1. Why are Switched Mode Power Supplies (SMPS) associated with radio interference?

Switched mode power supplies (SMPS) employ high
frequency switching and thus, are a source of radio interference, a
recipient of radio interference and a conduit of radio interference.
(Older linear type transformer based power supplies do not employ high
frequency switching voltages and will be quieter as compared to
switching type of supplies).


2. What are the typical sources of radio interference in a SMPS?

The primary emission sources originate in the switching devices due
to their fast switching current transitions: harmonics of the switching
frequency and broadband noise created by under-damped oscillations in
the switching circuit. The secondary source is from the bridge
rectifier, both rectifier noise and diode recovery. The AC input
rectifier / capacitor in the front end of the switching power supplies
(excepting those with power factor correction) are notorious for
generating power supply harmonics due to the non linear input current
waveform. The noise is both conducted and radiated through the input
power cord and the DC output wiring to the radio.


3. How is a SMPS affected by received radio interference? How can the problem be solved?

Switching power supplies are also recipients of radio interference.
The normal operation of the power supply can be disturbed due to RF
noise getting coupled into the power supply. Thus, the power supply may
generate excessive RF noise and lose output voltage regulation due to
excessive transmitter energy being coupled through the AC / DC lines to
the power supply’s regulator feedback path. This may be due to antenna
being too close or due to the antenna or feed system not radiating
properly. First check the antenna system SWR. Then, if necessary,
relocate either the antenna or the power supply farther apart.


4. What is the source of the “buzzing” sound heard at the receiver? How can this be avoided?

The receiver may “hear” the power supply. A slowly moving, slightly
buzzing carrier heard in the receiver may be caused by the antenna being
too close. As with the transmitter related noise pick up, a loose
coaxial connector or a broken or a missing ground may aggravate this
problem. Normally these noises will be below the background or “band”
noise. Increase the separation between the power supply and the
receiving antenna. Use an outdoor antenna. This will reduce the amount
of signal picked up from the power supply and also increase the amount
of the desired signal.


5. What are the standards pertaining to RF noise in SMPS?

The conducted and radiated noises are limited as per the applicable
national / international standards. In North America, the applicable
standard is as per FCC Part 15(B) for Class “B” digital devices. The
European standard is as per EN55022, Class “B” & EN610000-3-2, 3. Thus, the RF interference is limited but not entirely eliminated.


6. How can conducted RF noise be limited in a SMPS?

The conducted RF noise from these power supplies is limited to the
maximum allowable levels by internal filtration. The filtered RF noise
currents (normally < 5mA) are bypassed to the chassis of the power
supply. The chassis is, in turn, connected to the earth ground pin of
the AC input power cord (for Class 1 units). Thus, the filtered noise
currents are intentionally leaked to the earth ground. This is termed as
the “Earth Leakage Current”. For safety against electric shock, this
earth leakage current is also required to be limited. It will be seen
that these two requirements are conflicting.

NOTE: In some cases, to prevent electric shock
hazard due to abnormal leakage current (like in marinas, spas, hot tubs,
wet spaces etc.), the AC outlet circuits / receptacles in these areas
are served through a GFCI (Ground Fault Circuit Interrupter).

This GFCI is normally set to trip when it senses an earth leakage
current > 5 mA. A single GFCI may be serving multiple AC outlet
circuits / receptacles and therefore, will be sensing the sum of all the
leakage currents of the devices connected to these. As the switching
power supplies have intentional leakage current as explained above, it
may trip a GFCI feeding multiple AC outlet circuits / receptacles. In
such cases, disconnect devices connected to the other AC outlet circuits
/ receptacles served by this GFCI.


7. What additional steps can be taken to reduce the effects of RF noise?

Following additional guidelines may be followed to reduce the effects of RF noise:

  • a. Use additional appropriate AC radio frequency interference (RFI)
    power line filter immediately before the ac input of the power supply. These
    cord sets, with integral line interference filters, reduce common and
    differential mode interferences over a wide frequency range. Because
    they are shielded, they are also effective against radiated
    interferences. In addition to the built-in filter networks, the cable
    conductors are coated with an RF absorbing ferrite compound. This
    provides additional attenuation at high frequencies that is lacking in
    most regular LC filters. The RF absorption of the ferrite-coated cable
    avoids resonance’s at high frequencies, reducing the conducted and
    radiated RF noises even further
  • b. Use additional appropriate DC radio frequency interference
    (RFI) power line filter immediately after the DC output of the power
    supply.
  • c. Twist the positive and negative wires from the output of the power supply to the radio
  • d. The DC side positive and negative outputs of these power
    supplies are isolated from the chassis. As explained at paragraph 6
    above, the noise currents are filtered to the chassis ground and the
    chassis ground is connected to the earth ground through the earth ground
    pin of the AC power outlet receptacle. Avoid connecting (referencing)
    the DC negative output terminal of the power supply to the earth ground
  • e. Connect a 1/4” wave length of wire on the negative terminal of
    the power supply. Connect one end of the wire to the negatvie terminal
    and leave the other end free. The wave length corresponds to the wave
    length of the interfering frequency. (May not be practical for long wave
    lengths)

[ Formula: Wave length (Meters) = 300 / frequency in MHz ]

By 9M2PJU

An amateur radio operator, Royal Signals veteran, jack of all trades and master of none.

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