Evan Mayerhoff
An often overlooked parameter is voltage coefficient (VC). That may be because for most component applications, the effect of voltage on a component’s electrical characteristics is very small. Thus, we generally tend to disregard this parameter, and may neglect to take it into account when high voltage is applied to the part. Under these conditions, the effects could be substantial. Here are a few examples of the effect of high applied voltage on component characteristics:
The capacitance of some types of ceramic capacitors can decrease substantially when full applied voltage is applied. For example, types Z5U and Y5V can decrease by 50% when full voltage is applied.
When voltage is applied to a resistor, it generally reduces in value in a non-linear fashion. When we first learned Ohm’s Law, we made the assumption that resistance was constant with applied voltage. That was a very good assumption. However, with high voltage applied, the change in resistance can be enough to result in a meaningful error. For example, some precision high voltage resistors have a voltage coefficient of 1 ppm/V, which initially looks small enough to be neglected. (Some HV resistor manufacturers even tout this as an excellent value.) A quick calculation shows otherwise. With, say, 20,000 volts across a resistor, the effect of VC caused the resistance value to change by 2%, which is quite a substantial linearity error when a precision measurement is needed. Thus, measuring a component’s resistance at low applied voltage is not sufficient when good accuracy and linearity is needed. One way to combat the effects of voltage coefficient is to use several resistors in series, thereby reducing the effect on each resistor, and the entire resistance string, in total.
The above discussion relates to actual resistors, designed to handle high voltages. Parasitic resistance of insulators can also affect a circuit. For example, an insulator can be a source of leakage current. The voltage coefficient of such a parasitic resistance can be orders of magnitude worse than an actual resistor, and in some precision applications can have a substantial adverse affect on circuit operation.
Another, perhaps more subtle, effect of applied voltage is the change it causes in a resistor’s temperature coefficient (TC). This is an important issue when you are concerned with temperature compensation. If you are looking to achieve the best performance of a voltage divider, you might measure the TC of both the top and bottom resistors, and then match resistors. If the measurements are only made at low voltages, there could be an error in the results. When high voltage is subsequently applied to the top resistor, the TC could be different than measured. Thus, the performance of the voltage divider will not be as good as you might have expected.
In a similar fashion, an applied electric field in a high voltage environment can affect component characteristics, sometimes permanently. Permanent change can occur when there is physical movement of molecules in a component or polarization, brought about by the presence of a high electric field. Sometimes there can also be a chemical reaction due to or aided by the electric field. These can occur when the component has not been fully passivated, or when a potting material is not fully cured. This is another reason to ensure that high quality components are used and that potting materials have completed the curing process before voltage is applied.
Contact Information
Evan Mayerhoff
Application Engineering
High Voltage Connection, Inc.
516-466-9379
evan@highvoltageconnection.com
© 2007 High Voltage Connection, Inc.