long term durability and drift of a zener biased NPN "pre-regulator" - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: January 28, 2017, 09:05:53 pm
content: I have a device that draws a small amount of current (~25mA) and is powered from a 28V supply, but that supply, being automotive, can spike up to over 100V occasionally.I don't want to use a switching type voltage dropper, due to noise considerations. So i am considering using a normal linear regulator, but putting it behind a high voltage NPN transistor, whose gate is biased to say 10V by a resistor and zener diode. This acts as a fairly course pre-regulator for the actual regulator.My question is "does this sort of arrangement have any long term durability issues"?? I've heard reports of the unreliability of zeners in this sort of arrangement, but i wonder if that's more of a drift type concern, which does't matter in this application, as long as the pre-reg output is suitably high enough above the linear regs drop out voltage.

author: Benta
author_rank: Super Contributor
timestamp: January 28, 2017, 09:39:19 pm
content: If you want to make it rugged, a wirewound series resistor followed by a MOV across the terminals of your device will do the trick.As you say, we're talking spikes here.

author: Ian.M
author_rank: Super Contributor
timestamp: January 28, 2017, 09:42:28 pm
content: Why would you want to put a silicon junction directly in the way of 28V system automotive spikes? That's asking for trouble getting a big enough SOA to survive a load dump and avoiding secondary breakdown. You'll have to severely over-specify the transistor. Also unless you add a high voltage blocking diode, large negative going transients will degrade the transistor by Zenering its b-e junction, discharging the regulator's input cap.KISS - pulse rated (10x overload for 5s) wirewound resistor as a series dropper, 50V decoupling cap before the regulator shunted by either a TVS with a voltage rating above the 28V supply if the regulator has enough headroom, or a parallel pair (in case one goes O/C) of 3W 1N5913 series Zeners to shunt pre-regulate to whatever you need.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 29, 2017, 05:19:29 am
content: Quote from: Benta on January 28, 2017, 09:39:19 pm"Spike" is kind of a ponderous term for something that swells up and down on a human time scale.MOVs can absorb the energy of a load dump, but they don't solve the excess voltage problem (typical MOV clamping is about triple the nominal operating voltage).If your load current isn't big, a high voltage pass regulator will ride through very nicely. And TIP31C are plenty cheap!Tim

author: max_torque
author_rank: Super Contributor
timestamp: January 29, 2017, 12:11:46 pm
content: That was my thought. I could fit a 250V transistor, which cost pennies, say something like this:http://docs-europe.electrocomponents.com/webdocs/1276/0900766b81276ac8.pdfDuring the spike (actually "load dump") that transistor would obviously have a large power dissipation (at say 50mA current a 100V spike means 4.5W) but those load dump spikes only last around 500ms, so total energy is just 2.25J.With a passive resistor, there is going to be a compromise in terms of the resistance that can be used, and hence the requirement to limit downstream (of the resistor) voltage with a large (expensive) MOV or TVS etc

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 29, 2017, 01:13:58 pm
content: Nice thing about a BJT is it doesn't drop much voltage, just Vbe. You can't use an NMOS the same way without losing Vgs(th), plus some change; and you can't use a PMOS that way at all (without additional control circuitry).That said, depletion mode MOS aren't unavailable, and can be used for inline current and voltage limiting applications, while dropping very little (only Rds(on)) normally. They're usually more expensive and not available in such ratings, but it's worth dropping the idea.The other downside about load dump is, even if your current consumption is small, the duration is long, so the temperature rise is considerable. Big transistors become very attractive, even if you don't need the extra electrical ratings. A DPAK can only absorb ~100mJ over such a time scale; D2PAK is good for several J. (IIRC -- check the transient thermal resistance plot to be sure.) So, you can't cheap out with tiny parts when you've got to handle high load currents, or for hot-plugging load switches, or electronic fuses, which work the same way.Once you've got that settled, an MOV on the input will save you from fast spikes, or from extreme load dump, should it occur. Don't forget a fuse!Tim

author: David Hess
author_rank: Super Contributor
timestamp: January 29, 2017, 04:31:17 pm
content: I have done this before and it works great. I went a step further and used the transistor/zener combination as a preregulator by referencing the low end of the zener to the regulator's output instead of ground but either way can work. Line and load accuracy and long term stability of the transistor/zener combination is not a problem.Improvements include using current bias for the transistor instead of a resistor; this prevents the zener current from increasing with input voltage which would be desirable to support higher input voltages. If high input voltages are possible for an extended time, then I would implement a circuit to shut down the input or lower the output current after a defined time sort of like fold-back current limiting or safe operating area protection.If a higher voltage drop is acceptable, then a MOSFET may be desirable for its lower gate current. Or a depletion mode MOSFET can be used but they are more expensive. In a high current design watch out for secondary breakdown of bipolar *and* modern vertical MOSFET devices.The first time I did a circuit like this, it regulated 28 volts down to 12 volts and I tested it by plugging it into the 120VAC wall socket. It would have withstood 240VAC just as easily.