Communication switching power supply technology introduction (1)

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Summary:

The traditional switching power supply has a power factor of only 0.45-0.75, extremely low efficiency, and high harmonic content. In order to improve the power factor of the switching power supply, in the early 1980s, the switching power supply correction concept was proposed abroad and became the initial form of power factor control technology. In the late 1980s and early 1990s, Europe and Japan introduced control standards for the harmonics of switching power supplies. There are currently two existing standards: IEC 555-2 and IEC 1000-3-2. Due to the increasing requirements for power efficiency and electromagnetic compatibility, power factor control technology has become one of the research hotspots of switching power supplies.

1 Basic principles of power factor control

If the input of the rectifier circuit is directly connected to the resistive load, the rectified waveform is a sine wave, the power factor is basically 1, and the harmonic component is very low. However, due to the action of L and C filtering in the actual circuit, the current and voltage cause a phase difference, and the charge and discharge current of the capacitor, the voltage of the inductor, etc. all cause sharp pulses, resulting in the generation of higher harmonics and a significant decrease in power factor. If a primary isolation circuit is inserted between the rectifier circuit and the converter such that the integrated load of the input circuit is close to resistivity, the power factor is expected to be improved.

There are two types of actual power factor control circuits: passive correction circuits and active correction circuits.

(1) Passive correction circuit

Passive correction circuits rely on passive component circuits to improve power factor and reduce current harmonics. The circuit is simple, but it is bulky and is rarely used today.

(2) Active correction circuit

The active correction circuit inserts a converter between the input circuit and the DC-DC converter. The specific control circuit causes the current to follow the voltage and feed back the output voltage to stabilize it, thereby pre-stabilizing the DC-DC converter. This scheme is complicated in circuit, but its volume is significantly reduced, thus becoming the main research direction of PFC technology.

For active PFC technology, a two-stage converter was originally used. The first stage is dedicated to the PFC pre-stage and the second stage is used for DC-DC conversion. Now I am going to study the single-stage converter, that is, to make the relevant parts that can be merged into the same level, which is similar in form to the first-stage converter circuit.

2 power factor control strategy

The power factor control (PFC) circuit plays an important role in improving the power factor of the power electronics network side and reducing the harmonic pollution of the power grid. With the popularity of PFC technology applications, PFC circuit topology has become increasingly mature.

2.1 Classic Control Strategy for PFC Rectifiers

The six basic topologies of power electronic circuits (Buck, Boost, Buck-boost, Flyback, Sepic, Cuk) can in principle constitute PFC, but due to the unique advantages of Boost circuits, they are most commonly used in practice. The PFC control strategy is based on whether the input inductor current is continuous, and the PFC is divided into discontinuous conduction mode (DCM) and continuous conduction mode (CCM). DCM control can be used in a variety of ways such as constant frequency, frequency conversion, and other areas. The CCM mode has direct current control and indirect current control depending on whether the transient inductor current is directly selected as the feedback and controlled amount. Direct current control with peak current control (PCMC), hysteresis current control (HCC), average current control (ACMC), predictive transient current control (PICC), linear peak current control (LPCM), nonlinear carrier control (NLC) Waiting for the way. The control of the current can also be realized indirectly by controlling the voltage at the input of the rectifier bridge, which is called indirect current control or voltage control.

(1) DCM control mode

DCM control, also known as voltage tracking method, is a simple and practical control method in PFC, which is widely used. Features of DCM control mode:

The input current automatically tracks the voltage and maintains a small current distortion rate;

The power tube achieves zero current turn-on (ZCS) and does not withstand the reverse recovery current of the diode;

The input and output current ripple is large, and the filter circuit is required to be high;

The peak current is much higher than the average current, and the device is subjected to a large stress;

Single-phase PFC power is generally less than 200W, and three-phase PFC power is generally less than 10kW.

(2) CCM control mode

The advantages of CCM over DCM are:

Input and output current ripple is small, THD and EMI are small, and filtering is easy;

The RMS current is small and the device conduction loss is small;

Suitable for high power applications.

There are two methods of direct current control and indirect current control in CCM mode. The advantage of direct current control is that the current transient characteristics are good, and it has overcurrent protection capability, but it needs to detect transient current and the control circuit is complicated. The advantages of indirect current control are simple structure and clear switching mechanism.

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