Transformer design

[Blackwarrior]

Hi everyone. Newbie to the forum

I'm looking for an easy tutorial on winding a stepup transformer. I need to generate 700vdc from a 9volt battery. The current requirements are 5-10mA output at 700vdc.

I've searched the web on how to design such a transformer but nothing is simple enough.

I know I need around 1:80 ratio, and will have to input around 0.6amps to achieve this output current..

Can anyone help with this please.

Thanks

[rob77]

you need a DC/DC converter not a step up transformer. transformers are AC devices.

[Kleinstein]

With only 700 V DC, you might get away with reusing a transformer from a normal 230 V switches mode supply in reverse direction, especially if you use a voltage doubling rectifier.  At the still rather low power flyback converters are the usual choice.

700 V and 5 mA are 3,5 watts. That is too much for a normal 9 V block  battery.

[hamdi.tn]

You may reuse an existing transformer in reverse , you can use a transformer from an SMPS giving the right duty-cycle you can get your 700V or whatever. but as Kleinstein said it will to much for a 9V battery.

[Blackwarrior]

Thanks for the quick replies guys. Firstly the battery source will be 6x1.5 volt C cells so I think these may cope ok. I did try using a standard 230v to 6v transformer in reverse, I fed that with a mosfet at around 8khz, varied this frequency with the micro but couldn't get anywhere near what I wanted as expected.
Thanks, if anyone has any circuits that could do the job I'd appreciate it

[hamdi.tn]

i did a circuit that i used it in the lab for a while, and it's just

STM32F0 dev board using one timer , once channel and it's complementary output, generating a 100KHz pwm signal
an H-Bridge L298
a switching transformer from an old ATX power supply, its output is wired to the H-bridge and powered up by a bench PS at 12V
i can get easily 350VDC from the primary, and it was good enough to power up small phone chargers and such small load, and to do the tests it was originally made for

[fcb]

In addition to the other answers you'll have a problem with a PP3 (9V battery) for your application.

700V @ 10mA = 7W

7W @ 9V = 0.77A

And that's assuming 100% efficiency, in reality you'll be luck to get to 85% efficiency (7W @ 85% = 8.23W input, or 0.91A @ 9V) - and at these sort of levels your 9V battery internal resistance will play start to become a dominant factor (further dropping the output voltage of the battery, and increasing the current required, etc....)

A couple of things you could try:
1. Build one and test it (although you'll probably need to drive it from a bench PSU whilst you do your inital development)
2. Replace the 9V with perhaps 6xAA or 6xAAA cells.
3. Try one of those Lithium 9V batteries - they are super expensive but if you must have the PP3 form factor they might help.

[Brumby]

.... The OP has already said the power will be supplied by 6 'C' cells.

[Performa01]

Thanks for the quick replies guys. Firstly the battery source will be 6x1.5 volt C cells so I think these may cope ok. I did try using a standard 230v to 6v transformer in reverse, I fed that with a mosfet at around 8khz, varied this frequency with the micro but couldn't get anywhere near what I wanted as expected.
Thanks, if anyone has any circuits that could do the job I'd appreciate it

It might be a good idea for you to show us your current circuit configuration first, so we get a chance to spot what's going wrong...

[kripton2035]

lots of infos : http://ludens.cl/Electron/trafos/trafos.html

[Blackwarrior]

thanks Guys, i think ive cracked it, i will generate a schematic later and pass this on. basically, i wound my own transformer using a very small bobbin and core from farnell. this wont be good enough for a finished version as there isnt enough room for proper isolation between the windings at this voltage. i think id be better off with a separator in there. but with a single stage Cockcroft Walton in im getting around 670VDC good enough.

circuit description:

using an arduino to generate 7.8Khz (experimental) oscillator at 50%. this feeds a MOSFET N channel that grounds one end of the transformers primary, the other end is connected directly to the 9 volt supply. one end of the secondary winding goes to ground whilst the other goes through the Cockcrofts single stage.   

circuit to follow
thanks
 

[Blackwarrior]

This is my basic circuit, ive omitted the arduino.. think i still need some kind of snubber in there maybe???

any comments would be appreciated

[Performa01]

A diode across the primary transformer winding (30T) would be a good idea indeed (cathode towards 9VDC). The MOSFET is only rated 75V, so I wouldn’t be surprised if it’s already fried…

The 2nd thing is the control signal. You indicate it’s PWM – in that case make sure (in software), it can never go beyond 50% duty cycle, i.e. the Transistor is never turned on for more than 50% of the PWM period.

