The first rule of good EMC design is to eliminate noise at its source. For a switching power supply, which often seems like a natural noise generator, eliminating noise at its source can be a tall order. Synthesis methods provide effective tools for power supply designers to use to eliminate noise at its source. Parts 1 and 2 of this series describe these synthesis methods that achieve improved EMC properties for nonisolated converters. In Part 3, we applied similar synthesis methods to isolated full- and half-bridge circuits to achieve equivalent converters with terminal ripple current and common mode noise cancellation. Now, let's look at isolated single-ended flyback and forward converters and push pull converters with the same focus.
Fig. 1 illustrates a conventional single-ended flyback converter with some of its parasitic circuit elements. The circuit has both pulsating input current and pulsating output current, which generates conducted differential mode noise at both line and load terminals. The high dV/dt nodes are unbalanced, so common mode noise from displacement currents is potentially a big problem. Fig. 1 illustrates two of the displacement current paths, although several other important displacement current paths aren't shown. The dotted terminals of the transformer windings are monopole generators for radiation of electric fields, and the primary side generator is in phase with the secondary side generator — so electric fields from the two generators add. We can create dipole generators from monopole generators simply by adding a solid copper ground (image) plane.
Dipole radiators are superior to monopole radiators, because they provide first-order electric field cancellation in the far field. We can improve the situation by applying synthesis methods that improve the terminal current properties, reduce the magnitudes of the high dV/dt generators, and create a balanced winding structure that will increase the order of the poles of the high dV/dt generators — thereby improving the degree of displacement current and electric field cancellation.
Synthesis of Single-Ended and Push-Pull Converters
Fig. 2(a), on page 48, illustrates input and output of two terminal networks that form the core of the single-ended flyback converter. We are deleting input and output capacitors and line and load from the circuit because they play no role in the synthesis. We are left with two magnetically coupled two terminal networks, an AB network that comprises the main switch and the primary winding, and a CD network that comprises the output rectifier and the secondary winding. In the AB and CD networks, the dotted terminal of the winding is the high dV/dt node. We can split each of the two windings into two equal series connected windings — each with half the number of turns of the original winding, as you can see in Fig. 2(b), based on the reasoning stated in Key Point No. 2 of Part 1 of this article series.
We can reverse the positions of the switches with their adjacent windings, based on the reasoning stated in Key Point No. 4 of Part 1 of this article series, as shown in Fig. 2(c), to achieve a circuit that is fully balanced and reduces the magnitudes of the high dV/dt generators to half of their original values and adds a pair of equal and opposite generators. Now, the high dV/dt generators are not only reduced in magnitude, but the electric fields created by the generator pairs of equal magnitude cancel in the far field, and the displacement currents created by the equal and opposite high dV/dt generators will also cancel. We can add two pairs of windings to the transformer and two pairs of capacitors to achieve input and output terminal ripple current cancellation, based on the reasoning stated in Key Point No. 1 of Part 2 of this article series and as you can see in Fig. 2(d). The new primary windings will have the same number of turns as the two primary windings that existed previously in Fig. 2(c). Similarly, the two new secondary windings will have the same number of turns as the secondary windings of Fig. 2(c). We connect the windings and capacitors so the capacitors connect winding terminals with voltage waveforms that are in phase and of equal magnitude. We'll assume, for analytical purposes, that the capacitors are large and that their voltages are invariant. The voltage of the primary side capacitors is equal to the line voltage. The voltage of the secondary side capacitors is equal to the load voltage. Capacitor C1 connects the undotted terminal of winding WP3 with the undotted terminal of winding WP2. Since (1) the dotted terminals of windings WP2 and WP3 are both connected to ac grounds, (2) the windings have the same number of turns, and (3) they are magnetically coupled we can assert that these two winding terminals have identical ac voltage waveforms. Therefore, they can be capacitively coupled, based on the reasoning of Key Point No. 6 of Part 1 of this article series. The capacitors provide a path for ac current to flow to the new windings and a path for dc line currents to flow when the switch S is not conducting.
For example, when switch S is closed, the undotted terminals of the primary windings become positive with respect to their dotted terminals. In each winding, current increases (ramps up) from the undotted terminal to the dotted terminal, as illustrated in Figs. 3(b) and 3(c), on page 50. Consider the network terminal A. Current is ramping up toward the A terminal in winding WP3 and ramping up away from the A terminal in winding WP1. The two current ramps at the A network terminal cancel so that the current at the A network terminal is dc. At each of the four network terminals the winding current ramps cancel and the net current is dc. During the time that switch S is on, all four capacitors are discharging and during the time switch S is off, all capacitors are charging.
