There are two common working modes of DCDC switching power supply, namely PWM mode and PFM mode, which we often hear. One is the common working mode, and the other is the low-power working mode. This section takes the buck structure switching power supply as an example to introduce their working characteristics and distinguishing methods.

Pwm:pulse width modulation is pulse width modulation. It is characterized by fixed switching frequency or switching cycle and variable duty cycle.

Pfm:pulse frequency modulation is pulse frequency modulation, which is characterized by the change of switching frequency or switching period.

See the following figure for the waveform comparison between the two.

Why use PFM mode?

This is because the load has different current consumption according to different working states, and the current varies greatly. The small current may be only a few Ma, and the large current may be hundreds of Ma or even a few A. However, the efficiency of the switching power supply is not high at low load current due to the fixed switching frequency. In order to improve the efficiency of the power supply and reduce the switching loss of the power supply itself at low current, the PFM working mode is proposed.

According to the above figure and the historical article "detailed analysis of DCDC buck circuit principle"

It can be seen that in PWM mode, the buck tube is always in the on-off state, and the inductor is constantly charged and discharged, while in PFM mode, the tube is only occasionally switched off, commonly known as intermittent switch, and the inductor is occasionally charged and discharged.

Because the control tube of PWM Mode Buck is a continuous switch, while the tube of PFM mode is an intermittent switch, this leads to the most significant difference between the two, that is, the ripple of PFM mode is greater than that of PWM.

How to judge whether the power supply works in PFM or PWM mode?

The following figure shows the test results of PFM and PWM waveforms of a power supply. The first line is the output voltage ripple. It can be seen that the ripple of PFM mode is greater than that of PWM mode. The second line is the voltage waveform at the SW position of the power switch node. It can be seen that the switch stops when the switch is on in PFM mode, while the SW node in PWM mode is always in the on state. This can be used as a symbol to distinguish PFM and PWM.

The third line is the charge and discharge current waveform of the inductor. It can be seen that the inductor has intermittent charge and discharge waveform in PFM mode, while the inductor in PWM mode has continuous charge and discharge, which can also be used as a symbol to distinguish PFM and PWM modes.

In the above waveforms, the PFM mode is intermittently switched once. In some cases, it is intermittently switched for many times. For example, in the following figure, the first line is the waveform of the switch node, that is, the intermittent switching pulse sequence is generated. From the third line, we can see the inductance. The inductance is repeatedly charged and discharged during switching, and the charging and discharging will stop when the switch stops.