조광 방식: PWM 조광
이 짧은 주문형 교육 비디오에서는 세 가지 조광 방법론, 주 FET, 시리즈 FET 및 션트 FET 조광 방법을 기반으로 PWM 조광의 기본 사항을 알아봅니다.
리소스
Hi. My name is Kelly Wallace. I'm a product marketing engineer working with TI's LED driver portfolio. This video will cover a basic overview of pulse-width modulation, or PWM dimming.
For an overview of the basic relationship between LED current and light output, as well as an overview of analog dimming, please see the first video in the Dimming Method series.
PWM dimming modulates the regulated current through the LED at a much lower frequency than the switching frequency of the converter. However, the dimming frequency must be faster than the human eye can detect, 200 hertz or above, so there is no visible flickering in the LED light output.
The average LED current is proportional to the duty cycle of the PWM dimming command. Well-implemented PWM dimming generally results in a wider linear dimming range than analog dimming. With PWM dimming, the LED current is either zero or at the nominal LED current level, which minimizes color shift that can occur during analog dimming. However, PWM dimming can add complexity to the design compared to analog dimming.
In this video, we will cover three types of PWM dimming-- main FET dimming, series FET dimming, and shunt FET dimming. Using main FET PWM dimming, dimming is achieved through the enabling and disabling the main switching MOSFET in the power converter. This dimming method is compatible with all converter typologies.
Many modern LED drivers have a specialized PWM dimming pin that accepts a dimming signal from a microcontroller or other external source. Main FET PWM dimming generally can achieve higher contrast ratios than analog dimming, but it has a lower performance compared to the other two forms of PWM dimming.
The challenge using main FET dimming occurs when trying to achieve high-contrast ratios. The rise-and-fall time of the LED current in response to the PWM command is through rate limited by the inductor and can be further limited by the output capacitor. This creates a non-ideal trapezoidal pulse, as seen here. As long as the rise-and-fall time are a small proportion of the total pulse width, the transfer function still remains highly linear. However, at low duty cycles, non-linearity will increase.
In addition to the trapezoidal pulse, additional non-linearity occurs when the falling PWM edge occurs during the MOSFET switching off time, as seen in the image on the left. Since the same MOSFET is used for both the switching and the dimming frequencies, the average LED current will only change if the falling edge occurs during the MOSFET on time.
This creates a stairstep effect in the transfer function of LED current versus dimming duty cycle. To minimize this effect, you can increase the switching frequency of the converter or choose another PWM dimming method.
Similarly to main FET dimming, series FET dimming is achieved through enabling and disabling the main switching MOSFET in the power converter in response to a dimming signal. However, in this method, there is also a FET placed in series with the LED string. The output drive and the dimming FET turn off at the same time, at which point the inductor energy is absorbed by the capacitor.
When the LED driver and dimming switch turn on, the capacitor discharges, which turns on the LED while the inductor is still ramping up. This ensures faster rise-and-fall times of the LED current in response to the PWM signal, which allows for faster PWM switching frequencies and higher contrast ratios than main FET dimming.
This also means it is important to maximize the output capacitance. An additional advantage of higher output capacitance is the reduction of the LED current ripple. This creates a sinusoidal LED current waveform that can even approach a DC value as the ripple reduces with increased capacitance. It is less efficient than main FET dimming, particularly at high light outputs due to the I squared R losses in the series dimming FET. This dimming method can also be used with all converter typologies.
The final method of PWM dimming we will go over is shunt FET dimming. If you are running up against limitations caused by delays in the shutdown and startup of the converter, shunt FET dimming may be a better option as it is the fastest method of PWM dimming with the highest potential contrast ratio.
In shunt FET dimming, an external FET is paced parallel to the LED string to quickly bypass the converter's output current around the LED. This means the inductor current stays continuous while the LED is off, so there is no extra delay due to ramping up and down the inductor current.
The only limitation to rise-and-fall times are due to the shunt FET capacitances. Capacitance is required across the LED but needs to be minimized due to overshoot current, a current from the instantaneous change in the voltage at the output. The order of magnitude of the overshoot is determined by the I equals CDVDT relationship.
Shunt FET dimming has the largest linear dimming range compared to other PWM dimming methods. However, one disadvantage is that, unlike the other forms of PWM dimming, shunt FET dimming cannot be used with a boost converter since the output would be less than the input every dimming cycle. This method also has lower efficiency, especially at low light output due to the additional I square R loss from the shunt FET.
To summarize, PWM dimming is a great alternative to analog dimming. It tends to be more complex to design, but it can provide higher PWM dimming frequency and higher dimming ratios.
There are three main types of PWM dimming. The first, main FET, uses a digital signal to turn the LED driver output drive on and off. Series FET is very similar to main FET dimming, except that it adds a FET in series with the LED stack to turn the LEDs on and off more quickly. Shunt FET dimming places a FET in parallel to the LED stack to shunt converter output current around the LED.
That's all for today. Please check out the first video in our Dimming Method series for a similar training on analog dimming. Thank you for watching.
이 비디오는 시리즈의 일부입니다.
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