How Is Allowable Ripple Current Measured?
There is no industry standard on how to measure allowable ripple currents. TDK specifies a maximum increase of 20°C from ambient temperature due to ripple current for all MLCCs. In actuality, increases in temperature are estimated indirectly when the ESR of MLCCs and thermal resistance are measured. Joule heating continues to warm an MLCC as long as the ripple current flows, but the part will also radiate heat, which cools the part. Temperature increases are determined with the balance between this heat generation and heat radiation.
The ratio of the amount of heat generation per unit time and temperature increase is called thermal resistance, and if the MLCC case sizes and material are the same, they will have the same thermal resistance. Therefore, if the ESR is known, the amount of power dissipation per unit time can be calculated. The amount of increased temperature can be calculated by multiplying the power dissipation figure with the thermal resistance. The maximum ripple current allowed for a temperature increase of 20°C is calculated with this method to find the allowable ripple current.
How Can I Measure a Capacitor’s Insulation Resistance?
Insulation Resistance (IR) is the extent to which the dielectric material in a capacitor resists leakage current. It is the resistance of the dielectric material itself*1. IR is measured by leakage current. Knowing the leakage current and applied voltage, the insulation resistance can be calculated based on the ohm’s law. There are two basic ways to measure the leakage current. First, apply an ammeter in series with the capacitor and voltage source (see Figure 1).
Second, apply a voltmeter in parallel with a resistor, and then connect in series to the capacitor and voltage source (See Figure 2).
The first method is usually applied to capacitors less than 1uF. Low capacitance capacitors have low leakage current; thus, a low current ammeter can measure the current accurately. If the leakage current is high, the ammeter will not able to measure accurately due to the noise and unstability of the charged capacitor. Therefore, the second method should be used for higher capacitance capacitors*2.
Figure 1: Series ammeter with capacitor.
Figure 2: parallel voltmeter with a resistor and series to a capacitor.
*1 “Glossary of Capacitor Terms”, Ray Ostlie （1989）
*2 Keithley Switching Handbook, 3rd Edition, 1995, Page 4-12
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