effiCient and affOrdable SChOttky diOdeS amazonaws com
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Efficient and Affordable Schottky Diodes SiC diodes in the 600V and above range enable virtually loss-less switching in combination with good conduction behaviour, allowing high efficiency and reduced complexity in today’s SMPS solutions
courtesy: infineon
S
ilicon carbide (SiC) as the ideal semiconductor material for power electronics applications is well known for decades. However, it was only in 2001 that first commercial devices based on SiC were introduced into the market. Reason for this long pre-development time was the difficult substrate wafer manufacturing process. In fact, the first commercial SiC devices were manufactured on a 5.1cm (2-inch) diameter wafer. Seven years later, the wafer diameter used in production was increased to 10.2 cm (4 inches). This made the formerly exotic and expensive SiC technology much more affordable. The unique benefit of SiC diodes in the 600V and above range is that virtually loss-less switching is enabled in combination with attractive conduction behaviour, allowing benchmark efficiency and reduced complexity in today’s SMPS solutions. The third generation of SiC Schottky diodes features the industry’s lowest device capacitance for any given current rating, which further enhances overall system efficiency, especially at higher switching frequencies and under low load conditions.
Optimal utilisation of SiC diodes in pfc Which diode to choose for a given power application depends on the following factors: 1. Efficiency, both at full load and light load w w w. e f y m ag . co m
Infineon’s third-generation SiC Schottky diodes
2. Surge current capability, start-up and cycle drop-out 3. Thermal behaviour Whereas the efficiency requirement clearly points towards the use of SiC Schottky barrier diodes, the surge current capability points towards a merged p-n/Schottky concept to cope with potential thermal runaway in over-current conditions. Thermal behaviour, which benefits from the improved Rth/Zth values, has been achieved by the third generation. The only remaining task is then to choose the current rating for the given power application. Similar to the RDSon optimisation choice for a power MOSFET, in a power-factor correction (PFC) stage the losses attributed to the diode can be split into conduction losses, which
come from the ohmic slope of the forward characteristic, and capacitive losses, which come from the capacitive displacement charge. As conduction losses decrease with a lower ohmic slope or higher current rating of the diode, capacitive losses increase with a higher current rating. As conduction losses are directly proportional to the square of the power drawn from the PFC stage, a higher current rating will be beneficial for full-load efficiency. Vice versa, a low current rating will be beneficial for light-load efficiency. Choosing the right diode is thus in the end an evaluation of how to optimise the trade-off between full-load efficiency and light-load efficiency. Let’s look at an evaluation example. Assume a 1000W continuousconduction-mode PFC application with
e l e c t ro n i c s f o r yo u • O c to b e r 2 0 1 0 • 9 3
Design Features 1. Lowest switching losses due to lowest Qc (Qrr) for any current rating in the market 2. Fully surge-current stable, high reliability and ruggedness 3. Lower cost
Fig. 1: Comparison of conduction (dotted line) and total losses (solid line) of boost MOSFET (blue) and diode (red lines), respectively, as a function of RDSon or slope of the ohmic forward characteristic, respectively. The ampere values indicate the rated current of diodes with a resistivity as given on the X-axis
Vice versa, conduction losses, which go absolutely with the square of the current or output power, change to a linearly increasing dependency in a graph where the relative losses vs output power are plotted. Fig. 2 shows the result. At light load, third-generation SiC diodes give 0.26 per cent better efficiency than competing 8A SiC Schottky barrier parts. Whereas, at full load, a small efficiency penalty of 0.08 per cent is seen. Due to better thermal coupling, there is practically no increase in junction temperature. Furthermore, thermal runaway is avoided by the merged p-n/Schottky concept. At light load, however, efficiency is worth a lot as it is becoming increasingly difficult to meet the everdemanding efficiency targets there. At input voltages higher than 90V, the advantage of this optimisation is becoming even bigger. Additionally, there are SiC Schottky diodes available in not only the TO-220 package (real two-pin version) but also the DPAK package for highpower-density surface-mount designs.
Applications and benefits Fig. 2: Contribution of diode losses to the efficiency (diode losses/output power) as a function of output power in a 1000W PFC stage at 90V input and 130kHz switching frequency
wide input voltage range running at 130 kHz. Fig. 1 shows the conduction losses (in dotted lines) and total losses both for boost MOSFET and diode in this example. An ideal match here would be 75 mOhm for the PFC MOSFET and the 8A third-generation SiC Schottky diode as both devices lead to an optimum, which is close to the minimum of losses but on the right hand side
of this optimum (which is important with respect to cost-effectiveness). Now taking this 8A diode and varying the load from 20 per cent to 100 per cent gives a parabolic behaviour of the losses relative to the rated output power. Capacitive losses, which remain in their absolute value independent of the output power, increase inversely proportional to the output power as shown in the graph.
