Transformer design

[Blackwarrior]

Thanks Performa for this info, I'm going to work on this tonight, I've brought a high spec function gen home from work to play around with different frequencies.. I also got a different core and bobbin to try out maybe in a day or so.

I will work on this transformer I've wound now though, and as you say, it will be a good learning curve.

I'll keep the thread up dated as I progress

Appreciate everyone's help with this...

Cheers

[Blackwarrior]

Got one more question guys to complete the above formuli you's have kindly simplified for me. That's wire thickness for primary and secondary windings... Any help would be appreciated. Thanks

[Blackwarrior]

ive been playing around with this transformer learning a few things before i start on my new one.

ive connected a 0.5R resistor between ground and the source of the MOSFET to get the current drawn as advised.

the first attachment is the scope readings at the 7Khz i was originally driving it at. i see excessive current being drawn at this frequency with the transformer saturating.

by increasing the frequency to 9Khz i was able to stop saturation and reduce overall current as seen on the PSU by half, from 0.46 A down to 0.22A

input voltage = 9VDC
output voltage @ 9Khz = 227AC
output voltage thru Cockcroft = 496 VDC
Primary Inductance of TX = 1.22mH
secondary Inductance of TX = 402mH

i am going to try and work out windings on the new transformer with the given formuli you guys have shown me.

the transformer core is the ETD N97, farnell No 1422745
the transformer bobbin is the ETD29, 13 pin, Farnell No 1422746

thanks for your help guys

Edit. Pink trace is the gate, yellow is on the 0.5R resistor

[Performa01]

Got one more question guys to complete the above formuli you's have kindly simplified for me. That's wire thickness for primary and secondary windings... Any help would be appreciated. Thanks

It's all about wire resistance and losses, which you want to both minimize and distribute evenly between windings.

Power dissipation is I² * R;

Assuming an ideal transformer, the current is indirect proportional to the turns ratio: Ip/Is = Ns/Np;
Ip = primary current [A]
Is = secondary current [A]
Np = primary number of turns
Ns = secondary number of turns

For equal losses in the primary and secondary windings, you want Ip² * Rp = Is² * Rs;

If you do the basic math, you'll find that this criterion is met with:

Ap = As * Ns/Np;

Ap = Cross-sectional wire area for the primary winding
As = Cross-sectional wire area for the secondary winding

Hence the diameter of the wire used for the primary winding is:

dp = ds * sqrt(Ns/Np);

It just so happens that this means, that the total cross-sectional area of the primary winding should be about equal to that of the sum of all secondary windings.

The resistance of a wire is calculated: R = rho * l / A;

rho = electrical resistivity [ohm*m] (depending on wire material) 
l = lenght of the wire [m]
A = cross-sectional area of the wire [m²]

see https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity


Example:
If you want two windings, say 10 turns for the primary and 1000 for the secondary, then you know the current ratio, i.e. primary current will be 100 times the secondary current.

Now you have to figure out what the maximum wire diameter is that fits the space. Assume that space is e.g. 20mm wide and 14mm high (just some random numbers not related to any particular core!), then you will subtract a few millimeters of the height to leave room for insulation wrappers plus a little spare, so let's say you take 4mm for that and then 10mm are left. That is a cross-sectional area of 20mm * 10mm = 200mm². That means 100mm² for each winding.

Primary side: 100mm² / 10 turns = 10mm²;
Secondary side: 100mm² / 1000 turns = 0.1mm²;

For calculating the wire diameters, do not use the common formula A = d² * Pi/4, as there is a fill factor that can never be 100% with round wires. Simply use d² instead, thus d = sqrt(A);

Now the associated diameters are 3.16mm and 0.316mm.
In practice, you might use 3mm for the primary and 0.3mm for the secondary in this example.

[Performa01]

ive been playing around with this transformer learning a few things before i start on my new one.

The N97 ferrite core seems to be optimized for 100kHz, and I think the screenshots hint in that direction also. So your final design should aim for a frequency in that range.

Would you mind doing some more experiments with the 'old' transformer at higher frequencies (20, 50, 100kHz) and showing us the results?

[Blackwarrior]

Performa, thank you so much for your input on this transformer design. invaluable stuff for sure, and im learning a great deal from it. thanks also to Chris_Layson too for the formulas. i am knocking up a spreadsheet putting all these formuli in at the moment.

Yes ill do those tests now Performa... no probs.

[Blackwarrior]

Reminder of what i am testing at various frequencies.

Input voltage = 9.0VDC
Input Signal to Gate is from a SDG 2122X Gen - Amplitude = 9VDC - Offset = 4V - Duty = 50% - Phase = 0

MOSFET is a P75NF75 N Channel.

Transformer is hand made, primary winding = 540 turns, secondary = 30 Turns
inductance Primary = 1.22mH - Secondary = 402mH

the following scope readings are: Top (Pink trace) = Gate - Bottom (Yellow Trace) = SOURCE pin (0.5R resistor connected between ground and Source of MOSFET)



EDIT: sorry primary winding is 30T and Secondary is 540T

[Performa01]

Well, that's pretty close to what I've expected...

