Drive LED CFL ballast IC

Christian Rausch, Unterhaching, Germany designers with CFL (compact fluorescent lamp ballast) in IC (such as the International Rectifier company IR53HD420) to the heating filament, light bulb, provides the current manufacturers to produce large amounts of the IC lamp, their price is about $2. The design example is how to use CFL ballast IC drive LED, rather than drive CFL. The ballast IC is essentially a self oscillating half bridge for off-line operation. It usually operates at 320VDC, whose power is roughly equivalent to the 230VAC main rectifier or a 120V voltage doubler. The IC generates a square wave voltage amplitude is 320Vp-p, the frequency of tens of hertz. Typically, this square wave voltage is connected to a series of CFL lamps and a current limiting inductor L1 (Figure 1). With a parallel capacitor and the use of LC resonator, you can heat, light the lamp tube, and to provide the current. This scheme works well, since the CFL tube has a high impedance at the time of shutdown and has low impedance at work. Lamp voltage is generally 150Vp-p. The number of LED in series, and then connected to a bridge rectifier, you can imitate a CFL, at least in the light state. It is not important to simulate the off state because LED does not require an ignition process. For given RT and CT values, the rectifier bridge operates at 70kHz. The circuit provides approximately 64 80mA of current for the LED. In the machine vision system, infrared LED is used to illuminate the field of view of the CCD camera. The prototype of this circuit uses a 2.7mH inductor removed from a damaged CFL. The LED current is composed of a DC current and a small ripple current. In order to obtain the high efficiency and long life of the LED, the ripple current should be kept as low as possible. LED manufacturers usually require several percentage points of value. Such low ripple current may be difficult to achieve with an electrolytic capacitor C5, such as a metal foil capacitor in parallel with the C4 is sufficient to cope with most cases of operation. The voltage at the input of the LED rectifier is basically stable during an oscillation period, so the inductor current is a triangular wave shape, which is favorable for EMC (electromagnetic compatibility). LED average current formula: ILEDAVG= (1/2 * VDC-N * VFLED) / (4 * f * L1), where VDC is the power supply voltage, N is the number of series LED, VFLED is a positive voltage of LED, f is the oscillation frequency, while L1 is the current limiting inductance value. Although the circuit in Figure 1 can work very well, but it also has some shortcomings, supplement the circuit in Figure 2, D5 and C6 by adding D6 and T1 T1 around a EPCOS EP13 coil frame, it uses no gap EP13 skeleton of T38 material, inductance is 7000nH. The primary and secondary windings are wound around 90 turns with a 0.2mm line, and the secondary winding is wound around the primary winding. Stray inductance is not important in this case, the inductance of the primary winding and secondary winding are 50mH. Figure 2 Figure 1 circuit circuit has some advantages, for example, the supply current in Fig. 1 ballast must flow through the IC R1, and then enter the IR53HD420, then it is clamped at 15.6V. When the supply current is higher than 6mA, the power consumption of R1 exceeds 2W. In Figure 2, R1 can take a higher value because it provides only a small start-up current. After starting, a charge pump consisting of C6, D5, and D6 provides sufficient current for the VCC to clamp the internal zener diode to 15.6V. The design formula of charge pump is ISUPPLY (AVG) =f * C62 * VDC-15.6V. R1 power consumption can be stabilized below 0.25W. In addition, the total forward voltage of LED in Figure 1 must be less than half the supply voltage. The circuit in Figure 2, by adjusting the transformer winding ratio, can be as much as possible to connect the LED, as long as the component does not exceed the rated value (LED voltage can be even higher than VDC). Figure 1 circuit has a less obvious problem, that is, the full bridge rectifier voltage swing occurs at both ends of the LED string. This situation does not cause problems when all LED are close to the bridge. However, in many lighting equipment, LED and electronic devices are expected to be separated. Due to stray capacitance, this method results in high capacitive currents from LED to ground, affecting efficiency and generating EMC problems. Using the transformer in Figure 2, the LED string can be either directly connected to the ground, or through the capacitive grounding. Now, you can use a long cable, easy to separate LED and electronic equipment.

Source: China Power Grid