Power resistor components are used in a wide variety of power supply applications to protect sensitive electronic components. As thick film technology offers high thermal conductivity, a robust construction, relatively low tooling and piece part costs, and high density compared to most other resistor types it is often the technology of first choice.
The main power resistor applications in power supplies are outlined below:
• Balancing resistor
• High voltage divider
• Snubber circuit
• Inrush current suppression
Each of these applications and the best choice of resistor are discussed below
It is often necessary to use parallel resistors in the power supply output stage to handle the high load currents, increase reliability and reduce thermal stresses. Due to variability in transistor manufacture and thermal issues the VBE of each transistor in the parallel arrangement may be different. As a result balancing resistors are used in the emitter line of each paralleled transistor to deliver an appropriate voltage drop and balancing effect. The resistors tend to be low value but high current.
High Voltage Divider
High voltage dividers are common in power supplies for regulation and control of the high voltage rail. The voltage is typically scaled down and fed back using a tap point between two resistors. Matched pair resistors are often used to minimise TCR effects. A low resistor tolerance value is also required to prevent voltage errors.
Snubber circuits are used to filter out spikes on the rising and falling edges of square waves during power supply switch on and limit current surge. Typically, a resistor is connected in series with a capacitor across a switch. Snubber circuit design involves compromises including voltage rate, peak voltage, turn off time and component cost. In some cases paper design is not sufficient and experimentation is required to achieve the optimum result. Choosing the perfect power resistor can compromise the capacitor choice and visa versa.
Inrush Current Suppression
A resistor in the main supply line limits the inrush current at switch on. The resistor must be able to withstand over 20x the nominal current generally for up to 10ms. The resistors tend to have a low resistance value to limit voltage drop although tolerance is not generally an issue.
Thick film power resistor components are therefore an excellent choice for most applications. The technology can deliver a wide range of resistor values, tolerances, power ratings and footprints to satisfy the needs of most power supply applications.