Power supply design should be aware of the two concepts of PWM and PFM

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-modulated PWM is the term used in switched regulated power supplies. This is classified according to the control mode of voltage regulation, in addition to PWM type, there are PFM type and PWM, PFM hybrid type. Pulse width modulation (PWM) switching voltage regulator is to adjust its duty cycle through voltage feedback to stabilize the output voltage when the output frequency of the control circuit is unchanged.

PFM: (Pulse frequency modulation) Pulse frequency modulation
    A pulse modulation technique in which the frequency of the modulated signal varies with the amplitude of the input signal and its duty cycle remains unchanged. Because the modulated signal is usually a square-wave signal with a change in frequency, PFM is also called square-wave FM

    PWM is the change of frequency width and narrow,PFM is the change of frequency and no, PWM is the use of wave pulse width control output,PFM is the use of pulse control output.

    Among them, PWM is the most widely used control mode in switching power supply, which is characterized by low noise, high efficiency at full load and can work in continuous conductive mode. Now there are many PWM integrated chips with good performance and low price on the market. Such as UCl842/2842/3842, TDAl6846, TL494, SGl525/2525/3525, etc. PFM has the advantage of low static power consumption, but it has no finite current function and can not work in continuous conduction mode. Integrated chips with PFM function include MAX641, TL497, etc. Pwm-pfm has the advantages of both PWM and PFM.

    DC/DC converter is boosted or depressurized by switching with internal frequency synchronization, controlled by changing the number of switching times, so as to obtain the output voltage of the same as the set voltage. In PFM control, the switch will be stopped when the output voltage reaches above the set voltage, and the DC/DC converter will not perform any operation before descending to the set voltage. However, if the output voltage drops below the set voltage, the DC/DC converter will start switching again so that the output voltage reaches the set voltage. The PWM control is also switched on and off in sync with the frequency, but it minimizes the current flowing into the coil when the boost setpoint is reached, adjusting the boost to align it with the set voltage.

    Compared with PWM, the output current of PFM is small, but because the DC/DC converter controlled by PFM will stop when it reaches the set voltage, the current consumed will become small. Thus, the reduction of current consumption improves efficiency at low loads. Although PWM is less efficient at low load, because of its small ripple voltage and fixed switching frequency, the noise filter design is easier and the noise elimination is simpler.

    If you want to have the advantages of both PFM and PWM, you can choose PWM/PFM switching control DC/DC converter. This function is controlled by PWM at heavy load and automatically switches to PFM control at low load, that is, the advantages of PWM and PFM are combined in one product. In systems with standby mode, products using PFM/PWM switching control can achieve higher efficiency.

    As far as DC-DC converters are concerned, the PFM in the industry only has a Single Phase and is implemented in Ripple Mode, so the Ripple at the output end is larger. There is no negative inductive current, so it can improve the light load efficiency. The Transient was fine because it looked at the output Ripple, and there was no under-shoot when doing Dynamic. PWM has Single Phase &Multi-phase, mostly implemented in Voltage Mode or Current Mode, and has no requirement for output Ripple. There is inductive negative current in light load, so the light load efficiency is poor. Compensation is slower than Ripple. When PWM is used in combination with PFM, Pulse Skipping occurs when inductive negative current is detected and is no longer controlled by the internal Clock. At this point, the controller will turnoff both h-mos & L-MOS, Coss & L will have damped oscillations.

    Each engineer's contact with the field is not the same, there may be some areas with PFM more, some with PWM more, but from the entire power supply industry, I believe that PWM is still used more. From the 1980s to the present, PWM began the "dynastic rule" status in the field of power conversion, because each method has disadvantages and advantages. The key is to see whether it is suitable for the needs of customers in the forum to see a netizen is so written, I think the comparative image of writing, he said that if the PFM and PWM power car to compare, PFM = Mercedes, with PWM = Volkswagen.

The main advantage of PFM over PWM is efficiency
    1. For PFM and PWM with the same peripheral circuit, the peak efficiency PFM is equivalent to PWM, but before the peak efficiency, the efficiency of PFM is much higher than that of PWM, which is the main advantage of PFM.
    2, PWM due to the influence of the error amplifier, the loop gain and response speed are limited, PFM has a fast response speed

The main disadvantage of PFM compared with PWM is the difficulty of filtering
    1, filtering is difficult (harmonic spectrum is too wide).
    2, before the peak efficiency, the PFM frequency is lower than the PWM frequency, which will cause the output ripple larger than the PWM.
    3. PFM control is more expensive than PWM control IC.

    One of the main reasons why PFM applications are not as many as PWM is that another reason is the huge advantage of PWM: the control method is easy to implement, and the PFM control method is not easy to implement.