. There ARE issues with scr failure, when improperly sized for large storage capacity and supply/fuse combinations. Parts intended for crowbar applications have I^2t ratings that are considerably larger that jellybean devices. MCR67,68 and 69 were examples of parts designed for this use. The latter two are still available. RL
Reply by ●December 1, 20122012-12-01
On Sat, 01 Dec 2012 09:02:20 GMT, Jan Panteltje <pNaonStpealmtje@yahoo.com> wrote:>On a sunny day (Sat, 01 Dec 2012 10:23:11 +0200) it happened >upsidedown@downunder.com wrote in ><1cfjb891c6qafs1hnce5ngtbu8q2t81lel@4ax.com>: > >>I once made a power supply for a 100 W RF transmitter. When used with >>an indoor antenna, the RF signal could get into the protection circuit >>and trigger the SCR and blow the fuse. > >LOL >very long time ago, my first job, we were making >big controlled rectifiers for the army, used to test their transmitters. >We got a call that the voltage was not stable, went way up, >and killed their transmission equipment. >We went there, and sure enough, voltage went way up, >measured some things, case was not grounded, electronics got the RF, >bad things happened. >After grounding the cases things worked normally.The dimensions of the human body match quite well with 1/2 or 1/4 wavelengths at VHF frequencies, thus the radiation limits for humans are quite low at VHF frequencies. However, at HF frequencies, the limits are quite higher, so in reality much higher levels are allowed, so in places into which people are allowed to freely enter, any equipment should also tolerate these field strengths. The situation gets worse, when long wires are attached to the device, such as speaker cables in an audio amplifier or power supply wires, forming antennas much longer than the human body. For this reason audio amplifiers and DC power supplies needs some filtering in the external wires (or at least in the feedback circuitry).
Reply by ●December 1, 20122012-12-01
On Sat, 01 Dec 2012 10:23:11 +0200, upsidedown@downunder.com wrote:>On Fri, 30 Nov 2012 15:50:55 -0600, Jon Elson <jmelson@wustl.edu> >wrote: > >>> 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. > >Please remember to put some ferrites in series with the SCR gate >drive. > >I once made a power supply for a 100 W RF transmitter. When used with >an indoor antenna, the RF signal could get into the protection circuit >and trigger the SCR and blow the fuse.Was that a gate issue or a problem with the control circuitry ?:-) ...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 ●December 1, 20122012-12-01
On 12/1/2012 5:22 AM, legg wrote:> On Fri, 30 Nov 2012 14:43:08 -0800, mike<ham789@netzero.net> wrote: > >> 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. > > With the battery attached, the overvoltage condition would be > difficult to detect, until the battery was toast. A current regulator > failure in this condition would likely blow the fuse without external > aid. > > If it's a back-up battery, it should be connected into the circuit in > such a manner that source failure (including the shorted condition) > doesn't interfere with the intended back-up function. > > There ARE issues with scr failure, when improperly sized for large > storage capacity and supply/fuse combinations. Parts intended for > crowbar applications have I^2t ratings that are considerably larger > that jellybean devices. MCR67,68 and 69 were examples of parts > designed for this use. The latter two are still available. > > RLI spent a significant portion of my career trying to train that kind of thinking out of engineers. Some engineers leap directly to telling you why your example is wrong. Many times, I've had to go back and fix their designs. The ones you want working on your project use the example to consider what real users might do with the product and come up with a more robust design. I don't remember the exact one, but I once had a commercial bench power supply that I wanted to use for charging batteries. Turns out, if you turned off the power switch before disconnecting the battery, it made a LOT of smoke.
