Schematic Analysis of Overvoltage and Overcurrent Protection Circuit

shaorc

[Ask if you don't understand] The principle of an overvoltage and overcurrent protection circuit is explained in the following way in the textbook: As long as there is an overvoltage or overcurrent Uo, the thyristor VT2 will be triggered by the transistor VT1, turning it on and clamping the output voltage. When the Uo output is greater than 36V, VDz3 is reversely broken down, and the thyristor VT2 is turned on by the transistor VT1 to short-circuit the output. After analysis, it is found that whether it is overvoltage or overcurrent, the output of the transformer (the one in the upper left corner) and VD10 and VT2 will eventually form a loop to form a short circuit. I have the following questions: [1] Can the L2 part in the figure be regarded as an ordinary inductor? This looks strange and like a transformer. [2] When there is an overvoltage, how does VT2 turn on? The conduction of the thyristor requires positive pressure. This is available, but the gate is not triggered? [3] When there is an overcurrent, how does VT2 turn on? How does the current flow at this time? Moreover, according to the conduction condition of the PNP transistor, the voltage on R10 increases, so Vb＜Ve may not be true. Then VT1 is not conducting at this time. How to trigger VT2?

maychang

【1】Can L2 in the figure be considered as an ordinary inductor? This is not familiar, it looks like a transformer. It is a transformer, the primary and secondary are not separated, the two parts have the same number of turns, and are wound on the same core. This kind of thing is often called a common mode inductor.

maychang

【2】How does VT2 conduct when overvoltage occurs? The conduction of the thyristor requires positive voltage. This is achieved, but why is the gate not triggered? When overvoltage occurs, VD23 is broken down, and the voltage across R14 triggers VT2.

maychang

【3】When overcurrent occurs, how does VT2 conduct? How does the current flow at this time? Moreover, according to the conduction conditions of the PNP transistor, the voltage on R10 increases, so Vb＜Ve may not be true. Then VT1 is not conducting at this time. How to trigger VT2? When overcurrent occurs, the voltage across R9 exceeds the base-emitter conduction voltage of VT1 (about 0.7V), VT1 conducts, and its collector current flows through R11 and R14. If the voltage drop on R14 exceeds the VT2 trigger voltage, VT2 conducts. The voltage across R10 can be ignored because the base current is not large.

maychang

This is not a very reliable protection circuit. What will happen if VT2 is triggered and turned on? It is nothing more than short-circuiting the winding on the upper right of the transformer T. But this is a switching power supply, and the power switch part still works. It is even possible that no current flows through PC817 due to VT2 being turned on, and the switching power supply will increase the PWM duty cycle.

leiannuo

Nowadays, overvoltage and overcurrent protection does not need to be designed so complicatedly. Overvoltage can be clamped directly with TVS tubes, and overcurrent can be protected directly with resettable fuses - the cost can also be reduced a lot!

maychang

leiannuo posted on 2018-7-11 16:45 Nowadays, overvoltage and overcurrent protection are not designed to be so complicated. Overvoltage can be clamped directly with TVS tubes; overcurrent can be protected directly with resettable fuses - ...

Using TVS tubes and resettable fuses are both overvoltage and overcurrent protection methods. However, TVS tubes and resettable fuses are not applicable everywhere.

shaorc

maychang published on 2018-7-10 19:54 This is not a very reliable protection circuit. What happens when VT2 is triggered and turned on? It is nothing more than short-circuiting the winding on the upper right of the transformer T. But this is a switching power supply...

That's right, I just omitted some of the original words just to analyze this circuit. It said that after overvoltage or overcurrent occurs, Uo triggers VT2 through VT1, turning it on and clamping the output voltage, and turning off the switching power supply PKS606Y after 30ms. The switching power supply part is not drawn in the book. As long as PKS606Y does not receive a feedback signal within 30ms, it will be latched off.