LLC LED Driver Simplifies Design

Today’s flat-panel digital TVs and monitors are much thinner than the old, bulky cathode ray tube (CRT) displays used in the past. These new thin flat-panel TVs are very attractive to consumers because they take up less space.
To help meet consumer demand and make these digital devices thinner, some manufacturers are turning to LLC resonant half-bridge converters to drive the light-emitting diode (LED) backlights of these devices. This is because the zero-voltage soft switching (ZVS) achieved with this topology allows for more efficient, high-power density designs and requires fewer heat sinking components than hard-switching topologies.
One problem with this type of topology design is that the LLC dc/dc transfer function can vary significantly with load. However, this makes it more complicated to build the LLC controller and compensation current loop in the LED driver. To simplify this design process, this article will discuss a design method called pulse-width modulation (PWM) LED dimming that allows the LED load to vary with dimming while keeping the dc/dc transfer function constant.
LLC resonant half-bridge dc/dc studying the transfer function (M(f))
The LLC resonant half-bridge controller dc/dc (see Figure 1) is a pulse frequency modulation (PFM) control topology. The half-bridge FETs (QA and QB) are driven 180° out of phase and the frequency is adjusted/controlled using a voltage controlled oscillator (VCO). This in turn adjusts the impedance of the voltage divider formed by the resonant inductor (Lr), the transformer magnetic inductance (LM), the reflected equivalent impedance (RE) and the resonant capacitor (Cr). Only the voltage developed in LM is reflected to the secondary winding through the transformer turns ratio (a1).

Figure 1 LLC resonant half-bridge/controller

Equivalent reflected impedance:

(Equation 1)

Transformer turns ratio:

(Equation 2)

(Equation 3)

We can normalize and simplify the transfer function M(f) using the first harmonic approximation [1]. In Equation 4 for M(f), the normalized frequency (fn) is defined as the switching frequency divided by the resonant frequency (fO). Although only an approximation, this simplified equation is very useful in understanding how M(f) varies with input voltage, load, and switching frequency.

Normalized LLC half-bridge gain:

(Equation 4)

Adjusting dc current to adjust LED brightnessOne way to achieve LED brightness adjustment in LLC resonant LED drivers is to adjust the dc current through the LEDs. There is a problem with this: the output impedance of the LLC changes with the DC current. If not carefully considered, this change will cause M(f) to change, making the LED driver design more complicated. 
Problems with load variationDesigning
a half-bridge converter is not an easy task. Designers have to choose the magnetizing inductance (LM) based on the ZVS requirement. They also have to adjust a1, Cr, and Lr to get the ideal M(f) and frequency operating range. However, M(f) changes with Q, which in turn changes with output load (RL). See Figure 2 for details. The
M(f) variation of the resonant LLC half-bridge LED makes voltage loop compensation and transformer selection more difficult, complex, and confusing because there are too many variations to consider during the design process.

Fig. 2 Variation of M(f) with load.

Figure 3 LLC half-bridge LED driver using PWM brightness adjustment technique.

This technique uses a fixed low frequency signal (DIM) that controls FET QC, which is logically added to the QA and QB FET drivers. When the DIM signal is high, the LED backlight string is controlled to a fixed peak current (VRS/RS). Once DIM goes low, QA, QB, and QC are turned off immediately. After QA, QB, and QC are turned off, the LED diode stops conducting, and the output capacitor (COUT) stores energy to start the next DIM cycle on time. For more details, see the waveforms shown in Figure 4.

Figure 4 PWM brightness adjustment waveform

The average diode current (ID) is adjusted by adjusting the duty cycle (D) of the DIM signal, thereby controlling the brightness of the LED.

(Equation 5)

Although the LLC resonant half-bridge powers the LED from primary to secondary, the load (RL) to LLC transfer function (M(f)) remains constant even though the average LED current varies with duty cycle.

(Equation 6)
With a fixed RL and given Lr, Cr, and LM, the equivalent reflected impedance (RE) is constant and Q remains constant. This results in only one M(f) curve that varies with frequency (see Figure 5), rather than the multiple curves obtained with the traditional LED dimming method using variable RL (see Figure 2). Having only one M(f) curve to deal with in the design simplifies the design process by making loop compensation and transformer selection much simpler. In addition, there is another curve to be aware of when setting the minimum switching frequency to ensure that ZVS is maintained. In this case, the minimum f is set to the peak of the single M(f) curve (see Figure 5).

Figure 5 M(f) of driving LED using PWM brightness adjustment technology

Designing an LLC resonant half-bridge converter for LED driving is not easy. The dc/dc gain of a traditional LLC varies over a wide range with load changes. Many gain curves need to be evaluated. This makes loop compensation and transformer design/selection more complex and confusing. To simplify the design process, combining LLC with PWM brightness adjustment technology is a better choice. This is because LLC is subject to a fixed load (RL) during power supply, but the LED current varies during brightness adjustment. As a result, LLC gain changes less, making loop compensation and transformer selection/design simpler.