Power Tip 47: Tame conducted common-mode emissions in isolated switchers (Part 1)
In 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.
In 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
drives common-mode 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.
This switching voltage creates current in the stray capacitance between the transformer’s 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.
This 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 specifications.
This 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 kΩ at typical power-supply switching frequencies. Simply adding an inductor in the current path to reduce the current is impractical.
For instance, if we wanted to reduce the current by a factor of 10, it requires 100 kΩ 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 system’s source impedance, so a common-mode series inductor, L1, now becomes realizable.
One 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 2:
Figure 2: C1 can become a shock hazard.
IEC 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 4700 pF.
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 power supplies.
For more information about this and other power solutions, visit: www.ti.com/power-ca.About the author
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