A common way to reduce the EMI radiation of the high dv/dt in a buck converter, is to slow down the SW node. By slowing down the SW node, voltage peaks from the device parasitic capacitances are less pronounced, reducing the associated radiating electric field. For many devices this is accomplished by a resistor in series with the boot capacitor. However, as the TPSM33620-Q1 has an integrated boot capacitor, the boot capacitor is not accessible. Furthermore, adding resistance in series with the boot capacitor can cause the device to generate more heat as this forces the high side FET to be on for a longer amount of time. With only access to SW, a snubber circuit is the only method on TPSM33620-Q1 to reduce the dv/dt speed.

Figure 3-2 shows the switching node of the TPSM33620-Q1. As is the case with any high dv/dt loop there are overshoots and undershoots in the high frequency range. These voltage spikes create correlated EMI peaks at these frequency ranges. Measuring the frequency of the dv/dt peaks yields frequencies in the 400MHz to 500MHz range, which matches the EMI spikes seen in Figure 3-3 and Figure 3-4. As a result of this noisy switch node, the TPSM33620-Q1 misses EMC by a couple dB in the 400MHz to 500MHz frequency range.

To combat these emissions, a capacitor and resistor is added to “snub” the high frequency dv/dt by dampening the parasitic inductance in this node. The snubber circuit siphons energy from the switch node, reducing the impact on EMI. This technical article can be used to calculate the required RC constant to effectively dampen the switch node. It is worth noting that by decreasing the resistance and increasing the capacitance of the snubber, the amount of energy that is attenuated can be increased. For the TPSM33620-Q1, the snubbing resistor was reduced to 1Ω and the snubbing capacitor was increased to 1000pF to compensate. Figure 3-5 shows the reduced dv/dt spikes in the switch node. As a result, Figure 3-6 and Figure 3-7 show an improvement in the high-frequency areas where the switching ring was generating noise. While effective at reducing EMI, the snubber circuit dissipates more energy decreasing the efficiency of the application. With this snubber circuit, TPSM33620-Q1 dissipates 30mW more energy at a 2A load.