Following these 4 basic tips can help ease your pain when it comes to EMI compliance. Of course, the topic of EMI is a broad one, and there are many other tips that could be mentioned.
Recall that in Part I of our discussion, we focused on what happens when parasitic capacitances of components in a supply couple directly to the input wires to the power supply. Now, let’s look at what arguably is the most common source of common mode EMI problems: the power transformer.
Problems arise from the parasitic capacitance between the primary and secondary windings, and the high dV/dt on the primary winding. This inter-winding capacitance acts like a charge pump, causing stray current to flow to the secondary side, which is usually tied to earth ground. Here are four very common tips to minimize this problem.
- Wind the primary so that the highest dV/dt is on the outer layer. The voltage potential varies with each turn. In a flyback, for example, the largest voltage swing occurring on the end that connects to the drain of the FET (see Figure 1). By keeping the “quiet” layer adjacent to nearest secondary layer, the dV/dt seen across the inter-winding capacitance is minimized. With this technique, be aware that the outer winding can now become a problematic noise source that could couple to other objects near the transformer. A shield winding might be needed around the outer winding.
- Use a shield winding between primary and secondary windings. Inserting a single-layer winding with one end connected either to the input or input return shunts the stray currents away from the secondary windings and back to their source. The trade-off with this technique is a slightly more complicated transformer design, and increased leakage inductance.
- Use a “Y- capacitor” across primary ground to secondary ground. This capacitor provides the stray currents a lower impedance path back to the primary. This local path within the power supply prevents these currents from otherwise finding another way back to their source through earth ground. However, there are safety limits on high large of a capacitance can be used here.
- Add a common-mode choke. Sometimes the other techniques mentioned are not enough to reduce EMI below required levels. Adding a common-mode choke increases the common-mode impedance and can be very effective at reducing the conducted noise. The trade-off here is the cost of an additional component. When selecting a common-mode choke, be careful to inspect the impedance curves versus frequency. At some point, all chokes turn capacitive due to their own inter-winding capacitance. Robert Kollman has an excellent Power Tip on this subject.
Check out this design from PowerLab as an example of where some of these techniques were implemented…
- PMP9144– Universal AC Input, 14V/2.4A Flyback With 88% Efficiency
Figure 1. The potential on the windings varies with each turn. (This figure is reproduced from the 2010 Texas Instruments Power Supply Design Seminar, Topic 1; “Under the Hood of Flyback SMPS Designs”, by Jean Picard. This is an excellent reference resource for flyback design.)