Did you measure the output voltages for the various test frequencies?

At 10kHz, it actually doesn't work at all. The current increases very slowly and only speeds up after ~40µs. After that, it runs up and into saturation very quickly. In this scenario, you weren't able to pump a lot of energy into the score. If you integrate the current over time, the resulting mean value will be very low obviously.

At 20kHz you can see the resonance of the transormer triggered, but it's still just one pulse followed by some ringing. But the ringing is at about 100kHz, indicating the frequency at which the transformer is willing to work

At 50kHz this effect is quite obvious, but you're still not there. You want to pump new energy into the transformer synchronous with its resonance frequency.

At 100kHz you see one complete period. You cannot see what's going on while the transistor is off, since the current flows through the diode during that time.

You could try adding a capacitor in parallel with the primary winding (30 turns), something in the range 470 - 2200pF; Once you've found an optimum value for the capacitor, you could also try to remove the diode and see how this affects the output voltage.

EDIT: Two more notes:

1) I wouldn't have recommended removing the diode without the nominal load at the secondary side if I had not learned by now that the P75NF75 MOSFET is fully protected (it intrinsic reverse diode is fully avalanche rated), so the drain voltage will be safely clamped to somewhere >75V.

2) By the process you were going through (crowned by finally adding a capacitor) you were moving towards a resonant converter, which is more efficient than the ordinary switch mode technologies. Another step would be going from single ended to balanced - maybe you should stick with the current transformer for a while and try this out also. This way you gain experience which will help to make good decisions for your final design...

[Blackwarrior]

I'll do the tests again and record the outputs on another channel, also take voltage readings on DMM..

I'm not quite getting the output I need but incorrect ratio seems the obvious cause..

As for the internal diode in the mosfet, you mentioned this earlier so what you've seen so far has been without the external one.

Certainly seems like I'm progressing, and certainly learning a thing or 2..  Thanks to you. And the other help im getting...

Working on those voltages now

Thanks Performa

[chris_leyson]

I just remember Farnell and RS stock a few CCFL lighting transformers..
http://uk.farnell.com/bourns-jw-miller/pm61300-4-rc/transformer-smd-ccfl-6w-13v-to/dp/1795421
http://uk.rs-online.com/web/p/chassis-mounting-transformers/2509495561/

[Blackwarrior]

voltages are taken with a Megger TPT320 voltage tester.. this one http://www.tester.co.uk/megger-tpt320-voltage-tester?gclid=CIz34Y6P8MoCFa0V0wodmuAJCw

At 100Khz - Voltage = 267VAC
At 50Khz - Voltage = 240VAC
At 20Khz - Voltage = 223VAC
At 10Khz - Voltage = 258VAC, current drawn is excessive.

Scope reading below, The Cyan is the output of the transformer with the Voltage tester as a load.

this is with a 1nF across Transformer

[Blackwarrior]

I just remember Farnell and RS stock a few CCFL lighting transformers..
http://uk.farnell.com/bourns-jw-miller/pm61300-4-rc/transformer-smd-ccfl-6w-13v-to/dp/1795421
http://uk.rs-online.com/web/p/chassis-mounting-transformers/2509495561/

they look like the solution i need, no longer manufactured though according to Farnell, and RS dont show any data for it...

thats a bumber, but a great solution i think.

thanks Chris

[Audioguru]

Some C cell batteries have a puny little AA cell inside.

[Performa01]

You didn't fit the external diode yet - actually I have wondered already when looking at your previous screenshots

So I would stongly recommend to repeat your tests with the diode fitted, so we can see (and discuss) the differences.

Your current measurements show basically the same as the previous ones, but now we can see what's going on at the secondary side.

At 10kHz, we see an excessive increase of current after some 40µs, to a peak level of 4A (2 volts/0.5 ohms). Then a huge kickback. The scope shows some -570V already, and it is actually much more, as the waveform is clearly clipped on the negative side. Duty factor of the output is not the same as the control signal, which also indicates that there is something going wrong.

Your scope probes are presumably rated for 600 volts maximum only. So this measurement did exceed the probe rating by quite some margin, so I would strongly recommend not to repeat any tests with this transformer at 10kHz without freewheeling diode across the transformer primary.

At 20kHz, it looks surprisingly good, apart from the strong ringing, indicating that the transformer would like to operate at a higher frequency. But duty factor is near perfect and negative voltage doesn't get excessive anymore.

At 50kHz, we don't see a ringing, but a clear indication that the transformer likes twice the frequency.

At 100kHz, all the wobbles are gone and the wavefor looks nive, even though symmetry is not perfect. An interesting experiment would be to further increase the switching frequency, maybe in 10kHz steps, in an attempt to optimize the waveform. The same effect should be achievable by increasing the capacitor across the primary winding, but it's certyinly much easier to adjust the frequency instead of swapping capacitors.

So could you please try that and then repeat the complete test with the external diode fitted?

[Blackwarrior]

Hi Performa. the only diodes i have that may me suitable are RHRP8120, 1N5818. so ill try the former as its spec is much higher.

doing tests now.

