There are 8 reasons why silicon carbide diodes are better than silicon diodes

August 4, 2023

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1--At the same rated voltage, SiC diodes take up less space than Si


The dielectric breakdown field strength of SiC is about 10 times higher than that of silicon-based devices, and at a given cut-off voltage, the drift layer of SiC is thinner and the doping concentration is higher than that of silicon-based devices, so the resistivity of SiC is lower and the conductivity is better. This means that, at the same rated voltage, the SiC chip is smaller than its silicon equivalent. An added benefit of using a smaller chip is that the inherent capacitance and associated charge of the device are lower for a given current and rated voltage. Combined with SiC's higher electron saturation speed, this enables faster switching speeds and lower losses than Si based devices.


2--iC diodes have better heat dissipation performance


The thermal conductivity of SiC is almost 3.5 times that of Si based devices, so it dissipates more power (heat) per unit area. While packaging can be a limiting factor during continuous operation, SiC offers a large margin advantage and helps design applications that are vulnerable to transient thermal events. In addition, high temperature resistance means SiC diodes have higher durability and reliability without the risk of thermal runaway.


3--Unipolar SiC diodes do not have a stored charge that slows down and reduces efficiency


SiC diodes are unipolar Schott semiconductor devices in which only a majority of charge carriers (electrons) can carry current. This means that when the diode is forward-biased, the junction depletion layer stores almost no charge. In contrast, P-N junction silicon diodes are bipolar diodes and store charges that must be removed during reverse bias. This results in a reverse current spike, so the diode (and any associated switching transistors and buffers) have a higher power loss, while the power loss increases with the switching frequency. SiC diodes produce reverse current spikes at reverse bias due to their inherent capacitive discharge, but their peaks are still an order of magnitude lower than P-N junction diodes, which means lower power consumption for both the diode and the corresponding switching transistor.


4--The forward voltage drop and reverse leakage current of SiC diodes match that of Si


The maximum forward voltage drop of SiC diodes is comparable to that of ultrafast Si diodes and is still improving (there is a slight difference at higher cut-off voltage ratings). Despite being a Schottky type diode, the reverse leakage current and resulting power consumption of high-voltage SiC diodes are relatively low at reverse bias, similar to ultrafine Si diodes at the same voltage and current levels. Since the SiC diode does not have the reverse charge recovery effect, any small power difference between the SiC diode and the ultrafine Si diode due to the forward voltage drop and reverse leakage current changes is more than offset by the reduction of the SiC dynamic loss.

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5--SiC diode recovery current is relatively stable in its operating temperature range, which can reduce power consumption


The recovery current and recovery time of silicon diodes vary greatly with temperature, which increases the difficulty of circuit optimization, but this change does not exist in SiC diodes. In some circuits, such as the "hard switch" power factor correction stage, a silicon diode acting as a boost rectifier can control the loss from the forward bias at high current to the reverse bias of a typical single-phase AC input (usually about 400V D bus voltage). The characteristics of SiC diodes can significantly improve the efficiency of such applications and simplify design considerations for hardware designers.


6--SiC diodes can be connected in parallel without the risk of thermal runaway


SiC diodes also have the advantage over Si diodes that they can be connected in parallel because their forward voltage drop has a positive temperature coefficient (in the application-relevant region of the I-V curve), which helps to correct all current uneven flows. In contrast, when devices are connected in parallel, the negative temperature coefficient of the SiP-N diode can lead to thermal runaway, requiring the use of significant derating or additional active circuits to force the device to achieve current equalization.


7--The electromagnetic compatibility (EMI) of SiC diodes is better than that of Si


Another advantage of the SiC diode soft-switching feature is that it can significantly reduce EMI. When Si diodes are used as switching rectifiers, the potentially rapid spikes in reverse recovery currents (and their wide spectrum) can lead to conduction and radiation emission. These emissions create system interference (through various coupling paths) that may exceed system EMI limits. At these frequencies, filtering can be complicated due to this spurious coupling. In addition, EMI filters designed to attenuate switching fundamental frequencies and low harmonic frequencies (usually below 1MHz) typically have a relatively high inherent capacitance, which reduces their filtering effect at higher frequencies. Buffers can be used in fast recovery Si diodes to limit edge rates and suppress oscillations, thereby reducing stress on other devices and reducing EMI. However, the buffer dissipates a lot of energy, which reduces the efficiency of the system.


8--The forward recovery power loss of SiC diode is lower than that of Si


In Si diodes, the power loss source of forward recovery is often overlooked. During the on-state transition from the off state, the diode voltage drop temporarily increases, resulting in overshoot, ringing, and additional losses associated with the lower initial P-N junction conductivity. However, SiC diodes do not have this effect, so there is no need to worry about forward recovery losses.