Author Topic: Transformer design  (Read 12891 times)

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Offline Performa01

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Re: Transformer design
« Reply #50 on: February 12, 2016, 08:08:42 pm »
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.

Yes, 100kHz should be the proper frequency and you certainly should try to get it at an optimum there.
Now you say it works best with 15nF bypass capacitor for maximum efficiency.
So has the bypass capacitor improved things? On the screenshots I can see that the peak current has increased, whereas you get more ringing, as the quality of the primary loop has increased too, as the loop is now nice and short, instead of including the cables to the power supply, which btw. will act as nasty little antennas...
 

Offline BlackwarriorTopic starter

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Re: Transformer design
« Reply #51 on: February 12, 2016, 08:22:24 pm »
The 15nf cap created a lot of noise in the current waveform, this is the second waveform on the latter 3 I've posted, the file name helps identify the readings. The last scope picture shows current wave looking pretty good with minimum noise.
I will order a load of mixed caps, polyprops and mylars.

In the meantime I will relay this part of the design out again, keeping tracks and wires as short as possible. Guess it's not helping using veroboard/strip board, but it will eventually be on a PCB.

I will carry on experimenting with what caps I find. So I'm looking for a good sinosoidal current waveform for best results?

I've noticed my gate drive signal is getting a bit distorted so I'm thinking of adding a non-inverting op-amp buffer in there, maybe give me more drive... Or something similar.

Thanks for all your help Performa.
 

Offline Performa01

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Re: Transformer design
« Reply #52 on: February 12, 2016, 09:03:11 pm »
I'm a bit confused now as you've stated that 108kHz seems to be an optimum. I wondered if you could see a difference with and without the bypass cap, in terms of output power and efficiency?
There clearly should be, as the high slew rate signals in this circuit need a short current path for all the high frequencies, which is provided ('closed') by that cap.

For the same reason an OpAmp isn't a good option, except for some ultrafast video/line drivers it will probably be too slow for efficient switching. A dedicated MOSFET lowside driver would be the much better choice.

Well, in a flyback converter, the current will never be a nice sinus, but it should at least not be neihter a narrow pulse nor a random ringing. What you've achieved so far doesn't look bad at all. So you're right, making the loops that carry high current as short as possible, using good bypass caps for that and providing a solid driver signal for the switch are key elements.

Another area for some experiments is the snubber circuit for the primary coil. A capacitor is a start, but you'd need to find out the best value for this, again for highest efficiency. A capacitor with a small series resistor (e.g. 100 ohms) might improve things even further.

If I were you, I'd set up a new test with a transformer temporarily made just for this: 3 equal windings, 30 turns each, so there is no change to what you've explored so far. This way the secondary is low voltage and much easier to work with in therms of measurements, and the third coil could be used as a 2nd primary if you attempt to make a push-pull converter (of course you can leave that out if you're sure you'll never go this route).
 

Offline BlackwarriorTopic starter

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Re: Transformer design
« Reply #53 on: February 12, 2016, 09:45:30 pm »
Sorry im confusing you, confuse myself at times, and thats when im making a coffee... :-//

the 2 scope readings at 108Khz, one was with the 15nF cap and one without. all the cap appeared to do was distort the current waveform. there was no difference at all to the output voltage, still 280VAC.

so to clarify, the cap is basically across the 9 vdc supply close to the transformer?

ive just checked my box of chips and found an MCP14e4 driver, so ill add this.

i dont have anymore bobbins or cores to wind more transformers, but ill certainly order a few more, they are cheap as chips as they say in UK.

im on with a new board now, ill put the new driver on it and keep tracks, paths as short as possible.

thanks for your help today, greatly appreciated.

 

Offline Performa01

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Re: Transformer design
« Reply #54 on: February 12, 2016, 09:59:15 pm »
Ok, don't worry about waveform distortion so much - it is more important to keep the power supply lines reasonably clean, so even if it doesn't appear necessary in terms of efficiency, you still need it in terms of emc ;)

Btw. if you see yourself digging deeper into SMPS design, I can recommend you a really great book:

http://www.amazon.co.uk/Supply-Cookbook-Second-Design-Engineers/dp/075067329X/

 

Offline BlackwarriorTopic starter

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Re: Transformer design
« Reply #55 on: February 12, 2016, 10:04:52 pm »
Thanks Performa, I'll make this new board up in the morning, had enough for one day......

