A Better Way to Manage Leakage Current Failures in LED Backlights

By Wonhwa Lee

Contributed By Digi-Key Corporation


A standard way to configure a backlight is to use two discrete devices: a 100 V MOSFET in a DPAK package and a 100 V Schottky diode, also in a DPAK package. The high leakage current of the Schottky diode can be a problem in LED backlight units, especially at higher temperatures. Some customers have experienced leakage failure issues of the Schottky diode during mass production. One way to improve the leakage failure is to increase the Schottky diode's voltage rating from 100 V to 120 V, but leakage current can still be an issue when the system temperature is high.

Our design team developed an alternative approach, using Fairchild's new 100 V BoostPak solution. The BoostPak family (Figure 1) uses a single package to house two devices: a 100 V MOSFET and a 150 V NP diode.

Image of Fairchild's 100 V BoostPak solution

Figure 1: The BoostPak combines a 100 V MOSFET and a 150 V NP diode into one package.

The BoostPak family is housed in a single 5-lead DPAK package. The N-channel MOSFET has been tailored to minimize the on-state resistance while maintaining superior switching performance. The NP diode is a hyperfast rectifier with low forward voltage drop and excellent switching performance. It has a much lower leakage current than a Schottky diode, so it delivers better system reliability in high-temperature applications.

Compared to the two-discrete solution, the BoostPak approach uses a smaller footprint and can save as much as 20 mm² of PCB space. Using one package instead of two also means there is less assembly work and lower system cost.

The BoostPak family is available in two versions, one with an output power rating of 25 W (FDD1600N10ALZD), the other with a rating of 40 W (FDD850N10LD). Table 1 gives the details.

Product ID BVDSS RDS(ON) max Package PO ID QG
FDD850N10LD 100 V 75 MΩ DPAK 5lead ~ 40 W 15.3 A 22.2 nC
FDD1600N10ALZD 100 V 160 MΩ DPAK 5lead ~ 25 W 6.8 A 2.78 nC

Table 1: BoostPak electrical specs.

Better Performance at Higher Temperatures We wanted to see just how much lower the leakage current is for the NP diode, so we ran some tests. The results are shown in Figure 2.

Image of comparison of diode leakage current

Figure 2: Comparison of diode leakage current.

Compared to a 100 V, 5 A Schottky diode, the 150 V, 5 A NP diode in the BoostPak family has much lower ratings for leakage current under all conditions, but the differences at higher temperatures are particularly dramatic. As the temperature rises, the leakage current of the Schottky diode goes up rapidly, while the leakage current for the NP diode stays proportionally lower.

The NP diode of the BoostPak family is produced using an excellent lifetime control process to get very fast reverse recovery time and reasonable forward voltage drop (VF (typ.): 0.9 V at IF=5 A, TJ=100°C) The comparison of reverse recovery time is shown in Figure 3.

Image of comparison of diode reverse recovery time

Figure 3: Comparison of diode reverse recovery time.

A real-world design

Our next step was to verify that the BoostPak family would limit leakage current in a real-world design, so we developed an evaluation board and tested it under various conditions. Figure 4 shows the basic design, with the BoostPak family highlighted.

Image of Fairchild BoostPak in an LED backlight

Figure 4: BoostPak in an LED backlight.

The design is for a 35 W boost topology that uses Continuous Current Mode (CCM) operation. The input voltage has a range of 20.4 V to 27.6 V and there is a single-channel DC output with a constant current of 640 mA at 55 V. We used the FDD8500N10LD version of the BoostPak family.

During CCM operation, the reverse recovery current of the diode increases the turn-on losses of the MOSFET. The NP diode offers a low reverse recovery current, so there is less impact on the MOSFET.

Testing for temperature and EMI

In designing our BoostPak products, we had two particular goals in mind. First, we wanted to keep the device case temperature below 65°C during operating. Second, we wanted to meet general guidelines for electromagnetic interference (EMI), with the aim of keeping EMI under the limits for CISPR22 Class B operation.

We measured the saturated temperature. As shown in Table 2, with an input voltage (VIN) of 24 V, the BoostPak family maintained a temperature of 61.5°C, below our target of 65°C.

VIN=20.4 V VIN=24 V VIN=27.6 V Remark
BoostPak (Q11) 66.9°C 61.5°C 59.3°C Pkg Top
Inductor 63.7°C 59.6°C 56.6°C
R8~R14 57.6°C 52.8°C 49.8°C
MOSFET (Q12) 50.8°C 50.0°C 52.1°C
R45~R49 58.9°C 57.3°C 60.0°C

Table 2: Test Results of VOUT = 55 V (35 W).

Next, we tested for EMI by examining radiated emissions for a load of five strings of LEDs. Figure 5 shows the results for a VIN of 24 V.

Image of radiated emissions

Figure 5: Radiated emissions: VIN = 24 V.

In the frequency sub-range of 30 MHz to 1000 MHz, the radiated emissions are well below the specified limits for CISPR22 Class B operation.

Conclusions

The test results show that our BoostPak family, which replaces a 100 V MOSFET and 100 V Schottky diode with a single 100 V BoostPak device, meets the necessary performance requirements for operating temperature and EMI with low leakage current of the diode. At the same time, using the BoostPak family serves to create a smaller, more compact design that is easier to assemble. In cost-competitive applications, such as LED TVs with a screen size less than 40 inches, those benefits can make a real difference. The BoostPak approach can also deliver savings in other applications, too, like LED lighting systems and DC/DC converters for step-up and step-down operations.

The BoostPak family discussed in this article is described in more detail in a user guide. The note provides a full schematic, a complete bill of materials, performance specs, and a summary of the test setup. To download the application note, visit the Fairchild website.
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