Last Updated on : 2022-11-24 09:20:28download
Pulse-width modulation (PWM) dimming controls the brightness of an LED by adjusting the relative ratios of the on time to the off time. When the switching frequency is greater than 100 Hz, the human eyes will average the on and off times, seeing only an effective brightness that is proportional to the LED’s on-time duty cycle. The more time the current is on, the brighter the LED appears to be.
There are two driver options you can choose from: cool and warm white dimming and correlated color temperature (CCT) dimming.
Mix two channels of PWM signals to control the brightness and color temperature. The cool white (CW) channel controls the cool white dimming (high color temperature). The warm white (WW) channel controls the warm white dimming (low color temperature). The CW channel is used for single-channel dimming.
Complementary PWM output
As shown in the above circuit diagram, PG0 and PG1 are complementary. Such PWM circuit design can provide benefits for power supply circuits in LED applications.
For PWM dimming mode, the MOSFET is on when the level is high. With the complementary PWM output applied, the outputs of the two channels cannot be high at the same time so drawing current from the input source will not happen. However, if you do not use the complementary output, the outputs of the two channels might be high at the same time. In this case, the two channels draw current from the input source at the same time. In terms of the power consumption, assume that one channel is 9W. The total power consumption when the two channels are both high equals the total output of 18W. Even if the average value is the same, the load on the input source is different.
Things to note:
In h-bridge PWM applications, if both the upper and lower power circuits are switched at the same time (one turned on, and the other turned off), both switches might be on for a short period of time due to delay in the circuitry. During this brief interval, a very high current (shoot-through current) will flow through both power switches, shorting the bridge supply.
To avoid this potentially destructive shoot-through current from flowing during switching, dead-band delay can be implemented to allow one power circuit to be completely turned off before the other is turned on.
The shadow areas in the
PG1_DT signals denote the configured dead time. Dead-time delays the on signals for the switching devices. During the dead period, both upper and lower switching devices are turned off in order to prevent a short-circuit. Usually, the dead time is only a few percent of the duty cycle.
The CCT dimming implements two channels for adjustment, one channel for brightness and the other one for color temperature.
As shown in the following circuit diagram,
U1 (BP1371) and
U2 (BP5926A) are LED drivers from Bright Power Semiconductor (BPS), used for brightness adjustment and color temperature respectively.
With built-in dual-channel 100V MOSFET, BP5926A provides high performance in the stepless color temperature adjustment. The light intensity ratio of the two channels can be adjusted by changing the duty cycle of the input PWM signal so that we can get the desired color temperature.
The outputs of the two PWM channels are complementary. The total current output remains unchanged during the color temperature adjustment, equaling the output of the constant current source.
The upper stage of the driver is charged and discharged as the capacitor is switched on and off. If the software cannot handle the process well, color temperature adjustment might cause flicker effects.
Make sure to observe the following principles when you work with the CCT driver:
The module outputs signals from PWMR, PWMG, and PWMB channels. The linear constant current LED driver implements the constant output of RGB current.
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