Take Advantage of the “Silent Switcher” Architecture

How Does It Work?

The first generation of “Silent Switcher” architecture used in IC LT8614, LT8640, and LT8641 reduces EM emissions by:
1. IC internal structure design and optimally placed package pins
2. well-controlled rising and falling edges
3. correct printed circuit board (PCB) design, which minimizes area of “hot loops” on PCB
4. minimization of GND impedance on PCBS
5. switching frequency spread spectrum modulation

Enhanced „Silent Switcher 2“ version implemented in IC LT8609S, LT8640S, LT8643S, LT8645S goes even further:
1. IC contains internal ground plane and the use of copper pillars instead of bond wires
2. IC integrates capacitors connected to pins BST and INTVcc, further minimizing the "hot loop"

Rise and fall time is in range of ns, yet the first circuit of this series, LT8614, has switching edges almost without ringing. The steeper the switching edges, the more the high frequency components signal contains. The higher frequency, the shorter antenna is required for the effective emissions. We don’t add antenna to circuit intentionally but many structures used as a part of system or PCB are antennas.

Radiation from Switching Voltage Regulator

Radiation can occur as either differential mode or common mode.
Differential-mode radiation is the result of the normal operation of the circuit and results from current flowing around loops formed by the conductors of the circuit. These loops act as small loop antennas that radiate predominately magnetic fields. Although these signal loops are necessary for circuit operation, their size and area must be controlled during the design process to minimize the radiation.
Common-mode radiation is the result of parasitics in the circuit and results from voltage drops in the conductors. The current that flows through the ground impedance produces a voltage drop. When cables are then connected to the system, they are driven by this common-mode ground potential, forming antennas, which radiate predominately electric fields. Because these parasitic impedances are not intentionally added to the system or mentioned in the documentation, common-mode radiation is often harder to understand and control.

Why Minimize Area of the“Hot Loops“?

Steep switching edges are required for normal operation of IC; the only way how to minimize radiation is minimizing the loop area by correct design of PCB.

Why Minimize GND Impedance?

Currents flowing through GND impedance create voltage drops. When frequency rises, impedance also rises due to inductance of GND tracks. When antenna is connected to GND, problem with EMC arises. Such antennas can be modeled as short dipole.

Intensity of electrical field E = K2 f L I_cm

• f – switching frequency and their harmonics
• I_cm – antenna current
• L – antenna length
When we combine spectrum envelope of trapezoidal signal with the frequency characteristic of antenna, we obtain common-mode radiation emissions envelope.


If the electric field intensity created by differential-mode emission is the same as for common mode, then K₁ f² A I_dm = K₂ f L I_cm. By solving I_dm/I_cm we get

I_dm/I_cm=K₂ L/ K₁ f A = 48e6 L/f A [Henry W. Ott, Electromagnetic Engineering Compatibility]

pre f=100MHz, loop perimeter 40mm => A=127,3mm²=127,3e-6m², and cable (antenna) length 1m je I_dm/I_cm= 3770

In other words, the common-mode radiation mechanism is far more efficient than the differential-mode radiation mechanism.
To minimize common-mode radiation, we have to minimize GND impedance.

How Spread Spectrum Helps?

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