Liquid crystal back light become smaller in size and lighter in weight every year. But the inverter circuit for use with a discharge tube is very difficult to miniaturize. However with the invention of an inductance isolation effect transformer, it is possible to make the inverter circuit to become smaller size, higher efficiency and lower cost.

Inverter Circuit is a step-up voltage circuit driving the Cold Filament Lamp. it is one of the important part of the liquid crystal back light panel device.@We used to using closed magnetic circuit type's step-up transformer in the conventional inverter circuit. And the products made by different manufacture are almost the same in it's structure.
In the other hand, the liquid crystal back light becomes smaller in size and lighter in weight gradually. With it, the size of a inverter circuit is also required miniaturize. However, because the structure of inverter circuit is very fundamental, it is impossible to have it smaller if we design it in a general way. It is a step-up transformer that occupies the largest space in an inverter circuit for a discharge tube, and the difficulty of miniaturization in size of the step-up transformer makes it impossible to miniaturize the whole inverter circuit in size. But this problem is overcame by the invention of inductance isolation effect transformer. The new style inverter circuit is named pencil inverter circuit for it's figure. Now ,we will introduce this pencil inverter circuit where a inductance isolation effect transformer is used. This new type inverter circuit not only has a small size but also is high efficiency, safe and low cost, low noise.(Picture 1,2)

*Picture 1. Pencil inverter circuit set up in a frame of back light unit set *Picture 2 Pencil inverter circuit (left) and conventional inverter circuit (right)

Conventional inverter circuit's structure and problem
1. the structure of a conventional inverter circuit Conventional inverter circuit is consist of a collector resonance type primary circuit, a closed magnetic circuit type transformer, a condenser ballast. We can not find the difference between the manufacture's structure. This part are indicated by wave signal in the circuit figure( Fig 1). Cold Filament Lamp which is connected as a load is a discharge tube. The general characteristic of a discharge tube is that the cathode dropping voltage has a negative-resistance characteristic. So in this case, a ballast is very necessary for keeping the stability of discharge current. Cb which is connected with the Cold Filament Lamp in the series circuit is for this purpose.(Fig 2) The current flowing to the Cold Filament Lamp because of the negative-resistance character is indicated by crooked triangle.

2. The obstruction of miniaturize and the parasitic capacitance caused in the circumference of the Cold Filament Lamp Generally if making the driving frequency higher, the size of the transformer can be smaller. But because the Cold Filament Lamp is a load, the parasitic capacitance caused in the circumference of the Clod Filament Lamp and caused by a secondary winding of a step-up transformer can not be ignored. The equivalent circuit of this combination is Fig 3. The resonance frequency decided by inductance of secondary winding and it's parasitic capacitance is very low. It is a value which can not be ignore in the inverter circuit design. The driving frequency of inverter circuit should be set up in the place lower than resonance frequency, and it also has a limitation to make the driving frequency higher. Thus, it caused obstruction to make the size of inverter circuit smaller.

*Fig 1. Inverter circuit and the voltage wave of inverter's each part *Fig 2 Crooked current wave caused by negative-resistance characteristic

*Fig 3. The equivalent circuit of conventional inverter circuit
the stray capacitance caused by the circumference of discharge tube

the stray capacitance caused by secondary winding

Cold Filament Lamp
*Fig 4. Pencil inverter circuit *Fig 5. Pencil inverter circuit's structure

*Fig 6. Conventional transformer(left) and isolation effect inductance (right)
*Fig 7. Isolation effect caused in the time when the lamp start
lamp is starting lamp is on magnetic flux

L1: isolation inductance
C1: stray capacitance
C2: stray capacitance

isolation effect transformer

In the pencil inverter circuit , the parasitic capacitance existed as a obstruction of the miniaturization is taken advantage of. It became possible to make the size of inverter circuit smaller.

Summary of the pencil inverter circuit
1.Structure of pencil inverter circuit
Fig 4.5. showed us the configuration and the diagram of the pencil inverter circuit. Basically there is not too much difference in structure between the pencil inverter circuit and conventional inverter circuit. The big difference is the inductance isolation effect transformer. This transformer's structure is very simple. We just turn the wire in order around the cylindrical shape ferrite core from one end of it. Fig 6 is a diagram showing comparison of inductance isolation effect transformer with conventional step-up transformer. Because the structure of inductance isolation effect transformer is very simple. It is possible to make it automatically. And since the characteristic of inductance isolation effect transformer is like the leakage magnetic flux type transformer's, there is no necessary to impregnate the insulation tape between conductive layer. So compare with the conventional transformer, it takes lower cost to make the inductance isolation effect transformer.

