AN836 STMICROELECTRONICS | Alldatasheet
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LOW FORWARD VOLTAGE SCHOTTKY DIODE REV. 2 INTRODUCTION In power supplies, the major portion of power losses is due to output rectifiers. The impact of these losses on the efficiency can be expressed by: η :efficiency drop due to the diodes. VF(IOUT) :voltage drop at the output current (IOUT) of the converter. VOUT :output voltage of the converter. This formula shows that the influence of the forward voltage increases when the output voltage of the pow- er supply decreases. This parameter becomes very important for the new standard of 3.3V output voltage. Another key param- eter is the leakage current which we have to take into account to develop high efficiency low forward volt- age drop Schottky with the best trade-off. TRADE-OFF The 3 most important application characteristics of a Schottky are: – forward voltage – reverse leakage current – reverse blocking voltage Generally for a given application, the first step is to fix the reverse blocking voltage. We then study the best trade-off (choice of a metal barrier) between the forward voltage and the leakage current. A decrease of the forward voltage increases the efficiency of the converter but increases at the same time the leakage current and limits operating range where we can keep the reverse losses under control. In the datasheet this range is defined by Tj max. To define the best trade off of a low forward voltage schottky we have to take into account its application condition. STMicroelectronics has developed two families of low forward voltage Schottky well suitable to two applications: the OR-ing Schottky and Schottky for 3.3V output power supply secondary. SCHOTTKY DIODES FOR 3.3V SWITCHED MODE POWER SUPPLY When a Schottky works in a switched mode power supp ly (forward, flyback,...), it sees during the same switching period conduction losses and reverse losses. In these configurations the trade-off between the forward voltage and the leakage current has to be cho- sen to have the best efficiency with a sufficient safety margin. This is to keep the reverse losses under control (Tj max = 125°C). η VF IOUT() VOUT
Table 1. Main Characteristics of Schottky To increase system reliability, power supplies are sometimes connected in parallel (Fig.1). Figure 1. OR-ing Schottky in redundant power supply continuous current (IOUT/n) flows in each Schottky. Obviously in this case there are no reverse losses. put voltage is not disturbed. The OR-ing diode then sees the reverse voltage VOUT. ficiency of the system. A breakdown voltage of 10V is sufficient for output voltage of 5V and 3.3.V.
Table 2 gives the main characteristics of the 10V and 15V schottky. Table 2. Main Characteristics of 10V and 15V Schottky ode for the secondary Power supply rectification. using 25V Schottky in a 5V forward converter, whereas today a 45V Schottky diode is needed.
REVISION HISTORY
Table 3. Revision History 10-May-2004 2 Stylesheet update. No content change.
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