Articles: Cases/PSU

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Output Voltage Ripple

The point of a switching power supply – and all PC power supplies are of the switching variety – is that the step-down power transformer operates at a frequency many times that of the alternating current in the mains. This helps reduce the dimensions of the transformer greatly.

The alternating mains voltage (at a frequency of 50Hz or 60Hz depending on the geographical region) at the PSU input is rectified and smoothed and applied to the switching transistor that converts the direct voltage back into alternating but at a much higher frequency (from 60 to 120kHz depending on the PSU model). This voltage comes to the high-frequency transformer that lowers it to the value we need (12V, 5V, etc) and then gets rectified and smoothed again. Ideally, the PSU’s output voltage should be strictly constant but it is impossible to get it from an alternating high-frequency current. The ATX12V Power Supply Design Guide standard demands that the output voltage ripple at full load be not higher than 50 millivolts for the +5V and +3.3V rails and not higher than 120 millivolts for the +12V rail.

During our tests we record oscillograms of the PSU’s output voltages at full load by means of a dual-channel oscilloscope Velleman PCSU1000 and show them all in a single diagram:

The top, middle and bottom graphs correspond to the +5V, +12V and +3.3V rails, respectively. To the right of the graphs the permissible maximums of the pulsations are given. As you can see, the +12V rail of this PSU meets the requirements easily. The +5V voltage is barely within the norm, and the +3.3V voltage violates the permissible limits. The high spikes of the oscillogram of the latter voltage indicate that the PSU doesn’t cope with filtering highest-frequency noise, which is usually the consequence of using poor electrolytic capacitors whose efficiency degenerates as the operating frequency is increased.

If the output voltage ripple is above the norm, it may affect the stability of operation of your PC system and interfere with audio cards and other such equipment.

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