How to achieve both high switching efficiency and low switching interference?

Fast switching transients in switching regulators are advantageous because this significantly reduces switching losses in switch-mode power supplies. Especially at high switching frequencies, the efficiency of the switching regulator can be greatly improved. However, fast switching transitions also have some negative consequences. Interference increases dramatically at switching frequencies between 20MHz and 200MHz. This leaves switch-mode power supply developers having to find a good compromise between high efficiency and low interference in the high frequency range. In addition, ADI has proposed innovative Silent Switcher? technology, which can produce minimal electromagnetic radiation even with extremely fast switching edges.

Figure 1 shows fast and slow switching transitions. Fast switching transitions can cause stronger interference coupling to adjacent circuit segments. PCB traces that experience sudden voltage changes can create capacitive coupling with adjacent traces that have high impedance. PCB traces with sudden changes in current flow can cause inductive coupling with adjacent traces. These effects can be minimized by slowing down switching transitions. Figure 2 shows a proven technique for asynchronous switching regulators. Here, a Schottky diode is used in one of the two switches. Placing a resistor in series with the bootstrap capacitor CBOOT (which provides the gate voltage of the high-side n-channel MOSFET) slows down the switching transitions of the switch. This technique can be used with integrated switching regulators when it is not possible to directly adjust the gate signal line of the power MOSFET. If using the switching controller with an external MOSFET, resistors can also be inserted into the gate drive traces. Resistor values ​​are usually less than 100Ω.

However, most modern switching regulators are synchronous switching regulators with high-side and low-side active switching. Here, using a resistor in the CBOOT path does not significantly slow down the switching transition. If a resistor in series with CBOOT is used here again (as shown in Figure 3), it will also slow down the switching transitions of the high-side switch. However, this may result in the low-side switch not fully closing. Therefore, the high-side switch and the low-side switch may turn on instantaneously at the same time. This will cause a destructive short circuit from the input voltage to ground. This is particularly critical because switching switching speed is also affected by parameters such as operating temperature and variability in semiconductor manufacturing. Therefore, even when tested in a laboratory, safe operation cannot be guaranteed. To slow down the switching transitions of a synchronous switching regulator with an integrated switch, use a synchronous switching regulator whose switching speed can be directly set via internal circuitry, such as Analog Devices' ADP5014. In these integrated circuits, it is internally ensured that when slowing down the switching transitions, both switches do not conduct at the same time, so a short circuit does not occur, and there is no resistance in either CBOOT path.

Regarding fast switching transitions, there has been a very important innovation in recent years that cannot be ignored. ADI's Silent Switcher technology significantly reduces electromagnetic radiation on fast switching edges by up to 40dB (10,000 times). therefore. Switch-mode power supplies can be developed with ultra-fast edges and minimal EMC issues. In most cases, Silent Switcher devices do not need to reduce switching speed to reduce EMI. With Silent Switcher technology, the trade-off between maximum conversion efficiency and minimum electromagnetic interference is largely eliminated.