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How UPS Power Rectifiers Work
- May 09, 2018 -

Modern rectifier chargers are divided into step-down and step-up types. The step-down type is mainly used when the voltage of the UPS battery pack is lower than the input AC peak voltage. The boost type is mainly used when the UPS battery voltage is higher than Enter the peak AC voltage.

First, input distribution system

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

(1) Lead to the early aging of all aspects of the input power supply line

The reason for the low input power factor is the large content of the input harmonic current components. When the harmonic current passes through the input cable, it causes the cable to generate additional heat, which leads to long-term heating, softening, brittleness, flaking, and breaking of the cable sheath material; When the wave current passes through the input circuit breaker (switch), the point of the switch will cause poor contact due to long-term heat generation. The effect of a positive feedback is that the switch is prematurely aged. When the harmonic current passes through the input fuse, the fuse is caused due to the long-term additional heat generation. Softening, drooping (causing the entire fuse to become uneven), and a natural breakage that causes a power outage.

(2) Cannot make full use of input power

Since the input power contains a large number 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 “congestion” of the line, the current density per unit sectional area is increased. Large, 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 equation that the power consumption on the line is proportional to the square of the current I, proportional to the resistance R of the wire, and the heat generation is a function of the power P and the time T, ie (Q=0.24 Pt). Such a long-term effect has caused the waste of electricity.

(3) Interference to the power supply network

When the input circuit is a thyristor (thyristor) rectifier, the thyristor opening is often accompanied by high voltage and high current, which not only destroys the input voltage waveform, but also forms a strong 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 (usually uncompensated value is 0.6 for single-phase diode rectifier for power, and full-wave three-phase thyristor full-wave rectifier for high power - 0.8 for 6-pulse rectification), which can result in the installation of a front-generator At least 3 times the power rating of the UPS.

Second, industrial frequency rectifier and high-frequency rectifier

From the foregoing discussion, it can be seen that the main reason for the low UPS input power factor lies in the circuit structure and working method of the input section. Modern rectifier chargers are divided into step-down and step-up types. The step-down type is mainly used when the voltage of the UPS battery pack is lower than the input AC peak voltage. The boost type is mainly used when the UPS battery voltage is higher than Enter the peak AC voltage.

1. Power frequency step-down rectifier

The buck rectifier has the power frequency and the high frequency, and the power frequency has the voltage regulator and the voltage regulator. The following section discusses the most widely used voltage regulator power frequency circuit in UPS. Three-phase rectification is generally used because the three-phase rectification has a low ripple factor and ripple factor. A three-phase thyristor full-bridge rectification circuit uses six thyristor rectifiers, which require six pulses to control each other, also known as six-pulse rectification. The three-phase full-bridge rectifier circuit works according to the line voltage, and the highest integer outflow voltage can be achieved when the commercial power is rated at 380V/220V (UDC=380V×√2=537V)

A typical battery pack rated voltage of 12V x 32 = 384V float voltage (approximately 438V) is sufficient. Because this kind of circuit works according to the frequency of the mains (so-called industrial frequency), it becomes an industrial frequency rectifier. Since the current capacity and the withstand voltage of the thyristor can be made very high, it has been widely used in medium and high power conventional double conversion UPS. And because the opening (phase) of this circuit rectifying device is controllable, it has the function of output voltage regulation. However, this function of output voltage regulation cannot be used as a basis for inputting large-scale changes in utility power. The reason is that thyristor has the potential to run out of control under certain conditions.

For example, a battery rated voltage of 384V, under normal circumstances the float voltage is less than 440V, if you think that in time is the voltage rating of the line Un rise to 135% Un can also ensure that the rectified voltage is lower than 450V, you can put this time The input voltage (135% Un) is provided to the user as an advantage of changing the UPS, and it will cause hidden dangers for the user's use. Of course, according to the principle of phased control, even if the input mains voltage rises to 150% Un, the battery float voltage can be stabilized under the normal condition below 440V. However, if the SCR is out of control at 135% Un, then 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 situation is: the filter capacitor behind the rectifier can withstand this high pressure, otherwise it will be no doubt fried; the other is: the original 12V section of the battery, now becomes the voltage of each section UB = 725/32 = 2.6V, which means that the battery is also scrapped! There are even other dangers, such as sulphuric acid and injuries caused by blown batteries.

On the other hand, since the work of the 6-pulse rectifier circuit is pulsed, the destructive effect on the input voltage of the mains is very significant, so that the harmonic content of the input current is more than 30%, and the input power factor is only about 0.8, in order to achieve The goal of the "green" power source must also be power factor correction. A rectifier using an ordinary diode does not have a voltage regulator function, and it is generally used in a low-power UPS circuit, and the charger is additionally provided.

2. High frequency step-down rectifier

In the general low-power UPS power supply, a diode rectifier is used to simplify the complexity of the circuit, but the diode rectifier has no voltage regulation function. In order to ensure the safety of the filter capacitor and the inverter, some use a BUCK type high-frequency step-down rectifier. BUCK (Buck) type high-frequency buck rectifier working principle: the control signal with high frequency pulse (general type fixed pulse width of 20kHz).

Add to the control pole of the switch power tube, when a control pulse arrives, VT opens, the current flows from the rectifier diode through the VT to the load and filter capacitor, which is the inductor L energy storage; after the control signal ends, VT ends, and the inductor L generates The back EMF maintains the original current flow and will release the village. 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. This circuit is a bit simple, the current to the load is continuous, but the input current is still pulsating.