Some experiments would be helpful – given the fact we don’t know much about the transformer, other than it’s probably meant for mains frequency (50Hz). So it will probably not work very well at higher frequencies and losses might be high, especially when driven by square pulses.

So you should insert a small resistor, say 0.5 ohm, in the source connection of the MOSFET and monitor the voltage drop across it with an oscilloscope. Now try to find out how log it takes for the current to reach 2A, i.e. 1 volt across the sense resistor. That is your maximum turn on time then – and it depends mainly on the inductance of the transformer.

Just recall the basics: When the transistor switches on, the current starts to rise and is only limited by the DC resistance of the transformer winding, which is supposedly <1 ohm. So the current could reach quite high values at 9V supply. Obviously, the control circuit has to be designed in a way that it switches off the transistor before that happens.

Now the transistor switches off and the transformer suddenly ‘sees’ a very high impedance at a very high current – according to Ohm’s law this means a very high positive voltage developing on the drain of the transistor. Way more than 75V, so the transistor is probably dead already. TO prevent that (in future, at least), the freewheel diode will kick in. It starts to conduct as soon as the drain becomes more positive than the supply voltage and will allow the current continue to flow. That’s how a simple forward converter works.

[Blackwarrior]

A diode across the primary transformer winding (30T) would be a good idea indeed (cathode towards 9VDC). The MOSFET is only rated 75V, so I wouldn’t be surprised if it’s already fried…

The 2nd thing is the control signal. You indicate it’s PWM – in that case make sure (in software), it can never go beyond 50% duty cycle, i.e. the Transistor is never turned on for more than 50% of the PWM period.

Some experiments would be helpful – given the fact we don’t know much about the transformer, other than it’s probably meant for mains frequency (50Hz). So it will probably not work very well at higher frequencies and losses might be high, especially when driven by square pulses.

So you should insert a small resistor, say 0.5 ohm, in the source connection of the MOSFET and monitor the voltage drop across it with an oscilloscope. Now try to find out how log it takes for the current to reach 2A, i.e. 1 volt across the sense resistor. That is your maximum turn on time then – and it depends mainly on the inductance of the transformer.

Just recall the basics: When the transistor switches on, the current starts to rise and is only limited by the DC resistance of the transformer winding, which is supposedly <1 ohm. So the current could reach quite high values at 9V supply. Obviously, the control circuit has to be designed in a way that it switches off the transistor before that happens.

Now the transistor switches off and the transformer suddenly ‘sees’ a very high impedance at a very high current – according to Ohm’s law this means a very high positive voltage developing on the drain of the transistor. Way more than 75V, so the transistor is probably dead already. TO prevent that (in future, at least), the freewheel diode will kick in. It starts to conduct as soon as the drain becomes more positive than the supply voltage and will allow the current continue to flow. That’s how a simple forward converter works.

Thank you for all this information, very valuable. The transformer I used is from farnell, this one... http://uk.farnell.com/epcos/b66208b1110t1/coil-former-e25-13-7-valox-420se0/dp/2077544

With the appropriate core. It needs to be as small as possible, but I think this one is too small..

I will put the diode on tomorrow, the mosfet hasn't fried yet and it's been running most of the day.

I have the circuit connected to a bench PSU, and the current drawn is no more than 0.6 amp at 9 volts..

The PWM never exceeds 50%. I will put the resistor in series with the source on the mosfet tomorrow to check the current flow and timing.

Thanks for the tips, appreciate your time

[Performa01]

Yes, your MOSFET seems to be a very rugged beast and the intrinsic diode acts as a zener clamp that seems to protect the transistor. Well, it all depends on the energy stored in that transformer. I still don't know what properties it has - the link to the coil-former doesn't tell much in this regard. The core is the main important thing, together with the wire used for the windings and the number of turns...

