Comparing Silicon Carbide Schottky Diodes to Silicon Diodes in Power Conversion Designs

Measurements shown in Figure 1 compare Bourns® SiC Schottky Diodes and industry-standard silicon Schottky Diodes for rectangular pulse widths of 10 µs. Bourns® SiC Schottky Diodes exhibit a negative temperature coefficient of resistance, illustrated by the decrease in forward voltage with temperature. This is beneficial when designers need to parallel two or more devices to increase the power capability to accommodate large area current sharing. The fact that the diode forward voltage and operating resistance decrease at elevated temperatures is also advantageous to help prevent thermal runaway in parallel connections.

SiC SBD Reverse Recovery Characteristics

For diodes made from silicon carbide semiconductor material, the faster reverse recovery speed and reduced reverse charge recovery is a result of the lower capacitance of their internal construction. Since the diode capacitance is constant over temperature as indicated by the constant stored charge, Q_rr, the reverse recovery time will be constant over temperature, which is better for stabilizing power switching applications.

Figure 2 shows the definition of the reverse recovery time. When forward bias is removed, the forward current decreases and keeps on flowing past zero at a rate of dI_f/dt, removing the excess charge stored in the depletion region. The reverse current is the flow of excess charge, reaching the maximum reverse recovery current, I_rrm. It decreases until all the stored charge is removed. The reverse recovery time is the time required to remove the excess charge. It is measured from the beginning of the reverse current to a linearized time when the current returns to zero and the stored charge is removed.

The reverse recovery of PN and Silicon Carbide Diodes is shown in Figure 3. Notice that the maximum recovery current of the PN diode is approximately six times larger than the SiC Diode. The reverse recovery time t_rr is a function of forward current and the rate of change as it is turned off. This function sets the maximum reverse recovery current, I_rrm.

In silicon high-speed junction PN Diodes, a high level of transient current flows when the current switches from forward to reverse, which leads to large losses when switching to the reverse bias condition. When current is applied in the forward direction, minority carriers accumulated in the drift layer contribute to electrical conduction until they disappear (also referred to as the diode storage time) when the diode is switched from forward conduction to reverse. This increases both the recovery time and the recovery current as the forward current flows and temperature increases, causing significant loss.

In contrast, SiC Schottky Barrier Diodes (SBDs) are majority carrier devices that do not use minority carriers for electrical conduction so, in principle, the minority carrier accumulation does not occur. As a result, only a small amount of current flows for discharging the junction capacitance, achieving considerably lower losses than silicon Fast Recovery Diodes (FRDs). This transient current is largely independent of temperature and forward current, making stable high-speed recovery possible under virtually any circuit condition. SiC SBDs also tend to deliver noise reduction to applications due to fast changes of reverse recovery current. Since Silicon PN Diodes are bipolar semiconductors that depend on the injection of minority charge carriers, they exhibit a large reverse recovery charge. During the conduction state of the diode, charge carriers are injected into the device and need to be removed from the device before a voltage can be blocked, or, in other words, before a space-charge region can be built-up. A higher charge carrier concentration will result in a high reverse recovery charge. Moreover, the reverse recovery charge is dependent on forward current and the device’s junction temperature. The advantage of using SiC Schottky Diodes is that because they are majority carrier devices, they show virtually zero reverse recovery charge. Looking at the switching waveforms in Figure 3, the reverse recovery current peak is minor compared to a fast Si PN Diode. Only the displacement current from the junction capacitance is visible. This leads to significantly lower turn-off losses. Furthermore, since the dynamic characteristic of a Schottky Diode is capacitive in nature, the reverse recovery characteristic of a SiC Schottky Diode is independent from forward current, di/dt, and device junction temperature.

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Conclusion

As shown in this article, Bourns® SiC Schottky Barrier Diodes have distinct advantages compared to the usual choice of Silicon-based PN Diodes. In most applications with operating voltages over 600 V, using SiC SBDs, with their inherent lower voltage drop and reverse recovery loss, will increase the design’s energy efficiency, particularly when employed in applications that operate at elevated temperatures. Utilizing SiC SBDs also helps to reduce heat sinking requirements, simplifies the thermal design and reduces the thermal impedance since the semiconductor material has much higher thermal conductivity.

SiC SBDs are particularly well-suited for high voltage and high current systems with frequencies of 100 kHz or below. This is because SiC SBDs have lower switching losses and lower forward conduction losses than similarly rated Silicon-based PN Diodes. Their overload durability is also enhanced and the waveforms during switching are softer, resulting in lower EMI from the application.

Silicon Carbide Diode technology is mature and proven, but has still been shown to have tremendous future potential. As a leader for more than 75 years in the development of innovative technologies and component solutions, Bourns’ new SiC Schottky Diodes are designed to deliver enhanced application efficiencies.

Each application is different and may require design tradeoffs. That is why it is helpful to have a good understanding of the differences between the various power semiconductor rectifying devices. This knowledge allows designers to select the power electronics device that is best suited to meet the specifications for their particular application, especially those developing DC-based EV chargers and On-Board Chargers. As shown in this article, energy efficiency can increase up to 2 % with 33 % higher power density when replacing silicon-based solutions with SiC Diodes. This allows engineers to design smaller applications while supporting higher power in the same space.