Cold Cathode Fluorescent Lighting (CCFL) Inverter Transformer

Overview of CCFL's

Cold Cathode Fluorescent Lamp (CCFL) Inverter Transformers are widely used to convert a low DC supply to a high voltage AC to drive, for example, LCD backlights. They employ a variant of SMPS technology to switch the DC using transistors (in a push-pull configuration) at high frequencies (40-80 kHz in our example) into the primary. A feedback winding is used to provide positive feedback to make the circuit oscillate Usually, the secondary winding is deliberately designed to have a set leakage inductance, which then resonates with a capacitor on the secondary to drive the lighting tube. As the transformer is such an integral part of the operation of circuit, measurement of parameters such as leakage inductance are important as well as the usual parameters of winding resistance and turns ratio. The transformer also needs to provide isolation, especially as the circuit first generates a higher “strike” voltage to start the tube, before settling into a constant running state.

Eaton make a variety of versatile CCFL transformers for these operations. Here, we will examine the CTX210607.

Note that the secondary is wound in 4 sections. This is so that the Volts per Turn drop is spread across 4 separate areas, which improves inter-winding isolation under the high potential differences without having to resort to heavily insulated wire.
The primary and feedback coils are also separate. This separation controls the deliberate introduction of leakage inductance discussed earlier.

CCFL Suggested testing

AT Editor Schematic for CCFL

The AT Editor software representation of the part is shown here to the left.

AT Fixturing for CCFL

The transformer package is a standard surface mount design, and as such is not suitable for Kelvin pins.

The fixturing shown here is a zero insertion force socket (ZIF) in which pairs of blades are closed onto each pin from the side.

This has the advantage of not putting the part under any mechanical strain whilst still maintaining a true Kelvin contact to each winding.

AT Test Program for CCFL

See our Test program here
or download the actual ATP test program here
(can be opened with free AT Series Editor software, see DOWNLOADS here)

The winding resistances are checked first to validate continuity and connection.
This is followed by and inductance check at the operating frequency on the primary.
This if followed by three checks of the turns ratios; half primary to half primary, primary to feedback and primary to secondary. Note that the last test is performed by energizing the secondary as for optimum accuracy it is best practice to energise the winding with the greatest number of turns.
We then test leakage inductance between primary and secondary; we find that the leakage is roughly 25% of the primary inductance as this is probably a deliberate design factor in the operation of the output LC circuit. For this we have used percentage limits around a nominal LL value, rather that checking LL is just below a maximum as is normally the case.
As the secondary is made from fine wire, and subject to high voltages and even higher “strike” voltages to start the gas tube, we next use the SURG test to check for any Interwinding coil weaknesses. This is performed by injecting high voltage pulses and measuring the characteristic ringing decay on the secondary winding. An empirical nominal from a known good transformer is selected for our limits. Any breakdown in the insulation will result in energy loss and hence a different decay response.
See the link at the end of this section for more information on SURGE testing.
Finally a HI POT test at 2KV AC is used to check primary to secondary isolation.

#	Test	Description	Pins and Conditions	Reason
1	R	DC resistance	pin 1-3, limits <135 mOhms	To check the total primary winding resistance is below a maximum. Also acts as a check of correct wire gauge and good termination.
2	R	DC resistance	pin 4-5, limits <100 mOhms	To check the feedback winding resistance is below a maximum. Also acts as a check of correct wire gauge and good termination.
3	R	DC resistance	pin 10-6, limits <175 Ohms	To check the secondary winding resistance is below a maximum. Also acts as a check of correct wire gauge and good termination.
4	LS	Series Inductance	Pin 1-3, 100mV, 20 KHz, nominal 27 uH +/- 10% (as per published spec)	Inductance as seen by the primary. To check the correct number of turns and correct operation of the core material
5	TR	Turns Ratio	Energize pins 1-3, 100mV 40 kHz, check Turns ratio and phase 1-2:2-3 to be 1:1 +/- 5%	To check correct ratio of windings between the two halves of the primary to the centre tap.
6	TR	Turns Ratio	Energize pins 1-3, 100mV 40 kHz, check Turns ratio and phase 1-3:4-5 to be 4.6:1 +/- 5%	To check correct ratio of windings from whole of the primary to the feedback winding.
7	TR	Turns Ratio	Energize pins 10-6, 100 mV 10 kHz, check Turns ratio and phase 10-6:1:3 to be 86:1 +/- 5%	To check correct ratio of windings from secondary to primary. The winding with the most turns is energised as this is best practice for optimum accuracy.
8	LL	Leakage Inductance	Pins 1-3 Hi, Pins 10-6 Low, 100mV, 40 kHz , check leakage is below 6.5 uH	To check leakage is below specified limit as a validation of correct placement and operation of windings.
9	SURG	Surge Stress Test	Energize pins 10-6, 4000V, 5 pulses. Check that mVs product is 166 mVs +/- 30%	To check for weak points in the secondary inter turn insulation, to prove longevity of the part over its operational life.
10	HPAC	AC Hi-Pot	2 kV AC,50Hz 1 second, Pins 1,2,3,4,5 High, Pins 10,6 LO. Check <15 mA current	To check isolation as per datasheet.
				AT5600 Run time 2.61 sec
				(AT3600 Run time 5.49 sec)

Notes:
As the Leakage Inductance is governed by turns ratio, and core response (already covered by TR and LS) and the physical positioning of the windings, some customers (using automatic winding methods) may have enough confidence in winding positioning to only audit this parameter occasionally, rather then on every part tested.

AT Test Results for CCFL

Links / References for CCFL Transformers

- The Eaton homepage
- The Eaton CTX210607 datasheet
- Wikipedia article on CCFL inverters
- Wikipedia article on the more general technical aspect of Royer Oscillators
- Our article on Detecting Shorted Windings (SURG impulse testing and STRESS WATT testing)
- Our Article on Leakage Inductance
- Our article on the Audit Test option on the AT5600