If you have all these data you should be able to calculate the primary inductance (or measure it), then it is possible to predict how fast the current will rise after the transistor turns on. Of course it also depends on the power supply and it sure is a wise move to set the current limit to some 2A.

As for the diode, it should be a reasonable fast 1A type, a UF4004 would be ideal, also a Schottky diode of course. But when at a pinch, an ordinary 1N4004 would do for the first steps as well, since the frequency isn't that high and efficiency is not a primary concern at this stage...

Good luck with your experiments!

[hamdi.tn]

did you wind it your self ?
i don't know why you need that capacitor in series with the output.
i go with what @performa said for the diode , you will need probably more rapid diode on the output. but i pick an UF4007.

[Blackwarrior]

Yes, your MOSFET seems to be a very rugged beast and the intrinsic diode acts as a zener clamp that seems to protect the transistor. Well, it all depends on the energy stored in that transformer. I still don't know what properties it has - the link to the coil-former doesn't tell much in this regard. The core is the main important thing, together with the wire used for the windings and the number of turns...

If you have all these data you should be able to calculate the primary inductance (or measure it), then it is possible to predict how fast the current will rise after the transistor turns on. Of course it also depends on the power supply and it sure is a wise move to set the current limit to some 2A.

As for the diode, it should be a reasonable fast 1A type, a UF4004 would be ideal, also a Schottky diode of course. But when at a pinch, an ordinary 1N4004 would do for the first steps as well, since the frequency isn't that high and efficiency is not a primary concern at this stage...

Good luck with your experiments!

Thanks Performa, I think I've got some Schottky diodes somewhere, I'll stick one of those in tomorrow, I'll also measure the inductance of the windings and note the wire I used, turns are on the diagram.

As I said the PSU never goes above 0.6 amps, current limit is set to 1 amp.
I will keep you all updated tomorrow with parameters of the circuit.

Thanks for your help and time.

[Blackwarrior]

did you wind it your self ?
i don't know why you need that capacitor in series with the output.
i go with what @performa said for the diode , you will need probably more rapid diode on the output. but i pick an UF4007.

Yes I wound the transformer myself, I have a great winder I bought off ebay, counts the turns for you, I used that for the 540 secondary winding, the primary I wound by hand as it was only 30 turns.

The series cap on the output forms part of the Cockcroft Walton circuit with the diodes....
Thanks for the tips

[chris_leyson]

EF25 is a big core for 7W but I guess you need a good layer of insulation. I notice you are driving the mosfet at 7.8kHz 50% duty cycle, so that gives you an on time of 64us, I think you need much bigger magnetic coupler for 7.8kHz, 10 Watt audio transformer spring to mind. Dave, where is the light bulb (Gru) emoji thingy ?

If you are using squarewave voltage drive then use the volt time product to calculate your minimum number of primary turns for a given core and winding size, mathematically you've got five variables which are assumed to be linear and you can translate or arrange them five ways, minimum.

(1) V*t = N*B*Ae, (2) B = V*t/(N*Ae), (3) N =  V*t/(B*Ae), (4) t = N*B*Ae/V, (5) Ae = V*t/(N*B), (6) f = 1/2t = V/2*(N*B*Ae) where

B = flux density (T)
V = applied voltage (V)
t = on time (us)
N = number of turns
Ae = effective core area (mm²)
f = switching frequency in MHz at 50% duty cycle

For the known variables

V = 9V, t = 64us, Ae = 52.5mm² for an EF25 core, B = 200mT, only because it's a good place to start from, then

Nmin = V*t/(Bmax*Ae) = 9*64/(0.2*52.5) = 55 turns. At Bmax = 250mT then Nmin = 9*64/(0.25*52.5) = 44 turns.

From astored energy point of view, assuming you wound the primary with 55 or 56 turns how much energy could you store in say TDK N87 ferrrite, Al for ungapped N87 is 1850nH and at 55 turns that's 5.6mH. Energy stored = 1/2*I²*L and I = V*t/L
Peak current I = V*t/L = 9*64E-6/5.6E-3 = 100mA, Stored Energy = 1/2*L*I²*L = 1/2*0.1²*5.6E-3 = 28uJ
Maximum power you could transfer P = operating frequency*stored energy per switching cycle = f*E = 7.8E3*28E-6 = 220mW.