*Fig 8. The resonance mode of isolation effect transformer *Fig 9. The distribution parameter circuit of secondary winding of isolation effect transformer

Resonator polyethylene
conductor coaxial cable with characteristic impedance 75Ħ

first resonance secondary resonance distribution parameter circuit

The step forward speed of wave in the coaxial cable is slow. Since L and C of secondary winding are very big, the speed become very slow.

Primary winding Secondary winding

Picture (a) Picture (b) Picture (c) Transformer

*Fig 12. Equivalent circuit of pencil inverter circuit

*Fig 10 Fig 11
*Fig 12 Equivalent circuit of pencil inverter circuit
DC Source ƒÎmatching circuit
L1: isolation effect inductance
C1: stray capacitance
C2: stray capacitance

*Fig 13(a) *Fig 13(b)
ƒÎmatching circuit ƒÎmatching circuit
antenna cold filament lamp
high frequency
source high frequency
In the radio set, it is a common sense to design a matching circuit between output of transmitting part and antenna

In the new design, it see the cold filament lamp as a antenna then design the inverter circuit(high frequency source).

*Fig 14(a) conventional closed magnetic transformer
input current

layer-short start


*Fig14(b) isolation effect transformer
input current

layer-short start3
*Fig15(a) the result of measuring the noise of conventional closed magnetic inverter circuit to the basic oscillation level, harmonic level is lower of -20dB/dec *Fig15(b) the result of measuring the noise of isolation effect inverter circuit to the basic oscillation level, the harmonic level is lower of -40dB/dec.

The working principle of inductance isolation effect transformer Inductance isolation effect is a phenomenon that will occur when the leakage magnetic transformer is excited by the frequency which is higher than itself. Secondary winding of the inductance isolation effect transformer for the Cold Filament Lamp have a large number of turns. Therefore a large inductance and parasitic capacitance are produced. In this case, inductance isolation effect occurs easily. In the closed magnetic transformer, the resonance point between the parasitic capacitance of transformer' secondary winding and the inductance of secondary winding itself is just one. Magnetic flux is going to permeate through the ferrite core which is much easier to be permeate through more than the air. So secondary winding is working as a inductance. But if we open two ends of core to make it working as a leakage magnetic circuit, magnetic flux reach the surface of the end at first, then permeate through the air which has the low permeability Therefor even with only a little of difficult condition for being permeated through, magnetic will leak in the halfway easier. So if taking excitation as a special condition, the secondary winding will divided by inductance winding in plural. The resonance point between the parasitic capacitance in the inductance which is divided in plural and the parasitic capacitance of this inductance winding is formed. Since the inductance seems like in the isolation condition, we call it "inductance isolation effect"(Fig 7). Higher the frequency is , smaller the resonance point is. Because the circuit 's secondary winding being turned long and thin, the inductance and parasitic capacitance as one unit will be produced. All these phenomenon decided the speed of wave. This wave speed also decide the size of resonance. When length of the secondary winding and the size of resonance point is the same, primary resonance will happen. Like the string, this resonance phenomenon also have the high resonance mode like secondary resonance and third resonance. Therefore the secondary winding of the inductance isolation effect transformer have plural resonance point.(Fig 8). We also can find that secondary winding of the inductance isolation effect transformer looks like a coaxial cable. If we look it as a coaxial cable, inductance is so large that the speed of the wave become slow, and the characteristic impedance become very large(With the difference of the wave speed, resonance also move on the secondary winding)(Fig 9) 3. The equivalent circuit figure of pencil inverter circuit. We measured the inductance isolation effect caused by secondary winding of transformer(primary resonance mode). Picture a. b. c. show us the result using pick up coil to measure three point a. b. c. on the circuit (Fig 10)as Fig 11. To comparing the phase, collector voltage wave on the T point (Fig 10) is showed at the same time.

The wave on T point is in the same phase with collector voltage. It shows the flowing of magnetic on this point is in the same phase with primary winding. There is no voltage being showed on b point. At this point the direction of each other's magnetic flowing is oncoming. . On c point, wave is in the opposite phase with collector wave. The flowing of magnetic at this point is in the opposite phase with the primary winding. In inductance isolation effect transformer, the magnetic of resonance point occurring far from primary winding is not influenced by primary winding. So it works like independent chock coil. Fig 12 shows us the equivalent circuit. If we see the part being around by the dotted line, We will find the independent resonance point ( inductance isolation effect) and the parasitic capacitance being produced by secondary winding forms aƒÎ circuit with the parasitic capacitance caused in the circumference of Cold FilamentLamp.

