Problems related to switching power supplies

All LCD monitors have a switching power supply and switching power supplies are a source of an incredible amount of electromagnetic fields and electromagnetic radiation, both radiated and conducted.

Reasons behind switching power supplies

The switching power supplies (SPS) have several technical advantages respect to other types of power supplies. SPSs are generally smaller and lighter, can give high currents at low voltages, can be quite stable and can also have a very good efficiency. SPSs in addition don't need an exact input voltage, the same SPS can be used with 230 V AC 50 Hz or with 120 V AC 60 Hz, which means that the same product can be sold in the same way in USA, Canada, Europe, South Africa, Australia, almost in any country of the world.

EMF created by switching power supplies

The technical advantages offered by SPSs are counterbalanced by their incredible amount of electromagnetic noise and radiation. The electromagnetic noise generated by a SPS is huge both in terms of intensity and in terms of frequency band.

An SPS gets an alternate voltage or even a DC as input, and by means of several oscillating circuits transforms it into a rapidly varying output voltage, which has a constant mean value but contains of a myriad of electric waves of different frequency and amplitude. The very low frequency waves can be blocked with some simple passive filter, but the high frequency waves cross the filters.

The EM noise generated by an SPS is partially radiated into the surrounding space and for the rest it travels along the output wires and reaches the device connected to the power supply.

Of course the manufacturers of these power supplies ensure that the device functionality is not affected by this electromagnetic noise. This is possible as long as the device works on a frequency band that does not intersects the band of the noise created by the power supply.

The environment problems arise when the device connected to the SPS has such a shape, extension and location that it brings the EMF radiation on the people. This is exactly what happens with LCD monitors.

All PCs have a switching power supply, which can be quite large and powerful, but the PC components are usually enclosed in a metal box, and usually people do not work or live with the PC box right in front of their face.

The problem of the SPS emissions then is particularly serious within LCD monitors.

Laptop computers also have an external SPS for using the laptop with a mains socket as power source, or to recharge the laptop's battery, so they too have the problem of EMF emissions coming from the SPS.

Filters are ineffective

We have tried many many ways of filtering the output voltage of a switching power supply but there is always a residual noise. The noise at low and middle frequency is virtually canceled by a simple passive filter, but the high frequency noise is not.

To eliminate the high frequency noise we have also tried some special components designed right for this purpose, but the results were not good enough in the case of a large LCD monitor.

Replacing the switching power supply with a linear power supply

The only solution we have found until now to avoid the effects of the switching power supplies in an LCD monitor or TV is to replace completely the original power supply with a customized linear power supply, designed and built very carefully to have extremely low EMF waves.

The reason for using the old linear power supply technology is that the linear power supply doesn't generate EM noise at high frequency. A linear power supply however creates some EM noise, in addition to the noise that comes from the electric network, but the EM noise generated by a linear power supply doesn't reach very high frequencies so it can be filtered enough well.

Linear power supplies are bigger, heavier and generally more expensive than SPS with corresponding electrical parameters, but they are needed to obtain a smooth DC power with almost no high frequency noise.

Custom linear power supply design and construction

We have collected many years of experience in designing and building low noise linear power supplies, including finding the right electronic components on the market, so if you are looking for some help you can contact us, write to mail@lowemfoffice.com or leave a message with this form .