4.2. Power
Introduction: What is Power for a connector?
There are several ways to define a power application. The most obvious are the current and voltage requirements that the connector must meet. Most people would agree that an application that specifies a current capacity of 30 A is a power application. Similarly, most people would agree that an application where the applied voltage is 440 V is a power application. In today’s marketplace with its emphasis on miniaturization, however, the voltage and current values that define a power application may be significantly lower. One way to account for this size related power relationship is to use the temperature rise of the contact as a function of current as the criterion for a power application. One such criterion comes from Underwriters Laboratories (UL) a standards organization directed towards ensuring safe operation in power applications. UL defines a temperature rise, T-rise, of 30 °C over the ambient temperature of the application as the limiting current capacity of a connector. This is an arbitrary but widely use criterion. Consider the factors that influence T-rise.
4.2.1 General Parameters
4.2.1.1 Temperature Rise
There are two components that determine the T-rise of a conductor carrying electrical current: the Joule, or I2R, heating and the heat dissipation to the application environment. Consider each component individually.
Joule Heating:
For a given conductor the Joule heating, JH, is given by:
where I is the current through the conductor, and R, the resistance of the conductor.
Fig. 4.12: Circular conductor
For a circular conductor, Figure 4.12, R is given by
where ρ is the resistivity of the conductor material, L the length, and A the cross sectional area of the conductor. For connector contacts the geometry becomes more complicated, but the Joule heating of the contact always depends on the resistivity of the contact material and its geometry as will be discussed later.
4.2.1.2 Heat Dissipation
The temperature rise caused by Joule heating is limited by transfer of heat to the application environment. For a connector there are two main mechanisms of heat dissipation, conduction and convection. Conduction dissipates heat by transferring the heat through the contacts and conductors to a cooler portion of the system, generally the terminating point of the connector, the wire/cable or the Printed Circuit Board, PCB. Convection dissipates heat by transfer to the application ambient from the conductor or connector surface. For a conductor, conductive heat transfer follows:
where q is the heat transfer rate, κ is the thermal conductivity of the conductor, ΔT is the temperature difference between the heat source and the sink and A and L are the cross sectional area and the length of the conductor respectively. This equation would be modified to reflect the contact geometry as noted previously. Convective heat transfer follows:
where h is the heat transfer coefficient, q, ΔT and A are as above. h is a parameter dependent on the ambient (air, water etc.) and the flow, if any, of the ambient over the over the surface of the conductor/connector. The temperature a conductor will achieve under a given current flow will be determined by the balance of Joule heat and the heat dissipation mechanisms. For a connector, this balance will, in turn, be dependent on the connector configuration as will be discussed later.
4.2.1.3 Contact T-rise vs Current
The following is a brief overview of the methodology for determining the T-rise versus current characteristic of a single mated pair of contacts in air. Recall that the current capacity is determined by the balance of Joule heating and thermal dissipation. Joule heating and conductive heat dissipation are essentially independent of the test conditions. Convective heat dissipation does depend on the test conditions. Free air convection heat dissipation will be greater than with the contacts in a housing, but less than that if air were flowing over the contacts. These differences must be taken into account when a test protocol for validating current rating is developed.
Figure 4.13 shows the temperature of the hottest point12 on the subject contact pair as a function of time for three different current levels, 3, 5, and 7 A. Note that the temperature rises rapidly initially due to Joule heating. The rate of T-rise slows and eventually levels off as the heat dissipation mechanisms become active. Note that it may take a few minutes for the connector to achieve its steady state, constant current temperature rise. The applied current is increased incrementally until the steady state temperature exceeds the current rating criterion, a T-rise of 30 °C above ambient. For the data shown, the 30 °C-T-rise would occur at a current over 5, but less than 7 A. This contact system could, therefore, be conservatively rated at 5 A. With this basic performance characteristic in hand, consider how materials and design choices determine the current rating of a contact system.
Fig. 4.13: T-rise vs time for Three current levels
4.2.2 Connector Effect and Parameters
4.2.2.1 Contact and Housing Considerations
The electrical considerations are different for contacts and connector housings. The contacts affect the Joule, I2R, heating, due to the R term, and the conductive heat dissipation due to the thermal conductivity of the contact. The housings are affected by the voltage side of power, which influences the contact spacing in the housing, and the convective aspect of heat dissipation through the geometry of the housing.
Housing Considerations
From a materials viewpoint, the polymer electrical parameters of interest include the volume and surface resistivity and the dielectric breakdown voltage. Suffice it to say that the polymers used in the manufacture of connector housings easily satisfy the electrical requirements.