Primary current in Half-bridge vs flyback

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grizedale

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In Half-Bridge SMPS, the peak primary current is, generally speaking, the same height as in continuous mode flyback......this is because with Half-Brdige SMPS's, the primary coil only gets half of Vin switched across it, due to the rail-splitting capacitors.

So why do we say that the Half-Bridge SMPS is better than flyback SMPS from an EMC point of view?
 

Hello
High di/dt loop acts as a loop antenna and is a source of radiated EMI.
One should not overlook the high dv/dt nodes in the converter topology. Parasitic capacitive coupling (like heat sink capacitance) is a major source of common mode conducted EMI. For a half bridge topology the max switch voltage stress is Vi as against Vi + n*Vo for a flyback. This can be one of the reasons for better EMC performance of a half bridge.

Regards
 

All converter topologies have discontinuous currents in some paths, but flyback tends to be the worst, since it is discontinuous on both input and output, regardless of its operation. Also as techoqwerty states, high dv/dt nodes are also a source of EMI, and flyback has the worst dv/dt as well (though this can be addressed with snubbers).
 

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