Power resistors are used in a variety of power electronic applications as load resistors, braking resistors and pulse management devices but one of the main applications is current sensing elements in power supplies and electric motor control.
When designing power supplies one of the key design challenges is minimising overcurrent and short circuit conditions that may damage sensitive electronic components used in the motor control circuitry. These overcurrent conditions can be caused by a number of factors including motor start-up conditions and high power switching.
A current sense resistor, combined with suitable measuring circuitry measures the current flow so that appropriate action may be taken if an overcurrent event occurs. By applying basic Ohms law the current through the resistor may be represented by the voltage drop across that resistor. A sense amplifier is used to measure the voltage drop.
To minimise power loss and heat generation the resistance of the sense resistor must be reduced (to typically <0.1 Ohm) this, in turn, means the resistor must have a sustained high current carrying capability and be able to withstand high current surges. A low value of resistance results in relatively low voltage drop across the resistor within the measuring capabilities of standard sense amplifier circuits.
Although resistance must be minimised for the reasons discussed above it can lead to serious measurement errors. Connection or wire resistance and temperature changes can cause significant errors. To minimise the impact of these errors “Kelvin resistors” may be used. These four terminal devices eliminate the IR-drop error voltage that would be present in the voltage sense leads if a standard two-terminal resistor were used. Four terminal power resistors separate the current leads from the voltage sensing leads.
The impact of temperature may be minimised by choosing the appropriate materials to construct the resistor. A low resistance value will also limit the heating of the resistor element and therefore the impact of TCR.
Manufacturing thick film power resistors for current sensing applications presents a number of challenges including:
•Very low resistance values.
•High current carrying capability and load life stability.
•Tight tolerance values.
A variety of standard power resistors are available for current sensing applications but when a standard device will not deliver the performance required by the application it is best to consult a specialist manufacturer with the capability to design and manufacture an custom power resistor device in low to medium volume.