Simulating an automotive load dump with a bank of charged capacitors?

Question

I've been working on a data logging device that will eventually go into my personal vehicle. I've scoured the interwebs and built what, in theory, should be a fairly stout power supply capable to handle fast transients and undervoltage and the fabled load dump. My problem is, though... I want to actually test the power supply.

In previous questions that I've asked related to the design of the power supply portion, I've been told that the "right way" to simulate a load dump is with a proper, preofessional testing device. I'm curious, though... what's stopping me from rigging up something that is "close enough"?

My idea is simply this: get a low-voltage DC power source, such as an old PC power supply. Get a DC-DC converter to step that 12V up to something in the 80V range. Assemble a bank of capacitors to get me in the neighborhood of a theoretical 12V load dump.. which from what I can tell is in the hundreds of joules range. Once charged, simply connect it to the battery input of the device - which wouldn't be connected to an actual power source lest I want to potentially screw that power source up - and see if explodes, and if not, see if it works normally afterwards.

Does this sound like the dumbest idea you've ever heard? Is it a somewhat decent idea that simply needs a lot of planning and number checking to in order to work roughly as intended?

Answer 1

A load dump is what happens to an automobile electrical system when a large load (such as the headlights) switches off. The problem is that the charging system (primarily the alternator) has significant inductance, and any attempt to rapidly reduce the current draw results in an "inductive kick" that creates a large voltage spike on the 12V bus. This kick is the same phenomenon used to create the spark in the ignition system, just a different manifestation of it. The point is, any equipment that's attached to the 12V bus needs to be able to withstand these occasional 100-200V voltage spikes without damage.

Since load dump is primarily an inductive phenomenon, it would probably be easier to simulate it that way, too. You don't really need to simulate the full energy of an actual automotive load dump; you just need to create the same voltage waveform across the supply terminals of your device.

Put a largish inductor (L1, on the order of 1H, perhaps the primary of a large power transformer) in series with your device (i.e., connect the device to the power supply through the inductor). This represents the inductance of the automobile charging system.

Put a few µF of capacitance (C1) across (in parallel with) your device; this represents the distributed capacitance of the automobile wiring, and helps to limit the risetime of the load dump event. Make sure this capacitor is rated for a few hundred volts.

Put a 120Ω resistor (R1) in parallel with your device, too. This represents other static loads within the automobile, and will set an upper limit on the peak voltage that the load dump creates. (This resistor will be drawing 100 mA and dissipating 1.2W.)

Now, connect a low-value, high-power resistor (R2) across your device, in series with a switch (SW1). This represents the load that is going to get "dumped". The value of the resistor should be such that the DC current doesn't exceed the power supply's capability, and you can adjust the value of the resistor to change the current with respect to the value of the inductor to dump a specific amount of energy (0.5×I²×L). For example, if your inductor is 1H and your resistor is 12Ω (@ 12W), you'll be drawing 1A, and the stored energy will be 0.5 Joules.

Close the switch to "charge up" the inductor, then open it — there's your simulated load dump event. With these resistor values, the peak voltage will be on the order of 100-120V. You can use different combinations of resistor values to simulate different kinds of events. The ratio of R1 to R2 approximately determines the peak voltage of the spike (relative to the power supply voltage). Scale both resistors downward to simulate higher-current (higher-energy) events. Make the capacitor smaller to get faster risetimes; 1H and 1µF resonate at 160Hz, which gives you a fairly leisurely 1.5ms risetime (1/4 cycle). For example, changing C1 to 0.01µF would give you a risetime of about 150µs.

Answer 2

You're on the right track, actually.

There aren't many generator designs out there, at least that I've seen, other than commercial test equipment of course; but I have seen two online. I will present them here.

JASO D 001-94 Type A-1

This standard specifies the RC network, generator circuit, and provides suitable values to use in it. It looks like copies may be available online. From p.8-9:

Interestingly, they also suggest using an alternator as such (Fig. 5)!

Here are the values. Type A is for 12V systems, B for 24V. Type 1 is the "load dump" surge in question.

Backfeeding is easily avoided: usually a diode-wired-OR connection allows the surge generator to drive the load during, while the main (12V) supply provides steady power otherwise. This diagram shows a diode from E1 only, but a diode from the generator side to the EUT (Equipment Under Test) could easily be added, making suitable adjustments for voltage drop.

Note that the risetime of the switch closure will be extremely short; this violates the specifications of ISO 7637-2 or ISO 16750-2, for which an RC filtering circuit (of low ~ms time constant) would have to be added, and the amplitude adjusted to account for peak loss through the filter.

On the other hand, perhaps you just want a representative idea of how well your device performs, and the fast risetime provides an opportunity to observe how tolerant it is of faster transients, or hard switching. Definitely some possibility to discover ringing or overshoot of ceramic capacitors this way.

