LED backlight leakage current fault solution BoostPak

A standard way to configure the backlight is to use two discrete devices: a 100V MOSFET in a DPAK package, and a 100V Schottky diode, also in a DPAK package. The high leakage current of the Schottky diode in the LED backlight unit can cause some problems, especially at higher temperatures. Some customers have encountered leakage failures in Schottky diodes during mass production. One way to improve the leakage failure is to increase the rated voltage of the Schottky diode from 100V to 120V, but the leakage current is still a problem when the system temperature is high.

　　Fairchild Semiconductor's design team developed an alternative approach using a 100V BoostPak solution. The BoostPak series (Figure 1) integrates two devices in a single package: a 100V MOSFET and a 150V NP diode.

　　Figure 1. BoostPak integrates a 100V MOSFET and a 150V NP diode in a single package.

　　The BoostPak series is available in a single 5-pin DPAK package. The N-channel MOSFET is designed to minimize on-resistance while maintaining excellent switching performance. The NP diode is an ultra-fast rectifier with a low forward voltage drop and excellent switching performance. Compared to Schottky diodes, it has much lower leakage current, providing higher system reliability in high temperature applications.

　　Compared to a dual discrete device solution, the BoostPak solution is smaller in size and can save up to 20mm2 of PCB space. Using a single package instead of two also means easier assembly and lower system cost.

　　The BoostPak series offers two versions, one rated at 25 W output power and the other rated at 40 W. Table 1 lists the details.

　　Table 1. BoostPak assembly specifications

　　Better performance at higher temperatures

　　We wanted to see how low the leakage current of an NP diode could be, so we performed some tests. The results are shown in Figure 2.

　　Figure 2: Diode leakage current comparison

　　Compared to the 100V, 5A Schottky diode, the 150V, 5A BoostPak series NP diode has a much lower rated leakage current under all conditions, but the difference between the two is very large at high temperatures. As the temperature rises, the leakage current of the Schottky diode increases at a very fast rate, while the leakage current of the NP diode remains low in comparison.

　　The NP diodes of the BoostPak series are made using an excellent life cycle control process to achieve extremely fast reverse recovery time and reasonable forward voltage drop (VF (typical): 0.9V, under the condition of IF=5A, TJ=100 degrees)

　　Figure 3: Comparison of reverse recovery times