Size vs Capability
A key advantage of these resistors is small size vs power rating and isolation voltage. Thick film resistors are designed to make easy use of thermal sinks. This makes them a quarter of the size of a wirewound equivalent – making them more suitable for systems with limited space.
Inductance is typically an unwanted property in resistors, particularly in filter circuits. All conductors have inductance, the value depends on its geometry. Generally a wide, flat and short geometry will have lower inductance than a thin, wound long conductor. This is where printed conductive ink allows resistor elements to be formed on wide flat ceramic insulating substrate. This delivers a very low inductance resistor.
Thick film resistors, with lower stray inductance, are therefore the preferred resistor type for high frequency applications like PWM filters or current shunts.
Dissipating heat from a resistor is all about surface area and the interface to a cooler medium. Thick film resistors enable a large flat surface area, meaning they are easy to attach to a heat sink or cold plate. Power dissipation of 2 kW is possible in 60mm square.
High Isolation Voltage
A power resistor printed onto a ceramic substrate has an intrinsic isolation from the terminals to the base plate. This can be is enhanced with coatings (polyimide for example) applied to the resistor element. The ceramic substrate is then placed in a supporting mechanical housing which is filled with a silicone gel to complete the electrical isolation solution. Isolation rating of up to 12 kVrms to the baseplate can be achieved in a range of ultra high power resistors.
Thick film resistors are more mechanically robust than wirewound resistors, which makes them less susceptible to damage as a result of poor handling or packaging, or rough insertion into a system.
Further, their construction involves constructive paint being screen printed and fired onto an insulating substrate, usually a ceramic base, making it a permanent assembly. This construction makes the resistors better at dealing with high power surges than their wirewound counterparts.
The ceramic layer – with the resistor printed onto it – is housed in a custom designed plastic case for protection, which also can incorporate mounting hardware. The housing also can have a spring installed to manage the pressure needed to reliably achieve good thermal contact without cracking the ceramics.
For more information on the different types of EBG resistors available from PPM Power, please contact us.