How to boost your embedded design's power supply efficiency

John Chiem

February 3, 2009

John Chiem

Schottky rectifiers have been used extensively in power supplies for the past 25 years. Schottky diodes feature a very low forward volt drop (V_F), at reasonable leakage current, switching speeds that are in the nanosecond range, and provide a competitive advantage over conventional epitaxial p-n diodes.

Consequently, Schottky diodes have become the dominant technology in the output stages of power supplies in the voltage range below 150V. Above this voltage the V_F of a Schottky can be equal to or greater than equivalent epitaxial p-n diodes.

However, compared with p-n diodes, Schottky diodes are known for their relatively high leakage currents in the off state and have lower maximum rated junction temperatures than conventional epitaxial p-n diodes.

In applications where high breakdown voltage or high reliability (high operating temperatures or high currents) are required, p-n diodes dominate the rectifier market. Unfortunately, p-n epitaxial diodes have a larger reverse recovery time than Schottky diodes increasing the switching losses within the power supply. Super Barrier Rectifier (SBR) technology combines the low VF performance of Schottky diodes while possessing the high reliability characteristics of epitaxial p-n diodes.

This article presents laboratory-derived data that demonstrates the benefits of SBR when used in the secondary stage of a power supply. In particular, it highlights how Super Barrier Rectifiers improve efficiency, reduce component temperature and increase the overall ruggedness of the power supply without the need for expensive re-designs.

Figure 1. 80PLUS power supply with multi-outputs two switch forward converter.

Improved efficiency
A desktop computer power supply commonly referred to as a 'Silverbox PSU' converts AC line to low voltage DC output for use by the hard drive, microprocessor and PC interfaces. It is comprised of a front end power factor correction (PFC) stage followed by an isolated DC/DC converter with several outputs.

The drive for higher efficiency means power supply designers now favor the two switch forward topology (Figure 1 above) to the conventional single switch topology as the DC/DC conversion stage. The additional transistor helps to reduce voltage stress enabling the overall system efficiency to reach the 80PLUS (greater than 80 percent efficiency) requirement, albeit adding complexity, increasing component count and cost.

Each power supply output is obtained by rectifying a pulse signal coming out of one secondary winding of a multiple output isolated DC/DC converter. Conventionally, output rectification has been achieved using 45V to 100V Schottky diodes. An 80PLUS computer power supply, rated at a few hundred watts, provides an excellent test bed to demonstrate the practical benefits of SBR diodes.

The 5V and 3.3V output rails of an 80PLUS power supply were used to evaluate the efficiency of the Schottky rectifiers 30CTQ045 and 30CTQ060. Efficiency and case temperature measurements were taken under room temperature and at an ambient temperature of 50°C, under 30 percent and full load conditions. The evaluation was then repeated with the SBR diode replacing the Schottky diode.

The output stage of the forward converter is operating at a switching frequency of 200kHz. At low load condition, the rectifiers work in discontinuous conduction mode (DCM) and traverse into continuous conduction mode (CCM) operation at high load to take advantage of lower RMS/peak current value.

The converter efficiency was measured at 115Vac. The output voltage used for efficiency evaluation is measured at the end of the power cable. Efficiency was measured across a range of loads, more critically at 20 percent, 50 percent and 100 percent.

Figure 2. Efficiency of 5V at 20A output at room temperature

The efficiency plots, taken at room temperature, of the 5V and 12V outputs of the 80PLUS power supply are shown in Figure 2 above and Figure 3 below. In both cases, the efficiency performance with the SBR diodes in situ was respectively 1.5 percent and 2.5 percent better than that achieved by Schottky solution.

Figure 3. Efficiency of 12V at 17A output at room temperature

The efficiency of the power supply with both SBR and Schottky as output rectifiers is shown in Table 1 below. Although the output of the evaluated power supply is capable of delivering higher load current than that shown in Table 1, the combined maximum DC output is limited to 300W. Nevertheless, the result shows that the SBR improves the efficiency of the overall system by more than 1 percent compared with that using a normal Schottky rectifier, enabling the power supply to achieve 80PLUS compliance. All measurements were taken with input voltage at 115V nominal.

Table 1. Efficiency for SBR diodes and Schottky rectifiers

The 80PLUS power supply was then placed inside a temperature chamber and the ambient temperature increased to 50°C. The efficiency of the 5V and 12V rail was measured from no load to full load with the Schottky diodes in the output rectification stage.

Figure 4. Efficiency of 5V at 20A output at 50 degrees Centigrade ambient temperature

The Schottky diodes were then replaced with the SBR30A45CT and SBR30A60CT and the evaluation repeated. The results are summarized in Figure 4 above and Figure 5 below. As in the previous case, the same conclusion can be drawn since the SBR diodes increase the efficiency of both outputs by approximately 1.5 percent.

Figure 5. Efficiency of 12V at 17A output at 50 degrees Centigrade ambient temperature

Keeping cool
As previously mentioned, Schottky diodes rapidly lose their performance when the operating temperature increases. Conversely, SBR offers the benefit of higher reliability at higher temperature.

To demonstrate this point, the 80PLUS power supply was again placed inside a temperature chamber which was set at an ambient temperature of 50°C. A type-K thermo-coupler was used to measure the case temperature of the Schottky diodes as the output load of the power supply was varied from zero to full load. The Schottky diodes were then replaced with SBRs and the procedure repeated.

Figure 6. Diode case temperature on the 12V output with d50 egrees Centigrade ambient temperature

Figure 6 above and Figure 7 below show the temperature plots measured from the 12V and 3.3V outputs. In both plots, the SBR diodes operate at a lower temperature than the Schottky diodes. In the former the temperature difference of 15° is consistent across the load range above 50 percent load conditions, whereas at the lower output voltage of 5V, the temperature difference is 60 percent to full load conditions.

Figure 7. Diode case temperature on the 5V output with 50 degrees Centigrade ambient temperature

In typical power supply applications, rectifier diodes are required to safely handle relatively large reverse power levels. As such, power supply designers will have to choose a reliable diode that can withstand the peak avalanche surges.

The choice, however, needs to be balanced with achieving the optimum power supply efficiency. This would then call for a diode rectifier that has the lowest forward voltage (V_F) with the highest peak avalanche rating, commonly denoted as the P_ARM(T_J) of the diode.

Due to the structural differences between SBR and Schottky diodes (the absence of a metal barrier in their structure), the SBR diodes have significantly greater avalanche capability compared to the standard Schottky diodes while offering the low forward voltage characteristic as previously described. This is yet another factor that makes SBR diodes attractive to power supply designers.

Table 2 below shows a comparison of the maximum peak-pulse reverse surge current (I_RMM) and the avalanche energy capability E_AS of one SBR diode compared to two equivalent Schottky diodes in the market. The SBR's reverse avalanche capability drastically outperforms both Schottky diodes, which means increased reliability for the SBR diodes against any large reverse surge currents.

Table 2 Peak-pulse reverse surge current

Critical improvement
This article has demonstrated how compared to Schottky diodes, the low VF of the SBR can increase a power supply's efficiency by up to 2.5 percent and reduce the temperature of the output stage by up to 15° C.

This can be a critical improvement when trying to meet the demands of an 80PLUS compliant PSU. SBR diodes have a higher avalanche rating than Schottky diodes, so improving the ruggedness of the overall power supply. Finally SBR's are pin for pin compatible replacements for Schottky diodes, avoiding the need for changes in the PCB.

John Chiem is technical marketing manager at Diodes Inc.