Constant current type LED drive power supply

Led constant current drive power supply is to convert the power supply to a specific voltage and current to drive the led led light voltage converter, citing the general situation: led drive power supply input including high voltage power frequency ac (namely mains), low voltage dc, high voltage dc, low voltage high frequency ac (such as the output of electronic transformer). The output of LED drive power is mostly a constant current source that can change the voltage with the change of LED forward voltage drop value.
The constant current mode needs to detect the switching current of the power switch tube by detecting the resistance, and can limit the current one by one, so as to realize the over-current protection. The clock pulse of fixed frequency sets the PVM latch, and the driving signal output from the Q end is high level, so that the power switch Vr is switched on, and the current at the primary side of the high-frequency transformer increases linearly. When the voltage drop U of the current detection resistance R core reaches and exceeds U, the current detection comparator flips, the output high level will reset the latch, and the driving signal from Q end is changed to low level, so that the switch tube is turned off, and the next clock pulse is directly used to set the PWM latch.
A. The constant current driving circuit output current is constant, while the output dc voltage varies within a certain range with different load resistance value. Small load resistance value leads to low output voltage, and the larger the load resistance value leads to higher output voltage.
B. Constant current circuit is not afraid of load short circuit, but it is forbidden to completely open the load.
C, constant current drive circuit drive LED is relatively ideal, but relatively high price.
D. Attention should be paid to the maximum withstand current and voltage value, which limits the number of LED;

Constant current LED drive power has the following advantages:
Constant current source is a kind of wide spectrum, high precision ac steady current power supply, with fast response speed, constant current accuracy, long-term stable work, suitable for various properties of load (resistance, sensibility, capacity) and other advantages. It is mainly used for testing thermal relay, plastic case circuit breaker, small short circuit device and production occasions such as need to set rated current, action current and short circuit protection current.
(1) it is a double closed-loop control system. The outer loop is composed of a voltage feedback circuit, and the inner loop is composed of a current feedback circuit, which is controlled by a voltage feedback circuit. Compared with voltage feedback circuit, the gain bandwidth of current feedback is larger.
(2) quick response to the transient change of input voltage. When the input voltage changes, the output voltage can be quickly adjusted to reach the stable value. This is because the change of the input voltage will lead to the change of the primary side inductance current, thus making U. The duty ratio of the output pulse can be changed directly by the current detection comparator without the need of error amplifier. It can simplify the design of error amplifier compensation network.
(3) under the common control of voltage control loop and constant current loop, the voltage adjustment rate index can be improved.
(4) with its own current limiting protection circuit, the current limiting threshold can be precisely set by changing Rs value. As long as the current pulse reaches the undefined threshold, the IWM comparator will operate to turn off the power switch and maintain the stable output voltage.

The main disadvantages of the constant current model are as follows:
(1) the existence of two control loops brings difficulties to circuit design and analysis.
(2) when the duty cycle exceeds 50%, it may cause unstable operation of the control loop, and slope compensation circuit needs to be added.
(3) poor noise suppression ability, because the primary side inductance is working in the continuous energy storage mode, the switch current signal increase slope is small, as long as the current signal add small noise, it is easy to lead to PWM controller error, need to add noise suppression circuit.
Constant current source drive, do not need in the output circuit series current limiting resistance, LED through the current is not affected by the external supply voltage change, environmental temperature change, and LED parameter dispersion, so that can maintain a constant current, full play of various excellent characteristics of LED.
LED constant current power supply is adopted to supply power to LED lamps. Due to the automatic detection and control of the current flowing through LED during the power supply, there is no need to worry about too high current flowing through LED in the moment of power supply, and no need to worry about the load short-circuit burning power supply.


With the wide application of LED, the performance of LED drive power supply will be more and more suitable for the requirements of LED. At the same time,LED drive power is the most important thing in the development of LED industry chain. The life and reliability of LED drive power directly affect the long life of LED light source. Therefore, in the continuous integration and development of the LED industry chain, the maturity of LED drive power technology plays a decisive factor.
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