I'll have a read of that now, thanks again
 

Offline BlackwarriorTopic starter

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Re: Transformer design
« Reply #56 on: February 14, 2016, 11:31:01 am »
Changed the circuit on this design to try and improve things, added a MOSFET driver and more decoupling.

circuit and 2 scope displays attached. the 9VDC frequency counter reading is incorrect, scope problem i guess. the 12VDC one is correct at 105Khz

EDIT: corrected schematic, readings

105Khz - 217VAC output at AC Test point - 486VDC at DC Test point - 9 VDC supply voltage
105Khz - 302VAC                                      -  651VDC                           -12VDC supply voltage

« Last Edit: February 14, 2016, 11:41:12 am by Blackwarrior »
 

Offline Performa01

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Re: Transformer design
« Reply #57 on: February 14, 2016, 12:09:53 pm »
Wow, you manage to pump quite some current into your transformer now, don't you?

What is the blue trace?

Schematic looks good - where did you get that snubber network from?

How did you measure the secondary AC voltage? Do you have a true rms meter for that? And what was the load?

we can assume to get just about 8Vp on the primary side during switch on at 12VDC, as there is a 4V drop on the current measurement resistor already. Did you test the output with that resistor shorted?
 

Offline BlackwarriorTopic starter

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Re: Transformer design
« Reply #58 on: February 14, 2016, 12:21:44 pm »
Hi Performa

the Blue trace is the VCC rail on the Transformer. drops quite a bit doesnt it at peak current.

too much current really for a battery operated device !!!

Snubber circuit i got from google search...

im still using the Megger voltage tester, its the Megger TPT320. it doesnt say in spec if its true RMS, just AC/DC up to 690V. load is <3.5mA.

i havent tested the design with the Current resistor (0.5R) shorted.

ive done some tests up to 150Khz, at 5KHZ steps... the best frequency is 115Khz.

i need to draw less current from the batteries to achieve the same results, but i dont see that possible !!??!?!


 

Offline Performa01

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Re: Transformer design
« Reply #59 on: February 14, 2016, 02:22:58 pm »
Well, the turns ratio on your transformer is 1:18 and you want some 700VDC if I remember correctly. This is 350VDC bevore the voltage doubler, and with no load, VDC will reach the peak VAC value, so 250VAC rms would be enough for that. Under load however, it is likely to drop a fair bit, so I would rather calculate 300VAC rms.

This in turn requires 300/18 = 16.7Vrms on the primary side, which is ~23.6Vp. Since your supply is only 9V, this would require some step-up process, i.e. even though you cannot apply more than 9V in the active phase, the coil voltage will get significantly higher in the opposite polarity when the transistor is off. I would aim for a more symmetrical primary voltage swing instead, as you probably have severe distortion, i.e. strong 2nd harmonics right now, which cause a DC component in your primary current, that compromises magnetic core properties and efficiency.

I would try to find the optimum working conditions for highest efficiency now. Measure the current consumption for different supply voltages (for the primary coil and MOSFET only, not the driver), e.g. from 1V to 9V in steps of 1V, and also measure the output voltages. As you presumably have a resistive load, you should be able to calculate input power (V_supply * I_supply), output power (V_out² / R_load) and see what efficiencies (P_out/P_in) you get.

The snubber network needs to be optimized for the individual design, so it would be better to leave it out for now. I wondered why the currents are so high all of a sudden, compared to what we've seen before. I suspect it is because of this network, which might not be well suited for your circuit after all.

Once you've found the best operating conditions in terms of efficiency, you get an idea what to change in order to make the circuit work for your particular requirements, i.e. 9V supply voltage and 300VAC output...

Good luck with your further experiments!
 

Offline BlackwarriorTopic starter

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Re: Transformer design
« Reply #60 on: February 14, 2016, 05:07:41 pm »
Thanks Performa

I ordered a new bobbin from farnell the other day but its on back order, hopefully it will be in on Monday. this one is slightly larger than the one im using so ill be able to increase seconday windings AND have room for insulation.

i did get a bigger core and former (some 35mm square), wound this with 800 sec and only 10 primary... definitely need more turns on primary. but i put that aside to get this one working..

ive started to lay a PCB out rather than this stripboard. this will be a better prototype. shorter/thicker tracks, isolated HV etc etc

thanks for all your help, i will continue with this transformer ive been testing but i think we have reached its capabilities maybe..

been a good insight into how these things work though. thanks to your explanations..

the quest continues.... :-+

EDIT: i took the 0.5R resistor out the source to ground earlier and got an extra 95VDC out of it.. took me to just over 600VDC, thats after the Cockcroft..
« Last Edit: February 14, 2016, 05:11:33 pm by Blackwarrior »
 


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