As the carrier of the light source, the drive circuit board directly affects the life and failure rate of the LED lamps. All electronic circuits of the LED lighting lamps are on the drive circuit board. The material and processing technology of the drive circuit board will directly affect the quality and life of the product. The drive circuit board materials are divided into three types: full glass fiber, half glass fiber, and cardboard. The standard material of the LED lighting drive circuit board should be full glass fiber double-sided board. If half glass fiber or paper single board is used, the later welding quality, moisture resistance, anti-aging ability and electrical properties will be greatly reduced. L.ED is a current type Driving device, a 1W white LED driven by a current of 350mA usually has a forward voltage Ur of 3.0~4.0V. The LED is a PN junction with very small dynamic resistance. Applying a voltage more than three times Uf to the LED will cause the current to be uncontrolled. If an LED is fed directly to a general off-line AC voltage, it will emit a very bright light and then quickly fail. The term driver is used to describe a power conditioning circuit that converts off-line voltage to a controlled DC current. LED light sources have long life, To last for tens of thousands of hours, the matching drive must also be able to last for the same amount of time. This requires that in the design of LED lamps, each design link of the LED driver should be fully considered, including the system structure to the selection of each circuit element.
(1) Topological structure of LED general lighting drive circuit. In the LED lighting application using DC/DC power supply, the LED driving methods that can be used include resistive type, linear regulator and switching regulator. In the resistive driving method, the forward current of the LED can be controlled by adjusting the current detection resistor in series with the LED. This driving method is easy to design, low cost, and has no electromagnetic compatibility (EMC) problems. The disadvantage is that it depends on the voltage and requires Filter LEDs, and are less energy efficient. Linear regulators are also easy to design and have no EMC issues. They also support current regulation and overcurrent protection, and provide an external current set point. The downsides are power dissipation issues, and the input voltage must always be higher than the forward voltage, and the energy efficiency not tall. The switching regulator controls the on and off of the switch (FET) tube through PWM, and then controls the current that drives the LED. The switching regulator has high efficiency, has nothing to do with voltage, and can control brightness, but its cost is relatively high and complex It is also high in intensity and has electromagnetic interference (EMI) problems. Learn more about electromagnetics at tycorun.com.
Common DC/DC switching regulator topologies include different types such as Buck, Boost, Buck-Boost, or Single-Ended Primary Inductor Converter (SEPIC). Among them, when the minimum input voltage is higher than the maximum voltage of the LED string under all working conditions, the step-down structure is adopted. For example, 24VIDC is used to drive 6 LEDs connected together. On the contrary, when the maximum input voltage is less than the minimum output voltage under all working conditions Using a boost structure, such as using 12VDC to drive 6 LEDs in series, and when the input voltage and output voltage range overlap, you can use a buck-boost or SEPIC structure, such as using 12Vdc to drive 4 LEDs in series, but this The cost and efficiency of this structure are the least ideal.
Figure 1 shows three basic examples of power supply topologies. The buck regulator shown in Figure 1(a) is suitable for situations where the output voltage is always less than the input voltage. In Figure 1(a), the buck regulator controls the LED current by varying the on-time of the MOSFET. Current sensing can be obtained by measuring the voltage across a resistor, which should be in series with the LED. An important design challenge for this approach is how to drive the MOSFET. From a cost-effectiveness standpoint, it is recommended to use an N-channel field effect transistor (FET) that requires a floating gate drive. This requires a drive transformer or floating drive circuit (which Can be used to maintain the internal voltage higher than the input voltage).
In the buck regulator shown in Figure 1(b), the MOSFET is driven to ground, which greatly reduces the drive circuit requirements. The circuit can optionally sense the LED current by monitoring the MOSFET current or a current sense resistor coupled to the LED. The latter requires a level-shifting circuit to get the power-to-ground information, but this complicates a simple design. The boost converter shown in Figure 1(c) can be used when the output voltage is always less than the input voltage. Such a topology is easy to design because the MOSFET is driven to ground and the current sense resistor is also ground referenced. One disadvantage of the circuit is that during a short circuit, the current through the inductor is unrestricted. Fault protection can be added by fuses or electronic circuit breakers.
