Research on a high-power LED power supply with high conversion efficiency and high power factor

introduction

Compared with other lighting, LED lighting has the following advantages: 1) high luminous efficiency, low energy consumption, good monochromaticity and narrow spectrum; 2) long service life, the service life of LED can be up to nearly 100,000 hours; 3) safe and environmentally friendly; 4) short startup time; 5) small size.

A large number of LED street lamps require corresponding driving power supplies, so the development of high-efficiency and high-power-factor LED driving power supplies also has huge demands and broad market prospects.

According to the requirements of a lighting fixture manufacturer in Hangzhou, this paper develops a driver power supply suitable for 80W~150W LED lighting fixtures, which requires high power input-output conversion efficiency and high power factor, high reliability and compliance with IEC-61000 standards.

1 Main circuit topology selection

When using a switching power supply topology, it must meet the current harmonic limit requirements for Class D devices in standards such as IEC61000-3-2, and must also meet the current harmonic limit requirements for Class C (lighting) equipment and the "Energy Star" specification that the power factor (PF) must not be lower than 0.9. To achieve this goal, LED street light power supplies must use power factor correction (PFC) and are also required to use a power supply topology that supports the corresponding power.

At present, AC/DC constant current source driver IC is shifting from flyback topology to high-efficiency resonant half-bridge (LLC) + PFC topology to give full play to the advantages of zero voltage switching topology (ZVS) and meet the increasing requirements of LED lamps for PFC (positive power factor), and require efficiency to be improved by >90%. Wide voltage input, short circuit and over-power protection, open circuit protection, and low total harmonic distortion (THD) are basic requirements.

Traditional power factor correction circuits are complex in technology, cumbersome in design steps, require many components, are large in size and expensive, so a compromise is often made between performance and cost during design.

BOOST uses an active power factor correction (APFC) circuit and works in continuous mode, with low harmonic current and switch tube voltage and current stress. The DC/DC uses a half-bridge LLC series resonant converter with a limited number of components. The resonant energy storage (tank) element can be integrated into a single transformer, so only one magnetic component is required. Under all normal load conditions, the primary switch can operate in zero voltage switching (ZVS) conditions, and the secondary diode can operate in zero current switching (ZCS) without reverse recovery loss.

A cost-effective, energy-efficient and high-performance solution especially for medium and high output voltage converters.

Therefore, the main circuit adopts FPC (using CCM mode) + LLC two-level topology structure.

Since PLC810PG integrates both CCM PFC and LLC controllers, it is particularly suitable for the needs of this system design.

2 Design of high-power LED power supply circuit based on PLC810PG

The circuit diagram of high-power LED power supply based on PLC810PG is shown in Figure 1.

The LED driver power supply is divided into several main parts including input circuit, PFC boost converter and LLC resonant converter.

Figure 1 Power supply circuit diagram

The input circuit is mainly composed of input filter, bridge rectifier (BR1), etc. C1~C6 and L1, L2 and R1~R3 ​​form EMI filter. C1 and C5 are connected between phase line L and neutral line N to protect the ground (E) and control high-frequency (>30MHz) noise. C3 and C4 provide differential mode EMI filtering. Common mode inductors L1 and L2 control low-frequency and medium-frequency (<10MHz) EMI, and C2 and C6 control the resonant peak in the medium frequency area. When the AC power is cut off, R1, R2 and R3 provide a path for EMI capacitor discharge to meet safety requirements.

F1 is a fuse that provides short circuit protection. RV1 is used for overvoltage protection. RT1 is an NTC thermistor that limits inrush current during circuit startup. When the circuit enters normal operation after startup, the relay operates to short-circuit the thermistor. Since no current flows through RT1, the circuit efficiency can be increased by at least 1%.

The main circuit of the PFC boost converter consists of L4, boost diode D2, PFC switch Q2, output capacitors C9, C11, etc. When the AC input voltage range is 140~265V, the PFC output DC boost voltage (VB+) is stabilized at 385V, and a sinusoidal current is generated at the input end of the bridge rectifier BR1, making the system present a purely resistive load, and the line power factor is close to 1.

The PFC section of the PLC810PG adopts a universal input continuous current mode (CCM) design that does not require a sinusoidal signal input reference, thereby reducing system cost and external components.

Q1 and Q3 form the buffer stage of Q2. Ferrite beads are connected in series to the gate and drain of Q2 to improve EMI.

The PFC buffer stage Q1 uses a 60V, 1A, SOT-23 packaged FMMT491TA NPN transistor. Q3 uses a 60V, 1A, SOT-23 packaged FMMT591TA PNP transistor. The bias power supply Q26 and Q17 use a 40V, 0.2A, SOT-23 packaged NPN small signal transistor MMBT3904LT1G.

The PFC switch Q2 uses STW 20NM50FD, 500V, 20A, on-resistance 0.22Ω, and uses TO-247AC package.

R6 和R8 是PFC 级电流传感电阻。连接在R6 和R8 上的二极管D3 和D4，在浪涌期间箝位（箝位电压为D3 和D4 的正向压降，约0.7V×2=1.4V），R6 和R8 上的电压以保护U1（PLC810PG）的电流感测输入。

When the system is powered on, the charging current for C9 passes through diode D1, and no surge current passes through L4, which avoids the possibility of L4 saturation. The small input capacitor C7 of the PFC stage is used to bypass the high-frequency components. C7 selects a low-loss propylene capacitor. Capacitor C11 is used to reduce the EMI of high-frequency loop components such as Q2, D2 and C9.

The LLC resonant converter consists of an LLC input stage and an LLC output stage. Q10 and Q11 are the half-bridge high/low-side MOSFETs of the LLC converter, which are directly driven by U1 through resistors R56 and R58. C39 is the primary resonant capacitor of transformer T1, which forms a resonant tank circuit with the primary of T1. Since the resonant inductor has been incorporated into the primary winding coil of T1, this circuit is still called an LLC resonant tank circuit instead of an LC resonant tank circuit.

Capacitor C40 is placed adjacent to Q10 and Q11 for bypassing.

The half-bridge switches Q10 and Q11 use IRFIB7N50LPBF type N-channel MOSFET, 500V, 6.8A, 0.32Ω, and TO-247AC package.

The secondary output of transformer T1 is rectified and filtered by D9, C37 and C38 to provide a 48V output to power the LED street light. The ferrite bead connected in series on the secondary of T1 is used to suppress high-frequency noise.

The DC-DC controller in the PLC810PG drives the LLC resonance. This variable frequency controller allows the MOSFET to switch at zero voltage, thereby eliminating most of the switching losses and improving efficiency. The core of the LLC controller is a current-controlled oscillator whose frequency control range supports the traditional operating frequency of TV power supplies.

To ensure zero voltage switching, the dead time of the LLC switches in the PLC810PG is tightly controlled within tolerance and adjustable by an external resistor. The duty cycle on both sides of the high and low voltages is closely matched to provide balanced output currents, thus reducing the cost of the output diodes.

3 LED driver power supply test results

The sample circuit board connected to the LED light module was tested in the laboratory, and the test results are: at full load, PFC level efficiency PFC> 95%, LLC level efficiency LLC> 95%, and total system efficiency total> 92% (AC200-265VAC). Since the LED street light power supply has power factor correction, within the range of 140VAC~220VAC, PF ≥ 0.98, the LED street light power supply conduction EMI meets the requirements of CISPR22B/EN55022B specifications, and the safety meets the requirements of IEC950/UL1950 Class II.