Ripple is defined as the amount of AC voltage that is superimposed on the DC output voltage. It is usually specified as the maximum AC voltage expressed as % RMS of the rated DC output voltage at full load. This is a worst-case condition because, for capacitor input filter networks, ripple is directly proportional to load current, decreasing in value with decreasing current. Glassman power supplies have two components of ripple voltage; a low frequency (mains related) voltage derived from the AC line and a high frequency related (30-70kHz) voltage generated by the switching circuits in the supply.

Test Setup

To measure ripple, refer to Figure 5. C represents the blocking capacitor required to prevent DC voltage from entering the AC measuring circuit. A resistor, R, and a spark-gap, SG, protect the measuring circuit from possible current surges. R and C must be chosen to pass the lowest frequencies of interest without attenuation. Note that the total value of R includes the DC probe resistance of the oscilloscope.  Because of the high DC voltages involved, it is not possible to, use a single capacitor. Instead, C is constructed from C's, all connected in series. In this way, the DC voltage on any one capacitor is reduced by a factor equal to the number of capacitors used. The maximum voltage across R is limited to the spark gap, SG, voltage. As mentioned earlier, Glassman offers a series of three DC voltage dividers that also include a ripple measuring circuit.

Test Method

With a suitable ripple measuring circuit in place, set output voltage, load current, and input voltage to specified values. Read the peak-to-peak ripple on an oscilloscope. To convert p-p values to approximate RMS values, divide by 2.828.

Figure 5

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