1. The failure analysis on common mode choke in circuit

2. 1. How common code choke works: It is a part of switch input power line filter, which is considered inhibiting common code interference from switch mode power supply and electricity grid, meanwhile, providing a input passage of current with low-resistance. It is illustrated by below drawings that how common code choke works to common code interference and power supply inputting current.

3. It can be seen from drawing (a), in the same direction, the same flux direction of common code interference signals equivalent with each one in cores is reinforced, forming large inductance which suppresses common code interference. In the drawing (b)，common code choke together with circuit input current with same circuit and opposite direction is mutually counteracted due to the magnetic flux produced by cores, at that moment, inductance value is approximately zero，resulting in a smallest input current resistance. The generated voltage drop is that the voltage reduction of windings DCR, then the loss is decreased to a lowest point.

4. 2. The influence of short circuit to common code choke There are two ways to cause the short circuit of common code choke. The first one is from two windings have a poor isolation which makes insulation of two windings damaged and causes short circuit. The second is two windings is in short circuit across cores（T-core common code choke easily generate in this circuit ）The short circuit of two windings of common code choke in its working will invite short circuit of input current. If no protection in power supply, common code choke will be burnt in one moment. Generally in this case, the burnt past only is common code choke itself and its magnetic components in front of input terminal, but the magnetic components backwards not..

5. 3. The impact of short circuit to common code choke The short circuit in loop in parallel of input terminal of common code choke will increase the output current flowing into common mode choke. More heavy short circuit, more large input current which leads to temperature rising of common mode choke coils. If no protective lines in circuit, it is likely to burn fiercely common mode choke. Meantime, components in short circuit crossed by current in loop will be destroyed because of overheat.

6. 4. Turn-to-turn short circuit to common code choke Common code choke has a turn-to-turn short circuit, that is short circuit loop on coupling inductance, will cause short circuited circulation and reduce largely inductance value and Q-value that damage that anti-disturbance ability of common code choke to common interference under this condition, and subsequently, EMC/EMI performance of switch power supply is descended to disqualification’s level. On the other side, The inductance reduction caused by turn-to-turn short circuit has no impact on input of common code choke , because two coils are linked in series connection in input current loop and common code choke shows a low-resistance status to input of power supply. For input current, it is required being smallest and then no phenomenon of heating and burning out.

7. 5. The influence of common code choke two coils non-equivalent on common code choke Physical working circuit of common code choke as below: A capacitor exists in input and output terminals of common code choke, below is the formula of capacitance: In formula： Xc  capacitance，unit：Ω f  current frequency，unit：Hz C  capacitor，unit：F

8. C1 and C2 are generally designed to≤0. 1uF=0
C1 and C2 are generally designed to≤0.1uF=0.1×10-6 F，for output current at this moment 50Hz or 0Hz(DC)，Xc is extremely strong，for high-frequency disturbance signals，XC is weak，More higher frequency，More weak XC，C1 and C2 in high frequency status make same common code choke input and output. Given that,，L1 and L2can be considered L1=L2，the dotted terminals of coupling inductance of K1is in series connection and series connected inductance is equivalent with L1 and L2. But if turns of L1 and L2 are unequal, common code choke works like L1≠L2，dotted terminal of K1 coupling inductance is in series connection，due to generated short circuited circulation, the inductance value and Q-value will greatly declined. In the condition of high-frequency, the influence on power supply input current won’t be there even through the lost of anti-interference ability to common code interference and that of the function of common code choke，no heating or burning.

9. 本课程讲解完毕，进入自由提问阶段 ***谢谢***
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本课程讲解完毕，进入自由提问阶段 ***谢谢***