7-8
Protection circuit: over-voltage protection:
1. The voltage output across terminals 5-6 of T901 increases when the voltage of electric net exceeds its
maximum. When the output exceeds 20V, ZD901 will conduct , and voltage will be applied across R912. Q902
will turn on when the voltage across R911 reaches 0.6V, and the voltage of the Base of Q901 will be pulled down,
Q901 gets through. 15V benchmark voltage of PIN7 connects to grounding through R909, R910, Q902. So it cuts
off the supply to IC and acts as over-voltage protection.
2. ZD902 or ZD903 will break down when 12V or 5V DC voltage increases, which leads to the light inside the
photo electricity coupling component gets brighter abnormally, PIN 2 connects to grounding through
photosensitive transistor, then it cuts off the pulse output. That is also effective way for over-voltage protection.
Temperature protection: When the working temperatures of components in the circuit exceeds a particular
temperature, the temperature-protection component inside the IC connects a resistor from PIN 5 to ground to test
the temperature. The pulse output is cut off instantly when the voltage of PIN 5 is lower than 0.65V. Completing
over-temperature protection.
Over-current protection: The voltage across R917 which is a current test resistor will be applied to the in-phase
output port of over-current test comparison unit in SG6841D. As long as the voltage exceeds 1V, the current
protection circuit inside SG6841D will start up; Then PIN 8 is closed, completing over-current protection.
Over-voltage protection circuit 1
Over-voltage protection circuit 2
12V
5V
TO U201
PIN2
5V
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