Green mode fly-back converters
Many power supply products are marketed under the “Green Power” label meaning that they are designed to maximise efficiency across the load range (known as average active mode efficiency) and minimise power consumed at no load. Active mode efficiency is the average of four measurements made at 25, 50, 75 & 100% of full load.
There are multiple pieces of legislation applicable to external power supplies (EPS) including the ErP directive (Energy related Products), CEC (California Energy Commission), EISA (Energy Independence & Security Act), NRCan (Natural resources Canada). Many power supply makers are also marketing component power supplies with similar specifications designed to enable users to meet green criteria for end applications.
The simplest approach is the “green mode” fly-back converter which is suitable for supplies up to around 100W.
The basic mode of operation is a fixed frequency design typically switching around 60 – 70 kHz. As the load reduces the switching frequency also reduces to minimise the number of switching cycles per second, reducing switching losses and maximising efficiency across the load range. The switching frequency reduction stops at around 22 kHz to remain in the ultrasonic range of the human ear. At very light or zero load the power supply enters a burst mode to minimise the power consumption.
The graph below shows the general concept & the oscilloscope traces show the switching waveform of XP’s ECS100 green mode component power supply at full load (switching at 62kHz) at 10% load (switching at 35kHz) and at zero load when the supply has entered burst mode to reduce the power consumed to <0.5W. Individual bursts occur at a repetition rate of 900Hz.
A side effect of burst mode operation can be audible noise at no load or very light load. Components with parts which can move under electrical stress can act as transducers and emit audible noise. These may be wound components, filter capacitors, line capacitors & snubber capacitors.
Steps are taken to mitigate audible noise such as varnish impregnation of transformers, changing ceramic capacitors to film types in key areas to avoid piezo electric effects and controlling burst mode frequency to avoid the area’s most sensitive to the human ear (2 kHz – 4 kHz). These steps may not eradicate audible under all conditions but go a long way to minimising the effects.
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