SYSTEM DEVELOPMENT
Multiphase Synchronization: Controlling Conducted Noise in a Military-Grade DC-DC Switching Power Supply By Marco Giudici, Guest Editor Reducing noise generated on the input lines of DC-DC converters is necessary to meet stringent Electromagnetic Interference (EMI) standards in military applications. When connecting the input of DC-DC converters in parallel to the same bus, the task becomes more difficult. In this article, Christian Jonglas, technical support manager at GAIA Converter, discusses the issue and explains how the multiphase synchronization of a DC-DC switching power supply can result in significant reductions in noise levels. Switching Mode Power Supplies (SMPS) are preferred to linear types due to their efficiency and consequently their lightweight. However, across all of the possible conversion topologies, SMPSs generate differential mode (DM) and/or common-mode (CM) electrical noise on inputs and outputs, and electromag-
netic radiation at some level, due to fast transitions of switching transistors and diodes.
ers with ‘discontinuous’ input current such as ‘buck’ or isolated forward types.
DM noise appears across the line and return connections and CM noise is from the line and return together to the ground (Figure 1). When there is a requirement to meet a specification for noise, whether conducted or radiated, it is advantageous to minimize DM first, as this is a source of radiation on leads and also factors into CM measurements. In addition, a standard Line Impedance Stabilisation Network (LISN), used for CM measurement, registers a combination of CM and DM signals. For military-grade DC-DC converters, the Electro Magnetic Compatibility (EMC) limits to be met are listed in Table 1.
Boost converters have continuous input current at higher power although they can be operated in a discontinuous mode at lower powers. In all cases, the current waveform into the switch combines a square and sawtooth or triangular waveform. A capacitor internal to the converter (at its input) levels out the waveform to approximate DC, sourced from the supply.
Table 1 - Military EMC limits
Standard
Parameter
Frequency Range
Max. level
Mil-STD 461 Mil-STD 461 Mil-STD 461 Mil-STD 461 DO160
CE101 CE102 RE101 RE102 Cond. noise power line Rad. noise
10hz to 150khz 10khz to 100Mhz 10hz to 100Mhz z 10kHz to 100Ghz 150kHz to 152Mhz
110 dBµA 94 dBµV 180 dBpT 102 dBµV/m 73dBµV
100Mhz to 6Ghz
102 dBµV/m
DO160
Different Switching Topologies, Different Input Current Waveforms The level of DM noise on the input of a DCDC converter is measured as current with a specified series blocking inductor or voltage into 50 ohms, AC coupled onto the power lines. The amplitude varies with the switching topology and is generally highest for convertFormula 1 Figure 1 - Differential and Common Mode noise 20
COTS Journal | July 2022
The maximum residual ripple and noise are what is set by the EMC standard limit lines. If the input current is decomposed into a square and sawtooth/triangle elements, their timing, and amplitudes, Fourier analysis can predict
the spectrum of harmonics and their amplitudes. This analysis is key to developing an effective conducted emission filter design to apply attenuation as required. For example, with a triangle current waveform typical of a flyback converter in discontinuous mode, the current I sin(n), the amplitude of the sine wave representing the nth harmonic of the current waveform into the switch is:
