Line Conducted Emissions
Exercises on Filters
In this exercise we shall simulate the performance of a differential mains filter that is used to reduce harmonics emitted into the mains supply by a product that uses a SMPS.
We shall start by considering an idealised filter and we shall assume that the LISN is perfect.
The current generator driving the filter is a simulated SMPS current waveform.
We will then consider a filter with a bulk storage capacitor possessing both series resistance (ESR) and series inductance (ESL).
The filter used in these exercises is made up of typical components, it has not been designed to meet a specification.
You will develop some of these ideas in assignment 2.
Contents
An Idealised Differential Filter
The circuit diagram of an idealised differential mains filter is shown in Fig X7.1. The mains supply and the LISN components have been omitted, which is equivalent to assuming the LISN is perfect.
Figure X7.1 An Idealised Differential Filter
The current waveform used in the exercises is a simulated SMPS current shown in Fig X7.2.
Figure X7.2 Simulated SMPS Current Waveform
The period of the SMPS switching frequency is 10uS, which corresponds to a switching frequency of 100KHz.
The current rises linearly to a maximum value of 50mA, in 5.0uS.
The current generator can be found at:
MB/Place/Components/Sources/Signal_Current_Sources/Piecewise_Linear_Current
The current generator setup is shown in Fig X7.3
Figure X7.3 Current Generator Setup
The transient analysis setup is shown in Fig X7.4
Figure X7.4 Transient Analysis Setup
The transient output node setup is shown in Fig X7.5
Figure X7.5 Transient Analysis Output Nodes
Note the use of expressions for the various voltages. We should remember to divide graphed values by 1000, to obtain actual values.
The simulated filter waveforms are shown in Fig X7.6 (a). Only the filter outputs voltages are shown.
Figure X7.6 Simulated Filter Output Voltages
Actual values of voltages are shown in Fig X7.6 (b). The filtered voltages are 2.1473mV with respect to earth. Note that they are of opposite polarity.
Differential Filter with Bulk Storage Capacitor ESR and ESL
Fig X7.7 shows a differential filter with a bulk storage capacitor that has equivalent series resistance of 100mOhms and an equivalent series inductance of 5nH.
Figure X7.7 Differential Filter with an Imperfect Bulk Storage Capacitor
Fig X7.8 shows the transient response of the imperfect filter.
Figure X7.8 Simulated Waveforms for the Imperfect Filter
Fig X7.8 (a) shows the current waveform of the signal generator and the filter output voltages. Fig X7.8 (b) shows actual values of the simulated voltages.
The peak current =50/1000= 50mA. The output voltages = 3.8341/1000= 3.8341mV and they are of opposite polarity.
The presence of the imperfect bulk storage capacitor has degraded the filter performance.
Fig X7.9 shows the Fourier analysis of one of the filter output voltages.
Figure X7.9 Fourier Analysis of the Differential Filter
We suggest you develop the exercises by altering filter component values and performing Fourier and transient simulations.
Updated 13/05/08 RJH