IEC 61000-4-5 Surge immunity test

[Mad ID]

Hello,
to obtain an EMC "Declaration of Conformity" my circuit needs to past 61000-4-5 test with 500V @ 2R internal resistance giving 8/20us 250A current pulse. During the initial testing the whole board got fried and I would like to get some advice from experts

First the info:
- Input voltage to the board is 9-30V, goes to a DC/DC +5V buck converter. Nominal voltage is 24V.
- Board is powered with 24V when 500V@2R surges with alternating polarity are applied on top of 24V. (1 minute pause between each, 10 times)
- The circuit which didn't withstand this is shown in the attachment.

Probably a very bad design decision, the DC/DC converter has an operating voltage range up to 32V (34V maximum rating). Nevertheless, I'm sure it's possible to design a surge protection which will give a only few hundred milivolt rise from 24V.

The varistor will clamp to 70V, by which time the capacitors will already be charged to 70V and the DC/DC fried. If I put a TVS diode in parallel with the varistor it dies during the first blow because it activates at much lower voltage than the varistor. TVS diodes are much better but there are only few which can withstand >200A @ 30V.

Here are my questions:
0. Would you use a DC/DC converter which has much larger operating voltage, like 50V? This increases cost of course..

1. Would you put a TVS instead of varistor? There are few which can take 250A but are very expensive.

2. How can I increase the impedance looking into capacitors which would drop this extra voltage and decrease the leftover current charging the capacitors?  10A of current during 10us will not charge the capacitors much, but 100A will. My first thought was to put an inductor, but won't the inductor saturate and loose all of its inductance when such a large current is applied?

3. Can a common mode choke help? In my understanding this is a differential surge and a common mode choke will have no effect?

4. Would you put the reverse protection diode before the varistors/other protection?

5. I think I also need a fuse up front, how can I be sure it will withstand the surge and not blow up? 

Thank you very much for your time.

[T3sl4co1l]

Try some resistance after the MOV (as much as you can tolerate, for acceptable losses at maximum load current).
Maybe even a second MOV after that.
Use bigger capacitors.
Current limiting devices (e.g., automotive load dump protection).
Crowbar type protection (thyristor type TVS, etc.).
Series fuse -- assuming it blows fast enough to matter, of course.  (Even the fastest fuses take nearly a milisecond to open, so that's probably not helpful here.  A fuse of any sort would be helpful to clear the fault and protect the wiring if a latching / thyristor type TVS is chosen.)

Series resistance helps by making a current divider.  So, the first MOV soaks up almost all the current and clamps the voltage to a more reasonable value, then your circuit only has to deal with the ~70V or whatever.  Which is still a lot more than 12V, but if you were to add another 2 ohms in series, you'd need to clamp only 30A, not 250A.

Note that a 20us time constant at 2 ohms suggests the test involves a charged 5uF capacitor; 100 times more, or ~500uF, will only be charged by 1/100th as much, or ~5V.  Assuming the ESR is equally low: 1/100th of 2 ohms is 20 miliohms, which might be practical using aluminum polymer type caps.  If you don't mind the price.

Correct, common mode doesn't matter here, and although you could filter the spike with inductance, it's not generally practical to do so.  Example: suppose you want to absorb the 500V spike, rather than shunt it.  It has a flux of about 5-10mWb, so for a current rise of say, 10A, that would require 500-1000uH, rated for saturation above 10A.  10A is certainly a lot more manageable, though still a lot of energy for an avalanche TVS to absorb (10A in 1mH is 50mJ, which -- maybe not too bad for an SMC size TVS, actually?).  A thyristor type would be great here, given the limited dI/dt and transient filtering action resulting from the inductance.  Or the dumb old MOV again.

Tim

[Precipice]

What actually fails? The anti-reverse diode or the switcher?
(I've got to admit, there's less headroom between 24V and 32V than I'd be happy with in a real product, if the power isn't completely under your control. )

I might be tempted by either a cheap but hair-trigger TVS diode after the input diode, where it will be somewhat protected (especially if you can sneak a resistor into the path), or just bite the bullet and fit a
http://uk.farnell.com/stmicroelectronics/sm30t28ay/diode-tvs-3500w-28-1v-smc/dp/2327845
at the front end. (Other TVSes exist, but, for all my ranting about ST, theirs do seem to mostly work)

I've pretty much abandoned anti-reverse diodes for this reason - they capture and prolong overvoltage. If it's acceptable to blow fuses, then a big single-ended TVS and a fuse protects most things. Polyswitch fuses if I need to make a pretence at recovering. Other than that, get a decent automotive rated PSU chip, and let them deal with the over/reverse voltage problems, and chop the top off big spikes with a TVS.

[qno]

Is the circuit Mains operated?

