Every LED has a rated current, which is the amount of current that needs to flow to get the
maximum light output. In a PWM driver, the current is switched at a high frequency between
0 and the rated output current. This means the LED load is either off or running at its rated
current. The ratio of on time to off time determines the LED brightness. See the following
diagram for an example of an LED that is dimmed to approximately 25% using PWM. The
25% level is a result of the current flowing for 25% of the time, then being turned off for the
remaining 75% of the time.
In a CCR supply, the current flows continuously at a set amount for a given light level. Since
the amount of light output is proportional to the current flowing through the LED, the current
is reduced to reduce the brightness of the LED. See the following diagram for an example of
an LED that is reduced to approximately 25% using CCR. Sometimes this type of dimming is
referred to as “Analog Dimming.”
Use of both CCR and PWM based drivers will have the desired effect of dimming the lights.
However, other factors come into play when each method is used.
Typically, LED specification sheets provide a particular characteristic, such as color or
efficiency (in lumens per watt), at a particular forward current. These characteristics will
vary based on the forward current. For example, the color temperature may be 3000K at
700mA of current, but only 2700K at 350mA. A PWM driver will only run the LEDs at the
rated current level or zero, preventing these characteristics from changing as the load is
dimmed. Practically, this means the same color temperature is maintained throughout the
dimming range