9 4 • O c to b e r 2 0 1 0 • e l e c t ro n i c s f o r yo u
SiC diodes can be used in SMPS (continuous-conduction-mode powerfactor correction), LCD, PDP, lighting, motor drive, UPS and solar applications. Benefits include system efficiency improvements at light and medium loads, lower system costs due to reduced cooling requirements, and broadest range of current ratings and lower costs/ampere for cost-effective performance improvements. Also, SiC diodes enable higher-frequency designs and increased power-density solutions. w w w. e f y m ag . co m
Efficient and Affordable Schottky Diodes SiC diodes in the 600V and above range enable virtually loss-less switching in combination with good conduction behaviour, allowing high efficiency and reduced complexity in today’s SMPS solutions
courtesy: infineon
S
ilicon carbide (SiC) as the ideal semiconductor material for power electronics applications is well known for decades. However, it was only in 2001 that first commercial devices based on SiC were introduced into the market. Reason for this long pre-development time was the difficult substrate wafer manufacturing process. In fact, the first commercial SiC devices were manufactured on a 5.1cm (2-inch) diameter wafer. Seven years later, the wafer diameter used in production was increased to 10.2 cm (4 inches). This made the formerly exotic and expensive SiC technology much more affordable. The unique benefit of SiC diodes in the 600V and above range is that virtually loss-less switching is enabled in combination with attractive conduction behaviour, allowing benchmark efficiency and reduced complexity in today’s SMPS solutions. The third generation of SiC Schottky diodes features the industry’s lowest device capacitance for any given current rating, which further enhances overall system efficiency, especially at higher switching frequencies and under low load conditions.
Optimal utilisation of SiC diodes in pfc Which diode to choose for a given power application depends on the following factors: 1. Efficiency, both at full load and light load w w w. e f y m ag . co m
Infineon’s third-generation SiC Schottky diodes
2. Surge current capability, start-up and cycle drop-out 3. Thermal behaviour Whereas the efficiency requirement clearly points towards the use of SiC Schottky barrier diodes, the surge current capability points towards a merged p-n/Schottky concept to cope with potential thermal runaway in over-current conditions. Thermal behaviour, which benefits from the improved Rth/Zth values, has been achieved by the third generation. The only remaining task is then to choose the current rating for the given power application. Similar to the RDSon optimisation choice for a power MOSFET, in a power-factor correction (PFC) stage the losses attributed to the diode can be split into conduction losses, which
come from the ohmic slope of the forward characteristic, and capacitive losses, which come from the capacitive displacement charge. As conduction losses decrease with a lower ohmic slope or higher current rating of the diode, capacitive losses increase with a higher current rating. As conduction losses are directly proportional to the square of the power drawn from the PFC stage, a higher current rating will be beneficial for full-load efficiency. Vice versa, a low current rating will be beneficial for light-load efficiency. Choosing the right diode is thus in the end an evaluation of how to optimise the trade-off between full-load efficiency and light-load efficiency. Let’s look at an evaluation example. Assume a 1000W continuousconduction-mode PFC application with
e l e c t ro n i c s f o r yo u • O c to b e r 2 0 1 0 • 9 3
Design Features 1. Lowest switching losses due to lowest Qc (Qrr) for any current rating in the market 2. Fully surge-current stable, high reliability and ruggedness 3. Lower cost
Fig. 1: Comparison of conduction (dotted line) and total losses (solid line) of boost MOSFET (blue) and diode (red lines), respectively, as a function of RDSon or slope of the ohmic forward characteristic, respectively. The ampere values indicate the rated current of diodes with a resistivity as given on the X-axis
Vice versa, conduction losses, which go absolutely with the square of the current or output power, change to a linearly increasing dependency in a graph where the relative losses vs output power are plotted. Fig. 2 shows the result. At light load, third-generation SiC diodes give 0.26 per cent better efficiency than competing 8A SiC Schottky barrier parts. Whereas, at full load, a small efficiency penalty of 0.08 per cent is seen. Due to better thermal coupling, there is practically no increase in junction temperature. Furthermore, thermal runaway is avoided by the merged p-n/Schottky concept. At light load, however, efficiency is worth a lot as it is becoming increasingly difficult to meet the everdemanding efficiency targets there. At input voltages higher than 90V, the advantage of this optimisation is becoming even bigger. Additionally, there are SiC Schottky diodes available in not only the TO-220 package (real two-pin version) but also the DPAK package for highpower-density surface-mount designs.
Applications and benefits Fig. 2: Contribution of diode losses to the efficiency (diode losses/output power) as a function of output power in a 1000W PFC stage at 90V input and 130kHz switching frequency
wide input voltage range running at 130 kHz. Fig. 1 shows the conduction losses (in dotted lines) and total losses both for boost MOSFET and diode in this example. An ideal match here would be 75 mOhm for the PFC MOSFET and the 8A third-generation SiC Schottky diode as both devices lead to an optimum, which is close to the minimum of losses but on the right hand side
of this optimum (which is important with respect to cost-effectiveness). Now taking this 8A diode and varying the load from 20 per cent to 100 per cent gives a parabolic behaviour of the losses relative to the rated output power. Capacitive losses, which remain in their absolute value independent of the output power, increase inversely proportional to the output power as shown in the graph.
9 4 • O c to b e r 2 0 1 0 • e l e c t ro n i c s f o r yo u
SiC diodes can be used in SMPS (continuous-conduction-mode powerfactor correction), LCD, PDP, lighting, motor drive, UPS and solar applications. Benefits include system efficiency improvements at light and medium loads, lower system costs due to reduced cooling requirements, and broadest range of current ratings and lower costs/ampere for cost-effective performance improvements. Also, SiC diodes enable higher-frequency designs and increased power-density solutions. w w w. e f y m ag . co m