[AndyC_772]

I'd like to seek confirmation that this test is even applicable.

Where does the 24V supply come from in normal service? Are 500V spikes representative of some real-world scenario?

Even the ISO pulses for 24V vehicle systems aren't this big.

[Jay_Diddy_B]

Hi,
Here is an LTspice model for the pulse generator and some ideas that you can explore to protect the DC/DC converter. You will probably need a 60V chip for this application.



I have attached a zip file with the model.

The Transorb models are from Littelfuse.

Regards,

Jay_Diddy_B

[Neilm]

I'd like to seek confirmation that this test is even applicable.

Where does the 24V supply come from in normal service? Are 500V spikes representative of some real-world scenario?

Even the ISO pulses for 24V vehicle systems aren't this big.

It could be relevant if the DC supply lines were long, or it was explicitly required by the standard to meet it.  By long I mean more than 30m.

[Gribo]

I have just managed to pass the same surge test with a similar circuit. I have used a varistor (clamping to 60V), reverse protection diode and a TVS (Clamping to 45V). However, I have used an automotive rated LDO (NCV4274 from Onsemi) which can tolerate 60V spikes and load dumps.

[Mad ID]

Hi.
You have all been very helpful!

Series resistance together with more capacitance should do the trick even without expensive TVS..just with varistor and fuse up front.

Regarding the reverse protection, I'm not sure that the circuit would survive reverse polarity if it has to wait for a fuse to blow... probably depends of the circuit and the current the source can supply. e.q. in a lab with current limited power supply.

As for my next designs, I'll definitely use a 40V+ supply. Looks like it all depends on how big series resistance can be fitted in the circuit.

[AndyC_772]

It could be relevant if the DC supply lines were long, or it was explicitly required by the standard to meet it.  By long I mean more than 30m.

I think I've found why I've not come across this requirement before; I found an app note from a PSU manufacturer which states that:

"The surge immunity test is applicable to products, which
are connected to mains supplies or other networks leaving
the building."

I've designed dozens of dc powered products before, but they're all powered from a local mains adapter or a battery, and never a dc supply from another building. The mains adapter gets surge tested, of course, but not the dc input to the product itself.

[Precipice]

Regarding the reverse protection, I'm not sure that the circuit would survive reverse polarity if it has to wait for a fuse to blow... probably depends of the circuit and the current the source can supply. e.q. in a lab with current limited power supply.

The race is between the (unidirectional, in reverse mode) TVS and the fuse. Your stuff downstream should only see a volt or so of negative, while that race is on. But yes - if the supply droops far enough to stop the fuse blowing, the fuse will have to sustain that current for longer, which might be a thermal problem...

As for my next designs, I'll definitely use a 40V+ supply. Looks like it all depends on how big series resistance can be fitted in the circuit.

High voltage tolerant (particularly automotive) regulators aren't too expensive, and make a lot of these problems go away. Be warned that some LDOs protect themselves by disabling the output during high voltage events, so if you can't tolerate a power dropout / reset, don't use those! (Switchers obviously come with a burden of filtering to stop noise leaving on the DC supply lines, but switching frequencies keep getting higher, making filtering cheaper, easier and smaller...)

[Richard Head]

I concur with AndyC_772
Trying to get a small DC-DC converter (or even a large one!) to comply with that is ridiculous.
I have designed mains PSU's that have to comply with that and that's tough enough, but for a 24VDC input it's crazy.
If I could show you the size of the capacitor bank that is used to generate the surge you would be shocked.
My first PSU came away with plenty of loose parts inside and a burned smell. My test was 3kA 8/20us pulse.
By the time the MOV's clamp the voltage is about 900V peak.

[Mad ID]

The race is between the (unidirectional, in reverse mode) TVS and the fuse. Your stuff downstream should only see a volt or so of negative, while that race is on. But yes - if the supply droops far enough to stop the fuse blowing, the fuse will have to sustain that current for longer, which might be a thermal problem...

Oh  my fault The circuit will survive.
I will definitely consider omitting reverse polarity diode (i.e. use unidirectional TVS) for designs which are susceptible to surge events.

[T3sl4co1l]

The race is between the (unidirectional, in reverse mode) TVS and the fuse. Your stuff downstream should only see a volt or so of negative, while that race is on. But yes - if the supply droops far enough to stop the fuse blowing, the fuse will have to sustain that current for longer, which might be a thermal problem...

I disagree: if that power supply is backed with a nice beefy capacitor, that diode might very well end up toasted.  The voltage drop might spike to 3-5V in the process, maybe even more.

It cannot be stressed enough that fuses blow in the milliseconds and semiconductors in the microseconds!

Diodes at least are generally quite robust, and occasionally can be saved by suitable choice of fuse.  Of course, they don't provide I^2*t for TVS diodes, so you'd have to guess at that one.

