Practical EMI Line Filters: Safety Issues in EMI Filter Design

• Safety Issues in EMI Filter Design (below)
• Practical Line Filters
• Safety Restrictions on the Total Y-capacitance
• Equivalent DM and CM Circuits
• Some Notable Industry Experiences in EMI

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 Section, 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.

Safety Issues in EMI Filter Design

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 exposed metal and parts of the circuit containing high voltage, and c) a layer of approved insulator between any exposed metal and the high voltage. Note that the insulator must have a minimum dielectric withstand capability of 1500 V ac or 2121 V dc.

++ To qualify the preceding slightly - connecting the metal enclosure of the equipment to earth can sometimes be considered as an acceptable level of safety protection - there are exceptions to this, as we will soon learn. However, assuming for now that earthing is acceptable, we know that to protect the user in case the earth connection fails (maybe due to something as simple as a loose contact), we need to provide one more level of protection. So this could simply be the "4 mm" of separation.

But consider the case of a high-voltage mosfet (switch) mounted on the (earthed) metal enclosure (for better heatsinking). Clearly, we can't provide any level of protection through physical separation. So in this case, we need to place one layer of approved insulator between the mosfet and the enclosure. Note that in this position, the insulator serves as 'basic insulation.'

++ What if we have an exposed conductor that is not connected to earth (such as for equipment with a two-wire ac cord), or if earthing is itself not an acceptable level of protection for that particular type of equipment as per safety regulations? Then, besides the layer of basic insulation, we need another insulating layer (with identical dielectric withstand capability). This is called 'supplementary insulation.' Together these two layers (basic + supplementary) are said to constitute 'double insulation.' We could also use a single layer of insulation, with dielectric withstand properties equivalent to double insulation (i.e. 3000 V ac or 4242 V dc). That would then be called reinforced insulation. So for example, if the equipment is by design, meant only for a two-wire ac cord, we would need two layers of approved insulators (or a single equivalent layer) from primary side to any exposed metal (e.g. output).

++ Why do we even bother to connect the enclosure to earth in the first place? In some cases, that is not even considered an acceptable level of protection. And besides, we could achieve two-level protection simply by double (or reinforced) insulation. The main reason for using an earthed metal enclosure is that we want to prevent radiation from inside the equipment from spilling out. Without a metal enclosure, whether connected to earth or not, there is very little chance that a typical off-line switching power supply can ever hope to comply with radiated (and possibly conducted) emission limits. That is especially true when switch transition speeds are dropping to a few tens of nanoseconds. Earthing further improves the shielding effect.

++ The metal enclosure is rather expectedly eyed by engineers as an excellent and fortuitous heatsink. So in practice, power semiconductors are often going to be mounted on the enclosure (with insulation). However by doing this, we also create leakage paths (resistive/capacitive) from the internal subsystems/circuitry to the metal chassis. And though these leakage currents are small enough not to constitute a safety hazard, they can present a major EMI problem. If these leakage currents are not 'drained out' in some way, the enclosure will charge up to some unpredictable/ indeterminate voltage, and will ultimately start radiating (electric fields). That would clearly be contrary to the very purpose of using a metal enclosure. So we then need to connect the enclosure to earth (other than for safety reasons!). We note that even if we didn't have power devices mounted on the enclosure, there could be other leakage paths present to the enclosure. And besides that, an unearthed enclosure would also inductively pick up and reradiate the strong internal electric/magnetic fields.

++ Therefore, a) providing a good metal enclosure, and b) properly connecting it to earth is the most effective method of preventing radiated EMI. However, by creating this galvanic connection (to earth), we also now provide a "multi-lane freeway" for the conducted (common mode) noise to flow "merrily" into the wiring of the building. So now, to be able to stay within the applicable conducted emission limits, we need to provide a common mode filter somewhere.

++ Generally speaking, if the equipment is designed not to have any earth connection at all (e.g. a two wire ac cord), there will usually be no metal enclosure present either. Ignoring the problem of meeting radiation limits for now, the good news here is that no significant common mode (CM) noise can be created either - simply because CM noise needs an Earth connection by definition. Therefore a CM filter need not be present in this case. However, we must remember that conducted noise limits include not only common mode noise, but differential mode (DM) noise too.

So irrespective of the type of enclosure and earthing scheme, DM filters are always required.

++ One of the simplest ways of suppressing noise is to provide decoupling (between the nodes involved). For CM noise this could mean just placing high-frequency ceramic capacitors between the L and E wires, and also between the N and E wires, possibly at several points along the PCB. But the problem is that each of these CM line filter capacitors also unintentionally passes some ac line current into the chassis (besides the CM noise). The ac component is not considered to be "noise," but it can certainly cause an electrical shock to the user. Therefore, safety regulations restrict the total amount of current that is injected into the earth/enclosure. And this in turn means that in any common mode filter stage we need to place an upper limit on the net CM filter capacitance. However, we know that if the "C" of an LC filter is made smaller, then to maintain the desired attenuation level (resonant frequency), we need to correspondingly increase the L. Therefore, it’s not surprising that the inductance used for the CM filter stage (in off-line applications) is usually fairly large (several mH).

Fgr. 1: Practical Line Filter and the CM and DM Equivalent Circuits

Practical Line Filters