You need to increase the operating frequency and use less turns on the inductor.

[Blackwarrior]

Hey Chris, thanks for all the formuli. I've seen some on Google that's just mind boggling, this has cleared things up a bit.
I'm going to order a few different bobbins and cores today from farnell or RS, and maybe a dedicated programmable oscillator as I can't really get above 60khz with the processors Pwm. It's the atmega328 running at 16mhz.

Really appreciate all the work you put into that post.

This needs to be as small as possible too.

Thanks again.

[Blackwarrior]

My suggestion is 2*ATB322524 from TDK, paralleled, operates at 500kHz in DCM, with voltage doubler rectifier.
You can use a MP3425 boost controller to control this thing.

Efficiency will not be too good since the high recovery loss at high frequency, but you can get a very small solution. The entire thing, if built properly, can be smaller than 2cm^2.

Thanks for these suggestions, I'll look them up today at work.

[chris_leyson]

Hi Blackwarrior, just a few scribbles on the back of an envelope, I find it helps to get a feel for what's going on.
You're probably better of running the switch at 50kHz, another approach might be, how many turns can you get into a single primary layer, say 10 turns for sake of argument, maximum on time t = N*B*Ae/V = 10*0.2*52.5/9 = 11.6us, and at 50% duty cycle thats 43kHz.
Al for ungapped N87 is 1850nH gives you L = N²*Al = 10*10*1850nH = 185uH
Peak current I = V*t/L = 9*11.6E-6/185E-6 = 0.56A, Stored Energy = 1/2*L*I²*L = 1/2*0.56²*185E-6 = 29uJ
At a switching frequency of 43kHz you could transfer 43E3*29E-6 Watts or 1.25 Watts.

If you had 10 turns on the primary that would give you 9/10 volts/turn 0.9V/turn so you wound need 700/.9 or 780 turns on the secondary, now you've got to watch out for leakage inductance, the mosfet will need protecting.

[Blackwarrior]

Hi Blackwarrior, just a few scribbles on the back of an envelope, I find it helps to get a feel for what's going on.
You're probably better of running the switch at 50kHz, another approach might be, how many turns can you get into a single primary layer, say 10 turns for sake of argument, maximum on time t = N*B*Ae/V = 10*0.2*52.5/9 = 11.6us, and at 50% duty cycle thats 43kHz.
Al for ungapped N87 is 1850nH gives you L = N²*Al = 10*10*1850nH = 185uH
Peak current I = V*t/L = 9*11.6E-6/185E-6 = 0.56A, Stored Energy = 1/2*L*I²*L = 1/2*0.56²*185E-6 = 29uJ
At a switching frequency of 43kHz you could transfer 43E3*29E-6 Watts or 1.25 Watts.

If you had 10 turns on the primary that would give you 9/10 volts/turn 0.9V/turn so you wound need 700/.9 or 780 turns on the secondary, now you've got to watch out for leakage inductance, the mosfet will need protecting.

Chris, your a star. thanks. im thinking of ordering the efd25 bobbin and former from farnell.
also the CD4536 oscillator instead of the uP.

thanks mate

[Blackwarrior]

ive checked the primary and secondary winding for inductance with an LCR meter.

primary = 1.227mH - DC resistance = 0.2R - Test freq = 15K
secondary = 400.2mH - DC resistance = 11.6R - Test freq = 1K

[Performa01]

As your goal is a primary current of 0.6A average, peak current should be about 1.2A.
With 1.227mH primary inductance, you need a turn-on time of about 1.227mH * 1.2A / 9V = 163.6µs.
The period of your control signal thus needs to be twice as much, i.e. some 327µs and the frequency is 3kHz.

So while you've already been given very comprehensive advise by chris_leyson, also on transformer selection/design, you could still investigate your current transformer and I think it would be a good learning experience.

So maybe you actually insert that current sense resistor in the source of the MOSFET and watch the current during the turn-on time at 3kHz. Does it go up to 1.2A as expected? If it goes much higher, then this is probably due to core saturation and would be a hint to lower the inductance, i.e. number of turns, and increase the switching frequency at the same time - which has been suggested already anyway.