Parasitic Inductance in Power Device Testing

In the world of semiconductor testing, understanding and managing parasitic inductance is crucial for ensuring the accuracy and reliability of test results. This article delves into the dilemma of parasitic inductance, its impacts, and effective strategies to minimize its adverse effects.

The Basics of Inductance

Inductance can be described as the tendency of conductors to oppose a change in current. A useful analogy is a garden hose with water flowing through it. When you suddenly increase the flow, it takes time for the water to stabilize due to the hose's inertia and resistance.

Similarly, electrical inductance resists changes in current, creating challenges in maintaining stable current flow in circuits. Parasitic inductance can be thought of as knots in the hose, creating all types of unexpected complications for the water current going through it.

The Importance of Minimizing Parasitic Inductance

When testing wide bandgap power devices (SiC and GaN), minimizing parasitic inductance is vital for several reasons:

• Minimizing Overshoot: It allows for a smooth, damped response curve with minimal overshoot. Such overshoot spikes can operate devices beyond their limits, risking permanent damage. Minimizing overshoot ensures reliable, accurate testing for devices- no one wants a water hose that abruptly bursts upon switching on!
• Fastest Rise Times: In applications such as automotive drive, devices need to be comprehensively tested to meet industry "zero defect" requirements. Testing at high currents with very fast rise times allows devices to be tested to the same (and harsher) conditions they will experience during their operational life. Fast rise times are particularly important for simulating stressful events such as shoot-through.
• Protection: Not all power devices pass the testing process and some may fail catastrophically. Minimizing parasitic inductance helps the system isolate failing devices faster, ensuring that energy is channelled quickly away before failing devices can cause damage to the test cell.

However, this is never the case in real life applications and designing, it's much more complex than that! Circuit and system designers would actively try to intertwine their "strays" to try and cancel out each other, leaving a much lower value than its constituents. Eventually, all of these tiny values will amount to one that might significantly affect the predicted theoretical output if not monitored closely (Every drop counts!).

Causes and Solutions

There are three main contributors to stray/parasitic inductance in semiconductor testing setups:

1. Cabling: Traditional cabling, also known as "soft docking", can introduce significant inductance due to the length and arrangement of the wires. Optimizing stray inductances is crucial when testing wide bandgap materials to fully utilize their fast switching speeds and exceptional electron mobility.
2. Test Type Parameters: Different types of tests (double pulse, short circuit, etc.) can impact the overall inductance measured in a system differently. For example, a test circuit that simulates one test condition such as a short circuit may have a lower parasitic inductance value than a multi-purpose circuit that can simulate other test conditions such as double pulse and reverse recovery.
3. PCB Design: PCB design affects parasitic inductance primarily through the length, width, and routing of traces (loops), as well as the use of ground planes and vias. Shorter, wider, and straighter traces, combined with proper placement of ground planes and minimizing the use of vias, can significantly reduce parasitic inductance and improve circuit performance.

Mitigating Strategies

In many applications, factors such as test types and PCB design aren't flexible due to the standardized nature of testing protocols and component designs. Therefore, one area where adjustments can be realistically made is cabling. Fun fact: having 100mm of cable could contribute up to 100nH (subject to cable type, frequency, etc..) of parasitic inductance! Strategically minimizing, or even eliminating, cabling would significantly help in reducing a system's overall parasitic inductance.

ipTEST’s Approach

Hard Docking is a method that eliminates traditional cabling, overcoming the aforementioned obstacle. By directly connecting the device under test to the testing equipment, hard docking minimizes the distance between the device and the test station, further minimizing parasitic inductance.

At ipTEST, we prioritize minimizing parasitic inductance through innovative design and advanced techniques. Our hard docking solution is a testament to this commitment, offering a robust alternative to conventional cabling. This approach not only enhances the accuracy of our test results but also enables us to optimize switching speeds, comprehensively testing the novel properties of wide bandgap materials.

In a Nutshell

Understanding and mitigating parasitic inductance is a cornerstone of effective semiconductor testing. By employing strategies like hard docking and optimizing test setup design, we can significantly reduce the adverse impacts, leading to more reliable and safer testing processes. At ipTEST, we are dedicated to advancing our methodologies to stay at the forefront of semiconductor testing technology.

For more insights into our testing solutions and to learn how we can help optimize your semiconductor testing processes, contact sales@iptest.com or visit our website.