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It is clear that C7 has higher Coss values in the low-voltage region it then drops more quickly compared to other SJ technologies as voltage increases and has significantly lower values in the high-voltage region. The higher capacitance at low voltage means that the C7 has lower turn off energy (Eoff).The lower capacitance at high voltage causes a faster dv/dt transition. Furthermore, the combination of COSS dropping at lower voltages and the reduction of COSS at higher voltages (which dominates EOSS, the overall value of the energy stored in the output capacitance) brings EOSS at typical DC link voltages down to around one half of previous generations. In hard-switching applications this energy represents a fixed loss that will be significant at light load conditions (whereas most other losses are load-dependent and decrease considerably as load reduces). The EOSS reduction, therefore, contributes to an improvement of light load efficiency in hard-switching designs that is comparable to that which could be achieved with much more expensive GaN technologies. Benefits in PFC and LLC Designs The improved performance offered by the 600V CoolMOS C7 technology translates into real benefits for both the efficiency- and cost/BoM-driven applications mentioned previously. Figure 3, for example, shows the efficiencies of the 600V C7 MOSFETs against devices from the CoolMOS CP family when deployed in a continuous conduction mode (CCM) power factor correction (PFC) SMPS operating at 65kHz. It can be seen clearly that in conjunction with the TO-247-4 package there is a 0.7% light load efficiency gain against CoolMOS CP and a 0.4% average efficiency gain over the entire load range. This PSU efficiency gain translates into a total cost of ownership reduction as a result of lower energy losses. In the case of a 2.5kW server PSU working with an average 50% load, for example, the improved efficiency could translate into an overall 9% reduction in energy costs. In addition, because there is less thermal loss the superior performance is achieved within a higher density form factor, which translates into additional board space savings. Figure 3: Efficiency improvement for hard switching PFC designs In Figure 4 we see a similar efficiency comparison for a highend 600W LLC resonant converter design, this time with respect to load current. Once again the 600V C7 technology delivers the highest efficiency across all load conditions. Figure 4: Highest Efficiency MOSFET for Soft Switching LLC Designs Doubling Switching Frequency The reduced switching losses delivered by improvements in Qg, COSS and EOSS allow the use of higher switching frequencies without any loss in efficiency. In tests, for example, it has been demonstrated that the C7 technology can be switched at 130kHZ - twice the frequency of the previous CP technology - with only a marginal penalty of around 0.2% loss at light loads. Doubling the frequency of operation translates into very real space and BoM cost reductions as it potentially halves the ratings (and size) of the inductive components needed, which reduces the cost of magnetics by as much as 30%. Boosting Full Load Efficiency Through Packaging In addition to the advanced semiconductor process, Infineon has also made advances in packaging technology. The 600V C7 CoolMOS family, for instance, includes an innovative TO-247 4pin package, which provides an additional connection for the MOSFET source. The TO-247 4pin package has been developed to address the issue that in common gate drive arrangements fast current transients can cause a voltage drop across the parasitic inductance of the source. This counteracts the driving voltage, reducing switching speeds and increasing associated energy losses. The additional ‘source-sense’ connection to the source in the TO-247 4pin package serves as a reference potential for the driver, eliminating source inductance in the gate driving loop and delivering an undisturbed signal to the gate. As well as enabling greater high load efficiency this innovation may, depending on application, also support BoM cost reduction by allowing the use of higher RDS(ON) MOSFETs. Portfolio and Support CoolMOS C7 600V MOSFETs will initially be available in TO- 220, TO-247 and TO-2474pin packages with maximum RDS(ON) ratings from 180mΩ to just 40mΩ depending on the particular device. As well as the devices themselves, Infineon is also providing comprehensive development and prototyping support in the form of application-specific evaluation boards and reference designs built around the CoolMOS C7 600V MOSFETs. These include a 130kHz 800W CCM PFC board, a 2.5kW PFC design, and a half-bridge 600W LLC board offering a 12V, 50A output. www.electronics-eetimes.com Electronic Engineering Times Europe June 2015 9


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