There are two main types of switching devices used in modern UPS, namely Thyristor (SCR) and IGBT. SCR is mainly used in multi-phase phase-controlled rectifiers and bypass switches. Let’s talk about the Thyristor (SCR)
Thyristor (SCR), also called silicon controlled rectifier, is the earliest semi-controlled switching device used in power electronics. It is a four-layer (PNPN) three-terminal high-power semiconductor switching device composed of three PN structures. In terms of performance, SCR not only has unidirectional conductivity, but also has more valuable controllability than silicon rectifier components. But it has only two states: on and off.
SCR has many advantages, such as low control power, power amplification up to hundreds of thousands of times, extremely fast response speed, can be turned on or off within a few microseconds, no contact, no spark, no noise, high efficiency, and cost Therefore, it is widely used in high-power UPS systems, mainly used in multi-phase phase-controlled rectifiers, static bypass switches, non-contact output switches, etc.
The disadvantage of SCR is that it has poor static and dynamic overload capability, and is easily misleaded by external interference. It is a semi-controlled device.
There are three shapes of SCR, namely bolt shape, flat plate shape and flat bottom shape.
SCR structure and working principle: SCR structure, equivalent circuit and device symbols are shown in Figure 1. It is a four-layer (PNPN) three-terminal switching device with three PN junctions. It divides the NP in the middle of the three PN junctions into two parts, thus forming a composite tube equivalent circuit of a PNP type triode and a NPN type triode. As shown in Figure 1(b). Figure 1(c) shows the device symbol of the SCR.
When a forward voltage is applied to the SCR, in order to turn on the SCR, the PN junction J2 that bears the reverse voltage must lose its blocking effect. The collector current of each transistor is also the base current of another transistor. When there is enough gate current Ig to input, it will form a strong positive feedback, so that the two transistors are saturated and turned on.
Assuming that the collector currents of the PNP tube and the NPN tube are Ic1 and Ic2, respectively, and the emitter current is Ia and Ik respectively, the current magnification should be α1=(Ic1/Ia) and α2=(Ic2/Ik). Suppose the reverse leakage current flowing through the J2 junction is Ico.. Then the anode current of the SCR is equal to the sum of the collector current and the leakage current of the two tubes, that is
Ia=Ic1+Ic2.+Ico=α1Ia.+α2Ik+Ico ——(1)
If the gate current is Ig, the cathode current of the SCR is
Ik=Ia+Ig——(2)
Therefore, the anode current of the SCR can be obtained as
Ia=(Ico＋α2Ig)/[1﹣(α1＋α2)]——(3)
The corresponding current amplification factors α1 and α2 of the PNP transistor and the NPN transistor change sharply with the change of the emitter current. When the positive anode voltage is applied to the SCR, and the gate has not been applied with the forward voltage, Ig=0 in formula (3), (α1＋α2) is very small, so the anode current of SCR Ia≈Ico , The SCR is in a positive blocking state; when the gate of the SCR is applied with a forward voltage, the current Ig flows from the gate G. Due to the large enough Ig, it flows through the emission junction of the NPN tube, thereby increasing the magnification α2, Therefore, a large enough collector current Ic2 will flow through the emission junction of the PNP tube, and the current amplification factor α1 of the PNP tube will be increased, and a larger collector current Ic1 will flow through the emission junction of the NPN tube. The positive feedback process proceeds quickly.
When α1 and α2 make (α1＋α2)=1 with the increase of emitter current, the denominator 1-(α1＋α2)=0 in formula (3), thereby increasing the anode current Iao of the SCR. At this time, The current of the SCR is completely determined by the voltage and resistance of the main circuit, and the SCR is in a forward conducting state. After the SCR is turned on, in formula (3) 1-(α1＋α2)≈ 0, even if the gate current Ig=0 at this time, the SCR still maintains the original anode current Ia and continues to conduct, and the gate has lost effect. When the SCR is turned on, if the power supply voltage is continuously reduced or the loop resistance is increased to reduce the anode current Ia to below the maintenance current IH, the SCR will return to the blocking state due to the rapid decline of α1 and α2.