Service Manuals, User Guides, Schematic Diagrams or docs for : . Various CCFL CIRCUITS CCFL Circuits CCFL Transformers CCFL Transformer Turns Ratio Selection (AN3375)
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Maxim/Dallas > App Notes > DIGITAL POTENTIOMETERS
Keywords: CCFL, cold cathode fluorescent lamp, transformer Oct 15, 2004
APPLICATION NOTE 3375
DS3984 / DS3988 Transformer Turns Ratio Selection
The DS3984 and DS3988 are multi-channel Cold-Cathode Fluorescent Lamp (CCFL) controllers. The key variable
in designing an optimal system with the DS3984 and DS3988 is the selection of the transformer turns ratio. This
application note describes how the transformer turns ratio varies with different CCFL lamps and DC supply
voltages and it provides a quick look-up table to help the user quickly select a transformer turns ratio that will
work well in their design.
The DS3984 and DS3988 are multi-channel Cold-Cathode Fluorescent Lamp (CCFL) controllers. These controllers
use a push-pull drive to convert a DC supply voltage to the high-voltage AC waveform that is required to power
the CCFL lamps. Figure 1 details this arrangement.
Figure 1. DS3984 / DS3988 CCFL drive schematic.
Each channel of the controller drives two logic-level N-channel MOSFETs that are tied between the ends of a step-
up transformer and ground. The transformer has a center tap on the primary that is connected to the DC supply.
The controller alternately turns on the two MOSFETs to create the high voltage AC waveform on the secondary.
The key variable in designing a push-pull drive scheme is selecting the turns ratio for the transformer. At a given
regulated lamp operating current, the transformer turns ratio is directly proportional to the CCFL lamp voltage
and inversely proportional to the DC supply voltage as shown in the equation below.
For a given DC supply voltage, higher operating voltage CCFL lamps require a higher transformer turns ratio. For
a particular CCFL lamp, a higher DC supply voltage will require a lower transformer turns ratio. The operating
voltage of CCFL lamps varies, based on a number of factors including lamp diameter and length. Lamps with
smaller diameters and longer lengths operate at higher voltages.
There is not an exact formula for calculating the transformer turns ratio. Table 1 lists some empirically derived
transformer turns ratio for a variety of CCFL lamps. As can be seen from the table, the transformer turns ratio
decreases as the DC supply voltage increases, and the transformer turns ratio decreases as the operating voltage
of the CCFL lamps decreases.
Table 1. Target Transformer Turns Ratios for Various CCFL Lamps
Lamp 1 Lamp 2 Lamp 3
Nominal DC
2.4mm x 438mm 2.2mm x 258mm 2.0mm x 218mm
Supply Voltage2
830 ~ 890 Vrms1 540 ~ 600 Vrms1 440 ~ 490 Vrms1
5V 80:1 70:1 65:1
6V 75:1 65:1 60:1
7V 70:1 60:1 55:1
8V 65:1 65:1 50:1
9V 60:1 50:1 45:1
10V 55:1 45:1 40:1
11V 50:1 40:1 36:1
12V 45:1 36:1 32:1
13V 40:1 32:1 30:1
14V 38:1 30:1 28:1
15V 36:1 28:1 26:1
16V 34:1 26:1 25:1
17V 32:1 25:1 24:1
18V 30:1 24:1 23:1
19V 29:1 23:1 22:1
20V 28:1 22:1 21:1
21V 27:1 21:1 20:1
22V 26:1 20:1 19:1
23V 25:1 19:1 18:1
24V 24:1 18:1 17:1
Notes: 1. Lamp operating voltage at 5mArms operating current.
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