No voltage rating for inductors

Powdered iron can get lossy if you drive it hard. Then it gets hot and everything gets worse. KoolMu or Sendust has become affordable.

I think a few of the Coilcraft parts have voltage ratings, but it's rare. The dual-winding guys, like DRQ127, are probably bifalar and have more ways to arc.

I usually test them.

How much voltage were you considering? That toroid looks pretty good.

Class D amp?

No. I was pondering why LED light fittings need to have a large electrolytic capacitor with its limited life.

I have a couple of small LEDs illuminating an otherwise frequently pitch black corridor. They're powered through a transformer, bridge rectifier and current limiter, with no capacitors at all. They obviously flicker at 100Hz, but it's not visible.

So then I was thinking that one could use a buck converter directly off the rectified, but not filtered, mains, hence the voltage requirement.

This has not moved past musings at this point, but I was looking at the voltage specs for inductors, hence my comment.

Sylvia.

Wound parts are mostly wound with double-enamelled copper wire, which is good to about 500V. Everybody is supposed to know this, which is why it won't get into the part data sheet.

It is in the wire data sheets, but if you've never designed or wound a transformer (or choke/inductor) you may not know. None of the the obvious text books I've got to hand say anything about it, which is a bit odd. When I was graduate student I got to know a guy from Croatia (Ales Strojnik) who spent a year designing and building a 600 kV scanning electron microscope for the Melbourne Physics department, and I may have picked up some stuff from him.

The problem with unfiltered mains is nearby lightning strikes, which can produce kilovolt spikes.

It is known that every last local area network ever invented got blown up during the prototype stage by a nearby lighting strike, and even the famous Intel/DEC/Xerox Park Ethernet Standard had to be modified after it was first published to put in proper lightning protection.

Transzorbs and spark-gaps can cope, if you are aware of the problem at the design stage.

Lots of LED bulbs have a bridge and a linear IC current limiter but no cap. A capless switcher is an interesting idea. The LED could run at constant current at a very high duty cycle.

An inductor like you showed would be fine at hundreds of volts. Test one to breakdown!

Low voltage LED lighting and hardware is not mains-rated for safety.

Their safe use is achieved through the isolation transformer, tested at the time of assembly to 3x book isolation ratings. The secondary circuit is SELV (Safe Extra Low Voltage).

Led lamps that sub for incandescent bulbs in mains hardware are constructed with sufficient insulation between the electronics and the end user, to achieve safe use.

Hardware running at SELV is subjected only to simple flammability and material/environmental regs.

Mains inductors are invariably core-loss or VT limited, below their current ratings or corona susceptibility.

If you crunch some numbers, you'll see why direct buck conversion off the mains, at low power, is not easily practised.

RL

Since the enamel insulation separates adjacent windings, it isn't the device rating at all; if there's 100 turns, at 1 kV, the adjacent turns have at least 10 volts across each two-layers, but possibly no more than that. The insulating former (for small power xformers) between windings and core is probably good for kilovolts. Spaghetti around the wires, and tape between layers, all add to the voltage-breakdown tolerance, as does pie-winding...

It seems Coilcraft anticipated your question Sylvia:

"Working Voltage Ratings Applied to Inductors" "Why Voltage Ratings Are Not Specified on Inductor Data Sheets"

However I accept someone could have a design that requires knowing the wire's dielectric breakdown (tested per IEC 60851-5:2008 test 13, documented per IEC 317-0-1, clause 13 ) The wire maker might declare this, but I suppose only in special cases would the inductor maker do so.

Thanks for the link - I read it all.

I'd have thought the same issues would arise in the context of mains transformer primaries, though the consequences of failure may be more predictable.

Sylvia.

Sylvia Else wrote: ===============

The is no similar way for a maker to rate an inductor, as the possible applications are endless. Up to the user to figure that out.

...... Phil

LTspice IV threw a 'singular matrix node error' here, but only if the Philips models for 2N2222 and 2N3906 were used. NSC models allowed the simulation to run.

I'm not sure that you're going to find a 'digital isolator' that's designed for PWM - they're usually pretty slow. You might consider an integrated gate driver + discrete nmos as a lower-parts-count solution.

If you check the input current, or the current in the freewheeling diode (probably not included inside the digital isolator, by the way), you'll see diode current spikes occurring when it turns off and the switch turns on. The amplitude of these spikes will vary with diode type and nmos switching speed (slower speeds are generated if a series gate resistor is present on M1).

These spikes are not simulation artifacts, they are modeling attempts to simulate schottky capacitance or rectifier reverse recovery.

The Bourns data sheet gives a flux density calculation in gauss using K * L * dI with a frequency-dependent core loss chart specific to the SRR1210 series.

In a ripple-regulated circuit dI will vary with frequency - in the sim it's between 200 and 400mAppk.

In a regulated circuit, the peak to peak flux swing in the choke is determined by Bppk = V * t / ( N * A) where Bppk = peak to peak flux density (Teslas) V = Vout + Vdfreewheel (volts) t = Tfreewheel (seconds) N = number of turns on the choke A = xsectional area of choke

If you've got a table for the choke's material, you can work out core loss (W/m^3 - mW/cm^3) and apply that to the volume of the core geometry being used.

Voltage is present in that equation. For peak or surge, it's either saturation or turn/terminal breakdown limited. Some core structures, when impressed with low impedance surges, will try to turn themselves inside out, in the attempt to force changes in the magnetic length/area ratio.

Lighting ballast design for the domestic market is not the easiest road to fame and fortune.

Considering the efficiencies of LED sources and the dominance of standard hardware formats, it turns into a marketing exercise.

RL

In any event, new luminaires do tend to try to build into the hardware any type of ballasting and safety barrier that may be required for direct sale.

RL

Voltage rating can be complicated. For instance, to be CE compliant, Transformer/Coil needs two different insulators between mains (230V AC) and low voltage side. And so on. I have noticed that there is usually a reason for those rules, but they are not very obvious and don't come by accident. And then there is China Export standards.

Two different insulators or one thicker one called "reinforced insulation" if its a class 2 (double insulated) product.

John

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The enamel coating on copper wire is the first, the second needs to be a non hygroscopic, high softening temp insulator with good stability over decades. Various plastics comply as does fiber sheet and treated cardboard.

....... Phil

as an example:

Thanks for taking a look. In the end this was never going to be more than an academic exercise. I don't even intend to attempt to build one. To be remotely viable commercially, most of the electronics would have to be bundled into an IC, and I don't see that happening.

While eliminating the electrolytic capacitor could benefit the consumer (assuming the high voltages don't produce other reductions in life), it's of little interest to manufacturers - even those making the hypothetical IC.

Sylvia.

Well, they wouldn't tell you, if they were, but there ARE application- specific ICs showing up in SEA - they just don't crow about it, or have any interest in the North American market.

That flat-topped line current waveform isn't any easier, w/r to harmonic distortion on the transformer-coupled grid, than the capacitive rectifier peak, but it is 'loss-reduced'. The linear LED lamps have a similar profile, when caps are removed.

RL

The real challenge would be to do it with 12 cents worth of discrete parts.