this Power Tip, we continue our discussion of common mode currents
which began in Power Tip 40. There we discussed how common-mode
currents are created by large voltage swings found in switching stages, which
drive currents into the capacitances to chassis ground.
isolated power supplies, this situation becomes worse because the secondary of
the isolating transformer eventually is connected to chassis ground. Hence,
there is considerable primary-to-secondary parasitic capacitance. Figure
1 presents a simplified schematic of the situation:
Figure 1: High-voltage
switching of Q1
current in C_STRAY.
(Click here to enlarge image.)
This is an
isolated flyback design which operates off-line. The input power of 110-to-220 V
AC is rectified and provides 100-to-400 V DC to the power stage. The power
switch quickly turns on and off, creating a 500-to-600 V switching waveform on
the drain of Q1, which is also applied to the primary of the power transformer.
switching voltage creates current in the stray capacitance between the
transformerís primary to secondary windings. This current flows either through
an intentional chassis ground connection at the load as shown, or simply may be
capacitively coupled to earth ground.
current has to complete the return path back to the noise generating switching
source. Without C1, it flows back to the AC input power source and then into the
input leads of the power supply, where more than likely it exceeds EMI emissions
current is particularly hard to filter because of its high source impedance. The
stray capacitance in the transformer is on the order of 100 pF, which has an
impedance of 10 k? at typical power-supply switching frequencies. Simply adding
an inductor in the current path to reduce the current is impractical.
instance, if we wanted to reduce the current by a factor of 10, it requires 100
k? of reactance, or 0.1 H, with less than 10 pF of distributed capacitance,
which is physically unrealizable.
Capacitor C1 presents an alternative solution. It provides a local return
path for the current to flow. Most of the common-mode current is returned within
the power supply through this capacitor rather than it returning through the AC
input power source. C1 also reduces the systemís source impedance, so a
common-mode series inductor, L1, now becomes realizable.
key in designing the common-mode filter is choosing the value of C1. From an
electromagnetic interference (EMI) point of view, the bigger it is the better.
Higher capacitance produces a smaller EMI signal with less source impedance, so
you win on a capacitance-squared basis.
However, higher capacitance also means larger line-frequency current in
the chassis connection. Note that there are safety limits to this current, to
reduce the possibility of shock in case the power-supply chassis connection is
broken and somebody ends up in the current path as shown in Figure
Figure 2: C1 can become
a shock hazard.
Std 601-1 limits this current to 0.5 mA RMS and there are stricter regulations
being discussed. For a 230-volt input, IEC effectively limits the value of C1 to
summarize, high dV/dt voltage waveforms driving parasitic capacitance to chassis
ground create common-mode currents. The currents are particularly hard to filter
because of their high source impedance. Filtering requires a chassis capacitor
which provides an alternate local return path and reduces the impedance. While
from an EMI filter point of view, more capacitance is desirable, total
capacitance is limited by safety concerns.
For a more detailed discussion on this topic, refer to
Topic 3 of the 2003 Unitrode Power Supply Seminar: www.ti.com/2003powerseminar-ca.
Please join us next month when we will discuss the common mode filter
inductor that completes the common mode filter for EMI reduction in offline
information about this and other power solutions, visit: www.ti.com/power-ca.
Robert Kollman is a Senior Applications Manager and
Distinguished Member of Technical Staff at Texas Instruments. He has more than
30 years of experience in the power electronics business and has designed
magnetics for power electronics ranging from sub-watt to sub-megawatt with
operating frequencies into the megahertz range. Robert earned a BSEE from Texas
A&M University, and a MSEE from Southern Methodist University.