This month's subject, dielectric absorption (DA), is somewhat esoteric in the world of capacitors. So, if you don't already know what it is, the odds are that you'll never need it. In fact, if you visit any capacitor manufacturer's website and search for DA, you'll get very few results. Most of them will be links to film capacitors — not because they exhibit dielectric absorption, but because they don't.
What is dielectric absorption? Let's return to our bucket analogy from Part 1A, envisioning the capacitor as a place to deposit and retrieve energy. If you fully charge the capacitor at rated voltage (fill the bucket) and then discharge it fully (empty the bucket), the dielectric absorption is a measure of how much charge reappears. In fact, one method of testing for DA is to charge the capacitor for a period at rated voltage DC, fully discharge through a resistor, wait for a period, and then retest the capacitor for the appearance of voltage across the terminals. Typically, the charge period is an hour, the discharge is through a 5Ω resistor for 10 seconds, and the recovery voltage is measured 15 minutes later. DA is the ratio of the recovery voltage to the rated voltage expressed as a percentage.
This may seem counterintuitive. If there's a perpetual motion mechanism in play, how does the capacitor recharge itself when it is disconnected? The answer is rather straightforward. When a voltage is applied, the electric field between the capacitor plates and the dielectric material polarizes, increasing the electric field. Polarization is an electrical stress applied to the molecules or ions in the dielectric material and causes mechanical distortion. When the capacitor is discharged and the stress is removed, the molecules relax, returning to a close approximation of their original position. If the capacitor is disconnected at that point, the small dipole movements that take place as the stress fully dissipates will generate a small voltage at the terminals. However, this voltage will decay over time due to DC leakage current (DCL).
In essence, what's going on is an electromechanical hysteresis effect. When the capacitor is charging, some of that energy is translated into a distortion of the dielectric material lattice via polarization. Some of this mechanical energy is retained after electrical discharge and subsequently produces residual voltage buildup as the dipoles de-align.
DA and DCL are the only two parameters that require a capacitor to be fully charged during testing. Most manufacturers perform a 100% capacitance test. However, this test is not conducted by charging. Instead, capacitance tests apply a small AC voltage to a capacitor and then compare the response of the tested device in a bridge circuit, which uses two or three balanced legs of precision resistors and capacitors to allow the test capacitor to be matched.
The DA of dielectric materials varies. Generally, dielectrics with low loss and low permittivity tend to have a lower DA than those with a higher loss or permittivity constant. The table below illustrates the various DA values for many different dielectric materials.
Interestingly, even though ceramic, tantalum, and aluminum materials — with their high dielectric constants — have the highest DA values (5-10%), they are usually referenced generically as either “N/A” or “high,” because DA is not an important factor in the majority of applications in which they are used, such as filtering or decoupling. However, if you do have a DA-sensitive application, you need to know the materials technologies with the lowest available DA values.
So, in which types of applications is DA important? Typically, in analog integrators, analog-to-digital converters, and sample-and-hold (S/H) circuits, which acquire and store charge. In fact, one good example of a DA-sensitive application is the sample-and-hold amplifier in the capacitance bridge mentioned above. Once a signal is sampled for use in the matching bridge circuit, the input voltage is matched to an internal reference. During the hold time of the S/H circuit, the voltage across the capacitor must not change due to either DA increase or DCL decrease. Consequently, S/H circuits are a prime example of the importance of capacitor selection with regard to both low DA and low DCL or high insulation resistance (IR).
Multilayer organic (MLO™) technology, one of the newest materials technologies available, is an unfired multilayer materials system based on affordable, organic, low-loss laminate cores and capable of highly accurate simulation. Capacitors made with this new technology (MLO capacitors) have been shown to exhibit the lowest dielectric absorption of all dielectrics: 0.0015%, versus all other technologies, including NP0 ceramics, which have DA values as high as 0.6%, and even low-DA polystyrene and polypropylene, which have DA values around 0.05% (see Table 1).
Dielectric absorption is something of an oddity in the capacitor industry. It is only of importance when you don't need it, as is often the case in many analog designs, such as the matching bridge circuits used to measure other capacitors. (This, by the way, should not be taken as a trend that these devices are advancing to the point of self-awareness).