SCR crowbar for DC Buck converter - Page 1
Forum section: electronics / beginners

author: Justanotherhobbyist
author_rank: Newbie
timestamp: December 06, 2019, 02:23:01 am
content: Hi,First of all, thanks for taking the time to look at my post. I have a servo (hitec hs-1100wp) in an R/C car that is intended to run off a 4s lipo. 14.8v nominal, or 16.8v when fully charged. The servo will draw 6.5a at stall. I have two 6s lipos in series (50.4v) for the ESC. I would like to use a buck converter to bring the main packs voltage down to 14.8v for the servo. This is the one I currently have in mindhttps://www.amazon.com/Diymore-Synchronous-Converter-Step-Down-Module/dp/B072BN43P8While looking at buck converters, I noticed a common failure mode is to allow full input voltage to go through the output. I'd rather not toast the servo. That brought me to the SCR crowbar.I think I understand the concept, but I'm having difficulty choosing the components. I'd like to use a 10a fuse, and have it blow around 16v. Browsing digikey for the SCR is where I've gotten stuck. The max gate voltage of their 12a SCRs don't come near 16v. Should I use a voltage divider between the zehner and the SCRs gate? Am I missing something here? Any input will be greatly appreciated.Reference specs:Battery voltage range: 42-50.4vBuck Converter: 8-60v input 1-36v output 10a "normal" 15a maxServo requirement: 11.1-16.8v (ideal would be 14.8v) 6.5a max

author: JustMeHere
author_rank: Frequent Contributor
timestamp: December 06, 2019, 03:35:38 am
content: Using a zener and a fuse should do the trick. Set it up so that if the zener starts conducting it will draw enough current to pop the fuse.Here's something on that:http://www.learningaboutelectronics.com/Articles/Overvoltage-protection-circuit.php

author: Ash
author_rank: Regular Contributor
timestamp: December 06, 2019, 04:28:19 am
content: Hi,Here is one I made for a local Amature radio club:https://github.com/AshleyRoll/HamProjects/tree/master/psu-crowbarit is adjustable, and maybe can give you some ideas..Ash.

author: Ian.M
author_rank: Super Contributor
timestamp: December 06, 2019, 05:07:57 am
content: The problem with a simple Zener and a fuse, is the fuse has to be on the output and the Zener has to be able to blow it. The fuse needs to be 7A so it doesn't blow on repeated servo stalls, and at a 16V clamping voltage will need to pass several times the rated current to blow it fast enough to do any good. At 7A its instantaneous dissipation at its clamping voltage would be 112W. Multiply that by whatever overload factor you get from the fuse's characteristic curves to blow it quickly enough . If its under-rated for peak power, it will fail before the fuse does and there's enough energy available to blow it open rather than shorted, which is 'GAME OVER!'Therefore you absolutely need that crowbar SCR.The voltage reduction ratio at min. input and max. output voltages is 2.5. Therefore, ignoring losses due to less than 100% efficiency,Iin=Iout/2.5so the input can be fused at a substantially lower current than the output. At higher input voltages and lower output voltages it will draw less current. A 3A fuse on the input should be able to supply >6.5A at the output and will be *MUCH* easier to blow reliably with a crowbar circuit.However IMHO a 16V crowbar threshold with a 16.8V abs max load voltage rating is unrealistic. You'll have difficulty getting a close enough tolerance Zener. Use a 15V 5% one and worst case it could be anywhere from 14.25V to 15.75V, a 1.5V range even before you add the tolerance of the SCR Vgkat its trigger point.You've only got 2V 'headroom' from nominal to max load voltage, and you've already used up 75% of that for zener tolerance!Personally I'd use a TL431A 1% adjustable shunt regulator and start with the crowbar circuit from itsT.I. datasheetfigure 22, (page 27) modified to separate the rail being crowbarred by the SCR, from that feeding the TL431 and being monitored, by adding a PNP transistor as per figure 21, with its collector driving a SCR gate, and a gate pulldown resistor, so the TL431 can be fed from the buck converter output as its abs.max. Vgkis only 37V, less than your battery. voltage. Put a low value resistor (a few tens of ohms) between the PNP collector and the gate to limit the gate current so tripping it during testing doesn't damage the PNP or the SCR.Test with a car bulb load and a 120V oven bulb in series with the SCR, so it can be tripped non-destructively, by cranking up the buck output till it trips, then if you are confident the SCR is beefy enough, do a final test with a real 3A fast blow fuse and no oven bulb to confirm the crowbar can clear the fuse without damaging itself.Ash posted while I was typing this and his crowbar circuit is very close to my suggestion except for where the SCR anode connects to.Add another TL431 Anode and Cathode in parallel to the over-voltage detection one, and feed it Ref pin from an adjustable (or carefully precalculated) potential divider including a NTC thermistor in its upper arm in contact with the buck converter heatsink and you can then trip on overheat hopefully before the buck converter blows. If you are 'gold-plating' it, use a Zener to stabilise the thermistor supply voltage so the trip point doesn't change with output voltage.

