The Catches: Boost Converter that don’t last

This happen about second year i started hands-on design – it was a 100V boost converter – most of the time the prototype works well – but every once a while – it stop working  after a large “blak” sound.

So tracing back on the switching waveforms – voltage, current I deduced that the inductance was not right – VL = L * (di/dt) relationship does not hold – drop in another piece – and it start working again.

Being curious as I am (luckily), I check the resistance of the inductor and compared to the new part (did not have LCR meter on hand). The resistance is much lower lower than the number stated in datasheet.

As the failure made me very un-comfortable – I went Google around – and it turns out that inductor does have voltage rating (although not mentioned in the datasheet of the one I was using), so the design was flawed in the first case – luckily it was found out earlier than later (it pays for being curious), else can’t even imagine what it would do in the field. Changing the part to high voltage part – and it work out solid!

For more details, refer to previous post: http://electroniccircuitdesignsharing.blogspot.com/2012/12/inductor-voltage-rating.html

Lesson –
Inductor does have voltage rating – due to the insulation coating of the windings, and the way wires being wound – and the inductor intended for high voltage operation will have working voltage stated in the datasheet – else don’t use it at high voltages.

Always look for the root-cause – it will save your ass

Inductor Voltage Rating

Have you ever wonder that when you read through datasheet for inductors, you rarely see the voltage rating?
Shown in picture below are some common inductors seen.

The fact is, inductor does have voltage rating, the winding wire have a fix thickness of insulation coating, if high enough voltage is applied across the inductor, although momentary and the current is within the specification limit, the insulation will breakdown and short to adjacent wire. From personal experience, normally inductor voltage is about 60V, unless specifically stated in datasheet. Maximum voltage allowed is usually depends on insulation thickness of wire used, and whether the windings overlap.
If you does wonder whether your inductor has insulation breakdown, the easiest way is to measure DC resistance, depending of actual short, the resistance will be a lot or somewhat lower than a good one. Alternately, if you have a LCR meter, you can use it to measure the inductance instead, in this case, expect faulty inductor to have lower inductance (less winding over the magnetic core).

Real Life Inductor : When Does An Inductor Stop Behaving Like One

1. When current is too high – because of magnetic saturation
2. When signal of interest is of too high frequency – higher or near inductor self resonant frequency. There are parasitic capacitance across inductor winding  so in a way, we have parallel LC circuit, at high enough frequency, the impedance of C will gets low enough to shunt out the impedance of inductor.