Tim

[Precipice]

I disagree: if that power supply is backed with a nice beefy capacitor, that diode might very well end up toasted.  The voltage drop might spike to 3-5V in the process, maybe even more.

Partly - but we've established that the use here has long cables feeding the power in - there's some useful resistance in there. (possibly enough resistance to stop the fuse blowing, of course, you can't win them all, but, on the upside, the fuse has resistance too...). In my experiences, at least 1.5V is transiently surviveable by most power devices. It's never guaranteed or specified, of course...
1.5V gets you 100A in figure 9 of http://www.farnell.com/datasheets/1716635.pdf , which should start most fuses popping.


In using beefy TVSes, I've seen similar I^2t for the forward and reverse directions - it's the same die after all.
OP is still definitely stuck with a hard problem - but it's a problem I'd attack with TVSes. Quite big TVSes, and not complete confidence. A 'proper' solution would have him gating the power off, but I don't know if the budget or space allows, or if his spec requires, or whether this is just a 'nice to have'.

[HackedFridgeMagnet]

Polyswitch fuses if I need to make a pretence at recovering.

Nice one.

BTW can't you do the surge stuff separately? Whats the application that needs to satisfy this test.

Would a Fuse and MOV do the job?

[Mad ID]

Whats the application that needs to satisfy this test.

Here in Europe it's a standard CE mark EMC Declaration of conformity for a DC powered device.

[Precipice]

Hang on - is this a product you're selling with a power adaptor, for a relatively normal indoor use?
If so, I normally see them tested as a pair, with the power supply - no directly injecting surge into the DC power. Standard immunity stuff, but not this...
There's not a consumer product I've ever seen that would survive what you're testing on the 5V or 12V supply.

Edit: other people have asked this before me in this thread. It's the big question...

[AndyC_772]

Whats the application that needs to satisfy this test.

Here in Europe it's a standard CE mark EMC Declaration of conformity for a DC powered device.

No, it's not. Having to cope with a 500V spike is anything but 'standard'. It's a very severe test indeed, and not one I've ever had equipment subjected to in nearly two decades of designing electronics for sale.

Please, before you go any further in bullet proofing your product, go back and check exactly which standard you're testing to, and that this surge test is actually applicable and required.

If the product you're making is an ordinary piece of consumer or industrial kit, and it'll be sold with a mains adapter, then I can virtually guarantee that your circuit doesn't need to meet this requirement.

The mains adapter it's sold with does need to pass a surge test, but that's another story - and 99.9% of the time you'll find the adapter has already been tested by its manufacturer. If your combination of adapter + product fails surge testing (in which the surges are applied to the ac mains, not the dc side), use a different adapter.

[Precipice]

Also: Does your test house think you should be applying this test, or are you doing it all in-house and interpreting the standards yourself?
If the latter, I'd have to recommend some reading other than the standards - I'm not sure what books are current and good, but there must be some.
Suggestions, anyone?

[AndyC_772]

I'd start by going back to the lab and asking them to show which standard they're applying to the product and why they believe this specific test applies.

Then, I'd go to another lab, and ask them for their opinion on which tests would be applicable. See what the correlation looks like between the two.

I'm happy to swap lab recommendations - or otherwise! - by PM.

[Mad ID]

Hang on - is this a product you're selling with a power adaptor, for a relatively normal indoor use?
If so, I normally see them tested as a pair, with the power supply - no directly injecting surge into the DC power. Standard immunity stuff, but not this...
There's not a consumer product I've ever seen that would survive what you're testing on the 5V or 12V supply.

Edit: other people have asked this before me in this thread. It's the big question...

Hi, sorry for not stating this clearly.
The answer is no, the product is not a consumer but an industrial one which is used in 24V trucks. DC power comes from truck's 24V internal power bus. No adapter involved.

[Mad ID]

Also: Does your test house think you should be applying this test, or are you doing it all in-house and interpreting the standards yourself?
If the latter, I'd have to recommend some reading other than the standards - I'm not sure what books are current and good, but there must be some.
Suggestions, anyone?

Hi, I'm not doing the tests.
The test are being done by a testing laboratory which is certified for EMC testing here in EU.

I asked multiple times, this surge is applicable in my case and they test many products with the same surge setup.

[AndyC_772]

To the best of my knowledge, the standard applicable to 24V vehicle powered equipment is ISO 7637-2.

I'd still strongly recommend checking with a different lab before incurring any more costs redesigning your product.

[Precipice]

If it's for an OEM, each has their own book of rules, and between you, you generate a test plan that calls for specific tests in specific ways, and you then test to that plan. I've just finished a GM product, that was hell on earth Next time, I shall campaign for having a say in the test plan...