PWM LEDs: Pulse Width Modulation for Dimming Systems and Other Applications

March 17, 2020 Cadence PCB Solutions

Display of PWM Signal wave form

 

Modulation in terms of a definition is the application of a controlling or modifying influence on someone or something. However, there is a second definition for modulation that I am sure most of us are familiar with as well. This secondary definition I am referring to is the variation in the tone, pitch, or strength of one's voice.

It is the second definition that my parents typically made use of. Times like those were when I really appreciated having test subjects, or should I say older siblings.

As defined earlier, modulation is, in essence, about control. Therefore, it is only logical that a method such as this will find a wide variety of uses. Furthermore, in terms of use, there is one variation of modulation control in use in the field of electronics, it is called Pulse Width Modulation, or PWM. Moreover, PWM finds itself in extensive use in today’s vast area of Electronics.

What is Pulse Width Modulation?

What prompts such extensive use of PWM in today’s field of Electronics is, of course, its ability to lend control over numerous devices within every subdivision of the area of Electronics. Overall, PWM signals are in use for dimming LEDs, controlling motors, and various other electronic devices. So, what does the functionality of the methodology of PWM entail?

PWM is a method that reduces the average amount of deliverable power of an applied electrical signal. Furthermore, the process is achieved by effectively chopping up the signal into distinct parts. Also, in terms of functionality, control over the average current and voltage delivered to the load is achieved by rapidly turning the switch between the load and the source, on and off.

Pulse Width Modulation Functionality and Applications

Furthermore, when comparing the on and off periods of the switch in use, increasing the on-time versus the off-time increases the total power supplied to the load. As you might imagine, this type of control has many useful applications. For example, PWM paired with maximum power point tracking or MPPT is one of the principal methods for reducing the output of solar panels to facilitate its use by a battery.

However, PWM is especially suited for running inertial devices like motors, which are not as quickly affected by this distinct switching. Also, this is equally true for LEDs because of the linear fashion in which their functionality is affected by their input voltage. Furthermore, the PWM switching frequency needs to be high enough as not to affect the load, yet the resulting waveform that the load perceives should also be smooth.

In general, the frequency in which the power supply must switch will vary widely depending on the device and its application. For example, the switching has to be done several times a minute in say an electric stove and well into the tens or hundreds of kHz for PC power supplies and in audio amplifiers. Furthermore, one of the significant advantages of using PWM is that power loss in the switching devices is substantially low. In fact, during the off phase of a switch, there is virtually no current. Also, during the on phase of a switch, while transferring power to its load, there is practically no drop in voltage across the switch.

Pulse Width Modulation Functionality and Applications Continued

Therefore, since power loss is a consequence of both voltage and current, this equates to a near-zero loss in power for PWM. Moreover, PWM is also ideally suited for digital controls as well. This is due to the nature of digital technology as a whole, i.e., 1’s and 0’s or On and Off states. In general, the intrinsic nature of digital technology lends itself effortlessly to the functionality of PWM, and thus it is easy to set the necessary duty cycle.

PWM is also currently in specific communication systems as well, and its duty cycle is in use to convey information over communications channels. PWM is a methodology or technique to generate low-frequency output signals from high-frequency pulses. By quickly switching the output voltage of say, an inverter leg between the upper and lower voltages (DC rail), the low-frequency output basically becomes the average voltage over the switching period.

If you recall, I briefly mentioned that PWM is ideally suited as a controlling methodology for LEDs due to the linear fashion in which their functionality is affected by their input voltage. Since we are, of course, referring to light output, then it is safe to say we are referencing some form of dimming. As I am sure you are aware, dimming in this case, is the process by which we vary the light output of an LED. We utilize this process to save energy, increase the lifecycle of the LED, afford other functionality, or even to set the ambiance in some cases.

Strip of LEDs

LEDs rely on PWM for maintaining their voltage.