In addition, some of the more complex topologies can provide such protection. Figure 2 shows two buck-boost circuits that can be used when the input and output voltages are high and low compared to each other. shown in the two buck topologies at the gate drive location). The buck-boost topology shown in Figure 2 shows a ground-referenced gate drive. It requires a level shifted current sense signal, but the inverse buck-boost circuit has a ground referenced current sense and level shifted gate drive. This inverse buck-boost circuit can be configured in a very useful way if the control IC is associated with the negative output, and the current sense resistor and LED are swapped. By choosing the appropriate control IC, the output current can be directly measured and the MOSFET can be driven directly.
One disadvantage of this buck-boost circuit is that the current is quite high. For example, when the input and output voltages are the same, the inductor and power switch currents are twice the output currents, which can negatively affect efficiency and power dissipation. A buck or boost” topology will alleviate these problems. In this circuit, the buck power stage is followed by a boost, and if the input voltage is higher than the output voltage, the buck stage will regulate the voltage when the boost stage is just powered on. If the input voltage is less than the output voltage, the boost stage regulates and the buck stage energizes. Some overlap is usually reserved for boost and buck operation so that there is no dead band when going from one mode to the other.
When the input and output voltages are nearly equal, the benefit of this circuit is that the switch and inductor currents are also nearly equal to the output current. The inductor ripple current also tends to be smaller. Even with four power switches in this circuit, its efficiency is significantly improved. The SEPIC topology shown in Figure 3(b) requires fewer FETs but more passive components. The benefit is a simple ground referenced FET driver and control circuit. Additionally, dual inductors can be combined into a single coupled inductor, saving space and cost. But like a buck-boost topology, it has a higher pulsating output current and switching current than a “buck or boost”, which requires a higher RMS current through the capacitor.
For safety reasons, isolation between off-line voltage and output voltage may be specified. In this application, the most cost-effective solution is a flyback converter, as shown in Figure 4. It requires a minimum number of components for all isolation topologies. A buck, boost, or buck-boost circuit can be designed based on the transformer turns ratio, which provides great design flexibility, but the disadvantage is that power transformers are usually custom components. Additionally, there is high component stress in the FETs and input and output capacitors.
(2) LED lighting drive circuit solution. According to different application requirements, the LED driver can work with constant voltage output, that is, the output is a clamping voltage under a certain current range; it can also work with constant current output, and the output design can be strict Limit current; can also use constant current and constant voltage output to work, that is, to provide constant output power, so the forward voltage of the LED as the load determines its current. In general, LED driver design needs to consider the following factors:
1) Output power. Involves LED forward voltage range, current and LED arrangement.
2) Power supply. For the application of wind-solar hybrid power supply, it should be DC/DC power supply.
3) Functional requirements. Dimming requirements, dimming method (analog, digital or multi-level), lighting control.
4) Other requirements. Efficiency, size, cost, fault handling (protection characteristics), standards to comply with, reliability, etc. More considerations are mechanical connection, installation, maintenance, replacement, life cycle, etc.
Driving LEDs faces many challenges. For example, the forward voltage of LEDs will vary with temperature and current, and the forward voltages of LEDs from different individuals, batches, and suppliers will also vary; The “color point” also drifts with current and temperature.
In addition, multiple LEDs are usually used in applications, which involves the arrangement of multiple LEDs. Of the various arrangements, driving a single string of LEDs in series is preferred because this approach provides excellent current matching regardless of forward voltage changes and output voltage (Uout) drift. Of course, other arrangements such as parallel, series-parallel combination, and cross-connection can also be used for applications that require matched LED forward voltages, and other advantages can be obtained. For example, in a cross-connection, if one of the LEDs is open due to a fault, the driving current of only one LED in the circuit will be doubled, thereby minimizing the impact on the entire circuit. The arrangement of the LEDs and the specification of the LED light source determine the basic requirements for the LED driver. The main function of the LED driver is to limit the current flowing through the LED within a certain range of operating conditions, regardless of the input and output voltage changes.
(3) DC/DC drive LED lighting scheme. In the design of the wind-solar hybrid LED lighting system, the nominal input voltage range of the driver chip should meet DC8-40V to cover the needs of the application, and the withstand voltage should be greater than 45V; if the driver 1C cannot adapt to the wide voltage range, it often increases when the power supply voltage increases. When it is high, it will be broken down and the LED light source will be damaged.
