CCM BUCK and DCM BUCK Circuits

Last time, I disassembled a power supply using BUCK PFC. The BUCK worked in DCM mode. I don’t understand why DCM BUCK was used instead of CCM BUCK. Judging from the materials used, the cost should be ignored. So it should be a trade-off between efficiency, PF value, power density, temperature rise, EMC, etc., right?

Personal opinion:

Compared with CCM BUCK, DCM BUCK can reduce the number of inductor turns and the current stress of the switch tube, but it needs to increase the inductor wire diameter, the working peak current will double, the effective current will also be larger, and the current stress of the output diode will be larger, which will lead to increased temperature rise and increased difficulty in EMC treatment.

It is difficult to judge the impact on power density, PF value and efficiency. I look forward to everyone's discussion.

Personal understanding:

This should be the same as BCM-BOOST PFC. The efficiency in BCM (DCM) mode is higher than that in CCM mode. Assuming the output power is not high, the current stress of each component is not large. Even for BUCK-PFC, its output voltage is still more than 100V. The selection of power semiconductor devices in CCM mode should be much more difficult than in BCM mode. There is no specific analysis or calculation, and it only represents a "temporary view". What can be determined is that BUCK-PFC's contribution to transformer design is unparalleled (especially flyback).

The output voltage of BUCK-PFC is generally lower than 100V, because the volume of capacitors below 100V is relatively much smaller, and the RON of the MOS tube is also much smaller, which can be said to be a hurdle.

The current stress of MOS tube and diode is almost the same, but the peak current of inductor is more than 1 times larger.

Is DCM mode more efficient than CCM? Is it true? Why?

According to the BUCK principle, the greater the step-down ratio, the lower the efficiency. So 220VAC input, +HVDC=300V,

The BUCK voltage should not be too low. Personal opinion:

1. BCM mode can use a smaller transformer (magnetic core);

2. The air gap of the BCM mode transformer is very small, and the edge flux loss is small (perhaps the radiation will be improved);

3. In BCM mode, there is almost no reverse recovery loss in the semiconductor (which also improves EMI);

4. In BCM mode, the current stress is indeed large, but the advantages and disadvantages are obvious after comprehensive comparison (low power).

The one I disassembled last time was a 90W one, with full voltage input, 84V buck output, and an efficiency of 94%~95%.

The DCM inductor should have a larger air gap, because the inductance is small, the DI/DT is relatively large, and the EMC should be worse, right?

In BCM mode, there is no DC flux in the core and theoretically no air gap is required.

① The applied volt-second value, number of turns, and core area determine the alternating magnetic flux;

VTon(n) + Np + Ae → △B

②The DC average current value, number of turns, and magnetic circuit length determine the DC magnetic field strength;

Idc + Np + Le(lg) → Hdc

It can be seen from calculation that in BCM mode, the number of core turns is very small.

If the current is too large, EMI will indeed become worse, but if there is no diode reverse recovery, EMI will also be better.

Common mode, differential mode? Conduction, radiation? Haha, I haven’t analyzed it.

Regarding EMI, DCM has high peak value, large switching noise, and high energy in the low frequency band; CCM has diode reverse recovery current, which can generate high-frequency noise, and high energy in the high frequency band. Qualitative consideration: When the input voltage, duty cycle, output power, and frequency are the same, the input port filtering part DCM requires a larger X capacitor or differential mode inductor, and CCM requires a larger common mode inductor; CCM also requires diode absorption.

Regarding power devices, DCM requires larger filter capacitors, FETs and diodes with larger peak currents; CCM requires larger inductors and diodes with faster recovery.