Fig. 3(a) shows the primary switch current that's unchanged from the conventional flyback converter. Fig. 3(b) shows WP1 and WP2 primary winding currents. Fig. 3(c) shows WP3 and WP4 winding currents. The WP3 and WP4 winding currents are equal to the ac components of the WP1 and WP2 winding currents. The line current, as illustrated in Fig. 3(d), has no ac component due to the ripple current cancellation mechanism. Notice that the winding current ramp slopes are less than the current ramp slope for the switch. The ac switch current is shared (split) between windings so the total net ac primary winding current is equal to the ac switch current, but the ac current in any primary winding is only half of the ac switch current. Fig. 3(e) illustrates the diode current. Fig. 3(f) illustrates the WS1 and WS2 winding current, and Fig. 3(g) illustrates the WS3 and WS4 winding current. Fig. 3(h) illustrates the load current.
Fig. 4(a) shows the core ingredients of a single-ended forward converter. We can apply the synthesis techniques described above to split each winding and rearrange the series connections to achieve a fully balanced equivalent circuit, as illustrated in Fig. 4(b). Compare the Fig. 4(b) circuit to the Fig. 2(b) and Fig. 2(c) circuits. In Fig. 4(b), we split and balance both choke and transformer windings. The Fig. 4(b) circuit achieves cancellation of displacement currents, but the terminal current properties are unchanged (i.e., the line current is pulsating and the load current is triangular). In Fig. 4(c), we add primary and choke windings and capacitors to achieve a circuit with input and output terminal ripple current cancellation.
The Fig. 4(c) circuit remains fully balanced so displacement current cancellation is also achieved. Notice that Fig. 4(c) includes an optional reset diode so the added windings can serve as a path for dc reset magnetizing current, if desired. The Fig. 4(c) circuit may not be a good candidate for applications requiring fast load current transient response due to the net zero output current slope. Another alternative, shown in Fig. 4(d), drives the additional primary windings with a second primary switch to form a push pull primary circuit. The secondary circuit is also altered in Fig. 4(d) to provide push pull secondary windings and a second diode associated with the second primary switch. The freewheeling diode is no longer necessary in Fig. 4(d).
Notice that the primary circuit of Fig. 4(d) is identical to the primary circuit of Fig. 6 of Part 3 of this article series, which was derived from a full-bridge forward converter. This suggests that we can start with a push-pull circuit and, by manipulating the windings and switches in a series of simple steps, change the circuit to an equivalent full bridge circuit, or vice versa. The secondary circuit of Fig. 4(d) can be used with the balanced bridge circuits revealed in Part 3 of this article series to form bridge circuits that are fully balanced from line to load and achieve ripple current cancellation at line and load terminals.
References ,  provide many additional examples of single-ended and push-pull circuits with EMC advantages using the synthesis methods described above. Reference  provides experimental results of circuit balancing on common mode noise performance. Reference  describes a push-pull forward converter with a fully balanced primary circuit with line terminal ripple current cancellation. References ,  provide additional examples of forward converters with balanced primary circuits and line terminal ripple current cancellation.
Pulling It All Together
By pulling each of the parts of this four-part series together, we can see how to apply circuit synthesis methods to a wide variety of converter types to achieve new equivalent circuits with much lower electromagnetic emissions. For most power converter circuits, an equivalent circuit that achieves terminal ripple current cancellation and displacement current cancellation can be synthesized using the methods described in this series. These synthesis methods provide effective tools for power supply designers to use to eliminate noise at its source.
Wittenbreder, E.H., “Power Electronic Circuits with Ripple Current Cancellation,” U.S. Patent 6,437,999.
Wittenbreder, E.H., “Synthesis Methods for Enhancing Electromagnetic Compatibility and AC Performance of Power Conversion Circuits,” U.S. Patent 6,507,176.
Shoyama, M., Ohba, M., and Ninomiya, T., “Balanced Buck-Boost Switching Converter to Reduce Common Mode Conducted Noise,” PESC 2002 Proceedings.
Herbert, E. “Analysis of the Near-Zero Input Current Ripple Condition in a Symmetrical Push-Pull Power Converter,” HFPC 89 Proceedings.
Leu, C.S., “Forward Converter For Off-Line Applications,” U.S. Patent 5,640,318.
Leu, S.C., “Integrated Filter Forward Converters,” U.S. Patent 5,907,479.
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