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Switched mode PSUs and noise

Why switching power supplies can cause interference and finding the culprit

Published : 09 October 2018


Why switching power supplies can cause interference and finding the culprit

Switched mode power supplies are wonderful things, they are extremely efficient in use, cheap to make and if you are careful, they don't produce so much noise that it kills you and your neighbours television, radio and broadband reception.

Too many words, skip to the end.

For the purposes of this article, we are assuming that the available power source is the main electricity grid, the stuff that come out of the sockets on the wall. This is generated remotely and distributed via a network of wires or cables to wherever it is needed. Another important thing to note is that this distributed electricity is, for the purposes of distribution, an Alternating Current, (AC), source. The low voltage power supplies described in this article are Direct Current, (DC), devices, designed to act like a perpetual battery.

A typical cheap Linear PSU

A typical cheap Linear PSU

A conventional low voltage power supply used to consist of a transformer, a rectifier and a smoothing capacitor. If you were looking for something with a little more quality, you could throw in a choke, a couple more capacitors and either a regulator or a Zener diode and a power transistor. These linear power supplies were used for years because they were simple and cheap to mass produce. As far as efficiency was concerned, they hovered around the 50% mark, which is to say that half of the input power was converted directly into heat and so only half the power that went in, came out in the form required. No one really cared about the efficiency because electricity was cheap and we were only talking about a sniff of it anyway. The heat produced from these linear power supplies was nothing in comparison to the heat produced from a radio, audio amplifier or television utilising thermionic valves as the active components.

The efficiency of the linear power supply was only considered on higher power, power supplies. Anything using 10 or 15 watts could quite happily generate another 10 or 15 watts of heat without comment, the comments only started when you were drawing a 100 watts or more, because of the 50% efficiency, another 100 watts would be created as heat, all of which had to go somewhere, otherwise things started smoking and melting.

In a switched mode power supply, (SMPS), the losses due to heat generation are substantially lower, which boosts the efficiency of the power supply. This is not to say that there is no loss due to heat, because every conductor of electricity barring a superconductor, has a finite resistance, and any current passing through this resistance generates heat. This was described by a German chap called Georg Ohm, way back in the early part of the 19th century, the effect being universally known as Ohm's law. The frequency of mains power is 50 to 60hz, depending on where you live. This frequency is good for generation and distribution of large amounts of power, but not so good for converting small amounts of power into other forms. The transformers needed for 50-60hz operation are physically large, but as the frequency of the AC increases, the size of the transformers needed drops dramatically. One of the ways that the SMPS can increase its efficiency is by increasing its frequency of operation. If the raw AC grid power is rectified, (made into DC), and then fed into an electronic chopper circuit, which then converts the power back into AC, but at a frequency dependent on the speed of the chopper circuit. The resultant high frequency, high voltage AC power can then utilise a smaller transformer with less losses to reduce the voltage down to the level required for output. With the smaller transformer, as well as a reduction in size, there is also a reduction in raw materials used. and a reduction in the weight of the finished product, all important factors if your market is the other side of the planet. The two photographs on this page are of the same scale, the transformer in the Linear PSU is 95% of the whole, whereas on the switched PSU, the transformer is the small yellow and black thing in the centre of the circuit board. The switcher in this case is rated at double the output power of the linear.

A typical cheap Switching PSU

A typical cheap Switching PSU

One of the side effects of using this approach is noise. I'm not taking about audible noise that will annoy you or your dog here, but electromagnetic noise, or radio waves, the kind of electrical noise that can cause havoc with radio and TV reception, baby monitors, WiFi signals and even your DSL Internet connection. Commonly called electromagnetic interference, (EMI). What is more, this noise or EMI is not confined to your equipment or even your house and garden, this is the kind of noise that can take out the radios, televisions, baby monitors and broadband connections for the entire street. Because of this side effect, a great deal of care must be used when designing and manufacturing a SMPS. There are design recommendations created by the International Electrotechnical Commission (IEC), the Federal Communications Commission (FCC) and the British Standards Institution (BSI), to name but a few of the national and international organisations that regulate and test electrical equipment including SMPS, to ensure that this doesn't happen. But it is not unreasonable to suspect that the cheap and cheerful, far eastern electronics factories, don't always stick to these recommendations when creating products intended for shipment to the western consumers.

TL;DR - Conclusion.

If you suspect that you might be suffering from the effects of EMI, the poor man's test involves the use of a small battery powered transistor radio with a MW Band AM setting. Tune your radio to some quiet unoccupied spot and turn the volume up so you can hear the static. By placing the radio in the proximity of something you suspect of generating EMI, you should be able to hear an increase in the amount of static on the radio if the thing in question is creating electromagnetic noise. Move the radio around all of your equipment, including the cabling, listening for any changes in the noise level of the static in the speaker. If you are not sure which piece of equipment is creating the noise, switch everything off and then power up each piece of separately until you find the culprit. If you can't find the source of your EMI in your own equipment, it may be coming from equipment belonging to one of your neighbours.

Almost all electronic devices will radiate some EMI, but the levels should be very low as to be undetectable with your transistor radio once you are over a few inches away. This is normal and usually not a problem, the problem devices are the ones you can hear close up and continue to hear in the next room. Try putting your transistor radio in front of your TV or next to your phone, this will give you some indication of what you are listening for during your testing.

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