TABLE OF CONTENTS When we start designing EMI filters, we will find that safety issues, thermal issues, and even loop stability concerns are intricately linked to the central issue of EMI. In particular, we must look closely at the aspect of safety, because even though we can attempt to sell equipment that doesn't function satisfactorily, product safety is a legal requirement without which we just cannot sell. So particularly in an off-line application, where the voltages are high enough to cause injury, safety becomes a major concern, even if we are just designing its EMI filter.
In this chapter, we will focus mainly on filters for (single-phase) off-line power supplies. However, tips for dc-dc converters will also be provided along the way.
The concept of safety and how it impacts the filter section is summed up as follows:
Any exposed metal (conducting) part (e.g. the chassis or output cables) is capable of causing an electrical shock to the user. To prevent a shock, such parts must be earthed and/or isolated from the high voltage parts of the power supply in some way.
No single point failure anywhere in the equipment should lead the user to be exposed to an electrical shock. There should be two levels of protection, so that if one gives way, there is still some protection available.
Levels of protection that are considered essentially "equivalent" are a) earthing of any exposed metal surface, b) physical separation (typically 4 mm) between any...
