Application analysis of Schottky diode in power management

Any asynchronous DC/DC converter requires a so-called freewheeling diode. In order to optimize the overall efficiency of the solution, it is usually preferred to choose Schottky diodes with low forward voltage. Many designs use diodes recommended by a converter design (network) tool. This is not always the optimal choice for diodes. Moreover, if the design tool does not consider the dynamic changes between thermal performance and leakage current, it is highly likely that the actual performance will differ from the analysis or simulation results of the design tool. This article will explore some typical parameters that should be carefully considered when choosing the right diode, and how to apply these parameters to quickly determine the correctness of the selection.

There is a trade-off in the design of any Schottky diode. This device is either optimized for low Vf or for low Ir. Therefore, if a low Vf is chosen, Ir will be higher, and vice versa. In practical application design, it is important not only to observe the values of Vf or Ir, but also to analyze what results they will produce in actual operation. Vf and Ir both change with temperature. As the temperature increases, Vf decreases, reducing thermal diffusion while the diode heats up. Unfortunately, Ir increases with the temperature of the diode. So, the higher the temperature of the diode, the more leakage current there will be, and the more internal power consumption there will be. This will cause the temperature of the diode to rise, thereby increasing the leakage current again, and so on.

If you insist on using a basic asynchronous DC/DC converter design case, you may want to conduct a basic analysis to determine the internal power consumption of the diode and the resulting device temperature. The operating duty cycle of a DC/DC converter is directly related to the ratio of voltage input to output (DC=Vout/Vin). The lower the ratio of voltage input to output, the longer the T2 time, and the greater the impact of PT2 on the power consumption of the entire diode. Conversely, the longer T1 (or the higher the ratio of and), the smaller the impact of PT2 on total power consumption, and the greater the role of PT1.

Taking two DC/DC converters as an example, both have an input voltage of 24V, but one has an output voltage of 18V and the other is 5V. Calculate the duty cycle using the ratio of Vin to Vout, and calculate the total power loss inside the diode using the Vf and Ir values in the data table. Then calculate the diode temperature caused by this based on the total power consumption, and find the actual values of Vf and Ir at this temperature. Finally, recalculate the internal power consumption based on the new diode temperature. This iterative process can be repeated multiple times to improve accuracy, but if you only want to roughly indicate the impact of different trade-offs between Vf and Ir, a single iteration is sufficient.

The device temperature can be calculated using the basic thermal equation that describes thermal properties, which is no different from the calculations used to describe voltage, current, and resistance. Once the internal power consumption (Ptot) of the device is known, it can be multiplied by the thermal resistance from the node to the environment (Rtja) to calculate the temperature change at the device node. Adding it to the ambient temperature gives the final node temperature of the device at this power consumption and ambient temperature.

The current common practice is to simulate power consumption design. Standard simulation tools can be used, as well as commonly used simulation tools online. Careful examination of thermal effects is essential. For the intended diode, it is highly likely that the tool used did not adopt the correct thermal model, or its thermal parameters (likely related to layout) do not match the design. Obviously, not every diode is identical, so we definitely do not agree with the use of "similar" diodes in analog design, and then assume that their thermal effects (and potential electrical effects) are also similar. Although not always feasible, it is still recommended to always create prototypes and verify their correct effects.