Backlight driver application design for large-size LED screens

Traditional boost LED driver

The traditional LED driver uses a boost circuit, and the simplified circuit form is shown in Figure 1.

Figure 1

The input voltage VIN of the boost type LED backlight driver is lower than the tube voltage VF of the LED Light Bar. It is boosted through V1, L1, and D1, and the LED constant current and current balance control module realizes current control.

In Figure 1, there is a VFmax (VF maximum value) for multiple LED Light Bars, and the voltage difference between each VF voltage and VFmax is △VF = (VFmax-VF). If the current flowing through each LED Light Bar is set to ILED, then the power loss of each channel is P = △VF × ILED. This loss is ultimately added to the LED constant current and current balance control module, generating heat, reducing the energy efficiency of the entire drive circuit, and causing energy waste.

There are two ways to solve this problem:

1. LED Light Bar manufacturers control the VF voltage of each Light Bar to ensure the consistency of VF voltage and reduce △VF, which means that they need to screen individual LEDs and strictly control the VF range.

2. Change the form of LED drive circuit, innovate drive technology, and adopt step-down switching circuit.

Buck LED driver working principle

The buck LED driver uses a buck switching circuit, and the simplified circuit form is shown in Figure 2.

Figure 2

The input voltage VIN is higher than the VF voltage of the light strip, and is stepped down through the switch tube, inductor, and freewheeling diode, and the LED constant current and current balance control module realizes current control.

In order to reduce the ripple current flowing through the LED, the inductor current generally operates in the current continuous mode. The inductor current waveform is shown in Figure 3 below.

Figure 3

Figure 4

Figure 5

The operating principle is as follows: When the switch is turned on, the circuit is simplified as Figure 4. The switch is turned on from time t0 to time t1. The time from t0 to t1 is TON time. During the TON time, the inductor current iL changes to △iL.

VIN-VO=L(diL/dt)=L(△iL/△t)= L(△iL/TON) (1)

When the switch is turned off, the circuit is simplified as Figure 5. The switch is turned off from time t1 and turned on at time t2. The time from t1 to t2 is TOFF time. During the TOFF time, the inductor current iL changes to -△iL. At this time, the voltage on the inductor L is (ignore the voltage drop of the freewheeling diode)

- VO = L(diL/dt)=L(-△iL/△t)=- L(△iL/TOFF) (2)

TON + TOFF=T (3)

The VIN voltage is known, the voltage VO and current of the light bar are also known, and the △iL of the iL current can be set to 10%~30% of iL. Considering the switching loss, the general switching frequency f can be set at 50~200K.

Therefore, according to formulas (1), (2) and (3), the inductance of the inductor L can be calculated.

If you think the inductance L is too large, you can increase △iL appropriately or increase the switching frequency f.