Switching power supply Buck circuit CCM and DCM working mode

1. Continuous Conduction Mode (CCM): In this mode, the inductor current never reaches zero during one switching cycle. The inductor flux doesn’t return to zero while the power switch is closed, meaning there is always some current flowing through the coil.
2. Discontinuous Conduction Mode (DCM): This mode is characterized by the inductor current reaching zero during the switching cycle. The inductor is properly “reset” as the power switch closes, resulting in zero current flowing through the inductor.
3. Boundary Conduction Mode (BCM): In this mode, the controller monitors the inductor current. When the current is detected as zero, the power switch is immediately closed. The controller constantly waits for the inductor current to reset before activating the switch. If the inductor current has a high value and a relatively flat cut-off slope, the switching period is extended. Hence, the BCM variator operates as a variable frequency system.

Figure 2 Three modes of inductor operation: CCM/DCM/BCM The midpoint amplitude of the current ramp is equal to the average value of the DC output current Io, and the difference between the peak current Ip and the valley current Iv is the ripple current. CCM working mode and characteristics According to the definition of CCM, the waveform of the step-down converter working in continuous mode is tested.

In the PWM pattern, waveform 1 represents the switching on and off of the switch (SW). When the switch is turned on, the common point voltage (SW/D) is Vin. Conversely, when the switch is turned off, the voltage at the common point swings to a negative value. In this state, the inductor current provides a bias current to diode D, resulting in a negative buck-freewheeling effect.

Waveform 3 illustrates the voltage change across the inductor. At the equilibrium point, the average voltage across the inductor (L) is 0, and the sum of the areas S1 and S2 is 0. The S1 area corresponds to the product of the voltage (Vin-Vout) and the time when the switch is on, while the S2 area corresponds to the product of the voltage (Vin-Vout) and the time when the switch is off. S1 can be calculated as the rectangle height (Vin-Vout) times D, and S2 can be calculated as the rectangle height (Vin-Vout) times (1-D)Tsw. By summing S1 and S2 and then averaging them over the period Tsw, we obtain the overall result.

The above formula can be simplified to the step-down DC transfer function of CCM: It can be seen from the above formula that Vout varies with D (duty cycle). In fact, let’s look at the last waveform above. When the switch is closed, the current waveform at point SW/D has a large peak. The voltage waveforms actually measured by the voltage chip ACT4065 and ACT4065A are shown in Figure 4 and Figure 5. The specific reasons are as follows.

First, because Vin is applied to the cathode of the diode when the switch is closed, abruptly interrupting the conduction period of the diode. For PN diodes, it is first necessary to change the PN junction back to the PN junction when it is electrically neutral during forward conduction, and remove all minority carriers. It takes a certain amount of time for the diode to remove all the injected charge to return to its off state, and before fully recovering, it exhibits short-circuit behavior. For Schottky diodes, there is a metal-semiconductor-silicon junction, which has no recovery effect, however, there is a large parasitic capacitance, and there is also a junction capacitance.

Also Read: Correct selection of capacitors in DC-DC converters

Square wave: It is composed of odd harmonics of sine waves, that is, composed of sine 1, 3, 5, 7…n and other frequencies. There is also a peak at the moment when the switch is turned off. I think it should also be related to the parasitic capacitance and junction capacitance of the diode and SW pin. Through the above, the characteristics of the CCM step-down converter can be summarized:1) D is limited to less than 1, and the output voltage of the buck converter is always less than the input voltage;2) If various ohmic losses are ignored, the conversion coefficient M has nothing to do with the load current;3) By changing the duty ratio D, the output voltage can be controlled;4) The step-down converter works in CCM, which will bring additional loss. Because the reverse recovery charge of the freewheeling diode takes time to consume, which is an additional loss burden for the power switch tube;5) There is no pulse ripple in the output, but there is pulse input current.

DCM working mode and related features

When the load current of switching devices is large, they operate in Continuous Conduction Mode (CCM). However, as the load current decreases, the overall ripple current decreases. In Figure 2, when the load current drops to half of the peak-to-peak value of the harmonic current (Io = (Ip – Iv)/2), the lowest point on the slope reaches zero. At this point, the inductor current becomes zero, and the energy stored in the inductor is also zero. If the load current in the inductor further decreases, it enters Discontinuous Conduction Mode (DCM). Here, both voltage and current waveforms undergo significant changes, as shown in Figure 6, resulting in a significant change in the transfer function.

In waveform 4, it can be observed that the inductor current reaches zero, causing the freewheeling diode to turn off. In theory, the voltage at the left end of the inductor should return to Vout because there is no longer any current flowing through the inductor and thus no oscillation occurs. However, due to the presence of parasitic capacitances (such as those associated with diodes and SW) in the circuit, an oscillation loop is formed. This leads to the appearance and disappearance of a sinusoidal signal after a few cycles, as shown by curve 2 and curve 3. The existence and behavior of these oscillations may vary in actual tests. For example, during the test of ACT4065A, I analyzed the SW/D waveform and observed oscillation occurring in the DCM mode.

Buck transformers are designed to maintain a constant output voltage throughout the entire load range, even if the inductor operates in Discontinuous Conduction Mode. As a result, it is easy to mistakenly assume that the inductor’s transition to Discontinuous Conduction Mode has no impact on the circuit’s operation. However, in reality, the transfer function of the entire circuit undergoes changes, and the control loop must adapt to these changes.

For Buck regulators, there is no problem with inductors entering discontinuous mode of operation. Before entering the discontinuous mode, the DC output voltage Vout=Vin•Ton/T. Note that this formula has nothing to do with the load current parameters, so when the load changes, there is no need to adjust the duty ratio D, and the output voltage remains constant. In fact, when the output current changes, the conduction time will also change slightly, because the conduction voltage drop of Q1 and the inductor resistance will change slightly with the change of current, which requires proper adjustment of Ton. After entering the DCM work, the transfer function will change, and the CCM transfer function will no longer be applicable, and the conduction time of the switch will decrease with the decrease of the DC output current. The following is the transfer function in DCM mode, the duty cycle is related to the load current, namely:

Because the control loop needs to control the output voltage to be constant, the load resistance R is inversely proportional to the load current. Assuming Vout, Vin, L, T are constant, in order to control the voltage constant, the duty cycle must change with the change of the load current. Current Logic is the biggest manufacturer of DC-DC Convertors in China.