NOTE 1.5
Introduction to Constant Voltage (C.V.) Mode and Constant Current (C.C.) Mode of LED Power Supply
Most of the traditional switching power supplies provide a constant voltage (C.V.) to the load. For example, MEAN WELL’s single output model RS-25-5 can provide 5V/0~5A to power all kinds of load. Its output voltage maintains at the constant value of 5V while its output current varies from 0~5A based on the load condition. When the loading is over 5A which is its rated current, the power supply unit will go into “overload protection” mode (generally speaking the activated range of overload protection is 105%~150% of rated output power). There are many different protection types that are often seen in the design of MEAN WELL power supply such as shutdown type (PSU will be shut off output), hiccup type (output voltage/current will be pulsing), and constant current type (output current will be fixed at a constant value between 105~150% of rated output current and the output voltage will go down to a low level). The power supply is under abnormal condition when the overload protection has been activated, so it is not suitable to let the power supply unit operate under this situation continuously. Equipped with “hiccup type” as their overload protection mode, the LPV, LPH-18 and LPL-18 series in MEAN WELL LED power supply family are considered to be “constant voltage power supply”. For LED related applications, it can be used in system incorporated with LED constant current driver IC (Fig. 1.5) or series connected with a ballast resistor (Fig. 1.4).
The so-called constant current (C.C.) LED power supply basically refers to the “constant current protection” concept and its output current is maintained at a fixed value by means of feedback control (this fixed current level will be close to its rated output current). The only difference is : Under constant current (C.C.) operation mode (or constant current protection mode), the C.C. LED power supply is still within the normal operating range and all the components have been chosen to continuously provide this constant current output. Generally speaking, the power supply unit will operate under constant current region only when the system design belongs to “LEDs drive by the power supply directly” (Fig. 1.3).
Figure 1.6: Typical LED power supply I-V curve
Fig. 1.6 is the I-V characteristic curve of a typical constant current LED power supply. The “constant current region” for each series of C.C. LED power supply may be different because of the different circuit design. As the figure shown above, the PSU will operate in the constant current region only when the output voltage is kept at 50% of rated output voltage or higher, so the sum of VF of all series connected LEDs should be higher than this limitation. If the sum of VF of all series connected LEDs can’t achieve 50% of rated output voltage, the power supply will go into the “hiccup region” and cannot drive the LEDs properly. Currently most of MEAN WELL’s LED power supplies can provide constant current output and the suitable operation region for directly connected LEDs is also specified in their SPECs (in the column of “Constant current range”). The constant current region can be 3V~100%VO, 9V~100%VO, or 70%~100%VO in different product series because of the difference of circuit design (concerning Harmonic Class C compliance, the lower limit may reach 75% of rated output voltage for some models with PFC function). When using power supply to drive LEDs directly, designer of lighting fixture should make sure that the VF sum of LEDs connected in series is high enough to operate the C.C. LED PSU in the correct region.
In fact, MEAN WELL’s constant current LED power supplies also possess the feature of “C.V. + C.C.”. That means before reaching the “constant current region”, they can be operated within the “constant voltage region” just like traditional power supplies. In this region, system design must incorporate suitable “LED drive IC” or “series connected with a ballast resistor”; after entering the “constant current region”, the PSU can provide constant current as the output and hence can drive the LEDs directly. We can find that the “C.V. + C.C.” characteristic can be used in many different LED installations and make the system design more flexible.
Below is the comparison chart of three different LED driving alternatives:
