Following on from the previous article, I will talk about conducted emissions (mains terminal interference voltage).

In a V-type line impedance stabilization network (LISN), it can only observe a sum of normal mode noise and common mode noise, but a Δ(delta)-type LISN shown in Fig. 1 can observe normal mode noise and common mode noise respectively by turning a switch.

 

Figure 1. Δ-type line impedance stabilization network (LISN)^*1

 

A normal mode noise can be measured as shown in Figure 2.

A normal mode current flowing in the primary side of the transformer is induced in the secondary side without change, so it can be observed with a measurement equipment.

 

Figure 2. Δ-type LISN (At normal mode noise measurement)

 

A common mode noise can be measured as shown in Figure 3.
A common mode current flowing from the power lines to the primary side of the transformer flows out from the middle tap of it to the ground line through a measuring instrument. Therefore, the measurement value shown by the measurement equipment is for the common mode noise.

In this way, Δ-type LISN can switch between normal mode noise and common mode noise with a single touch, making it possible to efficiently and correctly examine suppression measures according to the noise mode.

 

Figure 3. Δ-type LISN (At common mode noise measurement)

 

^*1 Partial excerpts from CMDM8700 data sheet (SCHWARZBECK)