This is our second section on noise countermeasures. As noise countermeasures, we will explain three different approaches to reducing noise, involving adding circuits and components for noise reduction.
Addition of a Snubber Circuit
Adding a snubber circuit is a commonly used method for reducing noise. Here we will describe addition of a snubber circuit to the output, but addition to the input is also possible. In this example, by adding an RC circuit to a switching node, the high frequencies of the ringing that occurs in switching are shunted to GND.
However, losses may occur depending on the snubber circuit added. If the capacitance of the capacitor is increased to enhance effectiveness, the resistor must be able to handle the power. Below is an example of calculation of snubber losses.
|Example of loss calculation:||Allowable losses of resistor for a case in which the snubber resistance is 10Ω, the snubber capacitance is 1000 pF, the input voltage is 12 V, and the oscillation frequency is 1 MHz
Snubber loss: P ＝ C ×V2 × fsw
1000pF × 122 × 1MHz ＝ 0.144W ⇒ a resistor rated power of MCR18(3216)： 0.25W or higher is necessary
Inserting a Bootstrap Resistor
An IC that uses an N-channel MOSFET as the high-side switch has a BOOT pin (the name may be different depending on the IC). This supplies the output voltage to a bootstrap circuit (often within the IC), and functions to provide a sufficient gate driving voltage to the high-side MOSFET. The BOOT pin is connected to the switching node, and so by inserting a resistor at this point, the voltage rise when the high-side MOSFET turns on can be made gradual. As a result, noise during switching-on can be alleviated. A disadvantage of this technique is a longer switching time, so that the MOSFET switching loss is increased.
Insertion of a Resistor at the High-Side MOSFET Gate
In this method, by inserting a resistor between the gate and the gate driver of the high-side MOSFET, gate charge is limited, and rising and falling of the high-side MOSFET are made gradual or "blunted", so to speak, to reduce noise during both turn-on and turn-off. As with addition of a bootstrap resistor, the MOSFET switching loss increases. However, this method cannot be used with an IC that has an internal switch. It is only available for use in configurations in which a controller IC is used with an external switch.
Next time, we will summarize our discussion of PCB layout.
・A snubber can reduce ringing in switching, but there is a trade-off with increased losses.
・When a bootstrap resistor is added, rising-edge noise can be reduced, but MOSFET switching losses increase.
・By adding a resistor at the gate, both rising and falling noise can be reduced, but MOSFET losses are increased. Moreover, this resistor cannot be inserted when an IC with an internal MOSFET is used.