| Test Method - DC / DC Converter |
Circuit Test Drawing
Basic usefully circuit |
 Figure 1 |
 Figure 2 |
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General Test Set-Up
Figure 3 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
 Figure 3 |
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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 4 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.
|  Figure 4 |
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Output Voltage Accuracy
Make and record the following measurements:
| 1.Output voltage at nominal input voltage (VON). |
| Output Voltage Accuracy = | VON - VO | X 100% |
| VO |
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| 2.Output voltage specified in the data sheet (VO). |
| 3.Output voltage accuracy is derived by the formula. |
| For example, an output voltage measurement of 12.01VDC yields |
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Line RegulationMake and record the following measurements:
| 1.Output voltage at nominal input voltage (VON). |
| Line Regulation = |  |
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| 2.Output voltage at maximum input voltage (VOH). |
| 3.Output voltage at minimum input voltage (VOL). |
| 4.Maximum Measurement of output voltage (VD). |
| 5.Line regulation is derived by the formula. |
| For example, VOH equals 12.02VDC, while VOL equals 12.005VDC using 12.02 VDC yields: |
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Make and record the following measurements:
| 1.Output voltage at nominal input voltage (VON). |
| Line Regulation = |  |
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| 2.Output voltage at maximum input voltage (VOH). |
| 3.Output voltage at minimum input voltage (VOL). |
| 4.Maximum Measurement of output voltage (VD). |
| 5.Line regulation is derived by the formula. |
Where △VIN % is the change in input line as a percentage
For example, VOH equals 12.02VDC while VOL equals 12.005VDC using 12.02VDC and input voltage 9VDC~12VDC |
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Load Regulation
Make and record the following measurements:
| 1.Output voltage at full load (VOF). |
| Load Regulation = | VOM - VOF | X 100% |
| VOF |
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| 2.Output voltage at minimum load specified in the data sheet (VOM). |
| 3.Load regulation is derived by the formula. |
For example, VOM equals 12.05VDC and VOF equals 12.01VDC
We derive: |
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Output Voltage Balance(Dual Output)
| 1.Plus Output voltage measured of nominal input voltage(+Vo). |
| Load Regulation = |  |
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2.Negative Output voltage measured of nominal input
voltage(-Vo). |
For example, if we measure the following
+VO=+12.01VDC
-VO=-11.999VDC
we derive: |
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Efficiency
Make and record the following measurements:
| 1.Output voltage at nominal input voltage (VON). |
| Efficiency = |  |
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| 2.Input Current at nominal input voltage (IIN). |
| 3.Efficiency is derived by the formula. |
| 4.VIN is nominal input voltage |
| 5.IO is output current |
For example, if we measure the following
VON=12.01VDC ; IO=755mA ; VIN=12VDC ; IIN=900mA
we derive: |
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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 5 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. |
 Figure 5 |
| 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 6)
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. |
 Figure 6 |
| This noise pickup is eliminated as shown in Figure 7 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. |
 Figure 7 |
| Another method of measuring the output voltage ripple & noise
that is specified for many switching power supplies is shown in
Figure 8. 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. |
 Figure 8 |
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Transient Recovery Time
| The time required for return to value of stabilization when a load in a step change cause output voltage of skew. |
 Figure 9 |
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Transient Response Deviation
| When a load was change in a very short time, the percentage of transient response amount is in output voltage. |
 Figure 10 |
| Transient variable rate must count “1” “2” respectively. |
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Peak-to-Peak Output Noise Measurement Test
Use a Cout ceramic capacitor. Please refer to capacitor value of every series)
Scope measurement should be made by using a BNC socket, measurement bandwidth is 0-20 MHz. Position the load between 50 mm and 75 mm from the DC/DC Converter. |
 Figure 11 |
 Figure 12 |
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Input Source Impedance
The power module should be connected to a low ac-impedance input source. Highly inductive source impedances can affect the stability of the power module.
In applications where power is supplied over long lines and output loading is high, it may be necessary to use a capacitor at the input to ensure startup.
Capacitor mounted close to the power module helps ensure stability of the unit, it is recommended to use a good quality low Equivalent Series Resistance (ESR < 1.0Ωat 100 KHz) capacitor, please refer to capacitor value of every series. |
 Figure 13 |
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Output Ripple Reduction
| A good quality low ESR capacitor placed as close as practicable across the load will give the best ripple and noise performance. To reduce output ripple, it is recommended to use reference of every serie’s capacitors at the output. |
 Figure 14 |
 Figure 15 |
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Test Method
DC / DC Converter
