Schottky rectifiers have been used extensively in power supplies forthe past 25 years. Schottky diodes feature a very low forward volt drop(VF ), at reasonable leakage current, switching speeds thatare in the nanosecond range, and provide a competitive advantage overconventional epitaxial p-n diodes.
Consequently, Schottky diodes have become the dominant technology inthe output stages of power supplies in the voltage range below 150V.Above this voltage the VF of a Schottky can be equal to orgreater than equivalent epitaxial p-n diodes.
However, compared with p-n diodes, Schottky diodes are known fortheir relatively high leakage currents in the off state and have lowermaximum rated junction temperatures than conventional epitaxial p-ndiodes.
In applications where high breakdown voltage or high reliability(high operating temperatures or high currents) are required, p-n diodesdominate the rectifier market. Unfortunately, p-n epitaxial diodes havea larger reverse recovery time than Schottky diodes increasing theswitching losses within the power supply. Super Barrier Rectifier (SBR)technology combines the low VF performance of Schottky diodes whilepossessing the high reliability characteristics of epitaxial p-ndiodes.
This article presents laboratory-derived data that demonstrates thebenefits of SBR when used in the secondary stage of a power supply. Inparticular, it highlights how Super Barrier Rectifiers improveefficiency, reduce component temperature and increase the overallruggedness of the power supply without the need for expensivere-designs.
|Figure1. 80PLUS power supply with multi-outputs two switch forward converter.|
A desktop computer power supply commonly referred to as a'Silverbox PSU' converts AC line to low voltage DC output for use bythe hard drive, microprocessor and PC interfaces. It is comprised of afront end power factor correction (PFC) stage followed by an isolatedDC/DC converter with several outputs.
The drive for higher efficiency means power supply designers nowfavor the two switch forward topology (Figure 1 above ) to theconventional single switch topology as the DC/DC conversion stage. Theadditional transistor helps to reduce voltage stress enabling theoverall system efficiency to reach the 80PLUS (greater than 80 percentefficiency) requirement, albeit adding complexity, increasing componentcount and cost.
Each power supply output is obtained by rectifying a pulse signalcoming out of one secondary winding of a multiple output isolated DC/DCconverter. Conventionally, output rectification has been achieved using45V to 100V Schottky diodes. An 80PLUS computer power supply, rated ata few hundred watts, provides an excellent test bed to demonstrate thepractical benefits of SBR diodes.
The 5V and 3.3V output rails of an 80PLUS power supply were used toevaluate the efficiency of the Schottky rectifiers 30CTQ045 and30CTQ060. Efficiency and case temperature measurements were taken underroom temperature and at an ambient temperature of 50°C, under 30percent and full load conditions. The evaluation was then repeated withthe SBR diode replacing the Schottky diode.
The output stage of the forward converter is operating at aswitching frequency of 200kHz. At low load condition, the rectifierswork in discontinuous conduction mode (DCM) and traverse intocontinuous conduction mode (CCM) operation at high load to takeadvantage of lower RMS/peak current value.
The converter efficiency was measured at 115Vac. The output voltageused for efficiency evaluation is measured at the end of the powercable. Efficiency was measured across a range of loads, more criticallyat 20 percent, 50 percent and 100 percent.
|Figure2. Efficiency of 5V at 20A output at room temperature|
The efficiency plots, taken at room temperature, of the 5V and 12Voutputs of the 80PLUS power supply are shown in Figure 2 above andFigure 3 below . In both cases, the efficiency performance with theSBR diodes in situ was respectively 1.5 percent and 2.5 percent betterthan that achieved by Schottky solution.
|Figure3. Efficiency of 12V at 17A output at room temperature|
The efficiency of the power supply with both SBR and Schottky asoutput rectifiers is shown in Table 1 below . Although theoutput of the evaluated power supply is capable of delivering higherload current than that shown in Table 1, the combined maximum DC outputis limited to 300W. Nevertheless, the result shows that the SBRimproves the efficiency of the overall system by more than 1 percentcompared with that using a normal Schottky rectifier, enabling thepower supply to achieve 80PLUS compliance. All measurements were takenwith input voltage at 115V nominal.
|Table1. Efficiency for SBR diodes and Schottky rectifiers|
The 80PLUS power supply was then placed inside a temperature chamberand the ambient temperature increased to 50°C. The efficiency ofthe 5V and 12V rail was measured from no load to full load with theSchottky diodes in the output rectification stage.
|Figure4. Efficiency of 5V at 20A output at 50 degrees Centigrade ambienttemperature|
The Schottky diodes were then replaced with the SBR30A45CT andSBR30A60CT and the evaluation repeated. The results are summarized in Figure4 above and Figure 5 below . As in the previous case, thesame conclusion can be drawn since the SBR diodes increase theefficiency of both outputs by approximately 1.5 percent.
|Figure5. Efficiency of 12V at 17A output at 50 degrees Centigrade ambienttemperature|
As previously mentioned, Schottky diodes rapidly lose theirperformancewhen the operating temperature increases. Conversely, SBR offers thebenefit of higher reliability at higher temperature.
To demonstrate this point, the 80PLUS power supply was again placedinside a temperature chamber which was set at an ambient temperature of50°C. A type-K thermo-coupler was used to measure the casetemperature of the Schottky diodes as the output load of the powersupply was varied from zero to full load. The Schottky diodes were thenreplaced with SBRs and the procedure repeated.
|Figure6. Diode case temperature on the 12V output with d50 egrees Centigradeambient temperature|
Figure 6 above and Figure 7 below show thetemperature plots measured from the 12V and 3.3V outputs. In bothplots, the SBR diodes operate at a lower temperature than the Schottkydiodes. In the former the temperature difference of 15° isconsistent across the load range above 50 percent load conditions,whereas at the lower output voltage of 5V, the temperature differenceis 60 percent to full load conditions.
|Figure7. Diode case temperature on the 5V output with 50 degrees Centigradeambient temperature|
In typical power supply applications, rectifier diodes are requiredto safely handle relatively large reverse power levels. As such, powersupply designers will have to choose a reliable diode that canwithstand the peak avalanche surges.
The choice, however, needs to be balanced with achieving the optimumpower supply efficiency. This would then call for a diode rectifierthat has the lowest forward voltage (VF ) with the highestpeak avalanche rating, commonly denoted as the PARM (TJ )of the diode.
Due to the structural differences between SBR and Schottky diodes(the absence of a metal barrier in their structure), the SBR diodeshave significantly greater avalanche capability compared to thestandard Schottky diodes while offering the low forward voltagecharacteristic as previously described. This is yet another factor thatmakes SBR diodes attractive to power supply designers.
Table 2 below showsa comparison of the maximum peak-pulse reverse surge current (IRMM )and the avalanche energy capability EAS of one SBR diodecompared to two equivalent Schottky diodes in the market. The SBR'sreverse avalanche capability drastically outperforms both Schottkydiodes, which means increased reliability for the SBR diodes againstany large reverse surge currents.
|Table2 Peak-pulse reverse surge current|
This article has demonstrated how compared to Schottky diodes, the lowVF of the SBR can increase a power supply's efficiency by up to 2.5percent and reduce the temperature of the output stage by up to 15°C.
This can be a critical improvement when trying to meet the demandsof an 80PLUS compliant PSU. SBR diodes have a higher avalanche ratingthan Schottky diodes, so improving the ruggedness of the overall powersupply. Finally SBR's are pin for pin compatible replacements forSchottky diodes, avoiding the need for changes in the PCB.
John Chiem is technical marketing manager at Diodes Inc.