 

Pulse Width Modulation in LED Dimming Systems

The majority of the dimming systems in use before LEDs, as well as those in use today, have designs that cater to incandescent light bulbs. Furthermore, these dimming systems typically use forward and reverse-phase dimming methodologies in which the dimmer chops or interrupts the line input (AC) to reduce the total power to the load (driver). Moreover, a reduction in the input power results in a reduction in the output of the driver, and subsequently, a decrease in brightness.

However, the side effect of this particular dimming method is a decrease in efficiency, in regard to the traditional incandescent light bulb. Furthermore, it also produces current surges, AC distortions, and unwanted EMI radiation. Yet, with an LED, this is not the case. In general, aside from an increase in cost, the LED is superior in every meaningful way.

An LED provides increased light quality, efficiency, lifecycle, and durability. Overall, as previously mentioned, in comparison to the incandescent light bulb, dimming an LED offers several advantages. Also, since an LED’s light intensity directly correlates to its input current and voltage, dimming saves energy. Moreover, as mentioned earlier, dimming an LED provides an increased life cycle since they operate at a lower temperature during periods of dimming. In general, LED’s have a wide dimming range compared to other light sources and affords dimming to less than 1% of their maximum output.

The Methodologies for Dimming LEDs

Overall, every LED device requires a driver to enable dimming, and currently, there are two standard methodologies that drivers use to accomplish this task. The first is PWM, which I have previously discussed, and the other is Constant Current Reduction or Analog Dimming.

Pulse Width Modulation (PWM)

With PWM, as stated, the LED is turned off and on at the LED’s rated current at a high frequency. Furthermore, this rapid switching is at speeds high enough for the human eye to see. Ultimately, it is the ratio of the time when the LED is ON (Duty Cycle) and the total time of one complete cycle that determines the LED’s brightness level.

Advantages of PWM Dimming:

  • Provides a more precise output level

  • Ideally suited for applications needing to maintain specific characteristics of the LED, for example, efficiency, color, or temperature

  • Provides a broader dimming range

  • Avoidance of color shifts by operating the LED at its proposed forward current and forward voltage operating point

Disadvantages of PWM Dimming:

  • Requires Drivers that are expensive as well as complex.

  • The rapid switching produces EMI radiation due to the quick-rising edge and falling edge of the switching cycles.

  • It creates performance issues in the drivers if using longer wires due to the inherent characteristics (inductance and capacitance) of the wire interfering with the fast edges of the PWM.

Constant Current Reduction (CCR)

With the constant current reduction method (CCR), the current continuously flows through the LED. Therefore, the LED is constantly on, unlike the PWM method, where the LED fluctuates between the on and off states. However, with CCR, you control the LED’s brightness by varying or changing the current levels.

Advantages of the CCR Dimming Method:

  • Ideally suited for applications with stringent EMI requirements as well as remote applications that utilize long wire runs.

  • The CCR utilizes drivers that afford higher output voltage limits (60 V) than those in use with PWM (24.8 V). These specifications refer to a UL classification of Class 2 drivers that are for use in damp and dry locations.

Disadvantages of the CCR Dimming Method:

  • The CCR method is not suited for applications that require dimming below 10% of maximum output. This is due to the inconsistent light production of LEDs at extremely low currents. In summary, the LEDs do not perform well at these current levels using this method.

  • The use of Low drive currents produces inconsistent color.

 

Composition of an LED

Understanding the makeup and function of an LED certainly helps your designing of one. 

 

PWM is a versatile control method with numerous applications in the field of electronics. Also, the linear fashion in which an LED is affected by current and voltage makes them the ideal candidate for PWM in terms of control. Although there are other methods of control, in terms of dimming systems for LEDs, the fact remains that the most advantageous method is pulse width modulation.

Working through PWM applications in your circuit designs is easy with the suite of design and analysis tools from Cadence. Allegro PCB Designer is both fundamentally powerful and can be easily integrated with any tool you need for your PWM design strategies.

If you’re looking to learn more about how Cadence has the solution for you, talk to our team of experts.

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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