We will place a series inductor and resistor in the ground leg between the power supply and the AC ground. The AC ground will then be connected to our chassis to complete the ground loop. Noise is coupled into the circuit through the stray capacitance between the switching transistor and its heat sink and through the inter-winding capacitance of the forward converter's transformer. In order to measure the noise, we will use a forward converter design that isbeing prepared for a future article featuring a complete power supply design using our Power Supply Designer package. The transient analysis model for the power supply is shown in Figure 14 .
Most coupling comes from the power transistor to heat sink path.
The heat sink could be isolated or connected directly to the chassis. Using a coupling of 5pf for an isolated heat sink .vs. 50 pf using the chassis results in the noise shown in Figure 15 . Taking the FFT of the transient response presents the data in a form that can be compared with various EMI specifications. Common mode filtering is needed to solve the problem.