AN10803

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Application note

Rev. 2 — 27 February 2012

7 of 30

NXP Semiconductors

AN10803

Triac dimmable CFL using UBA2028/UBA2014/UBA2027x

3.

Designing the application

3.1 Block diagram

Figure 6 shows the block diagram of the dimmable CFL with charge pump. The phase cut

mains voltage is supplied to the CFL. The input voltage is rectified and supplied to the

inverter, and also used for detecting the phase cut adjustment of the dimmer. This is done

by measuring the average input voltage and supplying it to the control input of the inverter

control IC. That is, the CSP pin of the UBA2014 or UBA2028 and the DCI pin of the

UBA2027x. The dimmer phase cut detection must be non-linear. That is, the lamp must be

dimmed to 10 % before the bus voltage becomes too low and the inverter switches off due

to low supply voltage.

In practice, the lamp must already be dimmed to 10 % at 120 degrees phase cut and

preferably maintain the lowest dim level to 130 degrees. Beyond that value, the lamp is

off, but the inverter is still switching and providing a charge pump current. Without the

charge pump current, the CFL input is high (ohmic), and the input voltage is no longer a

reliable representation of the dimmer position.

3.2 Resonant tank

Once the gas in the CFL tube is ionized however, its impedance becomes negative. This

means that the more current is flowing through the tube, the more conductive it becomes

The increased current increases the degree of ionization of the gas. Therefore, add some

form of current limiter to the lamp to prevent the current increasing to a level where the

tube is destroyed.

An inductor is placed in series with the tube therefore, the tube is supplied with a current,

rather than with a voltage. A capacitor is placed in parallel to the tube that creates a

resonant circuit or resonant tank in combination with the series inductor to ignite the tube.

Before ignition, the lamp impedance is almost infinite, so the quality factor of the resonant

tank is high. At start-up and before lamp ignition the switching frequency is above the

resonance frequency and goes down in frequency until the tube ignites.

Fig 6.

Block diagram of the charge pump CFL dimmer

019aab963

voltage source charge pump

DIMMER

CFL

mains

FILTER

INVERTER

DIMMER

PHASE CUT

DETECTOR