Efficiency versus EMC: a trade off eliminated

Whatever the application, power supply users want products that are small and efficient, yet have excellent EMC performance.

Until recently, it was difficult for designers to reconcile these requirements but, as Andy Skinner of Lambda UK explains, new developments in switch-mode technology now provide a complete solution.

In recent years, switch-mode power supplies have almost completely displaced traditional linear supplies in the majority of applications. The reasons are not hard to find: for a given rating, switch-mode supplies are much smaller and lighter than their linear counterparts, and they are also much less costly to manufacture.

Switch-mode supplies clearly have a lot to offer, but they do have one potential shortcoming. Because of the way they work, they are capable of generating radio frequency interference (RFI) or, to put this in the terms most often used, it is possible for them to have poor EMC performance.

To understand why this is, and what can be done about it, we need to take a brief look at how switch-mode supplies work. In essence, they’re not very complicated. They take power from the mains then rectify and smooth it to produce a high voltage DC supply of, typically, 380V in the case of a power supply with power-factor correction.

This DC supply is applied to an inverter stage which converts it back to AC, but at a much higher frequency than the mains supply. The exact frequency used depends on the design of the power supply, but is typically in the range 50-300Khz . The high-frequency AC from the inverter is then applied to a transformer which, because of the high frequency, can be very small, light and inexpensive.

Finally, the output from the transformer is rectified to produce the output from the power supply itself.

There are, of course, other facets of the power supply, including some form of feedback arrangement from the output to the inverter stage to ensure that the output voltage remains constant even if operating conditions vary. The major sections described, however, are enough to explain the potential trade-off between efficiency and EMC performance.

As far as EMC is concerned, the key section of the power supply is the inverter stage which converts DC to AC by using semiconductors, usually MOSFETs, as fast switches operating at relatively high power levels. This stage presents power supply designers with something of a dilemma.

If they arrange for the MOSFETs to switch from on to off and vice versa very quickly (hard switching), little power is lost in them, since they spend almost all the time either fully on, with little voltage drop across them, or fully off, with little current flowing through them. Unfortunately, fast switching unavoidably means that high levels of harmonics are generated or, in other words, that EMC performance is poor.

One solution is to stick with fast switching and suppress the harmonics by adding screening and filters to the supply. If they are to be effective, however, these measures add significantly to the size, weight and cost of the supplies, reducing the very benefits which have made switch-mode supplies so popular.

What about slowing down the on-off and off-on transitions in the MOSFETs? This so-called soft switching is certainly effective in reducing the generation of harmonics, but it brings another problem. Now, during the much longer switching transitions, the MOSFETs spend a considerable amount of time when they are neither fully on nor fully off.

During this time, they have both voltage across them and current flowing through them and, since voltage multiplied by current equals power, they generate heat. In other words, while soft switching addresses the EMC problem, in its usual form it also significantly reduces the efficiency of the power supply.

Not only does this mean that energy is wasted, it also means that the power supply runs hotter than necessary. This is a big problem in today’s applications, where compact designs mean that thermal management is always an important issue.

Clearly, what’s needed is a switch-mode supply which combines the efficiency of hard switching with the EMC performance of soft switching. Lambda has now developed a way of achieving this, and has patented its solution under the name Multi-Resonant Topology (MRT).

The key to MRT is to arrange for the MOSFETs to switch only when the voltage across them is close to zero. This is achieved by first of all arranging the operation of the inverter so that the current in the primary circuit of the transformer is continuous. This can be done, for example, using variable frequency control techniques to produce a primary current in the form of a distorted sine wave where the current lags the voltage.

Under these conditions, adding a small capacitor across each MOSFET gives it time to switch off with almost no voltage across it. Because there is no voltage across the MOSFET when it switches, the losses are small and because the switching is soft with a smooth voltage transition, harmonic generation is minimised. A bonus is that zero-voltage switching also reduces the peak stresses on the MOSFETs.

At this point a word of caution is necessary. For this arrangement to work properly, the load seen by the inverter must always be inductive. If it is not, the inverter will revert to hard switching and must now also charge and discharge the additional capacitors across the switches leading to even higher switching losses. It is relatively easy to arrange for the inverter to see an inductive load when the power supply is working near its maximum capacity, but much more difficult to arrange this when it is lightly loaded.

For this reason, some currently available power supplies give excellent results in terms of efficiency and EMC performance at full load, but have very poor EMC performance at reduced loads. This is a point well worth checking when specifying power supplies as shortcomings of this type are not always readily apparent from a data sheet.

With switch mode power supplies it has long been necessary to choose between efficiency and EMC performance. Now, this trade-off is no longer necessary. The latest developments give users the best of both worlds – power supplies which are small, light and cool running, and which deliver EMC performance so good that the need for external filters, even in sensitive applications, is a thing of the past.

Source: Lambda.