Classified by driving method
(1) Constant current type
a. The output current of the constant current drive circuit is constant, but the output DC voltage varies within a certain range with the size of the load resistance. The smaller the load resistance, the lower the output voltage. The larger the load resistance, the output The higher the voltage;
b. The constant current circuit is not afraid of load short-circuit, but it is strictly forbidden to open the load completely.
c. It is ideal for a constant current drive circuit to drive LEDs, but the price is relatively high.
d. Pay attention to the maximum withstand current and voltage value used, which limits the number of LEDs used;
(2) Regulated type:
a. When the various parameters in the voltage regulator circuit are determined, the output voltage is fixed, but the output current changes with the increase or decrease of the load;
b. The voltage regulator circuit is not afraid of load opening, but it is strictly forbidden to short-circuit the load completely.
c. The LED is driven by a voltage-stabilizing drive circuit, and each string needs to be added with a suitable resistance to make each string of LEDs show an average brightness;
d. The brightness will be affected by the voltage change from rectification.
(3) Pulse drive
Many LED applications require dimming functions, such as LED backlighting or architectural lighting dimming. The dimming function can be realized by adjusting the brightness and contrast of the LED. Simply reducing the current of the device may be able to adjust the LED's light emission, but letting the LED work under the condition of lower than the rated current will cause many undesirable consequences, such as chromatic aberration. An alternative to simple current adjustment is to integrate a pulse width modulation (PWM) controller in the LED driver. The PWM signal is not directly used to control the LED, but to control a switch, such as a MOSFET, to provide the required current to the LED. The PWM controller usually works at a fixed frequency and adjusts the pulse width to match the required duty cycle. Most current LED chips use PWM to control LED light emission. In order to ensure that people will not feel obvious flicker, the frequency of the PWM pulse must be greater than 100HZ. The main advantage of PWM control is that the dimming current through PWM is more accurate, which minimizes the color difference when the LED emits light.
(4) AC drive
According to different applications, AC drives can also be divided into three types: buck, boost, and converter. The difference between an AC drive and a DC drive, in addition to the need to rectify and filter the input AC, there is also a problem of isolation and non-isolation from a safety point of view.
AC power supply (mains drive) is applied to LED drive, generally through steps such as step-down, rectification, filtering, voltage stabilization (or current stabilization), etc., to convert AC power to DC power, and then provide suitable LEDs through a suitable drive circuit The working current must have high conversion efficiency, small size and low cost, and at the same time solve the problem of safety isolation. Taking into account the impact on the power grid, electromagnetic interference and power factor issues must also be resolved. For low- and medium-power LEDs, the best circuit structure is an isolated single-ended flyback converter circuit; for high-power applications, a bridge converter circuit should be used.
