1. Home
  2. Electronics Design
  3. Cheap pulse transformer as a ni-crhome wire heater

Cheap pulse transformer as a ni-crhome wire heater

formatting link

The 670uH 1.5:1 version.

E-dt is rated for 40V-us.

I ran a test where I applied a step through a 50 ohm resistor and watched to see how long it took for the voltage to nosedive back to zero (saturation). I reversed the primary windings and repeated the test in the opposite direction and got the same reaponse.

It saturated at ~50v-us so 20% margin between datasheet rating and actual. good job TDK.

I checked this against a spice model (simetrix) and got very similar results EP5- 3c96 with 42 turns on the primary (Got teh same inductance so I called it close enough). TDKis probable using N45 material which is similar to 3c96.

The application is a small half-bridge SMPS topology running at 200KHz and 50Vin. The secondary is attached to a 4 ohm Ni-chrome wire that I need to drive with 1A RMS for a few seconds at a time at low duty cycle. Seconds on, minutes off.

I understand the transformer resistance is far from ideal for this kind of load and high pulse current but I need galvanic isolation low cost and short non-custom design. It is still reasonablly efficient for what i need.

My question is, I am able to apply almost double the 50V-us I measured in the initial test. with 100Vin I can apply a 2uS pulse which should be 50V * 2us or 100V-us, and not see the primary current spike upward.

The lower I make the secondary resistance, the higher I can go in voltage and still not have the primary current show signs of spiking.

If course I need to limit the time to 10s of mS when testing to not burn the transformer up sice the load is experienceing much higher currents than 1A at this point.

If Increase the resistance on the secondary, the primary will saturate as a lower v-us product. Open circuit on the secondarty would have it saturate at near the 50Vus.

Why does a load on the secondary change the amount of V-us that can be applied to the primary without saturating? I thought the peek flux density was only determined by the Volt seconds applied to the primary, number of turns, perm, and Ae.

Since the currents are so high is this bucking the flux generated by the primary voltage?

Reply to
mook Jonhon
Loading thread data ...

Given an ideal transformer (no DCRs, no leakage inductance) with a shorted secondary, you can't saturate it because you can't apply any voltage across the primary; it's a dead short.

To see what's happening, you can Spice an ideal transformer and then add the copper resistances and leakage inductance. The volt-second saturation happens at the leads of the internal, inaccessable ideal transformer.

Your internal ideal transformer probably still saturates at 50 uv-s. But if the secondary is nearly shorted, the voltage across the ideal internal primary is not what you apply at the terminals; there's voltage drop in the copper. It just looks like the saturation v-us has gone up.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard
Reply to
jlarkin

Easy: primary voltage is lost to DCR and leakage. As these drop more voltage, flux in the core drops.

At some point, you'll be dropping more across the transformer than across the load, giving terrible efficiency. Then you absolutely need a bigger and better transformer.

It's not clear from the numbers given, just how much you're dropping, or if this is on the right slope to be correct.

There's also the difference between single ended and full-wave saturation: it's not clear which one you tested.

Leakage can be dealt with by going to a lower frequency -- of course, you need more flux for that, too: in other words, a bigger transformer.

Lunch, free, etc. :)

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website:

formatting link

Reply to
Tim Williams

Thats what I was thinking. The current in vs current out ratio is held even at the high V-us numbers so I think the "internal ideal transformer" is not saturated but being redced in voltage by the high DCR of the windings and the high primary current.

I was thinking it was something more "magical" that that. :)

You know what they say about transformers. Theres more than meets the eye. :)

Reply to
mook Jonhon

Vt = BNA

V=volts t=seconds B=teslas N=turns A=cross-sectional area of core in meters^2

B is limited by saturation, so deltaBmax = Vt / NA

If the core flux is saturated in opposite polarity with each pulse, then deltsB max = Bsat x 2.

Left to its own devices, an open circuit winding will only reset flux to a remenance value, under the influence of stored magnetization energy. So you don't get full Bsat swing in single ended pulse repetition.

Bsat reduces with temperature in most pulse grade core materials.

RL

Reply to
legg

There's magnetization and hysteresis and eddy currents and capacitance and skin effect and even prop delay. It just gets worse.

I want to make many channels of 150 volt isolated exponentially-decaying fast-rise pulses into 50-ohm coax. There is some secret, probably exploding device a few km away on the other end. I need an exotic pulse transformer, fast rise and lots of volt-secs. Turns out that a cheap Coilcraft dual-winding inductor is perfect. The way to find that out was to get one and test it.

Coilcraft is great. Heather answered my email on a Sunday and sent 10 samples on Monday.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard
Reply to
jlarkin

This is my experience as well with coilcraft. They also have a duplicate of the TDK at 1/2 the price.

Reply to
mook Jonhon

Thanks, That what the gray beards taught me long ago. working for 20 years but never ran across or even thought to look at intentionally pushing a transformer to the Bsat limts before with various loads.

never too old to learn.

Reply to
mook Jonhon

:

It's not the eyes that are failing here, but the fundamental grasp of what is going on.

If you refuse to think carefully about the basics of what's going on in the core, the elaborations are even more difficult to keep track of.

It's called a transmission line transformer. If you wind a transformer with twisted pair, the twisted pair acts as a transmission line - not a particu larly perfect transmission line, since minature coax is a lot more nearly p erfect - but quite good enough for lots of applications.

There 's a large market for transformers that can carry a fast switching dr ive across an isolation barrier. They all seem to be wound with twisted pai r.

He could have wound something for himself in ten minutes, if he'd had a ree l of the right wire and a few ferrite cores and formers handy.

He seems to have been frightened by a transformer at an early age and has s teered clear of them ever since.

--
Bill Sloman, Sydney
Reply to
Bill Sloman

That is only inductive component. There is also ohmic part. Denoting by V1 (time dependent) voltage on primary we have

V1 = B_tNA + IR

where B_t is time derivative of the magnetic flux. Flux is produced by both primary and secondary winding. With load on secondary magnetic fields from primary and secondary mostly cancel, so primary current is much larger than in case of unloaded secondary. Conseqently resistive voltage drop is much larger and to get the same flux one needs larger primary voltage. Of course, there is also voltage drop in secondary, so if goal is to get maximal power to the load then one needs to balance increasing current with resistive losses in transformer: both unloaded and shorted transformer deliver no power to the load and all power goes into losses. Assuming reasonable transformer model optimum is closer to 50% increase in primary voltage than to 100% increase.

--
                              Waldek Hebisch
Reply to
antispam

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.