Electrical pulse events are caused by switching operations, load transients and electrostatic discharges. Unless suppressed, they can damage a wide range of electrical equipment.
Pulse resistors are one solution, but it is important to select the right resistor to deal with the threat. Engineers must consider multiple factors that can influence resistor performance during electrical pulse events.
The Causes of Pulse Events
There are many causes of pulse events; they include:
- Switching of inductive loads like motors and transformers, where sudden changes in current can create voltage spikes.
- Load transients: Common in systems with varying power loads.
- Fault conditions: Electrical faults leading to rapid changes in current.
- Power supply instabilities.
- Strong magnetic fields can induce pulses in nearby equipment through electromagnetic induction.
- Electrostatic discharge (ESD).
When designing a resistor to withstand (or mitigate) transient events, understanding the root cause is an important first step.
Comparing the Performance of Various Pulse Resistor Technologies
Thin film and foil resistors (often found in precision applications) are most susceptible to damage via pulse events. The best-performing technologies are wirewound and carbon resistors. Unfortunately, both have performance limitations.
Wirewound resistors are relatively large and have high inductance. Carbon resistors can withstand high peak currents, but only for relatively short pulses, and their overall performance is poor.
Metal film and thick film resistors generally perform well when subjected to pulse events. They deliver good performance combined with small size at a relatively low cost.
Design Considerations for Pulse Resistors
The primary pulse resistor design issues to consider are:
- Thermal issues.
- Pulse energy capacity.
- Surge and pulse stability.
- Resistance value and tolerance.
- Voltage rating.
- Inductance.
These factors affect the geometry of the resistor track (e.g. serpentine or parallel paths) and its thickness. They also have a role in substrate selection, including its material and dimensions.
Thermal Considerations
Effective thermal management techniques are essential in managing the heat generated by pulses. These include optimising thermal paths, incorporating heat sinks and improving encapsulation to enhance heat dissipation. It is also important to consider the operating environment.
There is a trade-off to be made between thermal performance and cost. High-energy pulses often demand thermal management solutions that increase manufacturing costs.
The peak power rating of a resistor defines the maximum power it can handle during a pulse. It is typically much higher than the continuous power rating, as it assumes the resistor has time to cool between pulses. To prevent overheating or damage, the peak power rating must match or exceed the power demands of the pulse.
The characteristics of the pulse, such as its duration and frequency, are equally important. Short pulses allow resistors to handle high power for brief periods. Longer or more frequent pulses require improved thermal management to prevent cumulative heat damage.
The thermal time constant, which indicates how quickly a resistor can dissipate heat, plays a key role. Materials with high thermal conductivity typically exhibit shorter thermal time constants. This makes them better suited for handling successive pulses with minimal temperature rise.
Another important consideration is the temperature coefficient of resistance (TCR). A pulse event can cause rapid temperature changes, making a low TCR desirable for maintaining stability and precision.
Thermal resistance is a key factor, as it influences heat transfer from the resistor and hence its ability to sustain high-energy pulses without degradation. Larger resistors often provide better heat dissipation, making them more suitable for high-energy pulses. However, this must be balanced against cost and space constraints.
Pulse Energy Capacity
The resistor must be able to absorb and dissipate the pulse energy without failing. This is typically specified in joules (J) and depends on the resistor material and construction. Calculating pulse energy involves knowing the pulse’s duration and power. Refer to our ‘Choosing a Pulse Resistor for High Power Applications‘ post.
Surge and Pulse Stability
Resistors for pulse applications should not significantly change their resistance value or show physical degradation (such as cracking or burning) after repeated pulses.
The critical issue to address is the generation of significant localised heat during pulses. To enhance heat dissipation, it is important to use resistive materials with high thermal conductivity and a low thermal coefficient of resistance (TCR). Optimising the resistor’s size and selecting an appropriate substrate material improves thermal performance.
Another major factor affecting stability is high current density during pulses. This can cause electromigration within the resistive film, leading to changes in resistance over time. To mitigate this, resistive film materials often have high melting points and strong electromigration resistance. Uniform current distribution can be achieved by optimising the thickness and geometry of the resistive layer.
Mechanical stresses also pose a challenge. These include thermal expansion differences between the resistive film and the ceramic substrate. To avoid cracking or delamination of the resistive layer, it is important to match the thermal expansion coefficients of the substrate and the resistive film.
Thick film resistors typically consist of conductive particles dispersed within a glass matrix. Repeated pulse-induced thermal cycling can cause microstructural changes in the glass matrix, which can degrade performance over time. Optimising the composition of the resistive film is important to enhance its resilience under pulse conditions.
Resistance Value
Higher resistance values can limit peak current in the circuit. Although this is beneficial in pulse handling applications, there are downsides to this approach.
A higher resistance means a larger voltage drop across the resistor for a given current. Higher-value resistors often have lower power dissipation capabilities because the resistive film becomes thinner or less dense to achieve higher resistance.
Producing high-value thick film resistors with tight tolerances can be challenging, as slight film variations have a greater impact on resistance. An additional problem is the thermal noise generated by higher-value resistors.
Voltage Rating
Choosing an appropriate voltage rating for a thick film resistor subjected to repeated pulse events is challenging. The decisions involved must address the pulse resistor’s ability to handle the applied voltage and the dynamic effects of pulse conditions.
Different manufacturers may specify voltage ratings based on varying test methods. Hence, when evaluating data sheets, it is important to understand the test conditions.
The pulse resistor must be rated to withstand the maximum instantaneous voltage of the pulse, including any overshoot or transient spikes. It is important to choose a voltage rating that is sufficient not only for a single pulse but also for long-term reliability under repeated stress. This might involve derating the voltage specification for continuous pulse applications.
The resistor’s substrate and passivation layers must have sufficient dielectric strength to prevent breakdown under high voltage conditions. The voltage-handling capability is often related to the physical length and separation of the resistor’s terminals. Hence, higher voltage ratings typically require larger components to reduce the risk of arcing.
Inductance
For applications where pulses involve high frequencies, inductance becomes an important issue. As wirewound resistors have inductive properties, thick film and carbon resistors are preferred where high-frequency pulses are an issue.
Pulse resistor design hinges on an understanding of pulse dynamics and material properties. By assessing thermal management, energy capacity and stability under pulse conditions, engineers can enhance the resilience and performance of resistors in applications where electrical pulse is an issue.
If a standard pulse resistor does not match the demands of the application, specialist manufacturers can produce custom pulse resistors for demanding applications.