In-depth 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 UPS battery pack voltage is lower than a certain value of the input AC peak voltage, while the boost type is mainly used when the UPS battery pack voltage is higher than the input AC peak voltage.

　　1. Input power 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. A low input power factor will cause the following adverse effects:

　　(1) The early failure of each link in the input power supply line

　　The reason for the low input power factor is that the input harmonic current component is large. When the harmonic current passes through the input cable, the cable generates additional heat, causing the cable sheath material to heat up for a long time, become soft, brittle, crisp, and broken; when the harmonic current passes through the input circuit breaker (switch), the switch output point causes poor contact due to long-term heating, and a positive feedback effect is the premature aging of the switch; when the harmonic current passes through the input fuse, the fuse becomes soft and droops (making the thickness of the entire fuse uneven) due to long-term additional heating, and naturally breaks, causing power outages.

　　(2) Inability to fully utilize input power

　　Since the input power contains a large amount of reactive components, the active power is absorbed, and the reactive power flows back and forth in the cable, narrowing the normal effective current channel. Due to the "crowding" of the line, the current density per unit cross-sectional area increases, and the power consumption increases. 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 the resistance R of the wire, and the heat generation is a function of the power consumption P and time T, that is,

　　Q=0.24Pt

　　Such a long-term effect results in a waste of electricity.

　　(3) Interference with the power grid

　　When the input circuit is a 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 strong transmitted interference and radiation interference, which affects 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 (typically uncompensated values ​​are 0.6 for single-phase diode rectifiers for low power and 0.8 for three-phase thyristor full-wave rectification - 6-pulse rectification for higher power) can cause the installed power of the front generator to be at least 3 times the rated power of the UPS.

　　2. Power frequency rectifier and high frequency rectifier

　　From the previous discussion, it can be seen that the main reason for the low UPS input power factor lies in 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 UPS battery pack voltage is lower than a certain value of the input AC peak voltage, while the boost type is mainly used when the UPS battery pack voltage is higher than the input AC peak voltage.

　　1. Power frequency step-down rectifier

　　Step-down rectifiers are divided into industrial frequency and high frequency, and industrial frequency can be divided into stabilized and unstabilized voltage. The following discussion takes the most widely used stabilized industrial frequency circuit in UPS as an example. Three-phase rectification is generally used because the pulsation coefficient and ripple coefficient of three-phase rectification are low. A three-phase thyristor full-bridge rectifier circuit uses 6 thyristor rectifiers, which require 6 pulses for separate control, and is also commonly known as 6-pulse rectification. The three-phase full-bridge rectifier circuit works according to the line voltage. When the mains power is rated at 380V/220V, the highest integer output voltage can reach

　　UDC=380V×√2=537V

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

　　For example, if the rated voltage of a battery pack is 384V, and the floating charge voltage is lower than 440V under normal circumstances, if it is believed that even when the rated value of the line voltage Un rises to 135%Un, the rectified voltage can be guaranteed to be lower than 450V, the input voltage (135%Un) at this time can be provided to users as an advantage of the UPS, which will bury hidden dangers for users. Of course, according to the phase control principle, even if the input mains voltage rises to 150%Un, the battery floating charge voltage can be stabilized below 440V under normal circumstances. However, if the thyristor loses control at 135%Un, the thyristor rectifier becomes an ordinary diode rectifier, and the output rectified voltage UDC becomes

　　UDC=380V×1.35×√2=725V

　　At this time, two dangerous situations arise: one is whether the filter capacitor behind the rectifier can withstand such high voltage, otherwise it will explode; the other is that the original 12V battery now has a voltage of UB=725/32=2.6V per battery, which means that the battery is scrapped! It may even bring other dangers, such as sulfuric acid sprayed out by battery explosion, which may injure people and property.

　　On the other hand, since the 6-pulse rectifier circuit works in a pulsed manner, it has a significant destructive effect on the AC input voltage, causing the input current harmonic component to reach more than 30%, and the input power factor to be only about 0.8. In order to achieve the goal of "green" power supply , power factor correction must also be performed.

　　Rectifiers using ordinary diodes do not have the voltage stabilization function. They are generally used in small-power UPS circuits, and the charger is set up separately.

　　2. High frequency buck rectifier

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

　　Working principle of BUCK (step-down) type high frequency step-down rectifier:

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

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