Is inductor winding insulation rating important for a high-voltage application if the voltage is not across inductor itself?
In many hobby and even commercial designs, people use generic wire-wound inductors or common-mode chokes that do not have a specified insulation or hipot voltage rating. Drum-core inductors are sometimes used as well.
Example circuit that generates more than 0.5kV based on switching duty cycle:
My question is: How important is the inductor’s insulation / dielectric voltage rating in this specific case?
Since the inductor never sees the full 500V across its terminals (This is an assumption, please correct if this is not true), is turn-to-turn insulation stress typically low enough that a non-HV-rated inductor is acceptable? Or can long-term insulation breakdown still occur?
I am looking to optimize for space, so if I can use tightly wound SMD inductor that would help.
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| User | Comment | Date |
|---|---|---|
| Elleanor Lopez | (no comment) | Dec 19, 2025 at 08:53 |
Since the inductor never sees the full 500V across its terminals (This is an assumption, please correct if this is not true)
The inductor sees $ V_{out} - V_{in} + V_{diode} $ during the part of the PWM cycle when the MOSFET is off. In this case it would be around 489V.
A typical magnet wire has high enough insulation. It's around 200 V/μm, and the insulation is a few μm thickness. That's how hobby projects get away with unrated inductors. Vanilla commercial [non hi-rel] designs probably do risk analysis for what would happen if the inductor shorts. It may be acceptable if inductor shorts are detected during QC, and a small percentage of devices stop working while still on warranty.
Strictly speaking, insulation rating of the magnet wire doesn't directly translate to the voltage rating of the inductor. The wire may be damaged during the winding of the inductor. There are inductors rated for high voltage. They might be hi-pot tested at the factory, I'm guessing.
By the way, I’ve got doubts about a single-stage boost from 12V to 500V. Practical boost factor $ V_{out} / V_{in} $ for this kind of circuit should be about 5 to 6 or smaller.
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Nick has already pointed out that most of the output voltage will be across the inductor during the output part of the switching cycle.
You can't really guess much from the insulation on the magnet wire. If the winding is a single layer, then the voltage between adjacent pieces of wire is the total voltage divided by the number of turns. This is rarely the limiting factor. It then matters how the wires from the ends are routed to the pads, how close the pads are to each other, and whether the core is conductive. None of these issues have much to do with the insulation rating of the wire.
The best way to be sure is to get an inductor from a reputable manufacturer where the maximum voltage is specified in the datasheet. A hi-pot test is not really applicable here. If you apply high voltage to the inductor, current will build up rapidly. Hi-pot tests don't work that way. They expect insulation, and usually specify 60 or more seconds dwell time.
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