The difference between PWM and PFM of power supply

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 role do they play in switching power supplies and what status do they occupy?

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

PWM: (pulse width modulation) pulse width modulation

Pulse width modulation (PWM) is the term used in switching regulated power supplies. This is classified according to the voltage stabilization control method. In addition to the PWM type, there are also PFM type and PWM and PFM hybrid type. The pulse width modulation (PWM) switching voltage stabilizing circuit adjusts the 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) Pulse frequency modulation

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

PWM is the change of wide and narrow frequency, PFM is the change of presence and absence of frequency, PWM is the use of wave pulse width to control the output, and PFM is the use of the presence or absence of pulses to control the output.

Among them, PWM is currently 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 PWMs with good performance and low price on the market. Integrated chip; PFM has the advantage of low static power consumption, but it has no 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 it reaches the set voltage or above, the current consumed becomes very small. Therefore, reduction in current consumption improves efficiency at low loads. Although PWM is less efficient at low load, its ripple voltage is small and the switching frequency is fixed, so it is easier to design a noise filter and eliminate noise.

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 when the load is heavy, and automatically switches to PFM control when the load is low, that is, it has the advantages of PWM and PFM in one product. In systems with standby mode, products using PFM/PWM switching control can achieve higher efficiency. The above are some differences between the control technology of switching power supply, PWM and PFM. I believe it can be a good reference for everyone when designing.