Looking for pulse-rated zener.

Hello -

First, I wish to thank everyone participating in this discussion. While I'm an EE, it has been a long time since some of this has crossed my mind.. and the technology has improved considerably. The discussion has poked a few embers.. Thank you.

the inductor?

Using the effective series resistance of the inductor does not dissipate the energy fast enough. For 8 uH and a series resistance of something like .050 ohms, tau is on the order of 160 microseconds.

The zener has the feature of placing an (almost) constant voltage across the coil during field collapse. Ideally, superposition applies.. and I(t) = I(0) - Integral(v(t), t)/L. Faster than exponential decay.

Think I looked at this when first thinking about it.. and wasn't happy at the response time. I'll go and look again. I am also not familiar with "avalanche mode" transient absorbers and will stare at those as well.

The current circuit (pun not intended) uses a pair of IRFB18N50K MOSFETs and a pair of series-connected BZW03D100 100V across the MOSFETs. Hmm.. maybe the MOSFETs should be across the coils with a series fast diode..

Another poster suggested using the MOSFET intrinsic diode as the zener clamp. I stared at that one.. is this feasible (I'm still climbing out of using BJTs) ? I like the idea of the snubber better.. but have to think a little more on it.

Especially as now the researcher wants the magnetic field to enclose a larger volume. The coils for THAT assembly work out to about 400 uH .. and still need 15A.

Your time is *really* appreciated.

Mark

Yes. And the magnetic field is instantaneously proportional to coil current.

No argument. The switch current goes to zero first, the coil current detours first to the coil capacitance, and to snubber, if any. Then the voltage rises till either the capacitance (and snubber) absorb all the energy stored in the coil, or a clamp voltage is reached. If the latter (as the op is doing) then the current detours to the clamp. The current continues to ramp toward zero as does the magnetic field. As the current and magnetic field pass through zero, the voltage starts to swing back toward that connected to the other end of the coil (toward zero volts across the coil). But to change the voltage across the coil requires current to charge the stray capacitance across those nodes, so this current passes through the coil and and this causes a reversal of the magnetic field.

Perhaps I misunderstood your last post when you said, "Allowing current to flow after the switch opens will mean that the magnetic field will continue to exist while it collapses, and if current continues to flow, the magnetic field will reverse."

How could the field not continue to exist while it collapses? And the coil current must cease and change directions before the field ceases and changes directions.

My only point was that the applicable generalization for this op is that the magnetic field is instantaneously proportional to the coil current at all times, and the field reverses exactly when the current reverses. If you agree with this than we are on the same page.

Yes, the t = L/R time constant is what's at work there. So can we assume your coil's resistance is about R = L/tau = 0.05 ohms? This means it should drop about 0.75V at 15A. How are you establishing the 15A current?

Yes, that's the appeal of any high-voltage coil flyback scheme.

You're familiar with zener diodes, same thing... Same junction, just added blocks of copper to absorb transients without heating.

Well, the IRFB18N50K are 500V FETs with a rather high Ron, I'd say, not the best choice for your 15A current.

??

Hey, I don't usually think of myself as just another poster....

Yes, fine.

That'll be fun. Now you'll have 45mJ, getting up into slightly more interesting territory... You can use larger TVS parts (5kW) or use lots of 1.5kW parts in series, or you can use this common approach, recommended by someone else here, IIRC.

. 8A |--+---||--- gnd . | | / zener, TVS p6ke150A etc. . | '-/\\/\\--|) What's the magnetic field and its rapid shutoff for anyway?

--
 Thanks,
    - Win

Mark, You may not need a "pulse rated" zener. Most zener data sheets give a parameter call "transient thermal impedence". This parameter is expressed as a family of curves. It shows the equivalent thermal impedance (resistance) taking into account both the duty cycle and the repetition rate of the applied power. For an example of a set of transient thermal imnpedance curves, go to:

For some devices, only a single curve will be given. This curve will be for the case where the pulse repetition time is much greater than the thermal time constant of the device. . Once you pick the appropriate value off the curves, you can calclate the junction temperature rise using the formula: . Delta T = Rt*Power. . Where: Delta T is the junction temperature rise above athe ambient temperature . Rt is the transient thermal impedance from the curves Power = AVERAGE power dissipated in the Zener. (Sorry about the shouting :-) . You must also observe the Maximum current rating of the zener.

