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17th January 2006, 03:37 PM  #1 
diyAudio Member

Designing transformer leakage inductance
That HID ballast is the ultimate opportunity to play around with some SMPS topologies.
I had the ballast going using a pushpull switchmode current source and a Hbridge for feeding the bulb AC current. Now, that is a very straightforward architecture, but unfortunatly is is also quite bulky. UCC28085based currentmode pushpull, bridge rectifier, inductor, capacitor, current source inner feedback loop, Hbridge, Hbridge control and finally the outer loop which steers the current source depending on the lamp warmup status. And then the igniter, of course. Quite hard to get all that into a very small package, even if the control logic is mounted on vertical PCB's and the power logic on the horizontal carrier. Much, much simpler is a resonant scheme: The resonant tank is formed by the leakage inductance (primary and secondary tranformed to primary) and capacitor. This can be made to work under ZVS and it can generate the high voltage pulses by lowering the switching frequency. For example: here is a LTSpice simulation with an 1:16 transformer with a leakage inductance of 500nH (and ~6uH magnetizing inductance, which would equal 3 primary turns on an ETD34, N87 ferrite), a 220nF capacitor and a 120 Ohm load on the transformer secondary. Switching frequency is 285kHz with 50% duty cycle. My fingers are itching to try this, but I have no idea how I can design a transformer for a certain leakage inductance. How to do this? I can increase leakage inductance by simply adding more space between primary and secondary, but I have no idea what a doubling of the distance does to the magnetizing inductance, and how to calculate it's magnitude. If this cannot be calculated easily, I guess I can also use a small inductance (aircore inductor?) in series with the transformer, right? 
17th January 2006, 04:44 PM  #2 
diyAudio Member

The easiest way to get it is to add an inductor in series with the secondary winding, or maybe even with the primary. However, note that I've tried a similar approach (pushpull actually) to light and dim compact fluorescent lamps directly from 12V without additional switching stages, and the major drawback that I've found is very high peak currents (up to 15A peak to light a 23W lamp!!).

17th January 2006, 05:51 PM  #3  
diyAudio Member

Quote:
Quote:
The entire simulation is based on rough calculations and fiddling a bit instead of proper calculations. I suppose it can be better, but I wanted to get a feel for these things first. How would I see the pushpull variant of this resonant thing? Standard pushpull with two switches pulling the ends of a centertapped primary to ground, the center tap conencted to +V, and a capacitor between the ends of the centertapped primary? 

17th January 2006, 08:08 PM  #4 
diyAudio Member

A friend of mine generated a transformer using Magnetics Designer. I specified ETD29 geometry with 1mm gap (I happen to have such a core lying around), Epcos N87 material, 7 primary turns and 112 secondary turns. It seems that Magnetics Designer is capable of calculating leakage inductance since it shows up in the SPICE file he sent me. I have no idea what else he used for input.
Then, I tuned the inductor at the secondary side in SPICE. 82uH seems to work nicely. Waveforms look like the screenshot above. Looks like it might be able to work. (Lt)SPICE file. I have included the IRF540 subckt (although it is not the most suitable MOSFET) and transformer subckt so the file is standalone in case anyone is interested: Code:
* ZVS ballast test .SUBCKT irf540ns 1 2 3 M1 9 7 8 8 MM L=100u W=100u .MODEL MM NMOS LEVEL=1 IS=1e32 +VTO=3.55958 LAMBDA=0.000888191 KP=28.379 +CGSO=1.23576e05 CGDO=1.77276e08 RS 8 3 0.0251193 D1 3 1 MD .MODEL MD D IS=1.13149e09 RS=0.0078863 N=1.32265 BV=100 +IBV=0.00025 EG=1.17475 XTI=3.00167 TT=0 +CJO=7.95433e10 VJ=0.5 M=0.374991 FC=0.5 RDS 3 1 4e+06 RD 9 1 0.00623556 RG 2 7 4.10175 D2 4 5 MD1 .MODEL MD1 D IS=1e32 N=50 +CJO=1.75616e09 VJ=0.513551 M=0.614054 FC=1e08 D3 0 5 MD2 .MODEL MD2 D IS=1e10 N=0.40002 RS=3e06 RL 5 10 1 FI2 7 9 VFI2 1 VFI2 4 0 0 EV16 10 0 9 7 1 CAP 11 10 3.86673e09 FI1 7 9 VFI1 1 VFI1 11 6 0 RCAP 6 10 1 D4 0 6 MD3 .MODEL MD3 D IS=1e10 N=0.40002 .ENDS .SUBCKT trafo 1 2 3 4 ** ** ** ** ** ** ** ** Rdc1 N41 N61 3.572m Lmag N41 2 6.217u Rac1 N61 1 75.46m Lac1 N61 1 26.79n ** ** ** ** L12 N41 in2 143.9n C1_2 in2 2 4.175n C2_23 2 4 23.85p C3_23 2 N42 65.18p Efwd2 N82 4 in2 2 16.00 Vsens2 N82 N42 Ffbk2 in2 2 Vsens2 16.00 Rdc2 N42 N62 1.398 Rac2 N62 3 31.25 Lac2 N62 3 10.53u .ENDS V1 N12V 0 12 Rser=0.001 V2 gd_br 0 PULSE(0 12 0 30n 30n 1.75u 3.5u) R1 gd_br mosgate 10 XQ1 mosdrain mosgate 0 irf540ns Cr mosdrain 0 220n Lrp mosdrain trafopri 1f Xtr1 trafopri N12V trafo_sec 0 trafo Lrs trafo_sec vlamp 82u R2 vlamp N12V 120 .tran 50u .backanno .end 
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