Q&A with TOREX - DCDC Converter
TOREX held a power management basic webinar "DC/DC converter Introduction to design" in August. The questions we received from the audience during the Q&A session and the answers to them are published here in two parts. We hope this information will be of use to you.
Q1. Regarding power consumption and Tj verification of DCDC converters, we can only measure the actual power consumption with basic N=1. Therefore, how should we consider component variations, especially efficiency variations of DCDC converters, for Tjverification?
?A1. It is difficult to consider the variation even by actual measurement or simulation, but for the efficiency of DC/DC converters for that purpose, please use the following as a guideline. High efficiency DC/DC with 90~95% efficiency -3~4% is considered. Pay attention to the DCR of the inductor so that efficiency does not drop. 80~90% efficiency step-up, or step-down DC/DC with light load. Consider -5% for step-up and -10% for light load.
Q2. What is the difference between FRD (Fast Recovery Diode) and SBD (Schottky Diode) on the output side of a DC/DC converter?
A2. SBDs are used in general DC/DC converters of 100V or less. Although synchronous rectification has been progressing, when a diode is required for medium and high voltage step-up, SBDs are the best choice because of their low VF and switching speed. FRDs are used for high-speed switching in converters that handle several hundred volts, such as AC/DC PFC (Power Factor Correction) circuits. In recent years, SiC-SBDs with lower losses have become popular and are increasingly used in place of FRDs.
Q3. Will power supply ICs be degraded or destroyed if operated above the operating ambient temperature Topr?
A3. ICs have two defined maximum values: Topr max, the maximum operating ambient temperature, and Tjmax, the maximum junction temperature, which is the temperature of the internal chip. Operation above Tjmax may cause degradation or destruction. Even if it is not destroyed once, it may be destroyed by accumulation if it is repeated.
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The situation between Topr max and Tjmax varies depending on the IC.
Low-Iq ICs may malfunction due to circuit leakage or other reasons when operating ambient temperature exceeds Topr, since the bias current of such ICs is reduced or the internal circuit is switched when the load current is small. This IC also operates without any problem until the internal temperature reaches Tjmax under a heavy load and self heat generation. ICs with constant operation regardless of load current and IC self heat generation will operate without problems even when the Topr max is exceeded, as long as the internal temperature of the IC is below Tjmax. However, since the datasheet does not clearly state whether such differences, the IC should still be used at or below Topr. For example, malfunction due to internal leakage when CE(EN) is off or VOUT rises due to leakage in the output stage when the load is light may also occur due to variations.
There is a long-term reliability issue regarding degradation due to high temperature operation.
As can be seen from the conditions of the long-term life test, e.g., 125°C operation test, operating the device near Tjmax is the same as conducting a long-term reliability test. Therefore, if such operation continues, it is possible that, for example, a device that was expected to last 10 years may fail after only a few years.
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