Generate power supply glitch - Page 1
Forum section: electronics / projects

author: DavidDLC
author_rank: Frequent Contributor
timestamp: May 09, 2012, 08:00:05 pm
content: I have a lab power supply, I want to build a circuit to generate a glitch to the power supply outputIt will be something like a power transistor or power MOSFET connected to the power supply and a signal gen or microcontroller to generate the glitch.My plan is to momentarily switch the device off, for 1 nS and up.Should I use a power transistor or a power MOSFET ?How do you determine if the power device is fast enough to switch 1 nS signal ?David.

author: Time
author_rank: Frequent Contributor
timestamp: May 09, 2012, 08:24:34 pm
content: depends on how much power you want to switch. n channel RF power mosfet is probably your best bet. IXYS, microsemi...etcLook at the turn on and turn off time characteristics.

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 09, 2012, 09:01:02 pm
content: Don't forget about IGBTs. Especially if you are switching a lot of power.

author: Rufus
author_rank: Super Contributor
timestamp: May 09, 2012, 10:09:08 pm
content: Quote from: DavidDLC on May 09, 2012, 08:00:05 pmGood luck with that. The inductance of a bit of wire is around 1.3uH per meter. If your supply leads total 1m and you are supplying 1A switching it off in 1ns will generate and inductive spike of around 1.3kV.

author: Time
author_rank: Frequent Contributor
timestamp: May 09, 2012, 10:30:21 pm
content: Quote from: Rufus on May 09, 2012, 10:09:08 pmThan it will be good test for the robustness of his psu design

author: Time
author_rank: Frequent Contributor
timestamp: May 09, 2012, 10:31:51 pm
content: Quote from: TerminalJack505 on May 09, 2012, 09:01:02 pmIGBTs are excellent for switching power but they are probably too slow for 1 ns.

author: Kremmen
author_rank: Super Contributor
timestamp: May 10, 2012, 06:19:16 pm
content: You can forget IGBTs. They are not at all suitable for timings like this. The slow turn-off of an IGBT is somewhat problematic already at the kilohertz frequencies where they are normally applied. Trying to use them at nanosecond timings is a non-starter.RF fets can operate at tens of GHz but as amplifiers, not switches. Also their parameters seem to be optimized for that kind of use. I suspect you will be hard put to find a sufficiently fast switch for the kind of power a typical supply puts out. But then it may be because i never looked for something like that. It would also be interesting to see the gate driver circuit that charges and discharges a power fet gate in 1 ns. Not saying it can't be done but it won't be easy.

author: robrenz
author_rank: Super Contributor
timestamp: May 10, 2012, 06:29:27 pm
content: What are you really trying to accomplish? Give us more specific details to work with. You may be stating specs that are way more than you need to accomplish your goal.

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 10, 2012, 06:33:14 pm
content: QuoteI threw IGBTs out there for consideration since the OP mentioned power transistors (presumably the OP meant BJTs.) When it comes to switching power, IGBTs are going to be faster than power transistors.From the looks of it, though, 1 ns is going to be MOSFET-only territory.

author: Kremmen
author_rank: Super Contributor
timestamp: May 10, 2012, 08:25:02 pm
content: Quote from: TerminalJack505 on May 10, 2012, 06:33:14 pmWithout wishing to start an argument here, if you check how it is, you will find that in fact IGBTs are the slowest of all switches. They can handle brutal amounts of current but their internal structure makes them inherently slow. There is the insulated gate as the name implies and just because it is insulated there is no practical way to externally speed the charge neutralization. That's why IGBT turn-off is slow. A decent power transistor is going to beat any IGBT in switching time, though not in current handling capacity.

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 10, 2012, 08:42:08 pm
content: Alright then. I'll take your word for it.I'm watching some power electronics lectures and I seem to remember the instructor saying one of IGBTs advantages over power BJTs was switching speed. I somehow got it in my head that an IGBT had the high voltage, high current capabilities of BJTs and the switching speeds of MOSFETs.I'll see if I can find the lecture where he talked about them to see what he was really saying.

author: Time
author_rank: Frequent Contributor
timestamp: May 10, 2012, 10:06:43 pm
content: Quote from: Kremmen on May 10, 2012, 06:19:16 pmAs long as hes not talking about too much power and voltage I have confidence that DEIC420 combo'd with one of their fets could possibly get him in the range he wants. Microsemi also makes some combo packages with 2 RF mosfets and 2 drivers all in one that are pretty slick (DRF1300 maybe for HV types?). None of these solutions would be cheap for a DIY budget though.modify:I just had another thought though...you can run your DC supply in constant current mode through an inductor forward biasing an FRD. Than, if you have it available, pulse the the diode with reasonably fast and large negative voltage (through some capacitance to isolate the pulser from the DC). Theoretically your PSU will see a very fast disturbance in current with time characteristics proportional to the Trr of the FRD.... I might go play with this idea though since it has me curious...hard to say if that will work just after these initial thoughts. Oh, youll also need some kind of cap and load on the other side of the diode to absorb the HV spike its going to generate. Check out application note 918 from HP back in the day.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 10, 2012, 10:39:38 pm
content: Generating a 1 nanosecond glitch is hard. Trying to get your general purpose power supply to switch like that is doing it the *really* hard way.If your circuit is relatively low power and voltage the easiest way to do this is to power it from a high speed CMOS logic gate and switch the logic gate. You can use an XOR gate with one input delayed to generate reasonably short pulses, and you can parallel a bunch of NOT gates to supply more current. For a higher current circuit you could use a power transistor right at the load to shunt the supply current to ground, but few power devices are fast enough.Another possibility is to couple pulses into your circuit via a bias tee. You can generate short pulses with an avalanche pulse generator or whatever and couple them into the circuits supply rail via the bias tee capacitor. This won't switch the supply to zero but will supply a positive or negative current transient. Remember, unless your circuit acts like a 50 ohm load you will get a strong reflection. If you want a single clean pulse make sure to terminate the source to absorb the reflected pulse.