ƒÎ match circuit is kind of circuit which is usually used as a impedance matching circuit of the antenna.(Fig 13a)At here, we look the Cold Filament Lamp as a antenna, then measure the impedance of Cold Filament Lamp. The impedance is 75kƒ¶ - 100kƒ¶ . We take this result as a reference then adjust the parameter of inductance isolation effect transformer 's secondary winding and match the impedance of Cold Filament Lamp. Then we supply the Cold Filament Lamp stable voltage, the efficiency of inverter circuit will improve(Fig 13b) Let's looking back the conventional inverter circuit. With using the reactance of the ballast condenser, enforce the voltage drop. We can find it is a mismatch power feeding circuit of impedance. Therefore reflected wave is caused by the load. The reflected wave come back to primary winding through transformer, then become the loss caused by the resistance to make the efficiency of collector winding worse. Until now, the liquid crystal back lamp and the inverter circuit is designed separately. Usually we drawn the long high voltage endurable silicon coating feeder line around in the designing. But with it, the brightness will lost stability. It is called "leak". In this instance, we just think to improve the endurance of the Silicon coating feeder Line. But it doesn't solve the problem. But if we can notice it is the mismatching of the impedance, this kind of phenomenon will much easier to be understood. If means inverter circuit design is entering a field where the high frequency theory is necessary. To developing the efficiency of back light, it is very necessary to put the theory of high frequency in it. The characteristics of pencil inverter circuit
1. small size and high efficiency
Because there is no necessary for the ballast condenser and the size of inductance isolation effect transformer is very small, the inverter circuit can miniaturize. Therefore it is possible put the inverter circuit in the frame of light introduce system for back light of liquid crystal. The back light unit using this inverter circuit is a very simple devise (Picture 1).
Just supplying the direct current to the back light, it will turn on. It's near the back light' original ideal. If the design is like this, we can control this device including the high frequency design as a unit. This back light unit's bright efficiency is very high when the same power is supplied.
2. Safety
Like inverter circuit used in Cold Filament Lamp, the worst thing of the transformer supplied of high voltage is layer-short. When one part of the coating of the wire have some insulation problem, this insulation problem will expand from that part to the whole winding. In closed magnetic transformer, the magnetic caused by primary winding permeate through the secondary winding. So when the layer-short happened, all energy will concentrate on the layer-short part. In isolation effect transformer inductance, when layer-short happened, the magnetic will not permeate through the part having the insulation problem, and it will leak in front of the layer-short part. Therefore layer-short will not expand. Fig14a,b is an experiment about this phenomenon. We drop one drop of nitric acid on the secondary winding of inverter circuit transformer, than compare the consumption current of closed magnetic transformer and isolation effect transformer inductance. The closed magnetic transformer's consumption current increased after the experiment start. And the inductance isolation effect transformer's consumption current decreased . The fail-save occurred. In the real instance, if we just leave the layer-short transformer alone, in the close magnetic transformer , layer-short will expand until the whole circuit damaged, and in the isolation effect inductance transformer, layer-short will not expand.

4. low noise
Since the leakage of magnetic in inductance isolation effect transformer , we usually think the noise will be very high. But as showed in Fig 15a.b ,the truth is opposite with the anticipation. In closed magnetic transformer, the decreasing rate of the high frequency noise is -20dB/dec. It hides in the 600KHz- 1MHz radio broadcasting wave which can be seen. In the other hands, the decreasing rate of inductance isolation effect transformer is -40dB/dec, it isn't be covered by radio broadcasting wave. If we see the corrector current of corrector resonance inverter circuit, we can find there is violent current change. The combination rates of closed magnetic transformer's stratum is high. So the violent change happened on primary winding will spread to the secondary winding In the inductance isolation effect transformer, the big part of di/dt will leak in the middle of the road which the di/dt spread to secondary winding. It's difficult to spread to secondary winding. So it is harmful to increasing the combination of stratum to decrease the radiation noise of high frequency,. Because the theory of transformer is easy to be understand, we think we have already mastered all of theory about transformer. But at the same time, we still didn't understand the induction phenomenon properly. Even the transformer also have the part which haven't been developed yet like inductance isolation effect. I think it is very necessary to confirm the theory of transformer once again.