LEDs has many advantages over the traditional light sources, including good efficacy, long life-time and environmentally friendly properties. However, the cost of LED systems is usually higher compared with traditional light sources. As essential off-chip components in the LED drivers, the inductors are not only expensive but also take up a lot of space, especially in high voltage applications, which makes the LED system bulky and less cost-effective. The aim of this research is to use small inductors in the LED driver to bring down the cost and size of the LED system. In the meantime, the LED driver should maintain a good performance. One way to reduce the value of the inductors is to increase the switching frequency. However, high frequency will result in huge switching loss due to pa...[ Read more ]

LEDs has many advantages over the traditional light sources, including good efficacy, long life-time and environmentally friendly properties. However, the cost of LED systems is usually higher compared with traditional light sources. As essential off-chip components in the LED drivers, the inductors are not only expensive but also take up a lot of space, especially in high voltage applications, which makes the LED system bulky and less cost-effective. The aim of this research is to use small inductors in the LED driver to bring down the cost and size of the LED system. In the meantime, the LED driver should maintain a good performance. One way to reduce the value of the inductors is to increase the switching frequency. However, high frequency will result in huge switching loss due to parasitic capacitor of the power switch. The quasi-resonant LED driver can eliminate the switching loss with the help of zero-voltage switching (ZVS).

The steady state analysis of the zero-voltage switching quasi-resonant (ZVSQR) LED driver that applies to small inductance is conducted. The conversion ratio equation and the conditions to achieve ZVS are derived. According to the analysis, a ZVSQR LED driver can be designed to operate within a wider input voltage range with smaller inductors. A rule of thumb for determining the boundary between small and large inductance is given. The small signal model of the ZVSQR LED driver is also derived.

A monolithic ZVSQR LED driver for lighting applications is designed. The LED driver is able to operate at high switching frequency because large switching loss is eliminated by ZVS. Inductor values are therefore reduced. A constant-duty-ratio frequency regulator (CDFR) is proposed to regulate the LED current. The guidelines for selecting components for the proposed LED driver are also given. The proposed design, including the controller and power transistor, was fabricated in MagnaChip 0.35-μm 700-V BCD process. The LED driver with 2 × 3.9-μH inductors switches at around 5.4 MHz and powers up 16-28 1-W LEDs. Experimental results show that the LED driver achieves peak efficiency of 92.8% within the input voltage range of 55-120 V.

To further improve the first design, an auto-zero-voltage-switching quasi-resonant LED driver with GaN FETs for general lighting applications is presented. The proposed LED driver switches at high frequency to minimize the inductance to μH range. ZVS can be automatically achieved with the proposed controller to eliminate switching loss. The GaN FETs enable high-frequency operation and improve the power efficiency. Fully integrated LED shunt protectors are proposed to bypass the failed LEDs in series-connected LED strings. The overall lifetime of the LED strings can be improved and the maintenance cost can be reduced. The characteristics of the ZVS quasi-resonant LED driver with small inductors and the conditions for ZVS are also discussed in detail. The LED driver is fabricated with a 0.35-μm 120-V high-voltage process. It can provide up to 25-W power to the LED with 2 × 3.3-μH inductors and achieves 91.4% peak efficiency and a 0.973 peak power factor from 60 Hz 100–120 V_AC input.

The efficiency of the linear LED driver is limited by the ratio of the input voltage to the output voltage. The switch-mode LED driver needs bulky and expensive inductors and its efficiency suffers from the parasitic drain capacitor of the power switch. Motived by the above mentioned problems, a multiple-string hybrid LED driver is proposed. The multiple-string hybrid LED driver is designed for 100-120 V_AC input and can adaptively switch between linear mode and switching mode according to the input voltage to enhance the power efficiency. This work achieves 97% power efficiency and 0.996 power factor at 120 V_AC 60 Hz input with a small inductor of 6.8 μH.