“Switching power supplies are widely used in aerospace, home appliances, communications and other fields due to their small size and high energy utilization. What are the common working modes of switching power supplies? What are the characteristics of working in this mode? This article will explain to you two common modes: CCM and DCM.
Switching power supplies are widely used in aerospace, home appliances, communications and other fields due to their small size and high energy utilization. What are the common working modes of switching power supplies? What are the characteristics of working in this mode? This article will explain to you two common modes: CCM and DCM.
1. Definition of CCM and DCM
1. CCM (Continuous Conduction Mode): In a switching cycle, the Inductor current never reaches 0. In layman’s terms, the inductor “never resets”, which means that the magnetic flux of the inductor is never 0 during the switching cycle. After the power tube is closed, there is still current flowing in the inductor.
2. DCM (Discontinuous Conduction Mode) discontinuous conduction mode or discontinuous conduction mode: in one switching cycle, the inductor current will always reach 0. In layman’s terms, the inductor is “properly reset”, and after the power tube is closed, the current in the inductor is 0.
2. CCM working mode and characteristics
1. According to the characteristics of CCM, measure the waveform of step-down transformer working in continuous mode. As shown in Figure 1 below.
Waveform 1 represents the waveform of the PWM, which triggers the switch tube into on and off states. When the switch SW is turned on, the voltage on the common point SW/D is Vin. When the switch tube SW is turned off, the voltage on the common point SW/D is a negative voltage, and the inductor current will generate a bias current to the diode D, resulting in a negative voltage drop – realizing the effect of freewheeling.
Waveform 3 depicts the change in voltage across the inductor. At the equilibrium point, the average voltage across the inductor L is 0, that is, S1+S2=0. The area of S1 corresponds to the product of voltage and time when the switch is turned on, and the area of S2 corresponds to the product of voltage and time when the switch is turned off. S1 simply uses the matrix height (Vin-VOUT) multiplied by DTSWS2 can also be simply used with matrix height -VoutMultiply by (1-D)Tsw . If S1 and S2 are summed, then the whole period TswInner average, we get:
Simplifying the above formula can get the step-down DC transfer function of CCM:
From the above equation, it can be seen that VoutIt varies with D (duty cycle).
Summarize the features of CCM step-down transformers:
l D is limited to less than 1, and the output voltage of the step-down transformer must be less than the input voltage.
l By changing the duty cycle D, the output voltage can be controlled.
l Working in CCM mode will bring additional losses. Because the reverse recovery charge of the freewheeling diode takes time to consume, this is an additional loss burden for the power switch.
l The output has no pulse ripple, but there is a pulse input current.
3. DCM working mode and characteristics
1. The switching devices work in CCM mode when the load current is large, but when the load current decreases, the ripple current will decrease as a whole. When the inductor current continues to decrease, the inductor will enter the DCM operation mode, and the voltage and current waveforms will change greatly as shown in Figure 2.
As can be seen in waveform 4, the inductor current drops to 0, causing the freewheeling diode to turn off. If this happens, the left side of the inductor is open. In theory, the voltage on the left side of the inductor should go back to Vout, because the inductor L no longer has current and does not oscillate. But due to the existence of a lot of parasitic capacitance around. Such as the parasitic capacitance of diodes and SW, it is easy to form an oscillation circuit.
For the buck regulator, there is no problem with the inductor going into discontinuous mode. Before entering discontinuous mode, the DC output voltage Vout=VinTon/T. It can be clearly seen from this formula that it has nothing to do with the current parameter, so when the load changes, there is no need to adjust the duty cycle, and the D output voltage remains constant.
After entering the DCM work, the transfer function will change, and the transfer function of the CCM will no longer apply. The conduction time of the switch tube decreases with the decrease of the DC current. The following is the transfer function in DCM mode, the duty cycle is related to the load current, namely:
Because the control loop wants to control the output voltage to be constant, the load resistance R is inversely proportional to the load current. Suppose VoutVin,L, T are constant, in order for the control voltage to be constant, the duty cycle must vary with the load current.
At the critical switching current, the transfer function changes from CCM to DCM. When working in CCM, the duty cycle remains constant and does not change with the load current; when working in DCM, the duty cycle changes as the load current decreases.
Summarize the characteristics of DCM step-down transformers:
l VOUT depends on the load current;
l For the duty cycle, the transfer coefficient under DCM is easier to reach 1 in deep DCM than when CCM operates at a low load current.
4. Comparison of CCM and DCM
l DCM mode has low power consumption and high DCM conversion efficiency, which belongs to complete energy conversion;
l Working in DCM mode, the output current ripple is larger than that of CCM;
l Working in DCM mode, when the inductor current is 0, oscillation will occur;
l Working in CCM mode, the output voltage has nothing to do with the load current. When working in DCM mode, the output voltage is affected by the load. In order to control the voltage to be constant, the duty cycle must change with the load current.
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