Comprehensive analysis of three control technologies of switching power supply

    A pulse modulation technique in which the frequency of the modulated signal changes with the amplitude of the input signal and its duty cycle remains unchanged. Because the modulation signal is usually a square wave signal with variable frequency, PFM is also called square wave. Fmpwm is the wide and narrow change of frequency, PFM is the change of frequency, PWM is the output controlled by the width of wave pulse, and PFM is the output controlled by the presence or absence of pulse

    PWM is the most widely used control mode in switching power supply at present. It is characterized by low noise, high full load aging rate and can work in continuous conduction mode. Now there are many PWM integrated chips with good performance and low price in 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 current limiting function and can not work in continuous conduction mode. The integrated chips with PFM function include max641, tl497, etc; Pwm-pfm has the advantages of both PWM and PFM.

    The DC / DC converter is boosted or depressurized through the synchronous switch with the internal frequency, and controlled by changing the number of switches, so as to obtain the same output voltage as the set voltage. During PFM control, when the output voltage reaches above the set voltage, the switch will be stopped. Before falling to the set voltage, the DC / DC converter will not carry out any operation. However, if the output voltage drops below the set voltage, the DC / DC converter will start switching again to make the output voltage reach the set voltage. PWM control also switches synchronously with the frequency, but it will minimize the current flowing into the coil when reaching the set value of boost, and adjust the boost to keep it consistent 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 operating when it reaches the set voltage or above, the consumed current will become very small. Therefore, the reduction of consumption current can improve the efficiency at low load. Although the efficiency of PWM is inferior at low load, it is easy to design noise filter and eliminate noise because of its small ripple voltage and fixed switching frequency.

    If you need to have the advantages of PFM and PWM at the same time, you can choose PWM / PFM switching control DC / DC converter. This function is controlled by PWM under heavy load and automatically switched to PFM control under low load, that is, it has the advantages of PWM and PFM in one product at the same time. In the system with standby mode, the products with PFM / PWM switching control can get higher efficiency.

    As far as DC-DC converters are concerned, PFM in the industry currently only has single phase and is implemented in ripple mode, so the ripple at the output end is large. There is no negative inductive current, so the light load efficiency can be improved. Because it looks at the output ripple, transient is very good. There is no under shot when doing dynamic. PWM has single phase & multi phase, which is mostly realized by voltage mode or current mode. There is no requirement for output ripple. There is inductive negative current under light load, so the efficiency of light load is poor and the compensation is slower than ripple. When PWM is combined with PFM, pulse skipping will appear when the negative current of inductance is detected, which is no longer controlled by internal clock. At this time, the controller will turn off both h-MoS & l-mos, coss & l will have damped oscillation.

    Each engineer is exposed to different fields. Some fields may use PFM more and some use PWM more. However, from the perspective of the whole power industry, I believe PWM is still used more at present Since the 1980s, PWM has begun its "dynasty ruling" position in the field of power conversion, because each method has its disadvantages and advantages. The key is to see whether it is suitable for the needs of customers. I saw a netizen write this in the forum. I think it is more vivid. He said that if PFM is compared with PWM power vehicles, PFM = Mercedes Benz and PWM = Volkswagen.