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GENERAL TEST-SET-UP
Figure
1 Show a general DC/DC converter test set-up. Except
where otherwise required, the following conditions apply:
¡E
The
input voltage is nominal DC input voltage.
¡EThe
load is set to the rated output load (full load).
¡EThe
ambient temperature is 25¢J.
All
connections to the converter 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 converter measurements. Figure 2 shows
a voltage measurement being made from the output terminals
of a DC/DC converter by means of separate contacts which
do not carry load current. If contacts carrying load
current are used for measurement, an erroneous reading
of many millivolts can result.
Make and record the following measurements:
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1. Output voltage at
nominal input voltage (VON).
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2. Calculate output
voltage accuracy from the
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following formula.
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VO
is output voltage specified in data sheet
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LINE REGULATION
Make and record the following measurements:
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1. Output voltage at nominal input voltage (VON).
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VD is maximum
output voltage deviation measured.
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2. Output voltage at maximum input voltage (VOH).
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3. Output voltage at minimum input voltage (VOL).
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4. Calculate line regulation from the following formula.
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LOAD REGULATION
Make and record the following measurements:
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1.Output voltage with full load (VOF).
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2. Output voltage with minimum load specified in data sheet (VOM).
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3. Calculate load regulation from the following formula.
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EFFICIENCY
Make and record the following measurements:
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1. Output voltage at nominal input voltage (VON).
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VIN is nominal
input voltage and IO is output current.
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2. Input current at nominal input voltage (IIN).
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3. Calculate efficiency from the following formula.
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 This
is an AC measurement at the output of a power converter
at rated load and 25] ambient temperature. The measurement
is made in either millivolts RMS or millivolts 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 is almost always specified as peak-to-peak
rather than RMS value. A 50 millivolt 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.
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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 (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 which 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 which is specified for many switching power supplies
is shown in Figure 6.
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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 lead should
connect right to the ground ring of the probe and
be as short as possible. The oscilloscope bandwidth
should be 20 MHz and the oscilloscope should be
connected to AC ground. |
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