DC1950A

An active generator design is available from the good fellows at Analog Devices (née Linear Technology):
DEMO MANUAL DC1950A: Load Dump Generator | Linear Technology
This circuit uses a fixed power supply (could also be a capacitor charged up by a much weaker supply), and a couple, rather beefy, MOSFETs to deliver the pulse. Given the power dissipation ratings, and specialized type (IXYS/Littelfuse Linear L2: suitable for linear amplification use, wide SOA), you'd have to spend a few bucks building one -- but it's a pretty straightforward circuit, and looks competently designed.

I would say IXTX90N25L2 and IXTX110N20L2 are both acceptable here. You might make do with classic HEXFETs such as IRFP260 or bigger, but you'll have to evaluate the SOA yourself -- a problem exacerbated by the fact that no datasheets beyond the 1980s originals specify DC SOA, and most likely you'll need to use many more in parallel. The circuit is easily scaled to more parallel devices, at least.

Do not use modern MOSFETs, or BJTs, by the way. Or, not just anything. As a linear application, DC (or at least ~100ms) SOA is required here, and most parts exhibit 2nd breakdown (localized spot heating leading to instability and runaway failure). Check the datasheet, and see how much power can be dissipated, continuously (or at least for 10s ms), at significant voltage; if there's no such rating, skip it and go to the next.

The main reason to avoid modern MOSFETs though, is they're just so much smaller for the same V, I ratings. Power dissipation capacity maps to die size, and they've done a tremendous job shrinking die area (maximizing power-switching density).

Even if you had wide-SOA BJTs (there are some still available for audio power amplifier use, that would work here, actually), they would need further circuit changes anyway. But not a whole lot; say, an IRF640 source follower into BJT bases, would solve that pretty neatly. In other words, making a sort of hybrid Darlington device, in place of the MOSFET(s) as shown. Or in still other words: to basically construct a variant IGBT. Oh, on that note: don't use IGBTs either -- same problem, no SOA. Although, I have actually seen a couple IGBTs with DC SOA, which, I don't have a clue how or why they do (why did they bother testing it? How does the design not runaway instantly?), but, there are indeed a few out there, so... that's interesting?

Cheaping Out

If you know, as a matter of design principle, or by measuring it directly, that your device does not draw significant current during the surge, then the generator impedance can be much higher, while still developing the specified voltage waveform. This greatly reduces the size of transistors and capacitors (or power supplies) required.

This is, in effect, the basis of Dave Tweed's answer; though the inductive method suggested, has the drawback of having to locate an inductor of rather unusual rating[1]. Still, it can produce such a waveform, given you don't need to draw much current in the process.

I would guess all commercial units use capacitive energy storage, and if any use magnetics, it's going to be with a real spinning alternator.

[1] A 1H 1A+ inductor will be quite large, and it can't saturate, you can't simply use a power transformer for it (there must be an air gap in the core to store energy). Though, one might profitably look into microwave oven transformers, where the core can be split open (with a hacksaw or grinder), and an air gap introduced; the high-voltage secondary can be removed, and even replaced with another primary, if one has a matched pair of transformers to scrounge from. (The secondary has far too much resistance to be useful here, unfortunately.) I still don't think the inductance will be quite high enough, but maybe with a pair of 240V primaries wired in series, it'll get close..?

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Since writing this answer, I've explored the subject in further detail on my website:
Article: Automotive Load Dump Introduction | Seven Transistor Labs
Some interesting findings include:

• The DC1950A generator, with transistors as given, is actually, probably, capable of full load dump, into any load including a short circuit. Here's the simulated transistor power curve for limiting conditions specified in the paper:

  
  ^{Source: https://www.seventransistorlabs.com/Articles/images/LoadDump8.png}

  A single cheap or salvaged transistor could be used at the described ratings, making this an attractive possibility for testing high-resistance loads (i.e., using none [full ride-through], limiter, or switch-off type protection).

• A passive RCRC network is a feasible solution, if one has a diversity of capacitor values (including some fairly large ones) and power resistors. 12V, 87V, 2Ω, 350ms example:

  
  ^{Source: https://www.seventransistorlabs.com/Articles/images/LoadDump2.png}

  Component values are also tabulated for other pulse impedance and timing parameters. The simulation circuit above shows a virtual switch of course, but an SCR will serve ably (probably ≥ 50A rating is desirable).

• Improving on the linear amplifier, a switching power supply can be used, greatly extending the operating area and allowing the use of relatively meager transistors, and extending the pulse energy available from given capacitors (i.e., energy isn't being wasted in resistors). This is a much more complicated solution, but I present a fully detailed design here:
  Article: Automotive Load Dump Switching Generator | Seven Transistor Labs
  Photo of the prototype:

  
  ^{Source: https://www.seventransistorlabs.com/Articles/images/LoadDump_Power1.jpg}