Design of constant current drive power supply for high power LED lighting

In today's global energy shortage environment, energy conservation has become a general trend. At the same time, the country also vigorously advocates energy conservation and emission reduction. The 2008 Beijing Olympic Games that have ended and the upcoming 2010 Shanghai World Expo both have green energy conservation as their theme, which has brought great historical opportunities for the development of China's LED lighting industry. High-power LEDs have the advantages of high light efficiency, low power consumption, long life, high stability, pure light color, good safety, and strong controllability. They are gradually replacing previous light sources and are widely used in full-color display screens, traffic lights , car lights, background light sources, landscape lighting, special work lighting, etc., becoming a new generation of green light sources in the lighting field. According to domestic relevant institutions, driven by the Olympics and the World Expo, the scale of China's LED lighting market will grow rapidly from 4.85 billion yuan in 2007 to 9.81 billion yuan in 2010. Relevant experts believe that China's LED lighting industry will usher in a new development peak around 2010.

Problem

Generally speaking, the power of high-power LED is at least 1W, and the most common ones are 1W, 3W, 5W, 8W and 10W. It is called "green light source" and is developing towards high current (300mA ~ 1.4A), high efficiency (60 ~ 120lm/W), and adjustable brightness. However, the luminous intensity of high-power LED is determined by the current flowing through the LED. Too strong current will cause the LED to attenuate, and too weak current will affect the luminous intensity of the LED. Therefore, LED driver needs to provide a constant current power supply to ensure the safety of high-power LED use, meet the expected brightness requirements, and ensure the consistency of brightness and color of each LED. Therefore, the power supply traditionally used to drive light sources such as light bulbs (tungsten filaments), fluorescent lamps, energy-saving lamps, sodium lamps, etc. is not suitable for directly driving high-power LEDs. Using AC power to drive high-power LEDs also requires solving the problems of step-down, isolation, PFC (power factor correction) and constant current, and also requires high conversion efficiency.

At present, there are thousands of dedicated chips for high-power LED constant current drivers on the market, including ADDtek, SITI, SCT, PT in China, and well-known manufacturers such as Supertex, TI, Maxim, National Semiconductor, and Zetex in the UK. Most dedicated chips use hysteresis converters, low voltage input range, can boost, buck, PWM control, built-in or external power switches, output current up to 1.5A, built-in overvoltage, undervoltage, open/short circuit and temperature protection circuits.

As shown in Figure 1, the key feature of the hysteretic converter is self-oscillation, which means that the frequency will change with the input voltage, LED current and the number of LEDs driven. However, this converter often runs in continuous mode, which means that the inductor will never saturate and the current will never be completely exhausted. After the MOSFET is turned off, there will continue to be current to maintain the brightness of the LED. But the disadvantage is that the impedance presented by the detection resistor RCS is different when the duty cycle and frequency are constantly changing. The current flowing through RCS is not completely consistent with the actual LED current, and there is a deviation in the detection data.

Figure 1 Hysteretic converter

In the field of high-power LED lighting engineering, a constant current drive power supply of more than 100W is required, and a higher efficiency and power factor are required. The LED drive power supplies for E27, B22 and GU10 lamp holders on the market are far from meeting the needs of the high-power LED lighting engineering field.

Design considerations for high-power LED driver power supply

From the development history of lighting fixtures , isolation is rarely used. Isolation design will inevitably affect the driving efficiency of the lamps and does not meet the requirements of future energy saving and consumption reduction, so LED lighting does not necessarily have to be designed with isolation.

In terms of the number of high-power LEDs in series, the current flowing through the high-power LEDs is no longer limited by the number of high-power white light LEDs in series. In order to meet different luminous brightness requirements, it can be achieved by flexibly driving multiple high-power LEDs. For the parallel use of high-power white light LEDs, this type of circuit still cannot guarantee the consistency of the luminous brightness of the parallel branch LEDs. However, multiple identical constant current power supplies can be used to drive different parallel branch LEDs in branches, thus ensuring the consistency of the properties of the parallel branch LEDs, thereby solving the problem of luminous brightness consistency.

