A Simple Yet Versatile Dimming Ballast Solution

Mar 1, 2010 12:00 PM
Andre Tjokrorahardjo Applications Engineer, Lighting Systems and Applications International Rectifie

The IRS2530D is simple to use with any analog and digital control method available for dimming fluorescent lamps. In addition, it can implement dimming control for LEDs.

Fluorescent dimming systems can satisfy visual comfort, and reduce utility costs through daylight harvesting, demand reduction, scheduled dimming, and other strategies. A dimming electronic ballast is an essential part of this system.

To perform dimming functions, the ballast must be configured to understand an input signal from the control device, and act upon the current flowing through the lamp. Typically, this is a challenging task for the ballast designer and is usually achieved using a complex, high-pin-count control IC.

The IRS2530D is a new dimming ballast control IC in a compact 8-pin form-factor (Fig. 1). This DIM8™ is a 600-V half-bridge driver IC that includes all the necessary functions for preheat, ignition, and dimming control of the lamp, and protects the circuit against line and lamp fault conditions. With only eight pins to accomplish all dimming ballast functions, the IC can minimize component count and simplify design, and is flexible enough to be used with various dimming control methods.

Several reference-design kits have been created to help with the evaluation of the IRS2530D. Each of these kits uses a different dimming control method, and they cover various input voltages and lamp types. A complete description of each kit is available at International Rectifier's lighting website (www.irf.com/product-info/lighting/).

ANALOG DIMMING

Fig. 2 shows the schematic of a dimmable electronic ballast for driving a 26-W quad-pin CFL from a 220-Vac line with an isolated 1- to 10-Vdc dimming input. The ballast comprises an EMI filter to block ballast-generated noise, a rectifier and bus capacitor to convert ac line input into a dc bus voltage, a control IC and half-bridge to produce high-frequency square-wave voltage, and a resonant output stage for preheating, igniting, and running the lamp. The current-sensing resistor and the isolated dimming reference input are additional components needed for dimming application. The IRPLDIM4E reference design kit does not include the isolation section of the circuit.

When power is initially turned on, the bus capacitor (CBUS) charges up, and resistors RVCC1 and RVCC2 supply the micro-power current to the IRS2530D. After the VCC voltage reaches above UVLO threshold, the half-bridge starts to oscillate at the maximum frequency. The charge-pump circuit (CVS, DCP1, and DCP2) takes over as the main supply circuit for the IC and keeps the VCC at the internally clamped 15.6 V.

An internal current source at the VCO pin charges up the external capacitor (CPH). Output frequency decreases as the CPH charges and, at the same time, the lamp filaments are preheated using the secondary winding from the resonant inductor. As the frequency decreases toward the resonant frequency of the output stage, the voltage across the lamp increases. After it reaches a high enough voltage to ignite the lamp, lamp current begins to flow. The resonant output stage transitions to a series-L, parallel RC circuit with the Q-value and operating point determined by the user's dim level.

Ac lamp current is sensed by the resistor RCS, and the resulting ac voltage is coupled with the dc dimming reference voltage from isolation through feedback resistor (RFB) and feedback capacitor (CFB). This dc + ac signal is then fed into the DIM pin of the IRS2530D and is regulated by the control loop, such that the valley of the ac voltage always stays at COM.

When the dc reference voltage at the DIM pin is decreased for dimming, the valleys of the ac voltage are pushed below COM. The dimming control circuit increases the frequency to decrease the gain of the resonant tank circuit, and thus the ac lamp current, until the ac valleys at the DIM pin are at COM again.

The opposite happens when the dc reference is increased to increase the brightness level. In this way, the dimming control circuit keeps the ac lamp current peak-to-peak amplitude regulated to the desired value at all dc dim level settings.

QUAD-LEVEL SWITCH DIMMING

Another dimming control method is quad-level switch dimming, which uses the on/off switch to control the dimming level. When the switch is turned off and then turned back on in less than one second, the dimming level is reduced by one level.

If the dimming level is already at the minimum, this action will cycle the dimming level back to the maximum. If the switch is turned off for more than one second, dimming will stay at the last level.

The IRPLDIM5E is a quad-level switch-dimming fluorescent ballast which drives a 26-W quad-pin CFL from a 220-Vac line. Fig. 3 shows the schematic of the IRPLDIM5E.

The circuit in Fig. 3 is similar to the IRPLDIM4E, except for the circuitry to generate the dc dimming reference voltage. A microcontroller is used to provide the reference voltage and to determine whether to switch to the next dimming level.

The micro-controller used here is the PIC12F629, which contains some EEPROM non-volatile memory that allows the microcontroller to store the last dim level setting before shutting down after power is switched off. This enables the ballast to start up at that same setting when power is restored, no matter how long the ballast has been off.

Pin 5 of the microcontroller generates a fixed-frequency square wave signal with four different duty cycles, which correspond to four dimming levels. The square wave signal then goes through the low-pass filter to produce the dc reference voltage.

Pin 6 of the microcontroller is connected to the bridge rectifier through a filter circuit with a very short delay. This allows the microcontroller to detect when ac power has been removed and restored quickly.

The VDD supply capacitor, C1, is large enough to allow the microcontroller to continue to run for more than one second after ac power has been removed from the ballast. The microcontroller starts a timer as soon as it detects that the power is switched off. If power is restored within one second, the microcontroller will reduce the duty cycle one level, and thus reduce the dc reference by one step. If the level is already at the minimum, it will cycle back to the maximum duty cycle.

THREE-WAY SWITCH DIMMING

The three-way dimming system is widely adopted in the U.S. The system consists of a special lamp socket that has a four-position switch and a bulb with a modified screw base. For incandescent lamps, the bulb has two filaments and two connections on the lamp screw base.

The IRPLCFL8U is a three-way switch-dimming electronic ballast driving a 32-W spiral CFL from a 120-Vac line. Fig. 4 shows the ballast, together with the three-way socket and the modified lamp base. The interface circuit includes a voltage doubler (D1, D2, D3, D4, C1, and C2) in place of a rectifier and a circuit to generate the dc dimming reference voltage (R3, R4, R5, R6, R7, RPU, Q3, Q4, DZ1, and C5).

The first socket switch position is “off,” in which no filaments are connected. In the second position, the first filament (PL1) is connected across the ac line for the lowest brightness setting. Resistor R5 pulls up the dc dimming reference across resistor R7 and capacitor C5 to set the minimum brightness.

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