Switching devices for UPS should be selected based on the following factors: switching frequency, switching capacity, switching voltage and current, etc. The switching frequency should be considered first. Figure 1 shows the capacity range and operating frequency of various switching devices. When making a specific selection, you should first determine the switching frequency of the device according to the operating frequency of the rectifier and inverter in the UPS, and then select the appropriate voltage and current level according to the capacity of the UPS, the voltage of the DC link, the output current of the inverter, etc. The switching device.
For large and medium-sized UPS, there are mainly two switching devices, one is SCR and the other is IGBT. SCR is mainly used in UPS’s multi-phase phase-controlled rectifier, static bypass switch, non-contact static input/output In the switch, the working frequency is 50Hz, and the working voltage is 220/380V. When the frequency and voltage are known, it should be noted that the capacity of the SCR should be selected according to the capacity of the UPS. The voltage of the ordinary SCR has reached more than 6000V, and the current has reached several thousand amperes. Its own forward voltage drop is 1.5-3V, working frequency is not more than 400Hz (there is a fast SCR with short switching time, its working frequency can reach 400Hz, the reverse recovery current is small, and the off time is less than 10μs), it has been able to Meet the needs of large and medium capacity UPS. IGBT is mainly used in UPS’s three-phase high-frequency SPWM switching rectifier (PFC) and three-phase half-bridge SPWM high-frequency inverter. Its operating frequency is 5-20kHz, and its operating voltage is 800V DC. When its frequency and voltage are known, it should be noted that its capacity should be selected according to the capacity of the UPS. The voltage of the ordinary IGBT has reached 1200V, the maximum current is 1000A, the operating frequency can be as high as 100kHz, and its own forward voltage drop is about 3V. The IGBT produced by a certain company has a maximum voltage of 6500V and a maximum current of 2500A. To meet the needs of large and medium-capacity UPS, plus the IGBT drive is relatively simple, is a voltage-type control device, so it has been widely used in UPS.
In order to improve the power density of the new generation of high-power UPS and reduce the noise of the UPS, high-power water-cooled IGBTs and new liquid-cooled power modules (as shown in Figure 2) will replace air-cooled devices and be widely used in UPS.
The following introduces several parameters related to the capacity of the switching device IGBT, which is mainly used by UPS, during safe operation.
1) Collector-emitter voltage UCES
The collector-emitter voltage UCES refers to the highest voltage allowed between the collector and the emitter of the IGBT in the off state when the gate of the IGBT device is shorted to the emitter. Under normal circumstances, considering that there is inductance on the circuit when IGBT is turned off, it is easy to produce voltage spikes due to the large current drop rate when turning off. Therefore, the collector-emitter voltage that the IGBT bears during operation should be selected as the highest voltage of the IGBT. Around 50%. This is a key to choosing IGBT.
2) Continuous working collector DC current Ic
The continuous collector DC current Ic refers to the DC current at which the IGBT can raise the junction temperature of the IGBT to the maximum allowable junction temperature under a given case temperature. Usually given is the DC current value when the IGBT case temperature is 25°C and the junction temperature is 150°C. Some products also give other case temperature values and DC current values when the junction temperature is 1509. In general, the case temperature of the IGBT during actual operation is different from the value given. Therefore, the IGBT switching device manual may give a curve of Ic with the case temperature or give the corresponding case under several cases of case temperature. At this time, the corresponding continuous working current can be estimated according to the allowable shell temperature (related to the cooling method and ambient temperature) during the IGBT operation. It should be noted that the current is a direct current, and when the IGBT switching device is used in the inverter in the UPS, since the current passing through it is usually a half-wave sinusoidal current, the IGBT switching device should be selected according to the IGBT flow during operation. Calculate the average current from the current. The solution method is as follows: Assuming that the peak value of the power frequency half-wave current is Im, then the average on-state current is
The effective value of sine half-wave current is
The ratio of the effective value of IGBT to the average on-state current is
I/I Ta =π/2=1.57
According to the principle of equal effective value, calculate the allowable average current Id when IGBT flows through other waveform currents. The ratio of the effective value to the average value is the form factor
Kf =I/Id
but
Id=1.57ITa/Kf
When IGBT is selected, the on-state average current ITa of the IGBT should be 1.2-2.0 times the average current in normal use. Only in this way can it work reliably. Therefore, the average current should be smaller than Ic. It is particularly important to note that when using the effective value current to calculate the average current, be sure to pay attention to the waveform shape when the current flows through the IGBT. Only in this way can the average current be calculated more accurately, so that an IGBT switching device with a suitable continuous collector DC current value can be selected, otherwise the selected IGBT capacity will be too large and cause waste.
3) Continuous peak collector current ICM
Under transient conditions, the maximum current ICM that can flow between the collector and emitter of IGBT is much larger than the continuous operating current. Generally, IGBT will give the maximum collector-emitter current that the IGBT can flow within 1ms, usually this value is twice the continuous working DC current. Of course, if the time is shorter, the maximum collector-emitter current that the IGBT can flow is greater.
When choosing IGBT, you should understand the situation of IGBT in detail. The performance of the switching device IGBT can be considered in comparison with the parameter table of the IGBT device, the safe working interval, the loss and the requirements for the drive, in order to select a more suitable IGBT switching device.