author: Justanotherhobbyist
author_rank: Newbie
timestamp: December 06, 2019, 05:26:23 am
content: JustMeHere,The problem with that, is that the servo won't always be at stall, it may never be. (Steering, with a servo saver) So I would need to account for the overvoltage situation occuring at the idle current of 130ma (Driving straight, sitting still). I'd need at least 160w worth of zener to blow a 10a fuse. I thought about that initially, but after seeing the size and price of larger zeners I ended up at the SCR. I do remember reading speed was a problem too.

author: Justanotherhobbyist
author_rank: Newbie
timestamp: December 06, 2019, 05:35:19 am
content: Thanks Ash,I'll see if it can give me some insight. Most of what I found regarding SCRs was based on crowbars for converters stepping down X to 5v at relatively low currents for USB devices. Your design goes above and beyond my current requirements, so that helps.

author: Justanotherhobbyist
author_rank: Newbie
timestamp: December 06, 2019, 06:14:04 am
content: Ian.M,I appreciate the recommendation, I wanted the input fused so that's two birds with one stone. I can use Ash's example along with the T.I. datasheet as a reference, that's great. My background in designing circuits is non-existent, thanks for working with me on this.Is over temp the most common cause of failure for these converters? I have a telemetry radio, monitoring the temp and having an alarm set would be simple. Steering isn't something I'd like to lose if it can be avoided.

author: Ian.M
author_rank: Super Contributor
timestamp: December 06, 2019, 08:01:10 am
content: It depends on if the module has adequate cooling. e.g. the first review of your module on Amazon says it overheated in 10 minutes bucking 26V to 12V @110W, which is just over 9A. Presumably that was on the bench in free air. Outside on a hot day with restricted ventilation in the electronics bay of a model, its likely to be good for far less than that without forced air cooling. If you exceed the Tj_max*of the switching device under load, the chances are it will fail shorted sooner rather than later. OTOH if its only supplying the steering servo, its unlikely to be heavily loaded unless you are doing tight figure 8's or the steering is failing mechanically, so normally it should run fairly cool.Ideally you'd have telemetry for the temperatures of the main battery pack, ESC, main and servo motors, and this buck module and possibly the ambient in the electronics bay. Battery pack temperature is probably the most important as that can give you enough warning to save the model if you are quick, if the pack starts cooking off, and temperature directly affects battery lifespan so if its running hotter than normal you may wish to back off a bit.* Tj of the device is the temperature of the silicon die inside it and if its dissipating significant power will be quite a way above the mounting surface temperature and even further above the heatsink's fin temperature, so set your over temperature trip point conservatively.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 06, 2019, 09:12:57 am
content: Choose an SCR with a fusing I^2t rating several times higher than that of the fuse.Expect peak fault current in the 100s A range. Design accordingly, for trace and wire sizing, etc.Place the fuse at the battery, not the servo; that way the converter (or what's left of it) gets disconnected under fault. Who knows what else in it could burn up. Doesn't need to be a whole-battery fuse, but at least for this branch I mean.Most crowbar circuits should suffice; I'm partial to a TL(V)431 with positive feedback path (uses a PNP and SCR) so the idle current is modest and turn-on is sharp. Can also be done with a transistor and zener in place of the TL431.Tim