The difference between PWM and PFM of power supply

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The difference between PWM and PFM of power supply [Copy link]

The development of science and technology has produced many power supply designers. Engineers working on switching power supplies should be familiar with the two control technologies of PWM and PFM. What roles do they play in switching power supplies and what positions do they occupy?

　　There are three main control technologies for switching power supplies: (1) pulse width modulation (PWM); (2) pulse frequency modulation (PFM); and (3) pulse width frequency modulation (PWM-PFM).

　　PWM: (pulse width modulation)

　　Pulse width modulation (PWM) is a term used in switching voltage regulators. It is classified by the voltage regulation control method. In addition to PWM, there are also PFM and PWM/PFM hybrid types. The pulse width modulation (PWM) switching voltage regulator circuit adjusts its duty cycle through voltage feedback while the output frequency of the control circuit remains unchanged, thereby achieving the purpose of stabilizing the output voltage.

　　PFM：Pulse frequency modulation

　　A pulse modulation technique in which the frequency of the modulating signal varies with the amplitude of the input signal, while its duty cycle remains unchanged. Since the modulating signal is usually a square wave signal with a changing frequency, PFM is also called square wave FM.

　　PWM is the change of frequency width and narrowness, PFM is the change of frequency presence and absence. PWM uses the pulse width to control the output, while PFM uses the presence or absence of pulses to control the output.

　　Among them, PWM is the most widely used control method in switching power supplies. It is characterized by low noise, high efficiency at full load and the ability to work in continuous conduction mode. There are many PWM integrated chips with good performance and low price on the market. PFM has the advantage of low static power consumption, but it does not have current limiting function and cannot work in continuous conduction mode.

　　Compared with PWM, the output current of PFM is small, but because the DC/DC converter controlled by PFM stops operating when the voltage reaches above the set voltage, the current consumption becomes very small. Therefore, the reduction of current consumption can improve the efficiency at low load. Although PWM is less efficient at low load, it is easier to design a noise filter and eliminate noise because of its small ripple voltage and fixed switching frequency.

　　If you need to have the advantages of both PFM and PWM, you can choose a PWM/PFM switching control DC/DC converter. This function is controlled by PWM under heavy load and automatically switches to PFM control under low load, that is, it has the advantages of PWM and PFM in one product. In a system with standby mode, products using PFM/PWM switching control can achieve higher efficiency. The above are some differences between PWM and PFM, the control technology of switching power supplies. I believe that it can be a good reference for everyone when designing.