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Use a CFL ballast to drive LEDs

Designers use ballast ICs, such as International Rectifier’s (www.irf.com) IR53HD420, in CFLs (compact fluorescent lamps) for heating the filaments, igniting the lamps, and supplying the lamps with current (Reference 1). Manufacturers produce these ICs in high volumes, and they cost approximately $2. This Design Idea shows how you can use a CFL-ballast IC for driving LEDs instead of CFLs. A ballast IC is essentially a self-oscillating half-bridge for offline operation. It typically operates from 320V dc, which is approximately the same power as that from a 230V-ac mains rectifier or a 120V voltage doubler. The IC generates squarewave voltages with an amplitude of 320V p-p and a frequency of tens ofkilohertz.

Usually, this square-wave voltage connects to a series combination of a CFL tube and a current-limiting inductor, L1 (Figure 1). Together with a parallel capacitor and using the LC resonance, you can warm up, ignite, and supply the tube with current. This approach works well because CFL tubes have high impedance when they are off and low impedance when they are running. Thetube voltage is typically 150V p-p.

By putting several LEDs in series and connecting them to a bridge rectifier, you can construct an imitation of a CFL, at least in the on-state. Imitating the off-state is less important, because LEDs need no ignition procedure. At the given values for RT and CT, the bridge runs at 70 kHz. The circuit supplies 64 LEDs with a current of approximately 80 mA. The infrared LEDs illuminate the field of view of a CCD camera in a machine-vision system. The circuit prototype uses a 2.7-mHinductor from a dead CFL.

The LED current comprises dc current plus a small ripple current; keep the ripple current low for high efficiency and long LED lifetime. LED manufacturers usually demand valuesof a few percentage points. Such a low ripple current may be difficult toachieve with one electrolytic capacitor,C5, but a parallel combination withan additional foil capacitor, C4, workswell enough in most cases. The voltageat the input of the LED rectifier is fairlyconstant during one oscillation period,so the inductor current has a triangularshape, which is good for EMC(electromagnetic compatibility). Theequation for the average LED currentis I LEDAVG
(½VDCNVFLED)/(4fL1), where VDC is the supplyvoltage, N is the number of LEDs inseries, VFLED is the LED forward voltage,f is the oscillation frequency, andL1 is the inductance of the currentlimitinginductor.

Although the circuit of Figure 1 works well, it has some deficiencies that the circuit of Figure 2 remedies by adding C6, D5, D6, and T1, wound on an EPCOS EP13 coil former, with an ungapped-EP13-core of T38 material with an inductance of 7000 nH. Both the primary and the secondary windings are 90 turns of 0.2-mm wire; the secondary winding is wound on top of the primary winding. Stray inductance is not important in this case, and the inductance for both the primary and the secondary windings is 50 mH. The circuitin Figure 2 has several advantages over the one in Figure 1. For example,the supply current for the ballast IC ofFigure 1 must flow through R1 and intothe IR53HD420, where it gets clampedto 15.6V. At a supply current of about6 mA, R1 must dissipate more than 2W.In Figure 2, R1 can have a much highervalue, because its function is to supplyonly a small start-up current. Afterstart-up, a charge pump comprising C6, D5, and D6 pumps enough current intothe VCC pin so that the internal zenerdiode clamps to 15.6V. The design equationfor the charge pump is I_SUPPLY(AVG)
fxC₆2xV_DC-15.6V. The dissipationof R1now stays below 0.25W.

Also, the summed forward voltages of the LEDs in Figure 1 must be smaller than one-half the supply voltage.For the circuit in Figure 2, by tailoring the transformer-winding ratio,you can connect as many LEDs asneeded, as long as you do not exceedthe ratings of the components. (LEDvoltages even higher than VDC arepossible.) A less obvious problem ofthe circuit in Figure 1 is that the fullvoltage swing of the bridge appearsat both ends of the LED string. Thissituation does not present a problemwhen all the LEDs are close togetherand the LEDs are close to the bridge.However, in many light fixtures, youwish to separate the LEDs from theelectronics. Due to stray capacitances,this approach would lead to highcapacitive currents from the LEDsto ground, corrupting the efficiencyand producing EMC problems. Withthe transformer of Figure 2, you canground one end of the LED string eitherdirectly, as shown, or through acapacitor. Now, you can use long cablesto easily separate the LEDs fromthe electronics.

REFERENCES

“IR53H420(D)420, Self- Oscillating Half Bridge,” Preliminary Data Sheet No. PD60140-K, International Rectifier, Aug 19, 2003, www. irf.com/product-info/datasheets/data/ ir53h420.pdf.