Conversion of DC power supply to low-voltage DC stable power supply

The DC power supply is an important part of Electronic equipment, and the DC power supply circuit is one of the basic circuits of electronic equipment. Its function is to convert the mains (220V or 380V AC voltage) into a low-voltage DC stable power supply required by the internal components of the electronic circuit.

The general requirements for the power supply circuit are: high stability (that is, the load regulation rate, the input voltage regulation rate is small, and the ripple coefficient is small), high efficiency, small size, low cost, and complete protection functions (overcurrent protection, overheating protection, output overvoltage protection, input undervoltage protection, etc.), good stability, can work continuously for a long time.

According to the working principle of the power supply circuit, the power supply circuit can be divided into two categories: linear regulator and switching regulator. It is roughly composed of power transformer (also known as power frequency transformer, switching voltage regulator circuit generally does not need power frequency transformer), rectifier, filter, voltage regulator circuit and other parts, as shown in Figure 1.

Conversion of DC power supply to low-voltage DC stable power supply

In the linear regulated power supply circuit, the power frequency transformer plays the dual role of voltage transformation (converting the 220V AC voltage into AC low voltage of several volts, a dozen volts, and tens of volts) and isolation, so that the circuit after the transformer secondary and the power grid can be realized. Electrically isolated, becoming an uncharged “cold plate”. Due to the low frequency of the mains (50Hz or 60Hz), the power frequency transformer has many primary windings, the parasitic resistance of the windings is large, and the copper loss is high; on the other hand, the magnetic core material composed of silicon steel sheets has low resistivity and large eddy current loss. That is, the efficiency of the power frequency transformer is not high. In addition, there are disadvantages of large volume, heavy weight and high cost. The volume and weight of the power frequency transformer seriously restrict the miniaturization and miniaturization of the linear regulated power supply.

The rectifier circuit is generally composed of diodes. The unidirectional conductivity of the diode is used to convert the AC voltage into a unidirectional pulsating DC voltage. Figure 1 shows the output waveform of the commonly used half-wave rectifier and full-wave rectifier (including bridge rectifier) ​​circuits.

In order to make the pulsation of the input voltage of the voltage regulator circuit as small as possible, the rectified output voltage is low-pass filtered by a passive low-pass filter composed of a capacitor or a capacitor-Inductor. It can be seen from Figure 1 that after filtering, the output voltage pulsation has been greatly reduced. Some circuits (such as audio power amplifier stages) that do not require high power supply stability can directly use the filtered voltage as the power supply voltage.

Although the pulsation of the filtered DC voltage is low, it cannot be used as a working power supply for circuits that are more sensitive to power supply stability, such as AD conversion circuits, weak signal amplifier circuits, TTL logic circuits, etc. It must also be stabilized by a voltage regulator circuit. A high-stability DC power supply that is not affected by factors such as grid voltage fluctuations, load fluctuations, and temperature fluctuations can be obtained after the voltage is applied.

Rectifier circuit

The so-called “rectifier” circuit uses the unidirectional conductivity of diodes to convert alternating current into one-way pulsating direct current. Commonly used rectifier circuits include half-wave rectifier, full-wave rectifier and bridge rectifier. The circuit structure and waveform are shown in Figure 2.

Working Principle and Output Voltage Waveform

The half-wave rectifier circuit is the simplest, using only one diode. In the positive half cycle of 2u, the diode D is turned on. If the voltage drop on the diode D is ignored, the waveform of the voltage ou on the pure resistive load LR is the same as that of 2u; while in the negative half cycle of 2u, the diode D is reverse biased and is in the cut-off state. The diode has reverse leakage current, no current flows through the load resistor LR, and the output voltage ou is zero. The output waveform of the half-wave rectifier circuit is shown in Figure 1. It can be seen that the output voltage of the half-wave rectifier has a large fluctuation. The secondary coil of the transformer only works in half a cycle, the utilization rate is low, and it is only used in power supply circuits with small output power (in power frequency circuits, ordinary rectifier diodes are used; in low-voltage switching power supply circuits, low on-resistance is generally used. schottky diode).

In a full-wave rectifier circuit, it consists of a secondary transformer with a center tap and two diodes. In the positive half cycle of 2u, diode D1 is turned on and D2 is turned off. If the voltage drop on diode D1 is ignored, the polarity of the output voltage ou obtained on the pure resistive load LR is positive and negative, the waveform is the same as that of 2u, and the current direction is shown in the figure (b ) is shown by the solid line in ); and in the negative half cycle of 2u, the diode D2 is turned on and D1 is turned off. If the voltage drop on the diode D2 is ignored, the polarity of the output voltage ou obtained on the pure resistive load LR is also positive and negative. The waveform is also the same as 2u, and the current direction is shown by the dotted line in Figure 2.

Conversion of DC power supply to low-voltage DC stable power supply

The output waveform of the full-wave rectifier circuit is shown in Figure 2. It can be seen that the output voltage ripple of the full-wave rectifier is smaller than that of the half-wave rectifier. Although full-wave rectification can supply current to the load in both positive and negative half cycles of 2u, the two secondary coils of the transformer are always in an alternate working state, and the utilization rate is not high (same as half-wave rectification).

Since the pulsating current only flows through one diode, the loss is small, and it has been widely used in low-voltage and high-current rectifier circuits, such as high-frequency low-voltage, high-current rectifier circuits of switching power supplies. In order to further reduce the power consumption of the rectifier diode, in the low-voltage and high-current rectifier circuit, a Schottky diode with a smaller on-voltage is used as the rectifier diode; when the voltage drop of the Schottky diode is too large, the synchronous rectification method can be used. In the synchronous rectifier circuit, a MOS tube with an on-resistance of mΩ is used as a rectifier tube.

The bridge rectifier circuit consists of four diodes. In the positive half cycle of 2u, D2 and D4 are turned off, D1 and D3 are turned on, and the polarity of the output voltage ou is positive and negative. When ignoring the voltage drop on the diodes D1 and D3, The waveform of ou is the same as that of 2u, and the current direction is shown by the solid line in Figure (c). In the negative half cycle of 2u, D1 and D3 are turned off, D2 and D4 are turned on, and the polarity of the output voltage ou is also positive and negative. When ignoring the voltage drop across diodes D2 and D4, the ou waveform is also the same as 2u, and the current direction is shown by the dotted line in Figure (c). The waveform of the output voltage ou of the bridge rectifier circuit is the same as that of the full-wave rectifier; in the positive and negative half cycles of 2u, the secondary of the transformer is in the working state, and the utilization rate is high, that is, the rectifier circuit of the same output power, the bridge rectifier circuit needs the work The frequency transformer is the smallest in size, so it is widely used in power supply circuits (common rectifier diodes can be used when rectifying power frequency voltage; fast recovery diodes with high operating frequency can be used when rectifying high-frequency signals).

However, since the unidirectional pulsating current has to flow through two diodes, the loss of the diode is larger than that of the full-wave rectifier circuit, so it is not suitable for the low-voltage high-current rectifier circuit whose output voltage is only a few volts.

Figure 3 Input/Output Waveforms

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