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The brightness of the LED changes, we control the current of the LED by pulse width modulation (PWM); the pulse current waveform generated is the culprit of EMI. LED dimming requires PWM control, but the EMI it produces must also be suppressed.
Well, your new design is very bullish, it can almost meet EMI requirements, but not exactly. To your surprise, the circuit produces conducted EMI just beyond what you have to meet, and at least some of this EMI comes from dimmable LEDs.
The brightness of the LED changes, we control the current of the LED by pulse width modulation (PWM); the pulse current waveform generated is the culprit of EMI. LED dimming requires PWM control, but the EMI it produces must also be suppressed.
The schematic below shows a simple way to suppress EMI.
Figure 1: EMI suppression of PWM-controlled LEDs
The yellow and blue portions in the figure are LED dimming circuits before and after suppression, respectively. The yellow circuit on the left is easy to understand. The switch of transistor Q1 controls the on and off of the LED current. However, the current drawn from the +5V supply also changes up and down, and EMI is generated.
The blue part on the right is the improved circuit. It adds a constant current source consisting of Q3 and Q4. The current of the constant current source flows through the LED when Q2 is turned off, and does not flow through the LED when Q2 is turned on. Like the yellow circuit on the left, the duty cycle of the pulse current on the LED controls the LED brightness, but the current drawn from the +5V supply remains nearly constant, so there is little EMI.
Indeed, one difference between the two circuits is the opposite phase. The driver on the right uses a lot more components than the one on the left, and it consumes more power than the left. However, if you can afford these costs, the benefits of EMI reduction may also be worthwhile.
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