author: amspire
author_rank: Super Contributor
timestamp: May 10, 2012, 11:41:20 pm
content: The regulator circuits of power supplies generally to not regulate much for any fault condition below about 10uS. For anything faster, the regulation properties are those of the output capacitor(s) and wiring inductances.I think to investigate the behavior of the active regulation of any power supply, nothing faster then 100nS will ever be needed, and that is very easy to achieve with almost any mosfet. A simple 20MHz microcontroller can accurately be used to generate pulses in 50nS steps from 50nS up to as big as you like, and that again is easily good enough.So if you are looking to test for performance of the supply with very fast glitches, it really just comes down to supply's output capacitor behavior and the inductance of the wiring in and out of the supply. In general, any load that may generate fact glitches will usually have on-board capacitors anyway, that due to their proximity will dominate anything in the power supply.In testing the behavior of capacitors, you can probably get as much from using a simple switch to switch in a load, and do a single shot capture or the voltage across the capacitor. I do not see any advantage of a 1nS glitch over a simple static off to on switch for determining the properties of a passive component like a capacitor. It would be OK to test the properties of the capacitors independently of the power supply. Mercury wetted reed relay contacts can provide an excellent sub-1nS switch-on.Testing the behavior of the power and ground rails and planes on a PCB board to 1nS transients is a different issue. PCB transient behavior and power supply behavior are two distinct problems.Richard.

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 11, 2012, 05:51:01 am
content: Quote from: TerminalJack505 on May 10, 2012, 08:42:08 pmSince I'm (informally) studying power electronics this has been rather insightful. I watched that lecture again and also did some research.In the lecture, the instructor stated that the turn-on times for IGBTs are on the same order as for power MOSFETs. He made the same points about slow turn-off times that Kremmen did. This is due to the so-called tail current which is caused by stored charge. Power BJTs suffer from tail current as well. Because of this, I assumed that an IGBT's turn-off time would be on the same order as power BJTs. The instructor never said one way or the other, however.So, is the switching speed of an IGBT faster or slower than an power BJT? I foundthis statementon International Rectifier's website regarding IGBTs: "They are faster and offer far superior drive and output characteristics than power bipolar transistors." Of course the statement is being made on one of their IGBT product pages, so caveat emptor.ThisIXYSIGBT Fundamentalsdocument states, regarding IGBTs: "Switching speed is inferior to that of a Power MOSFET and superior to that of a BJT."Interestingly,thisWikipedia article states that IGBTs have "already completely replaced the bipolar transistor in power applications." Apparently power MOSFETs and IGBTs are the only players in power electronics so far as fully controlled switches go.I didn't find any evidence to support the claim that a Power BJT could switch faster than an IGBT. Perhaps with a Baker clamp circuit they can. Apparently, Baker clamps can be used with IGBTs as well, however.

author: poorchava
author_rank: Super Contributor
timestamp: May 11, 2012, 02:37:00 pm
content: This might be a crazy idea, but generally TVS diodes (tranzorbs) are extremly fast reacting devices. Maybe this effect could be utilized. Dunno.... Connect TVS across power supply outputs, but use such that triggers at slightly higher voltage. Now connect a BIG capacitor to GND and charge it to voltage somewhat higher than the TVS trigger voltage. Now use something like SCR to dump the capacitor into positive supply rail. If you adjust voltages correctly the voltage on positive supply rail will increase triggering the TVS and momentarily shorting the whole thing to GND. This idea may be completly broken, it got into my mind 10s ago. I dunno if this is feasible, so don't blame me if something blows up.

author: Kremmen
author_rank: Super Contributor
timestamp: May 11, 2012, 05:07:44 pm
content: Quote from: TerminalJack505 on May 11, 2012, 05:51:01 amI admit that my statement was only based on knowledge of the fact that IGBTs have the tail current phenomenon that is absent from BJTs. I have never had a reason to compare BJTs and IGBTs of identical collector current specs because the IGBTs i have played with max out in the region of 350-400 amps. No BJT comes near that, but those IGBTs truly are slow.Since the BJTs are not subject to the tail current it would be reasonable that things being the same otherwise, a BJT would be faster. But then, as you pointed out, BJTs are on the way out from the power switching scene. Could be there is no recent comparable development on that side vs companies like Ixys, Infineon and others who have introduced some impressive devices during the recent times.

author: robrenz
author_rank: Super Contributor
timestamp: May 11, 2012, 05:09:17 pm
content: What happened to the OP?

author: Kremmen
author_rank: Super Contributor
timestamp: May 11, 2012, 05:14:48 pm
content: He probably expired a while ago reading our OT babble.