Working Voltage
Ratings Applied toInductors
Why Voltage Ratings Are Not Specified on Inductor Data Sheets

Introduction

Test Methods

Voltage ratings are often specified for many electroniccomponents, including capacitors, resistors and integrated circuits, but rarely for inductors. This article
addresses the reasons why working voltage ratings are
not typically published for inductors.

A hipot (high potential) test is not sufficient to determine
inductor working voltage. Hipot tests are used to determine primary to secondary isolation within transformers, and can be used to determine isolation from windings
to case or windings to ground. However, traditional
hipot tests cannot be used to determine a terminal-toterminal voltage rating for inductors.

There are challenges to determine voltage ratings for
inductors, either by testing or calculation. Inductors do
not support dc or low frequency working voltages unless the inductance is high (typically >1 mH). Testing toverify working voltage can be difficult and should beapplication dependent. The various ways inductors are
made, and the stresses of processes like wire bending,make calculating a theoretical voltage rating infeasible.
This article presents these issues to make it easier tochoose an inductor most appropriate for the specificapplication.

Definition of Voltage Rating
The voltage rating of an inductor is the maximum voltage that can be applied to the terminals without causing
arcing or insulation breakdown. Voltage above the maximum rating may cause short circuits between turns,
through the insulation, or from the windings to core orframe.

Calculation
The large number of construction variables – turn count,wire insulation type and thickness, the layering of thewindings, bending or forming crossover leads – makes
it difficult, if not impossible, to calculate the theoreticalvoltage limit of an inductor.
For example, there are standard test methods for determining the voltage rating of film insulated magnet
wire, which generally has a voltage rating in the hundreds, if not thousands, of volts. This rating can easily
be compromised by the wire forming process necessary to wind an inductor. Magnet wire can be stressed,crazed or nicked, even in a carefully controlled manufacturing environment. A small scratch in the wire cancause arcing or breakdown at a voltage much lower
than the wire "catalog" rating. To calculate a rating that
includes these variables is not practical.

For inductor operation, V = L × di/dt clearly implies thatlarge dc voltage (or ac line voltage) cannot be impressed across the terminals of an inductor.
To illustrate, the 10 µH power inductor Coilcraft
MSS1038-103 has a dc resistance rating of 0.035 Ohms.
If a traditional 500 Vdc hipot test were applied to theterminals of this inductor, it would be like applying
500 V to a length of wire that has a resistance of0.035 Ohms. By Ohm's Law, to supply 500 V across
0.035 Ohms requires a source capable of providing
more than 14,000 Amps. While this seems silly, it does
illustrate the problem. Testing for voltage breakdown isreally determining when high voltage causes a shortcircuit. To a traditional hipot tester, the inductor isalready a short circuit!
In order to test for insulation breakdown or arcing, it isnecessary to induce a voltage pulse, spike or ringing
across the inductor. Such induced voltage or surge
testing is possible. The technique is well establishedand commercial testers are available. However, theinduced voltage depends on the inductor storing andsustaining a resonant energy, which is generally only
effective for inductance values greater than 1 mH, leaving out the vast majority of inductors except those
designed for line frequency applications.
The induced voltage that an inductor can withstanddepends on the type of wire insulation, the number andlayering of turns and other factors such as proximity toa conductive core, distance between terminals and theproperties of insulating materials such as the CTI (Comparative Tracking Index). Therefore it is likely that even
within a series of inductors, each inductance value
would have a different voltage rating.
Furthermore, it is difficult to extrapolate from induced
voltage testing to predict actual insulation breakdown inDocument 712-1

Revised 07/25/08

a specific application. Voltage arcing or breakdown
varies depending on environmental factors such as
dust or humidity and on operating conditions such as
the shape of the pulse wave. Generally signals withsharp rise times can cause arcing at a lower voltage
than constant or slowly rising voltages.

Conclusion (The Good News)
It might seem that choosing an inductor is nearly impossible. The good news is that all the factors thatmake it difficult to test or calculate an inductor voltage
rating make it unnecessary to test. The vast majority ofapplications require inductors to be operated at verysmall working voltages - usually just a few volts. Remember, most inductors work by impeding current flow,
not through high voltage induction. Almost all commercial inductors are good for a few volts or even tens ofvolts with no worries. So for most applications, inductor
voltage rating is not a concern.
For applications that operate from high line voltages orwork by induction of high voltage across the winding, itis necessary to consider the possible breakdown of theinductor windings. In these cases it is recommended
that the specific application be discussed with the inductor designer/manufacturer and a test plan developed to ensure the suitability of the inductor.

Document 712-2

Revised 07/25/08