BALUN transformer for capacitive loads

(snip)

That's right. with a capacitive load, the load impedance falls as frequency rises. Using a transformer to change an unbalanced source to a balanced pair of outputs does nothing to solve this mismatch. The largest capacitance the unbalanced 50 ohm load can drive to nominal output voltage at 1 MHz is about 3000 pF. If the balun steps the total voltage up 2:1 then the capacitance each side can drive is half of that.

So what capacitive load are you trying to drive to 100 v p-p at up to

Hi, In brief, i have 2 electrodes each with ~20pF capacitance to ground. I need to drive each of them with an out of phase RF signal i.e. one with A cos wt and other with A cos(wt-180). The RF amplitude and frequency range is around 0-100Vp-p and 1KHz to 1MHz.

So if you need to apply positive 100 volts to one electrode, while applying -100 volts to the other electrode, and each electrode has 20 pF to common (10 pF in series) the impedance across the differential output is about 16 k at 1 MHz. Neglecting stray transformer and wiring capacitance, you should be able to use a transformer with an impedance ratio of up to about 318:1. Of course, those stray capacitances may not be at all negligible, compared to 10 pF, so the real limit is probably about half that or less. 100:1 may be achievable. I think I would try a toroidal core transformer, with a center tapped secondary having 10 times as many turns as the primary. At this frequency range, I doubt that worrying about transmission line transformers is worth the trouble. This is more in line with switching power supply components. The core size will be dominated by the low frequency extreme (the primary will have to have an inductance of at least 8 mHy to stay above 50 ohms at 1 kHz), and winding capacitance and leakage inductance will dominate the high frequency problems. With a 100:1 impedance ratio, the implied minimum secondary inductance is .8 Hy. Winding a tightly coupled .8 Hy inductor with low losses at 1 MHz and low capacitance is a fair trick.

John, Thank you, this all makes sense to me. I have started to find the right torroid for this. I haven't made any transformers before in house, wondering if there are any practical guidelines. Do i have to worry about the saturation of the core, keeping in mind that i have 100 Vp-p at the secondary?

At the low frequency end, saturation is the prime consideration. This is because the integral of voltage over time (for each half cycle) is proportional to the flux swing of the core. So if it will handle 1 kHz at 100 volts, it will not saturate at any higher frequency. Here are some tutorials on transformer design:

John, So, i got myself a ferrite Core torroid with permeability 5000 and OD = .87in and ID = .25 inch. I wound up couple of turns and connected it to the output of the 50 ohm wavetek. The output dropped from 1 volt to few mVs at 10 KHz. So i kept on increasing the number of turns untill i get half a volt which is 12 turns now, this implies 50 ohm impedance at 10 KHz. Calculating it back, tell me the inductance is arpprox. .8mH. Now, to get higher impedance at 1KHz i have to wind more turns on the primary say 20 turns. That will imply around 200 turns for secondary, which i am not sure i can fit on this small core. I use a 30 Gauge magnet wire which is already too small. And winding 200 turns on this torroid will almost cost me a whole day. Is there a better way to get this done?

John, So, i got myself a ferrite Core torroid with permeability 5000 and OD =

.87in and ID = .25 inch. I wound up couple of turns and connected it to the output of the 50 ohm

wavetek. The output dropped from 1 volt to few mVs at 10 KHz. So i kept on increasing the number of turns untill i get half a volt which is 12 turns now, this implies 50 ohm impedance at 10 KHz. Calculating it back, tell me the inductance is arpprox. .8mH. Now, to get higher impedance at 1KHz i have to wind more turns on the primary say 20 turns. That will imply around 200 turns for secondary, which i am not sure i can fit on this small core. I use a 30 Gauge magnet wire which is already too small. And winding 200 turns on this torroid will almost cost me a whole day. Is there a better way to get this done? Any commercial products, i can buy quickly?

Something like that. So to get that same effect at 1kHz, you will have to increase the inductance by at least a factor of 10, so the turns will have to increase by at least the square root of 10 or 38 turns, minimum. Lets say 40. Then you need about 10 times that many turns on the secondary, center tapped. Of you stack two of these cores and cut the turns in half, etc.

Use a larger core. Perhaps a LFB360230-300, $1.35 from Digikey.

I suspect there might be something you could use, but you would probably be better off asking the makers than me.

Tim

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Now, multiply that factor times the 12 turns he has tested.

Sorry about that. We'll now return you to our usual programming.

Tim

snipped-for-privacy@gmail.com wrote: (snip)

Shouldn't take an hour. The trick is to wind all the wire you need and a bit extra over a soda straw, end to end. Notch the ends with scissors to help the wire stay on. Then you have a neat and thin object to pass through the core (instead of pulling a long lose piece of wire), with minimum fuss and tangle.

After you have something that works, you can take the design to a custom magnetics house for a quote on automated production.

Thanks John, I see the part # LFB360230-300, why not get the one which has the higher Net impedance at 1 MHz, say LFB43064-000? That will ensure i have high enough reactance with fewest number of turns (to make my life easy).

Thanks John, I see the part # LFB360230-300, why not get the one which has the higher Net impedance at 1 MHz, say LFB43064-000? That will ensure i have high enough reactance with fewest number of turns (to make my life easy).

I picked one with a large cross sectional area, but also a large window area, in case you needed space for lots of turns, especially if you go with Kynar or Teflon insulated wire, yo lower the inter winding capacitance. For all they cost, you might as well get an assortment.

You might also consider placing a pair of the tall, narrow form factor beads side by side, and winding through both, cutting the number of turns in half.

Works like a charm. I just finished winding and testing. Thanks a lot John, the core from the digikey was sufficient for the job. regards,

Congratulations!