The full series connection method requires the LED driver to output a higher voltage. When the consistency of the LEDs varies greatly, the voltages distributed across different LEDs are different, but the current passing through each LED is the same, and the brightness of the LEDs is consistent. If a constant current LED driver is used, when a certain LED is of poor quality and short-circuited, the output current of the driver remains unchanged, and the normal operation of all the remaining LEDs is not affected. When a certain LED is of poor quality and disconnected, all the LEDs in series will not light up. The solution is to connect a Zener tube in parallel across each LED. However, the conduction voltage of the Zener tube must be higher than the conduction voltage of the LED, otherwise the LED will not light up. For example, ADDtek's high-power LED protectors A716, AMC7169 and A720 are 350mA, 500mA and 700mA LED protectors respectively. As shown in Figure 2, it is connected in parallel with a high-power LED when used.

Load disconnection when the power fails is critical in two situations: power failure and PWM dimming. As shown in Figure 2, during the power failure of the boost converter, the load is still connected to the input voltage through the inductor and diode. This will continue to generate a small leakage current even if the power has failed, greatly shortening the life of the LED. Load disconnection is also important during PWM dimming. During the PWM idle period, the power has failed, but the output capacitor is still connected to the LED, and it will discharge through the LED until the PWM pulse turns the power on again. When implementing the load disconnect circuit, it is best to place a MOSFET between the LED and the current sensing resistor. In street lighting design, it is generally required to have an automatic shutdown function during the day. A photoresistor can be added in the middle of the circuit. The resistance value changes under the daylight to stop the MOSFET from working, and of course, the subsequent DC/DC can also stop working.

Figure 2 High-power LED protector

In many cases, it is very convenient to use low-frequency (50-200Hz) PWM to adjust the LED current, and adjust the brightness by controlling the pulse width. The advantage of this adjustment method is that the spectrum remains unchanged, while when amplitude adjustment is used, the spectrum will change with the change of the current flowing through the LED. Generally speaking, the efficiency of low-frequency PWM dimming circuits is higher than that of linear LED dimming circuits. In street lighting design, it is generally necessary to reduce the street light illumination by half at some point in the middle of the night to save energy and reduce consumption. A timer can be added in the middle of the circuit, and the power can be halved by outputting a 50% duty cycle at the time.

Waterproof design is divided into outdoor and indoor according to the use environment. Most of the current waterproof power supplies use epoxy resin as waterproof sealing filling material, and the color is mainly black, of course there are also white and some other colors. A few manufacturers use other waterproof filling materials. The important thing is that it can withstand high temperature, freezing, rain and some corrosive substances.

100W LED street light Can replace 250W high pressure sodium lamp or 300W mercury lamp. The output luminous flux of 100W LED street light is about 6250lm (after secondary optical design, there will be some loss), and the number of lumens reaching the road surface is still 6000, while the average illumination of the road surface can reach 16Lux (pole height 12m). The output luminous flux of 250W high pressure sodium lamp is 20000lm, but the number of lumens reaching the road surface is only 7000, and the illumination of the road surface is about 30~40Lux. Due to the difference in color rendering coefficient, the illumination correction factor of LED is 2.35 times, and the correction factor of high pressure sodium lamp is 0.94 times. Therefore, after the correction of 100W LED, the illumination of the ground is 37.6Lux, while the illumination of high pressure sodium lamp after correction is 28.2~37.6Lux, which are equivalent. Therefore, a 100W LED street light can replace a 250W high-pressure sodium lamp, and LED street lights can save 60% energy.

If secondary optical design is not performed, the LED illumination is relatively concentrated, so secondary optical path design must be performed to make the light intensity bat-shaped and the illumination range can reach 66m.

Main circuit design

The main circuit of high-power LED lighting constant current drive adopts an excellent two-stage combination of BOOST and DC/DC, which has good dynamic response and current stabilization characteristics, solves the harmonic pollution problem of the power grid, and makes the high-power LED drive power supply more green and environmentally friendly.

BOOST uses an active power factor correction (APFC) circuit and operates 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, while the secondary diode can operate in zero current switching (ZCS) with no reverse recovery loss. A cost-effective, energy-efficient and EMI-excellent solution particularly suitable for medium and high output voltage converters

Traditional power factor correction circuit technology is complex, the design steps are complicated, many components are required, the volume is large and the cost is high. Therefore, the design often has to compromise between performance and cost. This design uses IR1150, which is a new single-cycle AC/DC power factor correction control chip. It uses IR's patented single-cycle control (OCC) technology. It does not require the analog multiplier, input voltage sampling and fixed triangle wave oscillator required by the traditional PFC circuit, which greatly simplifies the design of the PFC circuit and reduces the size of the device.

