Before discussing how to eliminate noise in switching power supplies, we first understand the origin of switching power supply noise from the source. Subsequently, noise suppression strategies will be formulated for switching noise and for two types of DC-DC converters: those with external MOS and those with integrated MOS.
Before discussing how to eliminate noise in switching power supplies, we first understand the origin of switching power supply noise from the source. Subsequently, noise suppression strategies will be formulated for switching noise and for two types of DC-DC converters: those with external MOS and those with integrated MOS.
01 Current Paths of DC-DC Converters
First, the equivalent circuit of a synchronous rectification step-down DC/DC converter is used to understand the current paths of a switching power supply:
As shown in Figure 1, Q1 is the high-side switch and Q2 is the low-side switch. When Q1 is conducting (Q2 is off), the input current Iin flows from the input capacitor Cin (which is fully charged in the previous phase) through Q1 and inductor L to the output capacitor Cout and load Rload.
As shown in Figure 2, when Q2 is conducting (Q1 is off), Cout has been fully charged during the Q1 conduction phase. As Q1 just turns off, the back electromotive force of inductor L maintains the output current Iout. As the energy of the inductor weakens, Cout starts to discharge to sustain Iout. Referring to the blue dashed current line of the capacitor, the current path is from L through the load Rload to Q2.
The green frame in Figure 3 indicates the differences between the current paths in Figure 1 and Figure 2. The current in the red line changes drastically every time the switch turns ON/OFF. The extremely rapid current variation in this loop generates
high-frequency ringing within the loop due to the inductive component of PCB wiring, and the resulting voltage can be calculated by the following formula:
V=L×dtdiFor example, if a 1A current changes in 10ns in a wiring with an inductive component of 10nH, a 1V voltage will be generated.
02 Parasitic Components of DC-DC Converters
The red parts mark the parasitic components in the loop where the current changes drastically as shown in Figure 4. Wiring inductance exists in the PCB traces, typically about 1nH per 1mm. In addition, capacitors have equivalent series inductance (ESL), parasitic capacitance exists between the pins of MOSFETs, and the rise/fall time of switching MOS is only a few nanoseconds.
Therefore, as shown in the waveform diagram of Figure 5, the switching node generates ringing in the frequency range of
100MHz to 300MHz. The resulting current and voltage can be obtained by the two formulas provided below.
The red waveform is the switching noise component, and the cyan waveform is the fundamental wave component. Intense ringing at 100MHz to 300MHz appears during the rise/fall edges.
Such ringing acts as high-frequency switching noise and causes various effects. As shown in Figure 6, even if the loop is optimized, the residual switching noise component is conducted to the power supply terminal as common-mode noise.
Measures must be taken to limit noise by inserting high-impedance components such as inductors into the transmission line, and attention must also be paid to crosstalk. Although corresponding measures are adopted, the parasitic components of the PCB cannot be eliminated from the power supply IC, so the problem can only be solved through PCB layout design and the use of decoupling capacitors.
03 Summary
- In the loop where the current switches ON/OFF drastically during switching, high-frequency ringing (= switching noise) is generated due to parasitic components.
- Such switching noise can be reduced by optimizing PCB wiring and other methods. However, even so, the residual noise is conducted to the input power supply as common-mode noise, so measures need to be taken to prevent noise leakage.
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