The nominal output current of the driver chip is required to be greater than 1.2~1.5A. As an LED light source for lighting, the nominal working current of an LED with a power of 1W is 350mA, and the nominal working current of an LED with a power of 3W is 700mA. Greater current is required, so the driver IC selected for LED lighting fixtures must have sufficient current output. When designing products, the driver IC must work in the optimal working area of 70%-90% of the full load output. The driver IC using the full-angle load output current will not dissipate heat smoothly in the small space of Dengxian County, and it is easy to fail early.
The output current of the driver chip must be constant, so that the LED light source can emit light steadily, and the brightness will not flicker. The same batch of driver chips is used under the same conditions, and the output current should be as consistent as possible, that is, the discreteness should be small, so that the production can be effective and orderly on a large-scale automated production line; for driver chips with a certain discrete output current It must be divided into grades before leaving the factory or put into the production line, and adjusting the resistance value of the current setting resistor (rs) on the PCB, so that the constant current driver board of the produced LED lamps has the same luminous brightness to the same type of LED light source, and maintains the consistency of the final product. .
The packaging of the driver chip should be conducive to the rapid heat dissipation of the driver chip. For example, the die (Die) is directly pasted on the copper plate, and one pin extends directly outside the package, which is convenient for direct welding on the copper foil of the PCB to quickly conduct heat. For example, on a silicon die like 4mm × 4mm, if a current of 300~1000mA is to be passed for a long time, there must be power consumption and heat. The physical heat dissipation structure of the chip itself is also very important.
The ability of the driver chip to resist EMI, noise, and high voltage is also related to whether the entire LED lighting product can successfully pass CE, UL and other certifications. Therefore, the driver chip itself must choose an excellent topology and high-voltage production process at the beginning of the design. The power consumption of the driver chip itself is required to be less than 0.5W, and the switching frequency is required to be greater than 120Hz to avoid visible flicker caused by power frequency interference.
LM2734 is a step-down converter. The constant current drive circuit based on LM2734 is shown in Figure 5. The LM321 operational amplifier is used to obtain the voltage on the sampling resistor Rset, and combined with other resistors and capacitors, a complete and high-efficiency large Power LED constant current drive circuit. In actual use, some LED constant current drive circuits can directly obtain the feedback voltage from the sampling resistor, as shown in Figure 6.
In the circuit shown in Figure 6, the sampling resistor Rset determines the design of the constant current drive circuit, and has an important impact on the efficiency of the entire system, so careful design of Rset is essential to improve efficiency. Generally speaking, if the variation of LED drive current is required not to exceed 5%~10% of the nominal value, then a resistor with an accuracy of 2% is sufficient. The typical fluctuation range of the LED drive current is ±10%. Due to the large power consumption of the sampling resistor, the use of chip resistors with low power should be avoided.
In LED lighting systems powered by DC/DC power supplies, common specific applications include 1~3W buck LED drivers, 1~20W boost LED drivers, and 20~60W high-power LED drivers. In the 1~3W DC/DC driving LED lighting application, the CAT4201 step-down LED driver can be used. This device is compatible with 12V and 24V systems, provides a driving current of up to 350mA, and can drive 7 LEDs in 24V systems. , the efficiency is as high as 94%. The device adopts a patented switching control structure, which can effectively reduce the system cost, and can also provide comprehensive protection features such as current limiting, thermal protection and LED open circuit protection. The LED circuit driving 1~3W is shown in Figure 7. And in the DC/DC boost application that drives the LED power range of 1~20w, NCP3065/6 or NCV3065/6 buck, boost, single-ended primary inductance converter (SEPIC) and inverter multi-mode LED can be used driver, and select the boost circuit mode among them. In addition, NCP1034 synchronous step-down PWM controller can be used when driving LED power range of 20~60W.
There are two types of DC/DC driving LED lighting solutions: boost type and buck type. In LED applications with a boost DC/DC driving power of 1~3W, the NCP1421 boost DC/DC converter with true shutdown function can be used, while a step-down DC/DC driving power of 1~3W can be used. In the application of LED, NCP1529 step-down converter can be used, and the application circuit of the two is shown in Figure 8.
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