The half-bridge LLC series resonant converter uses the highly integrated green FPS power switch FSFR2100 launched by Fairchild Semiconductor. It uses zero voltage switching (ZVS) technology to significantly reduce the switching losses of MOSFETs and rectifiers. With this technology, this FPS switch can handle up to 200W of power without a heat sink, and up to 450W of power with a heat sink. Compared with the traditional hard-switching converter topology, the efficiency of FSFR2100 is improved by 10%. It can adjust the output under wide changes in input and load, while the switching frequency changes relatively little. In addition, it can achieve zero voltage switching (ZVS) over the entire operating range. Finally, all parasitic components, including the junction capacitance of all semiconductor devices and the leakage inductance and excitation inductance of the transformer, are used to achieve ZVS.

The main circuit of the lighting constant current drive power supply is shown in Figure 3. The input voltage of the front-stage APFC experimental circuit is AC 220V, the rated output is DC 380V, the switching frequency f is selected as 70kHz, and the rear-stage half-bridge LLC series resonant converter. The output voltage range is DC 300~360V, the output rated current is 350mA, the resonant frequency f0 is selected as 100kHz, the transformer turns ratio n=Np/NS=0.6, and the power meets the output power range of 150~300W. The main circuit is 85V~264VAC→rectification→PFC→380VDC→DC/DC (isolation) constant current→multiple LEDs in series. APFC can use power factor correction controllers IR1150, L6562 and FAN7527B, etc., and the half-bridge LLC series resonant converter uses FSFR2100.

Figure 3 Main circuit of high-power LED lighting constant current drive power supply

Key technology design

In the LED lighting driving mode, since directly connecting RSET to the FB terminal will cause excessive power consumption of RSET, a low-power LED constant current driver often places an operational amplifier between the FB feedback terminal and RSET to reduce power consumption. As shown in Figure 4, the operational amplifier obtains the voltage on the sampling resistor RSET, and combined with other resistors and capacitors, a complete, efficient, high-power LED constant current drive circuit can be formed. In this way, while ensuring that the LED obtains constant current power supply, the power consumption of RSET can be reduced to an acceptable level, so that the voltage across the LED is as large as possible, and the current flowing through is as large as possible.

Figure 4: Constant current drive of LED with low power

The high-power LED constant current drive power supply adopts a hybrid method of first stabilizing the voltage and then limiting the current. To meet the needs of the load, the voltage needs to be guaranteed to be within a certain range. The Vf value of the LED is between 3 and 3.6V, so the voltage range that needs to be adjusted by the power supply can be determined according to the actual number of LEDs. The high-power LED constant current drive is shown in Figure 5. Set the maximum setting value VSET of the voltage stabilizer (for example, DC360V), set the setting value ISET of the current stabilizer (300mA~1.4A), and sample the voltage on RSET. If it exceeds the setting value of the current stabilizer, the output voltage will drop accordingly. According to the number of LED lamps in series, the output voltage can be reduced to the minimum value (such as DC 300V).

Figure 5 High-power LED constant current drive

Compared with the resistor current limiting method, the switch regulation control mode has a higher circuit cost; the control loop can accurately adjust the LED current; it can realize amplitude and low-frequency PWM adjustment; it can realize automatic temperature compensation of LED characteristics; it has a wide input voltage range; basically no heat sink is required, which can save costs. For high input voltage and large operating current, other driving schemes will cause very high losses, but this mode can still maintain efficient operation.

Technical indicators

According to the above design scheme, the main technical indicators of high-power LED lighting constant current driver are: input voltage 85～264V; frequency 47～63Hz; output power 100W; output current: 350mA±5% or 700mA±5%; output mode: multiple high-power LEDs above 1W in series; output voltage range: DC 300～360V, efficiency ≥90%, power factor ≥0.99, harmonic ≤5%, steady current accuracy ≤5%; with timing, current regulation, shutdown functions; with overvoltage, overcurrent, short circuit and overtemperature protection functions; fully sealed, waterproof requirements IP65, external dimensions (L×W×H) = 185mm×70mm×45mm, weight 1.5kg. Working temperature -40～+70℃, storage temperature -50～+85℃, in line with relevant safety regulations, ROHS and electromagnetic compatibility standards, lightning protection design (ICE-61000-4-5 Class 4). It meets the requirements of lighting engineering well.