How to slow down switching transitions in an asynchronous buck converter

Fast switching transients in switching regulators are advantageous because they significantly reduce switching losses in switch-mode power supplies. Especially at high switching frequencies, the efficiency of switching regulators can be greatly improved. However, there are also some downsides to fast switching transitions. Interference increases dramatically for switching frequencies between 20 MHz and 200 MHz. This makes it necessary for switch-mode power supply developers to find a good compromise between high efficiency and low interference in the high frequency range.In addition, Analog Devices has come up with innovative Silent Switcher™ technology that enables even extremely fast switching edges to produce the smallest

Fast switching transients in switching regulators are advantageous because they significantly reduce switching losses in switch-mode power supplies. Especially at high switching frequencies, the efficiency of switching regulators can be greatly improved. However, there are also some downsides to fast switching transitions. Interference increases dramatically for switching frequencies between 20 MHz and 200 MHz. This makes it necessary for switch-mode power supply developers to find a good compromise between high efficiency and low interference in the high frequency range. In addition, Analog Devices has proposed innovative Silent Switcher™ technology, which can generate minimal electromagnetic radiation even with extremely fast switching edges.
 

How to slow down switching transitions in an asynchronous buck converter

Figure 1. Switching a switch-mode power supply with an input voltage applied at the switch node.

Figure 1 shows fast and slow switching transitions. Fast switching transitions create stronger interference coupling to adjacent circuit segments. PCB traces with voltage abrupt changes can capacitively couple with adjacent traces with high impedance. PCB traces with current abrupt changes can inductively couple with adjacent traces. These effects can be minimized by slowing down switching transitions. Figure 2 shows a proven technique for asynchronous switching regulators. Here, one of the two switches uses a Schottky diode. Putting a resistor in series with the bootstrap capacitor CBOOT, which provides the gate voltage of the high-side n-channel MOSFET, slows the switching transitions of the switch. This trick can be used to integrate switching regulators when it is not possible to directly adjust the gate signal line of the power MOSFET. If the switch controller is used with an external MOSFET, resistors can also be inserted into the gate drive traces. The resistance value is usually less than 100 Ω.
 

How to slow down switching transitions in an asynchronous buck converter

Figure 2. Using a bootstrap resistor to slow down switching transitions in an asynchronous buck converter.

However, most modern switching regulators are synchronous switching regulators with high-side and low-side active switches. Here, the use of resistors in the CBOOT path does not significantly slow down the switching transitions. If a resistor in parallel series with CBOOT (as shown in Figure 3) is used here again, it will also slow down the switching transitions of the high-side switch. However, this may result in the low-side switch not being fully turned off. 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 especially critical since switching transition speeds are also affected by parameters such as operating temperature and variability in semiconductor manufacturing. Therefore, even in laboratory testing, safe operation cannot be guaranteed. To slow down the switching transitions of a synchronous switching regulator with integrated switches, use a synchronous switching regulator that can directly set the switching transition speed through internal circuitry. For example, the ADP5014 from Analog Devices. In these ICs, it is internally ensured that when slowing down the switching transitions, the two switches do not conduct at the same time, so there is no short circuit, and there is no resistance in the CBOOT path.

How to slow down switching transitions in an asynchronous buck converter

Figure 3. A synchronous buck converter that can be shorted due to slowed high-side switching transitions.

Regarding fast switching transitions, there has been a very important innovation in recent years that cannot be ignored. Analog Devices’ Silent Switcher technology dramatically reduces electromagnetic emissions from fast switching edges by as much as 40 dB (10,000 times). As a result, switch-mode power supplies with ultrafast edges and minimal EMC issues can be developed. In most cases, Silent Switcher devices do not require reduced switching speeds to reduce EMI. With Silent Switcher technology, the trade-off between maximum conversion efficiency and minimum EMI is largely eliminated.

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