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Capacitance test method

By：JASENCAN

Time：2020.08.27

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A5. In order to accurately test large-capacity capacitors, the recommended test instrument is HP4284A, HP4278A or the same level instrument.
The HP4284A with the high output option (option 001) increases the rms current source rating from 10 mArms to 100 mArms. The higher the current range, the greater the output power of the meter will be applied to the rms voltage level required by the DUT. HP4284A is also attached to a monitor with voltage and current levels, which displays the actual voltage and current levels seen on the DUT. One disadvantage of using HP4284A is the prolonged measurement time. HP4284A uses feedback configuration with level monitor to maintain all rms levels.
The measurement time of HP4284A can be calculated according to the following formula:
[(meas_timeshort) + (~ 115msec.)] x n (Formula 2)
Where n = 2 to 6.
If measurement speed is more important than accuracy, HP4278A is recommended. This meter does not have ALC (Automatic Level Control) function or voltage and current level monitor, but it can still use a voltmeter to verify AC voltage. HP4278A is an earlier version of HP4284A. Because of the faster measurement speed, it is sometimes called a "production" meter.
If measurement speed is more important than accuracy, HP4278A is recommended. This meter does not have ALC (Automatic Level Control) function or voltage and current level monitor, but it can still use a voltmeter to verify AC voltage. HP4278A is an earlier version of HP4284A. Because of the faster measurement speed, it is sometimes called a "production" meter.
Again, the point is that the meter must be able to provide accurate voltage to the DUT. By switching to a low output impedance range in order to compensate for the low impedance of the large-capacitance MLCC, or using a function that can automatically maintain the test level of the DTU.
A6. HP4284 has a function called ALC. By activating this function, the set voltage level can remain unchanged in the DUT. When using this instrument to measure large capacitance MLCC, make sure that the ALC function is “ON” (Figure 3-4). Failure to enable this function may cause the capacitance reading to be incorrectly read low.

Figure 3: Set ALC to On

Figure 4: ALC on the indicator
1. Confirm that the voltage and frequency settings of the capacitance tester are correct (table 1, unless otherwise specified by the manufacturer).
2. Use a voltmeter to test the DUT in conjunction with a capacitance tester to determine whether the actual voltage of the component meets the 1.0 ± 0.2 Vrms requirement.
3. If the instrument has ALC or equivalent function, make sure it is turned on.
4. Refer to the tester manual to determine what impedance settings can be used and whether the meter can automatically switch settings to compensate for capacitor impedance.

Figure 5: HP4263B impedance

Figure 6: HP4278A impedance

Figure 7: Capacitance and AC voltage
A8. As mentioned earlier, as the capacitance and frequency increase, the impedance of the DUT will decrease. To further illustrate this point, the following example shows by calculation how the impedance of the DUT affects the actual voltage seen on the DUT.
Use HP 4263B tester and HP4278A tester to test capacitors (C3216Y5V1A106Z). Putting a frequency of 1kHz and a capacitance of 10 µF into Equation 1 produces a capacitor impedance of approximately 16 Ω.
When 1.0 Vrms should be added to the capacitor from the test equipment, the voltage is divided between the meter impedance and the DUT impedance. The figure shows that the impedance of HP4263B remains 100 Ω, but for HP4278A, the calculated capacitor impedance becomes 1.5 Ω. As a result, for HP4263B, most of the applied voltage drops in the tester's impedance, while HP4278A enables the capacitor to receive most of the voltage (Figure 5-6). The result is that the indicated value displayed by HP4263B will be lower than the true value.
As shown above, setting the OSC voltage of the test instrument to 1.0Vrms does not guarantee that all external voltages will be printed on the DUT. It is not surprising that the voltage on the DUT is about 10% of the set value. Figure 7 shows how low VAC affects the measured capacitance.
The figure below shows the difference between ALC on and off. When the ALC function is off, the actual voltage on the DUT is about 10% of the set voltage of 1.0Vrms. When the ALC function is on, the voltage on the DUT is almost 100% of the set voltage. HP4284A has voltage and current level monitors, but you can still use a voltmeter to verify the actual voltage.
ALC function of HP4284A capacitance meter