LED Car Lamp Drive Mode.

- Jan 16, 2018 -

The LED drive mode can be used in three types: resistance limit current, linear voltage regulator and switching converter. The resistance limit flow scheme is suitable for low efficiency applications, so it is not used in automobile lighting with high efficiency requirements and wide input voltage range. The linear regulator is suitable for low current or LED forward pressure drop slightly lower than the power supply voltage, but there are also problems of efficiency and low input voltage range. The switching converter has the characteristics of flexible circuit topology, high efficiency and wide input voltage. Therefore, after considering the factors such as working efficiency, installation size, static current, working voltage, noise and output regulation, the driving circuit is mostly switched converter. The topology of the switch converter is divided into Buck, Boost and buck-boost. At present, LED application in automobile lighting, its drive power supply must be lead acid battery.

Because the input voltage range of the accumulator is very different from the normal range, the driving circuit generally satisfies the voltage requirement of the LED array with the Buck-Boost topology. The dc gain (ratio between output voltage and input voltage) of the circuit topology is related to the ratio of the occupied space to D(the ratio of the opening time to the cycle in a switching cycle). When the battery voltage is lower than the required voltage of the LED, adjust D > 0.5, so that the circuit is in a state of pressure. When the battery voltage is higher than the required voltage of the LED, adjust D < 0.5, so that the circuit is in a state of de-stress. LED is the current type element controlled by the current, and the brightness is proportional to the current flowing through it. If the LED is not a constant current drive, the current fluctuation will cause the brightness change of the LED even if the voltage is constant. To ensure that the brightness is stable and reliable, the LED needs a constant current to drive it, and it needs to be able to control the ripple current at acceptable levels in any case. Therefore, the output of LED drive circuit must be constant current output rather than constant voltage output.

The Buck Boost circuit will power the battery to the LED array, and the sampling circuit will sample the current flowing through the LED and transmit the signal to the control circuit. The control circuit analyzes the sampling information and regulates the duty ratio of the switch tube in the buck-boost circuit, ensuring that the LED current is constant; When the circuit is abnormal, the power is cut off by the control circuit to ensure that the LED is not damaged. Generally, LED drive circuit must meet the following requirements:

(1) lifting pressure function. When the input voltage or the LED itself is reduced and fluctuated, the output voltage is adjusted to meet the constant demand of the output current to ensure that the LED is stable and reliable.

(2) high power conversion efficiency. In order to reduce the drive loss, save the cost, reduce the battery charge times and prolong the battery life.

(3) brightness adjustment function. When the ambient environment is very dark, the signal lamp often does not need the maximum current drive, which can control the driving current and change the brightness of the LED, thus reducing the power consumption of the LED. The common way to adjust the driving current is to use PWM signal control.

(4) it has perfect protection circuit. Various protection measures should be set up to protect themselves and LED reliable work. For example, low pressure lock, overvoltage protection, overheat protection, output open circuit or short circuit protection, etc.

(5) good heat dissipation function. According to the thermal characteristics of LED, temperature is one of the important factors affecting LED work. When driving at night, the LED is in a long light state. Therefore, it is necessary to have good heat dissipation function to ensure the life and reliability of the LED.

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