Analysis of the working principle of UPS power rectifier

Modern rectifier chargers are divided into two types: buck type and boost type. The buck type is mainly used when the voltage of the UPS battery pack is lower than a certain value of the input AC peak voltage, and the boost type is mainly used for the voltage of the UPS battery pack Input AC peak voltage.

1. Input distribution system

In the UPS power supply system of the data center, one of the most important indicators of the input circuit is the input power factor. Low input power factor will cause the following adverse effects:

(1) Early old paintings that lead to various links on the input power supply line

The reason for the low input power factor is the large content of input harmonic current components. When the harmonic current passes through the input cable, the cable generates additional heat, which causes the cable sheath material to heat up, soften, become brittle, crisp, and shatter for a long time; When the wave current passes through the input circuit breaker (switch), the outlet of the switch causes poor contact due to long-term heating. A positive feedback effect is the premature aging of the switch; when the harmonic current passes through the input fuse, the fuse is caused by long-term additional heating It becomes soft, sags (makes the thickness of the entire fuse uneven), and naturally breaks to cause a power outage.

(2) The input power cannot be fully utilized

Because the input power contains a lot of reactive components, the active power is absorbed, and the reactive power flows back and forth in the cable, which narrows the normal effective current channel, and the current density of the unit cross-sectional area is increased due to the congestion of the line. Increased power consumption. According to Ohm's law. The power consumption P on the wire is

P = I2R

It can be seen from the above formula that the power consumption on the line is proportional to the square value of the current I and proportional to the resistance R of the wire, and the amount of heat generation is a function of the power consumption P and time T, that is

Q = 0.24Pt

Such a long-term effect has caused a waste of electricity.

(3) Interference to the power supply grid

When the input circuit is a thyristor (thyristor) rectifier, the opening of the thyristor is often accompanied by high voltage and large current, which not only destroys the input voltage waveform, but also forms a strong transmitted interference and radiated interference. It should be the normal operation of other electrical equipment on the same line.

(4) Increase the installed power of the front generator several times

Low input power factor (generally the uncompensated value is 0.6 for single-phase diode rectifiers for power, 0.8 for three-phase thyristor full-wave rectification and 6 pulse rectification for larger power), which can result in at least 3 times the rated power of the UPS.

2. Industrial frequency rectifier and high frequency rectifier

It can be seen from the previous discussion that the main reason for the low input power factor of the UPS is the circuit structure and working mode of the input part. Modern rectifier chargers are divided into two types: buck type and boost type. The buck type is mainly used when the voltage of the UPS battery pack is lower than a certain value of the input AC peak voltage, and the boost type is mainly used for the voltage of the UPS battery pack. Input AC peak voltage.

Power frequency step-down rectifier

The buck rectifier is divided into power frequency and high frequency, and the power frequency is divided into regulated and unregulated. The following discusses the most widely used voltage-stabilized power frequency circuit in UPS as an example. Generally, three-phase rectification is used because the pulsation coefficient and ripple coefficient of three-phase rectification are both low. Six thyristors are used in a three-phase thyristor full-bridge rectifier circuit, which requires 6 pulses to be controlled separately. It is also commonly known as 6-pulse rectifier. The three-phase full-bridge rectifier circuit works according to the line voltage, and the highest integer outflow voltage can be reached when the commercial power is rated 380V / 220V

UDC = 380V × √2 = 537V

Generally, the rated voltage of the battery pack is 12V × 32 = 384V float voltage (about 438V) is enough. Because this circuit works according to the rhythm of the frequency of the commercial power (so-called power frequency), it becomes a power frequency rectifier. Because the current capacity and the withstand voltage of the thyristor can be made very high, it has been widely used in the traditional double-conversion UPS of medium and high power. And because the opening (phase) of this circuit rectifier device is controllable, it has the function of output voltage regulation. However, this function of output voltage regulation cannot be used as the basis for a wide range of input mains changes, because the SCR has the hidden danger of losing control under certain conditions.

For example, a battery pack has a rated voltage of 384V, and the float voltage under normal conditions is lower than 440V. If it is considered that the wire voltage rating Un rises to 135% Un in time, the rectified voltage can be guaranteed to be lower than 450V. The input voltage (135% Un) is provided to the user as the advantage of changing the UPS, which will bury hidden dangers for the user's use. Of course, according to the principle of phase control, even if the input mains voltage rises to 150% Un, the float voltage of the battery can be stabilized below 440V under normal conditions, but in case the thyristor is out of control at 135% Un, it can be The silicon controlled rectifier becomes an ordinary diode rectifier, and the output rectified voltage UDC at this time becomes

UDC = 380V × 1.35 × √2 = 725V

At this time, there are two dangerous situations: one is whether the filter capacitor behind the rectifier can withstand this high voltage, otherwise it will definitely be exploded; the other is that the original 12V battery is now changed to each voltage UB = 725/32 = 2.6V, which means that the battery is also scrapped! It may even bring about other dangers, such as sulfuric acid sprayed by the battery bursting to hurt people and injuries.

On the other hand, because the operation of the 6-pulse rectifier circuit is pulsed, the destructive effect on the mains input voltage Boxing is very significant. The input current harmonic component reaches more than 30%, and the input power factor is only about 0.8. The goal of the green power supply must also be power factor correction.

Rectifiers using ordinary diodes do not have a voltage stabilizing function. It is generally used in low-power UPS circuits, and the charger is provided separately.

2. High frequency buck rectifier

In general low-power UPS power supplies, diode rectifiers are used in order to simplify the complexity of the circuit, but diode rectifiers have no voltage regulation function. For the safety of filter capacitors and inverters, some use BUCK type high-frequency step-down rectifiers.

Working principle of BUCK (buck) high-frequency buck rectifier:

The control signal is pulsed at high frequency (generally fixed pulse width of 20kHz)

Added to the control pole of the switching power tube, when a control pulse arrives, VT is turned on, and the current flows from the rectifier diode to the load and the filter capacitor through VT. This is the energy storage of the inductor L; after the control signal ends, VT ends and the inductor L produces The back EMF continues to maintain the original current flow to release the village energy, and its path is: Lb → C, R → VD → La, so that the input forms a continuous current. After the energy in the inductor is released or reaches a certain level, the power tube is opened by the next trigger pulse, and the above process is repeated. The point of this circuit is simple. The current sent to the load is continuous, but the input current is still pulsating.