Thermal runaway in Schottky rectifier diodes and how to solve it

What causes thermal runaway?

Thermal runaway starts with heat generated by the leakage current of the rectifier. When not managed, this heat causes the temperature to rise faster than the thermal system can remove it. Once the heat has risen above a critical temperature, the component fails. While many factors are at play, not generating unnecessary heat and not removing heat fast enough is a key issue.

Figure 1: Illustration of thermal runaway for a Schottky rectifier. The system becomes thermally unstable once the junction temperature (shown on the x-axis) exceeds a critical temperature where the reverse power losses generated within the diode exceed the power dissipated by the package (ratio shown on the y-axis). Source: NexperiaThermal runaway tends to happen more often in applications in which components are exposed to ambient temperatures of 100° C and higher, and where dissipating excess heat can be difficult. An example of this is an automotive application involving heat created from combustion and contained under the hood of a vehicle. LED lighting, where the heat from the LEDs is dissipated close to or on the same heat sink with drive electronics, is another. Without proper design, this heat can lead to component failure. Luckily there are some options to solve and improve thermal runaway in Schottky diodes, but there is no one-solution-fits-all approach. The best solution often varies by application.

Planar Schottky diode

Schottky diodes are used as rectifiers in switching power supplies and other applications due to their fast switching speed and low forward voltage drop. For Schottky diodes, a lower forward voltage drop generally comes at the price of an increased leakage current. This means that Schottky rectifier diodes are more prone to thermal runaway than p-n diodes because of the higher reverse power losses. For this reason thermal management and thermal runaway becomes an important issue in power applications.

Figure 2: Equi-potential lines in a planar Schottky rectifier (left) and in a trench Schottky rectifier (right) in reverse direction. Source: Nexperia

Trench Schottky diode

One approach to managing thermal runaway is a trench Schottky diode, which has a wider safe operating area. This diode is specifically designed to prevent thermal runaway by a trench structure etched into the silicon. This structure acts as a field plate and results in lower leakage current and less heat generated as compared to a similar planar diode. The design change results in a component with a wider safe operating temperature and less chance of thermal runaway in high heat environments.

The trench design has some tradeoffs that must be considered in a circuit design. While all Schottky rectifiers have some amount of parasitic capacitance, the trench design creates additional parasitic capacitance as a result of the thin dielectric structure. For designs which are sensitive to parasitic capacitance, a planar Schottky rectifier may therefore be more appropriate.

Thermally optimized packaging

Figure 3: CFP5 package utilizing a clip improves the thermal performance of the package. Source: NexperiaAnother factor in thermal runaway is efficient heat transfer into the heatsink. Even the best heatsink will be inadequate if the heat from the component is not moved into it fast enough. For this reason, reducing thermal resistance is a key design consideration. The thermal system is composed of the heat sink as well as the component’s own thermal resistance and heat dissipation. Better component packaging designs can help reduce thermal resistance.

Nexperia’s CFP packaging

Nexperia’s clip-bonded flat power (CFP) design has a low thermal resistance to curtail thermal runaway. It has a smaller package size with thermal performance on par with traditional surface-mount designs such as SMA and SMB. The key design difference compared to wire-bond packages is the use of a solid copper clip in contact with the die to reduce thermal resistance. The design also has the advantage of reduced electrical resistance and package inductance. This reduced size packaging is available for both planar and trench Schottky diodes. In Schottky rectifier diodes, the CFP design can move a diode out of the thermal runaway danger zone.

Figure 4: The CFP3 and CFP5 packages offer electrical performance on a much smaller footprint. Source: Nexperia

Summary

Nexperia carries a line of both planar and trench Schottky rectifier diodes to meet circuit design criteria. They both feature the CFP package design that reduces the thermal resistance in a smaller form factor.

Thermal runaway in Schottky rectifier diodes is a problem in locations with high environmental heat, but it can be managed. Solving thermal runaway includes careful component choices and component package selection. Selecting components like the Nexperia line of planar Schottky diodes and trench Schottky diodes with wide operating temperatures — both of which have small form factor, low thermal resistance packaging — is the best ways to do this.

For help selecting the right Schottky rectifier diode and avoiding thermal runaway for an application, contact Nexperia.