General Test Set-Up
Figure 1 shows a general equipment set-up for testing DC/DC converters. Except where otherwise required, the following conditions should be applied: ● Nominal DC input voltage
● +25°C ambient temperature
● Full rated output load
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Measurements
| All connections to the converters should be made with great care, especially to the output pins. Standard four-terminal or Kelvin, measurement practices should always be observed in making DC/DC converters measurements. Figure 2 shows a voltage measurement being made from the output terminals of a DC/DC converter by means of separate contacts that do not carry load current. If contacts carrying load current are used for measurement, an erroneous reading of many millivolts can be resulted. | | |
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Output Voltage Accuracy
Make and record the following measurements: | 1.Output voltage at nominal input voltage （VON）. | | Output Voltage Accuracy = | VO - VON | X * | | VON | | VO is output voltage specified in the data sheet. | | | 2.Output voltage accuracy is derived by the formula. | |
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Line Regulation
Unregulated
Make and record the following measurements: | 1.Output voltage at nominal input voltage （VON）. | | Line Regulation = | VO - VD | X * | | VON | | ?VIN% | | VD is maximum output voltage deviation measured. | | | 2.Output voltage at maximum input voltage （VOH）. | | 3.Output voltage at minimum input voltage （VOL）. | | 4.Line regulation is derived by the formula. | |
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Regulated
Make and record the following measurements: | 1.Output voltage at nominal input voltage （VON）. | | Line Regulation = | VO - VD | X * | | VON | | | | VD is maximum output voltage deviation measured. | | | 2.Output voltage at maximum input voltage （VOH）. | | 3.Output voltage at minimum input voltage （VOL）. | | 4.Line regulation is derived by the formula. | |
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Load Regulation
Make and record the following measurements: | 1.Output voltage at full load （VOF）. | | Load Regulation = | VOM - VOF | X * | | VOF | | | | | 2.Output voltage at minimum load specified in the data sheet （VOM）. | | 3.Load regulation is derived by the formula. | |
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Efficiency
Make and record the following measurements: | 1.Output voltage at nominal input voltage （VON）. | | Efficiency = | VON x IO | X * | | VIN x IIN | | | | VIN is nominal input voltage and IO is output current. | | | 2.Input Current at nominal input voltage （IIN）. | | 3.Efficiency is derived by the formula. | |
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Output Ripple & Noise | This is an AC measurement at the output of a power converter at rated load and +25°C ambient temperature. The Measurement is made in either millivolts RMS or millimvolts peak-to-peak. Figure 3 shows the typical voltage waveform. | | In the case of DC/DC converters, the output ripple voltage is a series of small pulses with high frequency content and for this reason, it is almost always specified as peak-to-peak rather than RMS value. A 50 millivolts peak-to-peak output ripple from a DC/DC converter can have a very low RMS value – perhaps just 5V – but this type of specification would be of questionable value to the designer who must specify the power supply for his system. | |
| Because of the high frequency content of this ripple, special measurement techniques must be employed so that correct measurements are obtained. First, a 20MHz bandwidth oscilloscope is normally used for the measurement so that all significant harmonics of the ripple spikes are included. | | The actual ripple voltage measurement must be carefully made in order not to induce error voltages in the test equipment. Therefore, the conventional ground clip on an oscilloscope probe （see Figure 4） should never be used in this type of measurement. This clip, when placed in a field of radiated high frequency energy, acts as an antenna or inductive pickup loop, creating an extraneous voltage that is not part of the output noise of the converter. | |
| This noise pickup is eliminated as shown in Figure 5 by using a scope probe with an external ground band or ring and pressing this band directly against the output common terminal of the power converter while the tip contacts the voltage output terminal. This makes the shortest possible connection across the output terminals. | |
| Another method of measuring the output voltage ripple & noise that is specified for many switching power supplies is shown in Figure 6. A 30cm twisted pair of no. 20 AWG copper wire is connected to a 10uF capacitor of proper polarity and voltage rating. The oscilloscope probe ground led should connect right to the ground ring of the probe and be as short as possible. The oscilloscope bandwidth should be at 20MHz and connected to AC ground. | | |
