High-efficiency LED backlight driver power solution based on AP3031

With the development of power electronics technology, more and more portable devices are beginning to use small and medium-sized (7'~10') LCD panels as display output devices. Due to the limited battery capacity of portable devices, inefficient backlight power solutions will seriously shorten the working time of the device, so how to improve the efficiency of backlight driving is crucial. At the same time, with the increasingly fierce market competition, production cost has also become an important indicator for considering driving solutions.

At present, the industry usually uses a two-stage power supply solution to provide backlight driving for LEDs, that is, the voltage is reduced to 5V from the input power through a first-stage step-down circuit, and then a first-stage boost circuit is used to provide a suitable driving voltage for the backlight LED. The disadvantage of this solution is that it uses two-stage power supply, which is inefficient and costly.

AP3031 is a new generation backlight driver IC developed by BCD based on the Poly emitter process. Its feature is that the maximum value of the chip power supply voltage is increased from the industry's common 6V to 20V. Based on the high voltage resistance of AP3031, this paper improves the backlight driver solution, hoping to improve the efficiency of the converter and reduce the solution cost.

Figure 1 is a common boost backlight driver, where the input voltage Vin = 5V, which is obtained by the battery voltage through a first-stage step-down circuit. The output voltage is about 10V, driving a 3x13 LED matrix. Using an oscilloscope to measure the voltage and current waveforms of each power device in the boost circuit, the power loss of each power device can be obtained. The power loss distribution of the boost circuit is shown in Figure 2.

As can be seen from Figure 2, conduction loss accounts for the largest part of the converter loss, and conduction loss is the loss generated when current flows through the power tube (Q and D in Figure 1). Taking Q tube as an example, the voltage and current waveforms on Q tube are shown in Figure 3.

Therefore, the conduction loss PQcon-loss of the Q tube is:

It can be seen from equations 1 and 2 that when the output power Pout is constant, the input voltage is inversely proportional to the conduction loss. Therefore, choosing a higher input voltage can effectively reduce the conduction loss of the power switch tube and improve the converter efficiency.

The experimental test results are shown in Figure 4. The efficiency of the converter increases with the increase of input voltage. It can reach up to 93%, which is 8% higher than the 5V input. It should be noted that the supply voltage in this solution must be lower than the output voltage. When the supply voltage is higher than the output voltage (such as using a three-cell lithium battery to directly supply power), the following single-stage Sepic converter solution can be used.

Single-stage Sepic converter solution

The Sepic circuit can achieve both voltage boost and voltage reduction, so it is very suitable for portable systems with large input voltage changes. At the same time, because the AP3031 has a high withstand voltage of 20V, the system can use a single-stage Sepic circuit to directly drive the backlight. Figure 5 is a single-stage Sepic backlight drive circuit diagram, and its working principle is shown in Figure 6.

The operation of the Sepic circuit in Figure 6 can be divided into two stages: a. When Q1 is turned on, the current flows through L1 and increases linearly, the C1 capacitor discharges through L2, and the L2 current also increases linearly; b. When Q1 is turned off, the current flows through L1 to charge C1, and the current decreases linearly. At the same time, L2 discharges to the load, and the current decreases linearly. The specific waveforms at each point are shown in Figure 7.

Combining the waveforms at each point, the volt-second product balance formula for the two inductors L1 and L2 in the converter is written as:

From formula 3, we can find:

It can be seen from equation 4 that the Sepic circuit can both boost and buck, and can adapt to a wide range of input voltage changes. Compared with the traditional two-stage conversion (Buck to Boost) circuit structure, the Sepic circuit saves one power conversion circuit and can significantly improve the backlight efficiency. The experimental results are shown in Figure 8.

Backlight power supply solution selection

As mentioned above, the choice of system backlight power supply solution mainly depends on the system power supply structure:

* For systems powered by 5V DC (such as digital photo frames, etc.), the AP3031 Boost circuit can be used to power the backlight.

* For systems powered by dual-cell lithium batteries (such as portable DVDs, etc.), the AP3031 Boost circuit can also be used directly to power the backlight, which can reduce the overcurrent capacity requirements of the power devices in the front-stage Buck circuit and reduce device costs.

* For systems powered by three or more lithium batteries (such as netbooks), the AP3031 Sepic circuit can be used to power the backlight, which can reduce the first-level Buck power conversion circuit, save costs, and improve system reliability.

Conclusion

From this article, we can see that using BCD's AP3031, we can design a more efficient LED backlight driver, and at the same time significantly reduce the cost of the backlight driver. These solutions are mature in technology, have obvious advantages, and have broad application prospects.