Overview of PWM and PFM Control

The DC/DC control method controls the switching cycle and on time and can be a PWM control, PFM control, or PWM/PFM automatic switching control. Among the differences between these methods, the light load efficiency, the noise characteristic, and the output ripple voltage have a significant impact.

Here, we describe the operation modes, characteristics, and disadvantages of PWM control, PFM control, and PFM/PWM automatic switching control.
The principles that the operation and control are based on are described on other pages.

Differences between PWM control and PFM control

The key difference between PWM control and PFM control is the difference in efficiency during a light load.The following efficiency data compare the efficiency when the control method is switched between PWM control and PFM control in identical products. We can see that the efficiency during a light load is significantly different.

PWM control (Pulse Width Modulation)

PWM control is a control method that turns the switching element on and off at a fixed frequency, and stabilizes the output voltage by changing the width of the on period (the pulse width).
If the output voltage has deviated from the target value, the on duration (duty ratio) is adjusted according to the discrepancy.

• Characteristics:
• Switching frequency is fixed: The frequency at which noise is generated is easy to predict, and so countermeasures are comparatively straightforward to implement.
• Output ripple is small: Because operation is always at a fixed frequency, the output voltage fluctuations (ripple voltage) are small and stable.
• Disadvantages:
• Efficiency is low during a light load: Because the switching operation continues even if the load is light, the efficiency falls due to the supply current loss because of the switching with a light load.

PFM control (Pulse Frequency Modulation)

PFM control is a control method that keeps fixed the on duration of the switching element, and stabilizes the output voltage by changing the switching frequency.
If the output voltage drops below the set output voltage, the switching element is turned on for the fixed on duration, the off duration until the next pulse is extended if the load lightens, and the frequency is reduced.

• Characteristics:
• Efficiency is high during a light load: By reducing the switching frequency when the load is light, the number of switching times and the power loss are reduced, achieving high efficiency.
• Output ripple is large: Because the switching frequency fluctuates, the fluctuations in the output ripple voltage tend to be great.
• Noise spectrum fluctuates: Because the switching frequency changes depending on the load, noise countermeasures are difficult to implement, and there is even the possibility of noise being generated in the audible range.

PWM/PFM automatic switching control

The PWM/PFM automatic switching control is a hybrid method that combines the advantages of the two control methods above.
Automatic switching between PWM control and PFM control occurs based on the size of the output current (load).
When the load is heavy, the system switches to the PWM control because stability and a small ripple are required, and when the load is light, the system switches to the PFM control because high efficiency is required.

• Characteristics:
• High efficiency across the whole range of loads: can maintain high efficiency across a wide range of loads.
• Balance between stability and efficiency: benefits from the advantages of both controls.
• Disadvantages:
• When switching between controls takes place, the output may fluctuate temporarily and noise may be generated.