Use TVS diode as a rectifier? - Page 1
Forum section: electronics / general

author: Circlotron
author_rank: Super Contributor
timestamp: April 28, 2022, 02:39:01 am
content: I've got a pile of about 1000 pcs of 1.5KE400 TVS devices. They are the same size and appearance of a 1N5404. Would there be a downside to using them as a 50Hz rectifier in a simple power supply? E.G would they have higher than usual forward voltage drop, or a snappy turn-off that would produce HF noise. Obviously they are optimised for TVS use so what compromises have been made internally compared to an ordinary diode?

author: timenutgoblin
author_rank: Regular Contributor
timestamp: April 28, 2022, 05:21:34 am
content: Quote from: Circlotron on April 28, 2022, 02:39:01 amYou didn't specify whether the TVS diode is the A variant (uni-directional) or CA variant (bi-directional). I'm assuming it was the former. According to the datasheet the breakdown voltage is 380V < Vb < 420V which means you'll need to determine experimentally what it actually is. Assuming 400V, it should behave like a 400V zener diode when reverse biased but 1V silicon diode when forward biased.If it's the latter the device won't conduct unless 400V + 1V is present across the device in either direction.On the other hand if you want it to conduct at 400V (for a uni-directional device - cathode to AC and anode to filter capacitor) make sure the AC voltage is already single-wave rectified (i.e., always positive voltage)! If the filter capacitor is rated for 10V and you want to charge it to 5V then the AC peak voltage would need to be 5V + 400V. That translates to 286Vrms. You will need to boost your mains from 230Vrms ~ 240Vrms up to 286Vrms. Otherwise the device may never conduct reasonably.Also the reverse leakage current is 1uA at 342V. Perhaps that is important.If you are particularly creative you could try damaging a bi-directional device by successfully shorting one of the zeners but leave the other intact and subsequently convert a bi-directional device into a uni-directional device. Maybe that's too creative?I cannot help you regarding the 'snappiness' of the device.

author: Circlotron
author_rank: Super Contributor
timestamp: April 28, 2022, 06:30:37 am
content: TVS is unidirectional version and I would arrange it to conduct polarity-wise like a conventional rectifier diode.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 28, 2022, 09:11:22 am
content: Indeed you can get "avalanche diodes" for the same purpose i.e. rectification with a bit of surge immunity. Though I don't think these are common for almost anything at all beyond alternators, where they clamp load dump and so are available in much lower voltage ratings. (Even then, I don't think you see them often from general suppliers, they're very much an OEM thing?)Oh hm, I wonder if TVS have lower Vf, since they can have thinner junctions -- you can't get a 1N5400 as such anymore, they're all massively overrated compared to what they're sold as, that's why 50-400V diodes have the same Vf and Cjo, then 600-800V are actually 1.2kV or whatever, etc. Still won't be as low as a schottky of the same value, but might be a few mV of justification in that rare situation where you have some to spare.At 400V, I doubt Vf is much less though, and yeah, you'll be using them like pretty much any other diode, hardly a difference in Vf,  surge, etc. Speed is probably faster than "general purpose" but not by a whole lot for those voltages, I would think. Not sure about softness. Capacitance will be higher, of course. And that's basically it; for general purpose applications, yeah, no problem.Another good application for zeners as rectifiers, is in current-sourcing supplies, like an offline capacitor dropper supply. Or a current transformer maybe, when you need a DC feedback signal and you don't have dual or quad diodes handy for rectification.Tim

author: SeanB
author_rank: Super Contributor
timestamp: April 28, 2022, 01:39:51 pm
content: I have been using 12V and 24V 0.4W zener diodes as regular diodes for a long time, seeing as I have a reel of each, and aside from using them as diodes, or turning into LED's, there is nothing really else I can do with them.

author: pigrew
author_rank: Frequent Contributor
timestamp: April 28, 2022, 05:14:16 pm
content: These TVS diodes are "stacked die", so a series of two diodes. That will mean the forward voltage is double what you'd expect (~1.3, based on the chart).(speculation below:)TVS diodes should have a much larger die area than the normal diodes, so they can support higher peak powers. This gives the devices a higher capacitance (looks like ~150 pF) compared to the standard diodes (~50 pF). This should reduce the efficiency of your rectifier, but I expect it'll work still. I'd suggest finding a spice model for these or similar diodes, and see how they act.TVS diodes could be designed for lower average power, but these seem to be pretty beefy so this isn't a concern here.There is likely also a difference in the reverse recovery time/reverse recovery charge, but isn't specified for either diode.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 28, 2022, 11:37:03 pm
content: If it's unidirectional, it should work fine as a rectifier. Still, you should check reverse leakage current and capacitance, possibly not as low as with regular rectifier diodes. Check datasheet.