Function of Capacitor in High Speed PCB Design(III)

3. Equivalent shunt resistance EPR RL: the leakage resistance of a capacitor, as we call it, RL is an important parameter in AC coupling applications, storage applications (such as analog integrators and samplers) and when capacitors are used in high impedance circuits. The charge in ideal capacitors should change only with external currents. However, the RL in the actual capacitor causes the charge to leak slowly at the rate determined by the RC time constant.

4. Two parameters RDA, CDA is also the capacitance distribution parameters, but  actually they should have a relatively small impact, we do not discuss here. So there are three important parameters of capacitance distribution: ESR, ESL, EPR. One of the most important is ESR, ESL, the actual analysis of capacitor model generally only used RLC simplified model, that is, the analysis of capacitor C, ESR, ESL, which we will do in the next week to focus on the analysis of capacitor simplified model.

5. On the basis of the detailed model, we talk about two kinds of capacitors that are often used in our high-speed PCB design:

6. Electrolytic capacitors (such as tantalum capacitors and aluminum electrolytic capacitors) have very large capacitance, because of their low isolation resistance, that is, the equivalent shunt resistance (EPR) is very small, so the leakage current is very large (typical value is 5? 20nA/ μ F, so it is not suitable for storage and coupling. Electrolytic capacitance is more suitable for the bypass capacitor of power supply and the power supply for stabilizing power supply. The most suitable capacitors for AC coupling and charge storage are PTFE capacitors and other polyester type (polypropylene, polystyrene, etc.) capacitors.

Monolithic ceramic capacitors are more suitable for decoupling capacitors in high frequency circuits, because they have very low equivalent series inductance, that is, the ESL of equivalent series inductors is very small and has a wide decoupling frequency range. This has much to do with its structure. Monolithic capacitors are made up of multilayer sandwich metal and ceramic films, and these multilayer films are arranged parallel to the bus instead of wound in serial.