# High Frequency Transformer

Discussion in 'The Projects Forum' started by twenglish1, Apr 1, 2014.

1. ### twenglish1 Thread Starter Member

Feb 25, 2014
38
3
I have been doing lots of research and purchasing components to build a high power switchmode power supply, first part i have decided to tackle is the high frequency power transformer, i am using an EE80 core, in the research i have done primary turns are calculated as follows:

Primary Turns = (voltage * 10^8) / (4 * Frequency(hz) * Max Flux Density(gauss) * Effective Core Area(cm^2)

Voltage = 330v
Frequency = 100khz
Max Flux Density of the EE80 core i chose = 5000gauss
Effective Core Area = 4.06cm^2

From my calculations and more research i will need 4 primary turns if i use a full bridge set up, or 8 primary turns for a half bridge setup, is this correct? i am still learning and would like to check my work as i go

EDIT: I caught my mistake, i used the Bsat(flux saturation) instead of Bmax(max flux density) did some more research and found that 1000 gauss is acceptable for the frequency i will be using, so i will need 20 turns for a full bridge setup, or 40 turns for a half bridge setup, correct?

EDIT EDIT: These are the cores i will be using: http://www.ebay.com/itm/271418168459?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

Last edited: Apr 1, 2014

Dec 4, 2009
310
46
Is 330V DC? It appears ok.
First step is to determine the power handling capacity of the core. You can refer to the charts provided by manufacturers. Then decide on the turns and select the wire size. Check whether it will fit. If not, then go back to selecting the core. This is an iterative process.

3. ### DickCappels Moderator

Aug 21, 2008
5,220
1,595
Don't you think a small gap to keep the core from saturating in case the drives become imbalanced would be a good idea? If you do, then the number of turns would increase.

By the way, more information would be helpful, such as the input and output voltages and currents and the mode of operation would be helpful in evaluating your design (continuous 50% duty cycle or PWM -and if PWM, what kind).

4. ### twenglish1 Thread Starter Member

Feb 25, 2014
38
3
Input voltage is gonna be about 330, output 330 at 40 amps

5. ### ifixit AAC Fanatic!

Nov 20, 2008
650
118
You mean 40 milliamps right?

6. ### twenglish1 Thread Starter Member

Feb 25, 2014
38
3
Nope 40 amps short circuit, and 330v open circuit

7. ### ifixit AAC Fanatic!

Nov 20, 2008
650
118
The input power is 330v x 40 Amps is 13.2KW. If 10% is loss in the transformer that's 1.32KW dissipated in the transformer. You need a cubic foot of core to dissipate that kind of heat and a bandwidth of 1 MHz or so and a self-resonance well above that. A tall order indeed.

At full load the output should drop no more than 5%. So 330 x 95% = 313.5V. What current do you want out at a full load voltage of 313V?

8. ### twenglish1 Thread Starter Member

Feb 25, 2014
38
3
I'm basing this design off of a store bought tig welder I have, the outputs of the tig are 80v open circuit at 140 amps short circuit, that is roughly the same power level as what I am working with, the tig also uses the ee80 core, I have a bunch of sets of cores so I could parrallel them if needed

9. ### ifixit AAC Fanatic!

Nov 20, 2008
650
118
The TIG pulsed welding frequencies are much less than 100 KHz, more like 5-10 KHz. The duty cycle is not 100% and they use forced air to keep the cores and winding from melting.

A 330 Volt welding voltage seems way too high and dangerous for a hand-held operation. Is this for a robot controlled operation inside an enclosure?

Why 100 KHz? Why 330V? What is the application?

Play Safe,
Ifixit

10. ### tcmtech Distinguished Member

Nov 4, 2013
2,808
2,703
DIY plasma cutter.

11. ### twenglish1 Thread Starter Member

Feb 25, 2014
38
3
It's for a plasma cutter I want to build, commercial made plasma cutters usally run around 300v open circuit, the tig I have is a cheap harbor freight inverter welder, I measured the switching frequency at 100khz at the half bridge output, the tig uses a half bridge inverter with an ee80 ferrite for the power transformer, it is not pulsed like a higher end tig, the power supply for the tig welder is just a constant current switch mode supply, 80v open circuit and 140amp max on the current knob

12. ### ifixit AAC Fanatic!

Nov 20, 2008
650
118
Is this transformer part of the arc starting circuit? This is a much less demanding requirement.

The main DC supply that provides the cutting current can be controlled with PWM, but I can find no reference to one that operates at 100KHz.

In any case I don't know enough about this to be able to help much. Perhaps another member has built a DIY plasma cutter.

Have fun,
Ifixit

13. ### twenglish1 Thread Starter Member

Feb 25, 2014
38
3
This transformer is for the main power circuit, it will be driven by a half bridge igbt module to supply the current for the torch head, now since I measured 100khz on the tig welder, at the igbt output, each igbt would only be running at half that right?

14. ### tcmtech Distinguished Member

Nov 4, 2013
2,808
2,703
Are you measuring the actual AC voltage going through the HF transformer or the ripple on the rectified DC output?

Most commercial units run in the 45 - 60 Khz range being power IGBT's tend to have some switching efficiency problems at higher frequencies.

Also for a 330 VDC input you should be running a full bridge switching system with a capacitor in series with your primary winding not a half bridge being that with a push pull half bridge you will be subjecting your switching devices to voltage peaks 2 - 4X your rail voltage with very little effective or efficient ways to snub them.

15. ### twenglish1 Thread Starter Member

Feb 25, 2014
38
3
I'll measure the frequency again at the igbt drive connectors. The tig welder uses two paralleled capacitors in series with the primary and the igbt E2, I could do a full bridge setup, I bought a lot of 10 half bridge igbt modules, 150 amp(rms) at 1000v

Last edited: Apr 1, 2014

Dec 4, 2009
310
46
This is not my domain, but I can safely say by the charts, the selected core seems to be atleast 3-4 times smaller than required. Maybe, it is a pulsed requirement. Like I said, I have no experience on this. Some other member could help you.
I was trying to post this yesterday, was continiously getting errors!

17. ### twenglish1 Thread Starter Member

Feb 25, 2014
38
3
This EE80 core is the same one my tig welder is using to push 130 amps at 80v open circuit, so if I drop my output current I am going for down to 30 amps at 330v open circuit, that is about the same power level, oh and I measured the frequency of the igbt drive on the tig welder at 40khz, don't know where I got the 100khz initially

Dec 4, 2009
310
46
During the open circuit voltage of 80V, there is no current and at 130A the voltage is far less. So the power should be pretty lower than the 10kW+ range. But that is alien to me.
At the rectified output of the transformer, the freq would be doubled at 80kHz.

19. ### twenglish1 Thread Starter Member

Feb 25, 2014
38
3
See that's one thing I was curious about, the power level, in arc welding say the open circuit voltage is 80v and the max current is 150 amps, but the voltage will fluctuate with arc length, so how can you calculate the wattage from that? Or would you just say 12,000 watts max since it will be much less then that, but could never go over

20. ### tcmtech Distinguished Member

Nov 4, 2013
2,808
2,703
Most stick welders have a arc voltage of around 20 - 30 volts at their rated current.

The other method to figure it out is to look at the input KVA rating or input volts and amps and calculate it out from there.