Electronics > Projects, Designs, and Technical Stuff
Exploding IC in buck converter
Siwastaja:
36.8V nominal breakdown TVS & 40V abs. max spec - not gonna work.
The actual clamping voltage, when current flows through the TVS, is much higher than the nominal value.
Add unit variation on the top.
Teun:
Ok thanks! Trickery is that I need a reverse voltage of at least 28V for the TVS. So that's just not possible?
Ice-Tea:
The SMAJ26CA is at least closer: 28.9 breakdown (satisfies your 28V minimum) and 42.1V max clamping voltage. Still beats your 40V max, but better at least and in a lot more cases it will not surpass the 40V max (when the current isn't too high).
Still, not a good way to do things. You simply need a higher rated DC/DC controller.
Siwastaja:
The problem with TVS diodes is, they are not precision devices. The clamping voltage depends on current and unit variation. This is why the downstream components need to be rated higher. If your input is x volts, and you pick nominal clamping voltage of, say, 1.25*x to ensure it's never unintentionally clamping, then the worst case clamping is at least 1.5*x or maybe even closer to 2*x.
The problem in automotive systems is, the voltage varies quite a lot even in normal conditions. Assuming this is a 24V automotive system, you don't want to clamp at below 30V, otherwise you'll be dissipating power in the clamp unnecessarily. (BTW, you should have a thermally coupled polyfuse to protect against long-lasting slight overvoltage.) Now, if you pick the closest minimum guaranteed clamping voltage (with acceptable leakage figure) from 30V, the nominal clamping voltage will end up somewhere between 35-40V, and the worst-case, under load clamping will be somewhere around 45-55V. There is no easy way around this; I'd strongly consider replacing the DC/DC controller IC to something that can handle at least 60V absolute maximum.
--- Quote from: Ice-Tea on April 01, 2020, 07:37:58 am ---The SMAJ26CA is at least closer: 28.9 breakdown (satisfies your 28V minimum)
--- End quote ---
For an even remotely robust system, you need to look the "working peak reverse voltage" spec, which is only 26V; it will likely conduct and leak excessively in a 24V automotive system when the engine is running, causing another set of problems (excessive heating, possibly only in some units, which may only become a problem after many hours of runtime...) . SMBJ30CA would be the lowest acceptable from that series, but then again, we are back to 48.4V max clamping voltage, way over the abs. max. rating of the IC. I think there is no way out by just replacing the TVS with another.
T3sl4co1l:
Put another way, this design process is backwards. It should be:
1. Start with nominal input voltage.
2. That selects TVS nominal voltage. TVS peak voltage is typically 33-50% above that.
3. That selects converter max voltage.
4. If adequate ratings are not available, consider higher order methods, like a switching or linear prereg to deliver ride-thru, or a switch to disable load during transient.
If the TVS needs to handle massive energy, enough that a TVS (as such) isn't economical: use a MOV instead, which will be selected in the same manner. Note the MOV will have a peak voltage typically 100-200% above nominal, putting much more pressure on the converter rating.
Tim
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