Use of a diode will result in a slower decay of a magnetic field.

It'd be nice if you could find the reference. It's a common piece of urban engineering wisdom, but it doesn't hold up to theory, to calculations using datasheets of real parts, or to measurements.

The predictability of an added gate-zener function may be desirable.

--
 Thanks,
    - Win

Actually, there is another possible reason. Higher inductor flyback voltages mean shorter inductor discharge times. Dissipating a fixed amount of energy in a shorter time means higher junction temperatures because the heat can't diffuse as far through metal frames, and into the heatsink, etc. So that's an argument both for lower FET clamping voltages and for more predictable voltages.

--
 Thanks,
    - Win

[snip]

On CMOS ASIC's it can get even trickier. Any "diode" will have a myriad of parasitics, thus using flyback diodes is risky business.

Years ago, on an Ethernet chip that also needed relay drivers, I developed a scheme that simply turns the driver transistor back on if it's drain voltage exceeds VDD by one volt or whatever.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Winfield Hill wrote: snip

I've seen an app-note, onsemi I think, that stated that the fets could handle more power with the active clamping

And you could can get transistors with the clamping buildin though I guess it's mostly ~40 volt FETs for automotive or ~400 volt igbts for ignition

snip

--

-Lasse

Yep, That's what I used back about 25-30 years ago, with bipolar MJ... something or other transistors.

I also had some inductive goodie that I used as well. I can recall getting near textbook switching load lines along each axis.

It was so effective at reducing the transistor dissipation that I removed the heat sinks from the transistor flags. Then made the mistake of grabbing the flags to check the temperature while it was powered.

The technicians were delighted... almost as much as when I got knocked off a lab stool while working on a CD ignition ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

I'm talking about the current inside the coil, are you talking about the HOT current in a TV flyback circuit?

Wouldn't help much, if at all; this reversal comes from the energy stored in the node's capacitance, of which only 1/6 was in the TVS.

--
 Thanks,
    - Win

**** Nope; i dug out a scope picture and we are both incorrect. Current drops rapidly toward zero, then flyback pulse rises, has flattish top, then falls. During flyback pulse time, the current is near or at zero. After flyback voltage gets to zero, thenone has rapid current change to negative value, etc. *** After the flyback pulse gets to zero, the inductor current rapidly goes to a negative value and ramps toward zero (and could continue to positive) during the FET body diode clamp time. *** And the resulting magnetic field might be unwanted. We have not heard about that one way or the other; which is one reason i suggested transferring the energy to another inductor or to a capacitor. Certainly the switches that would be doing that would dissipate some of the energy to the good. *** Maybe, and maybe not; depends on desired result of the magnetic field and how picky one wants to be. *** Now that is to the good!

** And the op has not mentioned that a reverse magnetic field is allowable or not, so reverse curent may be a no-no.

(snip)

Does your flyback circuit have more than one winding on the coil?

Hello Win..... you don't have to get the energy out of the coil AND dissipate it in the same uS. Think about transferring the coil-energy into capacitor-energy in the uS, then discharging the capacitor at leisure. 8uH 15A +---+---+---////////---+--

If the RF feedback path is wholy internal how would this affect the method of using a higher gate drive resistance to slow the current fall to limit the voltage to less than the breakdown voltage ?

or does the zener just add more parasitics to make the difference ?

Colin =^.^=

Yes that's very nice.

A friend of mine used a similar configuration in his ignition design, but with an added inductor that returned the energy to the battery. That proved to be VERY efficient. Perhaps that's what you did way back when. Did you keep a copy of the drawing?

--
 Thanks,
    - Win

[snip]

I hope. That was in my BC days (before CAD, ~1977-1980). I'll make a trip to the archive storage (I rent an off-site, air-conditioned,

10'x10' storage "cell" that I have filled with file drawers) and take a look.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

I never could understand why Western houses eschew basements. A full basement is a VERY useful thing to have. Get double or half-again the floor area of the house in one quick step.

--
 Thanks,
    - Win