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 the switching regulator can be greatly improved. However, fast switching transitions also have some negative effects. Interference increases dramatically when the switching transition frequency is between 20MHz and 200MHz. This forces developers of switch-mode power supplies to find a good compromise between high efficiency and low interference in the high frequency range. In addition, ADI has proposed the innovative
Silent Switcher
™ technology
, which can generate minimal electromagnetic radiation even with extremely fast switching edges.
Figure 1. A switch-mode power supply switches on and off with the input voltage applied at the switch node.
Figure 1 shows fast and slow switching transitions. Fast switching transitions can produce stronger interference coupling to adjacent circuit segments. PCB traces with sudden voltage changes can produce capacitive coupling to adjacent traces with high impedance. PCB traces with sudden current changes can produce inductive coupling to adjacent traces. These effects can be minimized by slowing down the switching transitions. Figure 2 shows a proven technique for asynchronous switching regulators. Here, a Schottky diode is used in one of the two switches. The switching transitions of the switch are slowed down by adding a resistor in series with the bootstrap capacitor C
BOOT
(which provides the gate voltage of the high-side n-channel MOSFET). This technique can be used in integrated switching regulators when it is not possible to adjust the gate signal line of the power MOSFET directly. If the switching controller is used with an
external
MOSFET, the resistor can also be inserted in the gate drive trace. The resistor value is typically less than 100Ω.
Figure 2. Using a bootstrap resistor to slow down switching 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 a resistor in the
C
BOOT
path does not significantly slow down the switching transitions. If a resistor in series with C
BOOT
is used here as well
(as shown in Figure 3), the switching transitions of the high-side switch will also be slowed down. However, this may result in the low-side switch not being completely turned off. As a result, the high-side switch and the low-side switch may be turned on at the same time and instantaneously. This will result in a destructive short circuit from the input voltage to ground. This is particularly critical because the switching transition speed is also affected by parameters such as the operating temperature and variability in semiconductor manufacturing. Therefore, even in laboratory tests, safe operation cannot be guaranteed. To slow down the switching transitions of synchronous switching regulators with integrated switches, synchronous switching regulators should be used, in which the switching transition speed can be directly set by internal circuitry, such as the
ADP5014
from ADI
. In these integrated circuits, it is internally ensured that when slowing down the switching transitions, both switches are not turned on at the same time, so no short circuit occurs, and
there is no resistor in the
C
BOOT
path.
Figure 3. A synchronous buck converter that can short due to slow high-side switch transitions.
Regarding fast switching transitions, there is one very important innovation in recent years that cannot be ignored. ADI’s Silent Switcher technology allows electromagnetic radiation from fast switching edges to be reduced by up to 40dB (10,000 times). As a result, switch-mode power supplies with ultra-fast edges and minimal EMC issues can be developed. In most cases, Silent Switcher devices do not require the switching transition speed to be reduced in order to reduce EMI. With Silent Switcher technology, the
difficult trade-off between maximum conversion efficiency and minimum electromagnetic interference is largely eliminated.
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