Schottky Diode Thermal Runaway

The problem looked at here is a real-world case of thermal runaway in a Schottky diode. I will use some ideas from closed loop feedback control to analyze the problem. Schottky diodes are used extensively in switching regulators that power nearly every piece of high technology electronics. Putting this in perspective consider the case of a switching supply that converts 5Vdc to -5Vdc in a inverting Cuk topology. Given typical numbers for the output as -5Vdc @ 30ma you might think a Schottky diode rated at 2Amps forward current and 20V reverse breakdown would be perfectly safe. However, Schottky diodes can suffer failure due to thermal runaway caused by excessive reverse current under certain conditions.

The reverse leakage current of a Schottky diode is very sensitive to temperature. In fact it is exponential as follows Ir=I₀e^n·(Tj-To). The power dissipated due to this current is P = Ir·Vrev and the steady-state temperature rise is given to be dT = P·Theta. You can see this is a case of positive feedback. As the temperature rises so does the current. This in turn increases the power dissipation which increases the temperature. This can be analyzed using small signal gain theory applied to the following block diagram.

The small signal loop gain is given by:

g= V_revØ·n·I₀e^n·(Tj-To)

The temperature at which runaway will commence is when the loop gain is greater than unity. Using the small signal gain equation the critical junction temperature is

Tj= T0 + (1/n)·ln(1/(Ø·n·Vrev·Ir0))

As an example consider the MBRM140 diode operating at 60C ambient environment. From the data sheet the maximum leakage Ir(10V,25C)= 0.1mA and Ir(10V,85C)= 10mA . Fitting this data to the exponential results in μ equal to 0.07675. In the inverting -5V application the reverse voltage will be nominally Vrev = 11V. The thermal resistance on the PC board is estimated to be 25C/W. From this data the junction temperature at onset of thermal runaway is Tj= 105.2C. This is a surprisingly low junction temperature that can lead to thermal run away. The reverse voltage is well below specification and the forward current was not even considered in this example. It is a simple exercise to set up an example in an Excel spreadsheet and view the iterations as they evolve.

As noted this analysis included only the reverse leakage current at 100% duty cycle. If there is significant power dissipated in the forward direction that too should be included in the calculation using the appropriate duty cycle. The objective of this note was to show how closed loop control theory can be applied to seemingly non-control problems.