Infineon Technologies AG introduced a brand-new family of close-to-ideal semiconductor rectifiers: the SiC Schottky diodes. Here, the basic properties of the novel semiconductor material silicon carbide (SiC) will be presented in relation to market requirements and application benefits. The unique behavior of the now commercially available SiC Schottky diodes with a blocking voltage up to 600V will be described with special emphasis to the ideal switching characteristics. This is enabled by the absence of reverse recovery charge and current in these diodes.
Developing a unipolar diode with high blocking voltage
System miniaturization, the reduction of system size and weight, is a trend in electronics mainlyÑbut not onlyÑdriven by the increasing amount of portable applications. Compared to its functionality, the power supply of these items is still a dominating part concerning dimensions and weight of the whole system. For example, in a typical portable computer the power supply causes more than 10% of the total weight of the system. All manufacturers of switch mode power supplies (SMPS) thereby have defined roadmaps to increase the power density in their products.
The two major approaches to realize these roadmaps are:
- reduction of the size of passive components (by increasing the switching frequency and/or reducing the size of EMI filter through low noise generation), and
- reduction of the power losses and correspondingly the cooling effort (heat sink and/or fan).
This results in a specific requirement for the main power semiconductor components: a significant reduction of switching power losses. For this reason the unipolar semiconductors like MOSFETs and Schottky barrier diodes are replacing bipolar devices. The beauty of unipolarity is the absence of stored charge carriers and, therefore, theoretically instantaneous switching transients limited by small parasitic capacitances only.
Power MOSFETs like CoolMOS & OptiMOS can be found in a wide blocking voltage range from 20 up to 1000V. On the other hand Schottky barrier diodes today have a Silicon (or GaAs) material related limitation of maximum 250V. The main reasons for this limitation are:
- very high leakage currents, especially at higher temperatures since the reverse losses are comparable to the forward losses,
- strong increase of the area specific on-resistance Ron, A with breakdown voltage VBR2.5, and
- an increase of the chip area cannot solve the problem since it increases the reverse losses.
The input stage of an off line SMPS usually requires devices rated in the range of 500 to 600V.
Thus, a strong and unsatisfied need exists for Schottky barrier diodes beyond the blocking capabilities of established semiconductors like Si and GaAs.
Silicon carbide Schottky diodes can satisfy these requirements
With silicon carbide, belonging to the so called wide bandgap semi-conductors, the voltage range for Schottky diodes now can be extended to more than 1000V. This is possible by the material related benefits of SiC:
- 2 times higher metal semi-conductor barrier offers low leakage currents,
- 10 fold breakdown field strength allows very attractive specific on-resistance compared to Si and GaAs Schottky diodes (Figure 1), and
- a thermal conductivity more three times larger than that of Si (i. e. comparable to that of copper) allows to dimension SiC Schottky diodes to high current densities, leading to very small die sizes.
Benefits of Schottky diodes in expanded voltage range
SiC Schottky diodes offer a very low specific on-resistance with high rated voltages. In Figure 2 the typical forward and blocking characteristics of 600V SiC Schottky diodes are given up to 225¡C.
Different to Si and GaAs Schottky diodes, there is only a moderate increase in leakage current with increasing temperature. The area specific differential on-resistances of a 600V SiC-Schottky diode increases from about 0.9m½cm2 at room temperature to 1.8m½cm2 at 150¡C. This positive temperature coefficient makes the Schottky diode well suited for paralleling without the risk of thermal runaway.
Dynamic performance of SiC Schottky diodes is closer to ideal diode
When switching a Schottky diode off, there is no need to remove excess carriers from the n-region as for pin diodes. Hence, no reverse recovery current will show up. Instead only a displacement current for charging the junction capacitance of the diode can be observed (Figure 3).
The current transient is only depending on the external switching speed up to very high frequencies.
The charge transported by this displacement current is very low compared to the reverse recovery charge Qrr of pin diodes. Due to the different origin of this charge we have named it capacitive charge Qc. This Qc and the switching power losses of SiC Schottky diodes are not only ultra low. Compared to Silicon ultra fast diodes, where losses depend strongly on dI/dt, current level and temperature, they are more or less independent on these boundary conditions (Figure 4). A dependence of Qc on these parameters can not be seen at
the same scale as with a benchmark Si diode approach. Again this is due to the capacitance like behavior of this device in reverse direction.
Applications
In the following, two application examples are discussed with a focus on the efficiency and system benefits induced by the use of a SiC diode instead of a conventional one.
Power factor correction in SMPS (boost converter)
Worldwide requirements for power factor correction are growing strongly driven by legal requirements. Boost converters are usually used to realize active power factor correction. They can be driven in discontinuous current mode (DCM) and continuous current mode (CCM). The DCM solution does not require an ultra fast diode, but has the following drawbacks:
- all circuit components have to be oversized because of the high peak currents,
- the system becomes unstable at light load, and
- a complex EMI filtering system is necessary.
The CCM solution doesn't have these disadvantages. The circuit components don't have to be oversized, the system operates stable at light load and the requirements on the EMI filter are less rigid than in the DCM case. However, the power losses in the MOSFET and in the diode due to reverse recovery dramatically limit the efficiency and switching frequency of a CCM boost converter (Figure 5) when a conventional or ultra-fast Si pin-diode is used.
As SiC Schottky diodes do not show a reverse recovery behavior, the stress on the MOSFET will be reduced due to very low current spike during turn on transient. A less expensive MOSFET can be chosen and simultaneously higher reliability of the entire system can be achieved.
The efficiency shows a virtually independent behavior regarding frequency due to very low total switching power losses (Figure 5), which is ideally suited for CCM. Boost converters can therefore be operated at much higher switching frequencies. The size reductions of the boost inductors open new horizons in power density. This also has a strong impact on the inductor cost.
Does higher switching frequency cause more EMI challenges? The EMI norm regulation begins at 150kHz,
so the main harmonic of the boost converter can be well inside this range. One will suspect complications achieving the EMI norm in case of higher switching frequency. Figure 6 shows the impedance of a typical current-compensated double choke inductor depending on the frequency.
Efficiency
As it can be seen, the filter has its maximum impedance (i.e. highest damping efficiency) in frequency range from 300kHz to more than 1MHz. The main and higher harmonics of a boost converter running at 300 to 500kHz will be filtered with the maximum efficiency and, therefore, the increase of switching frequency to 300 to 500kHz does not cause a need for additional EMI filtering, whereas the electrical noise can be even reduced.
With the properties described above circuit designers now have a new degree of freedom in optimizing his PFC applications by:
- increasing the switching frequency,
- reducing the size of passive components like inductors,
- shrinking the size of semiconductor switches,
- shrinking or avoiding heat sinks,
- increasing the reliability, and
- increasing the power density
Secondary side output rectification voltage of SMPS
SMPS with an output voltage of 48V (e.g. for Telecom applications) require 250 to 300V rated rectifiers on the secondary side. The trend to higher switching frequencies in the main converter is in strong contrast with the poor dynamic properties of the existing Si diodes on the secondary side. The GaAs Schottky diodes used today are commercially available up to a blocking voltage of 250V. Replacing the GaAs with SiC Schottky diodes is now very attractive since:
- SiC Schottky diodes have low leakage currents and only a small temperature dependence of this leakage,
- their higher blocking capability (300 V instead of 250 V) causes an improvement of the reliability, and
- SiC diode is a very cost competitive alternative.
