Introduction To Linear Power Supply

The topology of a conventional linear power supply, as shown figure 1.1, uses a low -frequency transformer comprising a core made of silicon steel sheet to convert AC mains to a desired voltage, and rectifies and filters the voltage to a DC level. Since general equipments require stable DC voltage, the rectified and filtered DC level should be regulated through a power regulator. The power regulator clamps excessive voltage at the predetermined level and dissipates unwanted voltage in the form of heat. In order not to greatly reduce the efficiency, linear power supplies should put a limit to the variable range of the AC input voltage. The concept of linear power supplies is to clamp the unwanted voltage along with ripple voltage and dissipate them in the form of heat, which result in quite small ripple voltage. Due to linear topology, there will be no problems of ripple noise and EMI in linear power supplies.

Figure 1.2 shows a typical linear power supply with the main features indicated: (A) transformer comprising a core made of silicon steel sheet, (B) the big input capacitor, and (C) the big transistor with its heat sink.


Introduction to Switching Power Supply (S.P.S.)

(1)Switching power supplies mainly improve the weaknesses such as the huge volume,

the heavy weight, and the low efficiency in linear power supplies. The topology of a switching power supply, as shown in figure 1.3, rectifies and filters the voltage directly from AC mains (off-line) without using a low-frequency transformer. The capacitance of the bulk capacitor can be quite small due to the high rectified voltage.

Switching power supply uses a high-frequency transistor to chop the high DC voltage (rectified from AC source) into high AC voltage and converts it into a desired voltage by a high-frequency transformer, and then rectifies and filters the voltage to a DC level. The high-frequency switching of the transistor and diodes results in generation of ripple noises at the transient of switching.

(2) The block diagrams along with their description in a switching power supply are as follows: 


(a)  EMI Filter: The built-in filter reduces the EMI generated from the S.P.S.

(b)  Inrush Current Suppressor: Inrush current occurs at the time of power-on of the

S.P.S. If the inrush current is too large, the S.P.S. might be damaged, and the power distribution system could be adversely influenced. Thus, an inrush current suppressor is installed inside each S.P.S. to limit the high current.

(c)  Input Rectifier: It rectifies the AC input voltage to a DC level.

(d)  Power Factor Correction (PFC): The active/passive PFC circuit upgrades the power factor (PF) figure and approximates the waveform of input current to a sine wave, so as to be compliant to the harmonic current regulation and reduce reactive power.

(e)  Converter: It converts the DC voltage to high-frequency signal, and the signal is increased or decreased by a high-frequency transformer.

(f) Output Rectifier: It rectifies the DC pulses to a steady DC level.

(g) Feedback Control: It regulates the output voltage to a steady level to respond to variation of loads and AC input voltage.

(h) Protection Circuit: When in abnormal situations, the S.P.S. will shut down or go into other protection modes by activating protection circuits. The protection circuits consist of over voltage, over temperature and over current (overload) protections, and etc.

Comparison Between Switching Power Supply and Linear Power Supply 

(1) Compared to linear power supplies, switching power supplies possess the advantages of high efficiency and light weight. The advantages correspond to the contemporary demands of being light and handy, and energy saving. Thus S.P.S. has gained much popularity in field applications.

(2) The following table is the comparison between S.P.S. and linear power supply:

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