Design Criteria for High Power Terminations

ABSTRACT:

In today’s modern world, the needs for high power terminations that exhibit both “sustaining” high power and excellent VSWR have greatly increased.  Due to this increase in demand, the ability to interface microwave signals effectively to passive components has been imperative. Correspondingly, the need for resistive terminations with these characteristics is in high demand for wireless and telecommunication applications. In this paper we discuss methods to improve power handling capability and VSWR while still maintaining small size geometries.

INTRODUCTION:

The wireless telecommunications, radar, and broadcast industries rely on their ability to transmit high power RF signals. The data traffic for these applications is on a constant rise. As the overall systems grow smaller in size, the corresponding passive devices making up these systems must also become smaller. Thus, the technical demands for these passive devices require the following:

  1. Higher operating power.
  2. Higher frequencies.
  3. Considerably smaller device “footprints”.
  4. Consistent low cost construction techniques that maintain device performance.
  5. Controlling device heating and the elimination of heat.
  6. High Z/Low Z transmission line impedance matching techniques.

Methods of improving a resistive terminations power efficiency:

Heat removal and the control of heat remains the largest obstacle regarding device efficiency. It is well known that excessive heat when applied to thick film terminations will result in resistive value shifting. If the excessive heat is allowed to continue, permanent increases in resistive values will occur.  Four (4) methods are employed to improve the heat dissipating quality of high power terminations:

  1. Design the resistive element to distribute the heat uniformly over the entire substrate surface.
  2. Design the resistive element so that there is a consistent current density distribution throughout the resistors volume (prevent current bunching).
  3. Lower the thermal resistance between the resistor, substrate, and mounting base.
  4. Advise the customer on effective ways to heat sink the termination in his application.

Methods to improve heat dissipation:

The predominant form of heat removal from a high power resistive termination is by conduction from the resistive element, through the substrate, and then through the mounting base when used. Enlarging the resistive area on the substrate (based on frequency requirements) or dividing the resistive element into several resistors distributed over the entire substrate will lower “hot spots” and provide more homogenous heat dissipation. Since odd shaped resistors can affect current density, these odd shapes will also increase the “hot spot” heating. Current density in this case refers to the maximum tolerated current per unit of cross-sectional area that is perpendicular to the current of the conducting resistive element.  This is consistent with electro-migration effects where atoms will drift in the conducting materials. This drift of atoms is due to the continuous momentum transfer caused by conducting electrons. This drift effect is in part responsible for the “spot” heating of irregular shaped resistive elements.

The ideal resistive material for thin film resistors is nickel chromium. Nickel chromium is known to have very high current density limits. Additionally, since the nickel chromium is a thin film, the thermal resistance to the substrate is reduced. Correspondingly, in thick film systems, ruthenium oxide resistive material is the second best material.

Methods to improve thermal resistance:

The component “footprint” is the obstacle in efficient heat removal. This applies to both surface mount devices and devices with metal mounting bases. Heat must travel from the resistive element, through the ceramic substrate, through the metal base (when used) and then out to a distant heat sink of the customers design. By working with the customer and optimizing the heat transfer through the footprint, the dissipation can be optimized. The designer of the resistive element can also aid in the optimization of heat transfer by using the largest most suitable conductor patterns for “on board” heat transfer of the resistive element to the terminals.

Improved heat transfer rate and the measurement of it:

For those applications where heat transfer is critical, it is advisable to run the termination at full power while viewing the resistive surface with an infrared camera. The distinctive color regions identified by the camera will clearly show the heat distribution and any “hot spots” that may occur during operation. In this way, the design can be optimized.

Methods of improving VSWR:

Probably the most important factor for a 50 ohm power termination is its ability to have a good impedance match with its corresponding RF/microwave signals. If the termination should have am impedance that is not 50 ohms, significant energy can be reflected back into the customers circuit by the unmatched termination. This reflection increases the VSWR (voltage standing wave ratio) beyond the ideal 1:1 match. Poor VSWR matches degrade the customer’s circuit efficiency and must be avoided. Simply stated, VSWR is an indicator of how well the termination absorbs energy instead of reflecting parts of lost energy back into the circuit. Current requirements insist that terminations should have a flat broadband response that can range from DC to >6 GHz.  In order to improve the VSWR, an integrated matching network is needed.  An effective method to accomplish this is the high Z/low Z transmission line impedance match method. By using this matching method, it is possible to achieve larger resistive element areas. These larger resistive areas contribute larger capacitances that can be matched with the high Z/low Z integrated method.  For really critical requirements, degenerative ground structures can be used to impedance match the conductors as well as the resistive elements.

CONCLUSION:

In this paper, we explored methods to improve the characteristics of high power terminations. These improvements included the importance of impedance matching and heat removal. They also included ways to remove heat and a method of transmission line matching. Together, all of these techniques combined will produce an acceptable innovative high power termination that will expertly conform to the customers expectations. And, of course, continuous improvement is always in mind as technology advances. It is also important to note that initial “upfront” collaborations with the customer is the best route at achieving the optimum low cost high performance resistive termination meeting their needs.

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