the backup power source 11 the control module 14 controls the seventh driving unit 137 and the eighth driving unit 138 to turn on the third bidirectional thyristor tr 3 and the fourth bidirectional thyristor tr 4 the time to conductivity of bidirectional thyristors is in the order of microseconds the backup power source 11 can be coupled to the power switching circuit 200 to supply power in microseconds then the fifth relay 131 and the sixth relay 132 are turned on the control module 14 controls the seventh driving unit 137 and the eighth driving unit 138 to turn off the third bidirectional thyristor tr 3 and the fourth bidirectional thyristor tr 4 the first terminal 111 of the backup power source 11 is coupled to the power switching circuit 200 through the seventh relay 133 and the fifth relay 131 the second terminal 112 of the backup power source 11 is coupled to the power switching circuit 200 through the eighth relay 134 and the sixth relay 132 similarly when the power switching device 200 needs to switch from the backup power source 11 to the primary power source 10 the control module 14 controls the third driving unit 127 and the fourth driving unit 128 to turn on the first bidirectional thyristor tr 1 and the second bidirectional thyristor tr 2 the time to conductivity of bidirectional thyristors is in the order of microseconds the backup power source 10 can be coupled to the power switching circuit 200 to supply power in microseconds then the third relay 123 and the fourth relay 124 are turned on the control module 14 controls the third driving unit 127 and the fourth driving unit.