The LED driver power supply topology theory

The AC - LED lighting applications of DC power supply, power transformation of the building blocks (including diode, switch tube ( 场效应晶体管) , such as resistance and capacitance and inductance discrete component is used to perform their respective functions, and the pulse width modulation ( 脉宽调制) Voltage regulator is used to control the power conversion. Circuit is usually joined the transformer isolation type AC - DC power conversion including the flyback, normal shock and half bridge topology, as shown in figure 1, which is less than 30 w flyback topology standard selection of low and medium power applications, whereas half bridge structure is best suited to provide higher energy/power density. In terms of the transformer isolation structure, its size and the size of the switching frequency, and the majority of the isolation type LED drive basically adopts the 'electronic' transformer.

figure 1: common isolation topology.

the DC - DC power supply of LED lighting applications, LED drive mode can be used with resistance model, linear regulators and switching voltage stabilizer, the application of basic diagram in figure 2. Resistance type drive mode, adjust and LED series in the current sense resistor can control the LED forward current, the drive mode is easy to design, low cost, and no electromagnetic compatibility ( EMC) Problem, the disadvantage is dependent on the voltage, need filtering ( 装箱) Leds, and the efficiency is low. Linear regulator is also easy to design and no EMC problems, also support the current steady flow and over-current protection ( 折回来) Set point, and provide external current, is lack of power dissipation problem, and the input voltage is always higher than the forward voltage, and the efficiency is not high. Switching voltage stabilizer through a control switch PWM control module ( 场效应晶体管) And off to control current flow.

figure 2: the common DC - DC LED drive mode.

switch voltage regulator has higher energy efficiency, has nothing to do with the voltage, and can control the brightness, shortage cost is relatively high, the complexity is higher, and electromagnetic interference ( EMI) The problem. LED DC- Including step-down DC switching voltage stabilizer common topology structures ( 巴克) , booster ( 提高) And buck - Booster ( 巴克- 提高) Or single ended primary inductor converter ( SEPIC) Such as different types. Among them, all the work under the condition of minimum input voltage is greater than the maximum voltage LED string structure of decompression, such as 6 driven by 24 Vdc LED series; In contrast, all work under the condition of maximum input voltage is less than the minimum is used when the output voltage step-up structure, such as 6 driven by 12 Vdc LED series; And when the input voltage and output voltage range has overlap step-down - can be used Booster or SEPIC structure, such as driven by 12 Vdc or 12 Vac four series of leds, but the cost of the structure and the worst of energy efficiency.

driven by ac power directly LED the way, also got certain development in recent years, the application of diagram in figure 3. This kind of structure, the LED string array in the opposite direction, work in half a cycle, and the LED in the line voltage is greater than the forward voltage conduction. This kind of structure has the advantages such as avoiding AC - DC switching power loss and so on. But, this kind of structure of LED in the low frequency switch, old friend eye could detect flicker phenomenon. In addition, in this kind of design also need to join the LED protective measures, to make it from the effects of line surge or transient.

figure 3: directly driven by ac LED schematic diagram.