Two important concepts of switching power supply control technology

The power supply design should know that there are three main control technologies for PWM and PFM.
(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 a term used in switching regulators. This is classified according to the control mode of the voltage regulator. In addition to the PWM type, there are PFM type and PWM and PFM hybrid type. The pulse width modulation type (PWM) switching type voltage regulator circuit adjusts the duty ratio by voltage feedback while the output frequency of the control circuit is constant, thereby achieving the purpose of stabilizing the output voltage. PFM: (Pulse frequency modulation) A pulse modulation technique in which the frequency of a modulated signal varies with the amplitude of the input signal, and its duty cycle does not change.

Since the modulated signal is usually a square wave signal with a frequency change, the PFM is also called square wave FMPWM is a wide and narrow change of frequency, PFM is the change of the frequency with or without, PWM is the output of the pulse width control, PFM is The use of pulse control output. Among them, PWM is currently the most widely used control method in switching power supply. It is characterized by low noise, high efficiency at full load and can work in continuous conduction mode. Now there are many on the market. PWM integrated chips with good performance and low price, 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 continuously. Conductive mode, integrated chip with PFM function is MAX641, TL497, etc.; PWM-PFM has the advantages of PWM and PFM. The DC/DC converter is boosted or stepped down by an internal frequency synchronizing switch, and is controlled by varying the number of switching times to obtain an output voltage equal to the set voltage. During PFM control, the switch will stop when the output voltage reaches above the set voltage, and the DC/DC converter will not perform any operation until it drops to the set voltage. However, if the output voltage drops below the set voltage, the DC/DC converter will start switching again to bring the output voltage to the set voltage. The PWM control also switches synchronously with the frequency, but it minimizes the current flowing into the coil when the boost setpoint is reached, and adjusts the boost to match the set voltage.

Compared with PWM, the output current of PFM is small, but the PFM-controlled DC/DC converter stops when it reaches the set voltage or higher, so the current consumed becomes small. Therefore, the reduction in current consumption can improve the efficiency at low loads. Although the PWM is inferior in efficiency at low load, the noise filter is relatively easy to design and the noise is eliminated because the ripple voltage is small and the switching frequency is fixed. 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 during heavy load, and automatically switches to PFM control at low load, that is, it has the advantages of PWM and the advantages of PFM in one product. In systems with standby mode, products with PFM/PWM switching control can achieve higher efficiency. As far as DC-DC converters are concerned, the current industry PFM has only Single Phase and is implemented in the Ripple Mode mode, so the Ripple at the demand output is large. There is no negative inductor current, so the light load efficiency can be improved. Since the output is Ripple, the Transient is very good, and there is no under-shoot when doing Dynamic. PWM has Single Phase & Multi-phase, which is mostly implemented by Voltage Mode or Current Mode. There is no requirement for output Ripple. There is negative current of inductor when light load, so light load efficiency is poor, and Compensation is slower than Ripple. When PWM is used in combination with PFM, Pulse Skipping appears when the negative current of the inductor is detected, and is no longer controlled by the internal clock. At this point, the controller will turn off both h-mos & l-mos, and the Coss & L will exhibit a damped oscillation. The field of contact with each engineer is different. There may be more fields using PFM, and some use PWM more. But from the whole power industry, I believe that it is still used by PWM. Since the 1980s, PWM has started. In the field of power conversion, the status of "ruling domination", because each method has its shortcomings and advantages. The key is to see if it is suitable for customers. In the forum, I saw a netizen writing this way. I think it is more image, he said If the PFM is compared with the PWM power supply, use PFM = Mercedes, use PWM = Volkswagen.

The main advantage of PFM compared to 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 the efficiency of PWM. This is the main advantage of PFM. 2. PWM due to error The influence of the amplifier, the loop gain and the response speed are limited, and the PFM has a faster response speed. The main disadvantage of the PFM compared to the PWM is that the filtering is difficult and the filtering is difficult (the harmonic spectrum is too wide).

2. Peak efficiency Before, the frequency of PFM is lower than the frequency of PWM, which will cause the output ripple to be larger than PWM.

3. PFM control is more expensive than PWM control IC. One of the main reasons why PFM is used without 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.