CFL Ballast IC Driving LED

Designers use ballast ICs (such as International Rectifier's IR53HD420) in CFLs (compact fluorescent lamps) to heat the filament, ignite the bulb, and provide current for the lamp (Reference 1). Manufacturers produce these ICs in large quantities, and they cost about $2. This design idea shows how to use a CFL ballast IC to drive an LED instead of a CFL. The ballast IC is basically a self-oscillating half-bridge designed for offline operation. It usually operates from 320VDC, which is roughly equivalent to the power of a 230VAC main rectifier or a 120V voltage doubler. The IC generates a square wave voltage with an amplitude of 320Vp-p and a frequency of tens of kilohertz.

Typically, this square wave voltage is connected to a series CFL lamp and current-limiting inductor L1 (Figure 1). Adding a parallel capacitor and using an LC resonator heats, ignites, and supplies current to the lamp. This works well because the CFL has a high impedance when off and a low impedance when on. The voltage across the lamp is typically 150Vp-p. Connecting

several LEDs in series and connecting them to a bridge rectifier can simulate a CFL, at least in the on state. Simulating the off state is less important because the LED does not require an ignition process. For the given RT and CT values, the bridge rectifier runs at 70kHz. The circuit can provide about 80mA for 64 LEDs. In machine vision systems, infrared LEDs are used to illuminate the field of view of a CCD camera. The prototype of this circuit used a 2.7mH inductor removed from a broken CFL.

The LED current consists of a DC current and a small ripple current; the ripple current should be kept as low as possible for high LED efficiency and long life. LED manufacturers usually require a value of a few percentage points. Such low ripple current may be difficult to achieve with a single electrolytic capacitor C5. For example, an additional metal foil capacitor C4 in parallel is sufficient to cope with most operations. The voltage at the input of the LED rectifier is basically stable within one oscillation cycle, so the inductor current is a triangular wave shape, which is beneficial to EMC (electromagnetic compatibility). The formula for the average LED current is: ILEDAVG=(1/2×VDC-N×VFLED)/(4×f×L1), where VDC is the supply voltage, N is the number of LEDs in series, VFLED is the forward voltage of the LED, f is the oscillation frequency, and L1 is the value of the current limiting inductor.

Although the circuit of Figure 1 works well, it has some shortcomings. The circuit of Figure 2 is supplemented by adding C6, D5, D6 and T1. T1 is wound on an EPCOS EP13 coil frame. It uses a gapless EP13 frame of T38 material and has an inductance of 7000nH. Both the primary and secondary windings are wound with 90 turns of 0.2mm wire, and the secondary winding is wound on top of the primary winding. Stray inductance is not important in this case, and the inductance of the primary and secondary windings is 50mH. The circuit of Figure 2 has some advantages over the circuit of Figure 1. For example, the supply current of the ballast IC in Figure 1 must flow through R1 and then enter the IR53HD420, where it is clamped at 15.6V. At supply currents above 6mA, R1 dissipates more than 2W. In Figure 2, R1 can take a higher value because it only needs to provide a small startup current. After startup, a charge pump composed of C6, D5 and D6 provides sufficient current to VCC to clamp the internal Zener diode to 15.6V. The design formula of the charge pump is ISUPPLY(AVG)=f×C62×VDC-15.6V. The power dissipation of R1 can be stabilized to less than 0.25W.

In addition, the total forward voltage of the LEDs in Figure 1 must be less than half the supply voltage. For the circuit in Figure 2, by adjusting the transformer winding ratio, as many LEDs as needed can be connected without exceeding the component ratings (the LED voltage can even be higher than VDC). A less obvious problem with the circuit in Figure 1 is that the full voltage swing of the rectifier bridge appears at both ends of the LED string. This situation is not a problem when all LEDs are close to the rectifier bridge. However, in many lighting fixtures, it is desirable to separate the LEDs from the electronics. This approach results in high capacitive currents from the LEDs to ground due to stray capacitance, affecting efficiency and creating EMC issues. Using the transformer in Figure 2, either end of the LED string can be connected to ground directly or through a capacitor. Now, long cables can be used to conveniently separate the LEDs from the electronics.