In-depth analysis of CFL ballast IC driving LED application circuit

　　Driving CFL ballast circuit

　　Designers use the ballast IC in the CFL to heat the filament, light the bulb, and provide current for the lamp. 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 at 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 can heat, ignite, and provide 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 then connecting them to a bridge rectifier can simulate a CFL, at least in the on state. The off state simulation is less important because the LED does not require the 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; to obtain high efficiency and long life of the LED, the ripple current should be kept as low as possible. LED manufacturers usually require a value of several percentage points. Such a low ripple current may be difficult to achieve with an electrolytic capacitor C5, such as 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.

　　尽管图1的电路能很好地工作，但它也有某些不足，图2中的电路通过增加C6、D5、D6和T1作补充，T1绕在一个EPCOS EP13线圈架上，它采用T38材料的无隙EP13骨架，电感为7000nH。初级绕组和次级绕组均用0.2mm线绕90匝，次级绕组绕在初级绕组的上面。杂散电感在这种情况下并不重要，初级绕组和次级绕组的电感值均为50mH。图2电路比图1电路有一些优点，例如，图1中镇流IC的供电电流必须流经R1，再进入IR53HD420，此时它被箝位在15.6V。在高于6mA供电电流时，R1功耗超过2W。在图2中，R1可以取更高的值，因为它只要提供一个小的启动电流。启动以后，一个由C6、D5和D6组成的电荷泵为VCC提供充足的电流，使内部齐纳二极管箝位到15.6V。电荷泵的设计公式为ISUPPLY（AVG）=f×C62×VDC-15.6V。R1的功耗可稳定在0.25W以下。

　　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.