Technology
capacitance coupling the source to the victim.
This is the most common phenomenon which is also mistakenly
ignored. It is the primary source for the common mode currents
in any system. Example: Most designers use TO-220 Mosfets for
better thermal management and tie them (electrically isolated) to
the chassis that act as large heatsinks. The Capacitive coupling
between the mosfet and chassis can be 10-12pF because of the
TO 220 package. Assuming the Mosfet switches 400V at200Khz
with a turn on/off time of 100ns then Ipk = 48mA (Irms = 9.6mA).
Measuring the conducted EMI using a 50 ohm impedance LISN on
the Chassis ground will give 114 dBuV. The CISPR class B limit for
200Khz is 56 dBuV. Hence 58 dB of attenuation is needed to pass
the standard.
The problem is aggravated with the presence of higher harmonics
of the currents. A design may be under the pass limit at the
fundamental frequency (switching frequency of the converter)
but may fail at some higher harmonic (figure 1c).
Figure 1c: Passes at the fundamental of 680Khz but fails at higher
harmonics
The capacitive and inductive coupling due to the Electric (E) and
Magnetic (H) Fields are just a near field phenomenon. The E and
H fields result in conducted emissions if the frequency is <
30Mhz.
These fields behave as plane waves when the frequency is > 30
Mhz and is seen as radiation or Radiated emissions and is also
known as the Far field phenomenon.
Figure 2 is the analytical representation of the radiated and
conducted EMI from an SMPS. The conducted EMI can couple
back to the source and act as noise to other loads on the same
source. The radiated noise is mostly a result of the selfoscillations of the inductive or capacitive elements (including the
parasitic ones) in the converters or the higher harmonics of the
switching frequencies.
Figure 1a: Simple switching of Mosfet
Figure 2: Conducted and Radiated EMI in SMPS
Critical Area in Switching Converters
Current always takes the path of the least impedance rather than
the shortest path. The current waveform in switching converters
is a combination of Low frequency currents and high frequency
currents. Refer Figure 1 for the Current waveform. Its Fourier
transform shows the odd sine harmonics (figure 3) where the
amplitude is prevalent in the low frequency harmonics and the
sharp rise and fall times are due to the high frequency content.
The higher harmonics will have a different path as shown in
Figure 3. Naturally the least impedance path for the higher
harmonics is via the input and output capacitors rather than the
source.
Figure 1b: V and I waveforms
ELE Times | 52 | November, 2016