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A Better Way to Manage Leakage Current Failures in LED Backlights

A standard way to configure the 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. High leakage current of Schottky diode can be a problem in LED backlight units, especially at higher temperatures. Some customers have experienced leakage failure issue of 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.

Figure 1. The BoostPak Combines a 100 V MOSFET and a 150 V NP Diode into One Package

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 20mm2 of PCB space. Using one package instead of two also means there's less assembly work and lower system cost.

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

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.

Figure 2. Comparison of Diode Leakage Current

Figure 2. Comparison of Diode Leakage Current

Compared to a 100 V, 5A 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.): 0.9V at IF=5A, TJ=100 degrees C).

The comparison of reverse recovery time is shown in Figure 3.

Figure 3. 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.

Figure 4. 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's 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 degrees 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 degrees C, below our target of 65 degrees 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.

 Figure 5. Radiated Emissions: VIN = 24 V

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 with a single 100 V BoosPak family, meets the necessary performance requirements for operating temperature and EMI with low leakage current of 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 the 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 at 35 W Boost Converter for LED Drive using BoostPak .

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7 comments on “A Better Way to Manage Leakage Current Failures in LED Backlights

  1. etnapowers
    April 24, 2014

    “Some customers have experienced leakage failure issue of Schottky diode during mass production.”

    True, the Schottky diode is very sensitive to the voltage and current spikes, and it may fail during the testing for mass production, because of the presence some spikes in the ground of the testing equipment or the power delivered to the switch by the ATE.

  2. etnapowers
    April 24, 2014

    “The N-channel MOSFET has been tailored to minimize the on-state resistance while maintaining superior switching performance”

     

    True, but the MOSFET can handle only a small amount of power to be delivered to the LEDs.

  3. Netcrawl
    April 25, 2014

    @etnapowers N-channel MOSFETS have a wide range of power, their power ability ranges from under 1 amp to all the ay up to 100 amps, when it comes to power they're probably the best. 

    But MOSFETS devices are only capable of swicthing large amount of power because they're designed to dissipitate minimal power when they're turned on, if its not fully turned on then the device will have high resistance and will dissipitate considerable power.

  4. geek
    April 25, 2014

    “But MOSFETS devices are only capable of swicthing large amount of power because they're designed to dissipitate minimal power when they're turned on, if its not fully turned on then the device will have high resistance and will dissipitate considerable power.”


    @Netcrawl: Doens't that limit their application in the scenarios that involve low power? What are the alternates then in those cases?

  5. SunitaT
    April 29, 2014

    According to the article, high current leakage is experienced when Schottky diode is used under high temperatures, and this can be reduced by increasing its voltage. This means that Schottky good and can perform well at low temperatures. I am wondering, is there any way of standardizing the voltage capacities of Schottky diode so as to enable it perform at high temperature? Another case is that of MOSFET, I wonder if they can perform better at low power levels according to the way they are designed, therefore, how can it be improved to accommodate both high and low power levels?

  6. etnapowers
    June 10, 2014

    @Netcrawl: The N-channel Mosfet can handle a large amount of power but  only for power discretes devices. If LEDs are the load of a consumer application board then some signal N-channel mosfets are required, hence these integrated switches can manage a small amount of power, compared to other  types of switches.

  7. etnapowers
    June 10, 2014
    “But MOSFETS devices are only capable of swicthing large amount of power because they're designed to dissipitate minimal power when they're turned on, if its not fully turned on then the device will have high resistance and will dissipitate considerable power.”
     
    @Netcrawl: that's absolutely correct, the driving circuit of the gate of a N-channel has to be effective and reliable, to ensure a good power conversion, avoiding power dissipation inside the switch.

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