|Themes > Science > Physics > Electromagnetism > Electrostatics > Capacitors & Dielectrics > Capacitors in Real-World Applications > Basic Considerations: DF, Q, and ESR|
Dissipation Factor (DF)
DF and "loss tangent" are largely equivalent terms describing capacitor dielectric losses. DF refers specifically to losses encountered at low frequencies, typically 120 Hz to 1 KHz. At high frequencies, capacitor dielectric losses are described in terms of loss tangent (tan ð). The higher the loss tangent, the greater the capacitor's equivalent series resistance (ESR) to signal power. In addition, the poorer its Quality Factor (low Q), the greater its loss (heating) and the worse its noise characteristics.
When a capacitor is used as a series element in a signal path, its forward transfer coefficient is measured as a function of the dielectric phase angle, (theta). This angle is the difference in phase between the applied sinusoidal voltage and its current component. In an ideal capacitor, (theta) equals 90°. In low-loss capacitors, it is very close to 90o. (See Figure 3)
For small and moderate capacitor values, losses within the capacitor occur primarily in the dielectric, the medium for the energy transfer and storage. The dielectric loss angle, ð, is the difference between (theta) and 90o and is generally noted as tan o. The name "loss tangent" simply indicates that tan ð goes to zero as the losses go to zero. Note that the dielectric's DF is also the tangent of the dielectric loss angle. These terms are used interchangeably in the literature.
Quality Factor (Q)
Quality Factor (Q) is the ratio of the
energy stored to that dissipated per cycle. For a reactive component, this
Equivalent Series Resistance (ESR)
Equivalent series resistance (ESR) is
responsible for the energy dissipated as heat and is directly proportional
to the DF. A capacitor should be depicted as an ESR in series with an
ideal capacitance (C). ESR is determined by:
Circuit designs employing low Q capacitors usually produce large quantities of unwanted heat because tan ð and DF (or 1/Q) typically increase in a non-linear fashion with rising frequency and temperature. With some capacitors, this effect is enhanced by the naturally occurring decreased capacitance at high frequencies. High currents also produce increase heat, which in turn again increases the ESR and DF.
Even with substantial changes in current flow, high Q (low DF) capacitors will not exhibit the value shifts common to equivalent components exhibiting high DF, ESR, and other parasitics. Low ESR reduces the unwanted heating effects that degrade capacitors. This is an important goal in designing these components for high -current, high-performance applications, such as power supplies and high-current filter networks.
As Figure 4 shows, the significance of loss-contributing factors depends to some degree on the value of the capacitor.