How does a synchronous buck converter achieve voltage step-down using pulse-frequency modulation (PFM)?
1 Answer
A synchronous buck converter is a type of DC-DC converter that can efficiently step down a voltage from a higher level to a lower level. It uses pulse-width modulation (PWM) to control the output voltage. However, pulse-frequency modulation (PFM) is another control technique that can be used in certain scenarios to achieve voltage step-down with improved efficiency under light-load conditions.
In a traditional PWM-controlled synchronous buck converter, the high-side and low-side switches (usually MOSFETs) are turned on and off alternately to regulate the output voltage. When the high-side switch is turned on, the input voltage is connected to the inductor, storing energy in it. When the high-side switch is turned off, the low-side switch is turned on, and the inductor releases its stored energy to the output load. The duty cycle of the switches (the ratio of on-time to the switching period) determines the output voltage.
In contrast, a synchronous buck converter with pulse-frequency modulation (PFM) operates differently, particularly under light-load conditions, to improve efficiency. When the output load is light, the converter may only need to deliver small amounts of power. In such cases, the traditional PWM control would result in excessive switching losses and reduced efficiency because the switches are turning on and off at a high frequency, even though the load demands a lower output voltage.
In PFM mode, the synchronous buck converter adjusts its switching frequency based on the output load demand. When the load is light, the converter reduces its switching frequency or enters a burst mode, where the switches are turned on and off less frequently. This helps reduce switching losses and improves the converter's efficiency under light-load conditions.
Here's a simplified explanation of how PFM works in a synchronous buck converter:
Under light-load conditions, the controller senses the reduced output load demand and decides to reduce the switching frequency or enter burst mode.
The converter might skip some switching cycles or extend the off-time between switching cycles, effectively reducing the number of switching events.
With reduced switching frequency, the converter spends less time switching, leading to reduced switching losses.
The converter maintains regulation by monitoring the output voltage and adjusting the pulse frequency accordingly.
By using PFM in a synchronous buck converter, it becomes more efficient at light loads, as it avoids unnecessary switching losses, reducing power consumption and increasing overall efficiency for battery-operated devices and other low-power applications. However, under heavy loads, the converter may revert to traditional PWM control to ensure precise output voltage regulation.
In a traditional PWM-controlled synchronous buck converter, the high-side and low-side switches (usually MOSFETs) are turned on and off alternately to regulate the output voltage. When the high-side switch is turned on, the input voltage is connected to the inductor, storing energy in it. When the high-side switch is turned off, the low-side switch is turned on, and the inductor releases its stored energy to the output load. The duty cycle of the switches (the ratio of on-time to the switching period) determines the output voltage.
In contrast, a synchronous buck converter with pulse-frequency modulation (PFM) operates differently, particularly under light-load conditions, to improve efficiency. When the output load is light, the converter may only need to deliver small amounts of power. In such cases, the traditional PWM control would result in excessive switching losses and reduced efficiency because the switches are turning on and off at a high frequency, even though the load demands a lower output voltage.
In PFM mode, the synchronous buck converter adjusts its switching frequency based on the output load demand. When the load is light, the converter reduces its switching frequency or enters a burst mode, where the switches are turned on and off less frequently. This helps reduce switching losses and improves the converter's efficiency under light-load conditions.
Here's a simplified explanation of how PFM works in a synchronous buck converter:
Under light-load conditions, the controller senses the reduced output load demand and decides to reduce the switching frequency or enter burst mode.
The converter might skip some switching cycles or extend the off-time between switching cycles, effectively reducing the number of switching events.
With reduced switching frequency, the converter spends less time switching, leading to reduced switching losses.
The converter maintains regulation by monitoring the output voltage and adjusting the pulse frequency accordingly.
By using PFM in a synchronous buck converter, it becomes more efficient at light loads, as it avoids unnecessary switching losses, reducing power consumption and increasing overall efficiency for battery-operated devices and other low-power applications. However, under heavy loads, the converter may revert to traditional PWM control to ensure precise output voltage regulation.