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Power Electronics - Si, Si Schottky or SiC Schottky?

  Knowledge

The growth in the use of silicon carbide (SiC) in power electronics is gaining pace, because it allows for reduced power and switching losses as well as more compact form factors compared to silicon – and falling prices make it an increasingly compelling option for power semiconductor developers.

In power electronics, the demand is for ever greater switching performance at ever higher voltages. Space requirements, weight and efficiency also play a key role in choosing components for applications such as industrial motor control systems, renewable electricity generation and electromobility. The aim is to minimize cost and workload while maximizing the quality of the applications.

While silicon (Si) diodes are usually the standard product of choice here, components based on SiC offer significant advantages, especially from a voltage of 600V upwards. As high-performance components in circuit applications are always used with pulsed currents, they also need to account for the switching losses as well as the EMI (electromagnetic interference) generated by the reverse recovery currents.

Switching and Forward Voltage Losses

Switching losses are incurred with each switching process, for example when switching components on or off. As switching frequencies increase, so too do the corresponding losses, and thus the overall power loss of the system. This is why, when switching frequencies are high, much of the total power loss of the system is due specifically to that. If Si components are used in these applications, the high power losses and resultant heat generation require the load current to be limited or costly cooling to be implemented.

In terms of network frequency, forward voltage losses play the greater role, while with switching frequencies from a few 100Hz upwards, the switching losses incurred with these are dominant.

With a very high reverse voltage, reverse voltage losses also play a role - especially at high temperatures. SiC Schottky diodes are ideal in these cases, as they offer very low reverse recovery currents and short reverse recovery times, which allows them to heavily reduce the associated energy losses.

To calculate total loss, we apply:

Pv = Ps + Pt + Pr (switching losses- + forward voltage losses + reverse recovery losses)

While power loss in the diode increases as the forward current increases with a forward bias, it remains constant with a reverse bias. This is why the leakage current IR of SiC Schottky diode in the booster accounts for a not-insignificant share of the total loss with a low output current. At high currents, on the other hand, the forward voltage UF is the dominant factor. Because the Schottky diode spends most of its time in reverse-bias operation, the reverse recovery current has a significant impact on the diode's power loss, so it is not enough to simply keep the forward voltage of the diode as low as possible. It makes more sense to consider IR and UF in tandem and to evaluate how they both contribute to the total loss of the diode.

The higher the output voltage of the booster, the higher the switch-on time and the longer the Schottky diode remains in reverse bias. Reducing the forward voltage with Schottky diodes increases the residual reverse current, making it necessary to find the ideal diode.

So when choosing diodes, it is essential to minimize forward voltage losses, switching losses and charge, while also maximizing breakdown voltage and soft commutation. To ensure good energy efficiency, considering total power loss instead of individual module parameters is often a more logical approach with Schottky diodes.

Due to their lower switching losses and the absence of reverse current spikes when switching the diode off, SiC Schottky diodes are much more efficient than Si diodes. Radio interference is reduced accordingly and the EM behavior of the system as a whole is improved.

Operating Temperature and Thermal Design

Thermal design plays a key role in power electronics systems to ensure a high power density, thus enabling the production of more compact systems. At high currents, Si Schottky diodes are susceptible to excessive heat output. The combination of high heat levels and elevated leakage current (IR) can result in an increase in package and ambient temperature. Improper thermal design therefore might generate heat levels that cannot be dissipated. A possible result of this is "thermal runaway", which is an extreme rapid build-up of heat that may damage the component and possibly even the system as a whole.

The temperature relationship of SiC Schottky diodes is massively different from that of Si Schottky diodes. The heat conductivity of silicon carbide is almost three times higher than that of silicon, making SiC ideal for higher operating temperatures. Less heat loss when operating SiC power semiconductors also entails greater efficiency and smaller heatsinks, which reduces the space requirements of the application and its weight.

As the forward voltage Vf increases with operating resistance at higher temperatures, this helps to prevent thermal runaway, enabling SiC Schottky diodes to also be connected in parallel. Due to their positive temperature coefficient, they are also more suitable than silicon diodes for parallel circuits at high voltages.

Power Factor Correction

European standard EN 61000-3-2 defines the limits for the harmonic content of the mains current for devices intended for sale to the general public and having active power. It also defines restrictions and exceptions where 75W is exceeded. In practice, this means that the use of a bridge rectifier with subsequent filtering is in many cases not permitted for basic AC/DC conversion, as the mains current in this case is pulsed and exhibits a higher harmonic content. A booster known as a "power factor pre-regulator" or "power factor corrector" (PFC) is used to keep it roughly sinusoidal.

A CCM-PFC controller (continuous conduction mode) is the preferred active topology for very high-performance PSUs. This design imposes the following requirements on the flyback diode:

low reverse recovery time/charge (trr/Qrr) in order to reduce the switch-on losses of the MOSFET and the switching losses of the diode.

a low forward voltage Vf in order to reduce conduction losses and

a soft reverse recovery curve to reduce electromagnetic radiation (EMI).

The SiC Schottky diode is thus the ideal solution in this case.

The Best Diode for Each Application

Si diodes are the first choice for low-voltage applications. In high-voltage applications of 600V to 1200V on the other hand, SiC diodes offer significant technical advantages that make the higher costs worth it. In the 200-600V range, switching frequency and current are the critical factors. SiC diodes are required in various applications, including in charging stations for electric cars and on-board chargers (OBCs), in power converters for electric and hybrid vehicles, switched-mode power supply units and PFC circuits, as flyback diodes for inductors and MOSFETs/IGBTs, and as inverters in DC/AC converters for solar and wind power.

Rutronik offers ultra-fast high-voltage diodes from STMicroelectronics. These are ideal for cost-sensitive applications, and with a low forward voltage Vf, they are suitable for the input of an AC rectifier bridge. These STTHxx Si diodes are designed for 600V to 1200V with a current capacity of 5A to 30A.

The Schottky rectifier diode STPSC10H12 with an SiC substrate offers a low forward voltage and a rated voltage of 1200V thanks to the large band gap of the material. Due to the Schottky design, it exhibits no reverse recovery time during switch-off and has a negligible oscillation tendency. Their minimal capacitive switch-off behavior is independent of temperature. The SiC diode STPSC10H12 is especially suitable for use in PFC and secondary applications and increases performance under hard switching conditions. It is specified for operation at junction temperatures of between -40°C and +175°C. An AEC-Q101-qualified version for automotive used is also available in the form of the STPSC10H12-Y, which also supports PPAP.

 

 

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