Overvoltage protection design ideas

[ben7]

Re: Overvoltage protection design ideas

The simplest thing I can see doing is to connect a suitably sized power transformer across the AC input before the UPS transformer, and connect the output of the power transformer in series, anti-phased, with the UPS transformer primary winding. This will reduce the input voltage that the UPS transformer sees, thus reducing it's output voltage.
(Note that connecting the output of the power transformer in-phase with the AC will INCREASE the voltage the UPS transformer sees.)

Of course, this might cause more voltage drop during high loads, but it will give you headroom so that if the mains input is higher, you won't violate the DC-DC regulator input voltage ratings.

As for other solutions, there are ICs made by some manufacturers that can drive FETS, used as electronic fuses, or surge stoppers.
Check the LTC4366 as an example.

-Ben

P.S. In the case of the LTC4366, you'll need to make sure the FET can handle the power dissipation if/when there is a surge. So pick a FET that has a large SOA (Safe Operating Area). You'll need to make sure you set the overvoltage timer, so that the transistor is kept under it's SOA. (SOA varies, depending on time and power dissipation.)

P.S.2. The LTC4366 is a small chip, dunno if you would be able to solder it! Linear Technology has some other similar chips, possibly in a larger package, so you should check out their site.

[Dannyx]

Re: Overvoltage protection design ideas

That LTC chip looks very interesting and fit for the purpose. Had a look at its datasheet - looks complicated in there but I'm sure I'd be able to pull through. Soldering hasn't been an issue to me for quite a few years on, so no worries there It actually HAS to be small, since space inside my enclosure is very limited....I'd still need the series pass element to be bolted to a heatsink which requires additional space, so I'm not sure if it's doable at the end of the day...

[Badee]

Re: Overvoltage protection design ideas

that sure is interesting. any new updates about it?

[redwire]

Re: Overvoltage protection design ideas

LTC4366 used on a 24V truck electrical system, and they all died.
Lovely datasheet and the IC looks good but it cannot take any negative input voltage spikes- common occurrence in a car or truck. Have to add a bunch of circuitry for that, Fig. 8.

You can either clamp an overvoltage spike, turning it into heat in some component (i.e. TVS, MOV etc.); or you can switch off power or ride-through the overvoltage like the LC4366. The MOSFET gets very stressed in linear-mode trying to regulate say 24VDC out and 250VDC input spike.

[Dannyx]

Re: Overvoltage protection design ideas

So basically there is no "ideal" solution and it's all about compromise from what I gather. The simplest form is to just put a zener diode across the thing you want to protect and have it blow the fuse if it exceeds the zener voltage. In a car, I assume this wouldn't work to protect a small electronic device because the thing would blow fuses every time when there's a spike over the zener voltage like when the car starts. Makes me wonder how headunits and amps are designed, since those connect straight to the battery, so they're constantly exposed to ramps and spikes.

[Longbow]

Re: Overvoltage protection design ideas

There IS an ideal solution to your problem, as described. A box of units cobbled together is not going to require high level engineering for you to get into operation. In fact, at this point you don't even know if your unit will function as expected over a long haul. You have a very common requirement for inrush protection. In this case you are not too worried about overloading the mains, but simply providing a soft start situation for your equipment. Thermistors are used industry-wide on all types of equipment and I recommend that solution because it is relatively simple, inexpensive and effective. The only difficult part is finding the correct resistance value and dissipation.