[Blackwarrior]

ive fitted the RHRP8120 diode in and the current drain is massive. cathode to VCC.

here is the scope at 100Khz, i tried higher frequencies but it just got worse. output voltage is on Cyan trace, very low.

[Performa01]

Are you sure you haven't accidentally reversed the diode?

[Blackwarrior]

Are you sure you haven't accidentally reversed the diode?

no, definitely cathode to VCC and Anode to drain. ive tried 4 of these, all with the same results. even tried the 1n5818. same results

[Blackwarrior]

if i reverse the diode its a lot worse...

attached are the results on scope of the higher frequencies, up to 200Khz

[Performa01]

if i reverse the diode its a lot worse...

attached are the results on scope of the higher frequencies, up to 200Khz

That's without the diode, right?

Unfortunately, you haven't included the output measurements.
From the screenshots, it looks like you manage to increase the transferred energy quite a bit and this should show in the output voltage. The initial idea was to find the optimum switching frequency, where we could optimize for either maximum output power or maximum efficiency. This only makes sense if the intended load is connected to the output, of course.

Once that optimum has been found, you can calculate a different capacitor in order to get the frequency down if so required. Higher frequencies are generally more convenient, because lower values for the inductances and capacitances can be used and ripple filtering gets easier, but switching speed of the transistor as well as the losses in the ferrite core will decrease efficiency, so there is indeed an optimum frequency.

So I assume the output voltage was still increasing and you got a new maximum at 200kHz?

[Performa01]

ive fitted the RHRP8120 diode in and the current drain is massive. cathode to VCC.

here is the scope at 100Khz, i tried higher frequencies but it just got worse. output voltage is on Cyan trace, very low.

Okay, just wanted to make sure. The diode seems to work, but it looks like 100kHz is still way too low for this transformer design, which was a little surprising.

The current rise is near instant now and reaches 4A after only 1µs. Then the current remains constant, probably because of the core saturating, maybe also because of the current limiting of the power supply - or both.

Do you have a suitable capacitor right between the Vcc end of the transformer primary winding and the ground end of the current sense resistor?
If not, you should add some 10 ~ 100nF capacitor there, preferably not a crappy Z5U ceramic, but at least X5R, X7R or also mylar, even better a polypropylene.

If that is sorted, you might want to try again, but this time starting at 500kHz...

[Blackwarrior]

I wrote all the voltages down but forgot to add them..

These are all with the voltage tester as a load.

120Khz - 306vac
130Khz - 328vac
140Khz - 349vac
150Khz - 368vac
200khz - 385vac current drain massive.

My PSU is rated at 2amp only. And set to max

Unfortunately I only have ceramic and electrolytic caps at hand.
I will try it at the higher frequency 500Khz

You have the patience of a saint Performa, but I appreciate all this help and guidance to get this performing at its best.. Thank you for that

[Performa01]

I wrote all the voltages down but forgot to add them..

These are all with the voltage tester as a load.

120Khz - 306vac
130Khz - 328vac
140Khz - 349vac
150Khz - 368vac
200khz - 385vac current drain massive.

My PSU is rated at 2amp only. And set to max

Well, the output power is rising ...
The massive current drain is quite visible in the screenshots. You still manage to get some 4 amps, probably because of the output cap in the PSU, so the current limit does not kick in as the average current still doesn't seem to exceed 2 amps.

Unfortunately I only have ceramic and electrolytic caps at hand.
I will try it at the higher frequency 500Khz

Ceramic is okay, let's just hope you have an X5R/X7R at hand. Any value >10nF will do  (and will be way better than nothing anyway)

[Blackwarrior]

ive tried it at 500Khz and the output VAC drops to 46volt, similar results down to 300Khz and up to 1Mhz, also my squarewave into the gate very distorted.

i put a 15nF cap from VCC on transformer to ground next to source on MOSFET, results below, these are at 108Khz which appears to be the best freq to use giving me the highest output voltage at minimum current.

thanks

[Performa01]

Just wanted to add one thought:

Switching frequency should not normally make that big a difference. As stated before, there is an optimum of course, depending on the properties of the core mainly and also on some other components, but apart from that, output should be reasonbably constant if the frequency changes a little. The fact that we see major differences indicates that it still doesn't work as intended, and this suspicion is well backed by the screenshots you've provided.

The goal is to get near sinosoidal primary current, i.e. neither longer periods of near constant current, neither short spikes. Unfortunately, we don't really see the current when the transistor is switched off, but the output power and efficiency are good indicators too. Once you've found the sweet spot, you will have gained a 'feeling' for the properties of the components, mainly the core, and as it looks now, you need either very high frequencies or you'd have to increase the inductance. Since the latter means more turns of wire for the windings, we want to avoid that and rather experiment with a parallel capacitor as additional energy storage element, thus lowering the resonance frequency.

Btw, for any further experiments, you should use either the capacitor or the external diode, not both at the same time.

 I would say you should stick with the capacitor for now.

If you want to add some output regulation (at a later point, first you should optimize the basic design), there are still several possibilities, it might just not be digitally controlled PWM at frequencies that high...