Advantages of using CoolSiC TM MOSFETs in bidirectional inverter applications



Bidirectional converters are increasingly being used in fields such as solar energy and electric vehicle charging. In these applications, the efficiency of energy conversion, storage, and recovery is critical. This article discusses the advantages of silicon carbide semiconductors in reducing converter losses, as well as in terms of cost, weight, and size.

As the industry continues to turn to renewable energy sources, people are paying more and more attention to power generation and energy storage technologies, with the goal of making fuller use of intermittent energy supplies such as wind and solar energy. Batteries are a common energy storage solution. Driven by technological progress in the electric vehicle market, the cost of energy storage is declining. This opens the way for energy storage applications of all different scales, from homes to utilities. As the energy supply mode turns to renewable, traditional fossil fuel power generation is decreasing. By using distributed storage to feed the AC power back to the grid through the Inverter for “peak shaving”, it can play its advantages and make power generation more cost-effective. Also more reliable.

In order to achieve this goal, batteries need to be able to be charged with cheap or convenient energy and then discharged to a local load or “fed” back to the utility grid. Both AC-DC chargers and DC-AC inverters are already very mature products, but if they can be effectively combined, costs can be saved more effectively. Therefore, people have a strong interest in “two-way converters”, and one of the high-volume application markets will be families with local renewable energy and energy storage equipment (perhaps electric car batteries).

Bidirectional converter requirements

One of the main issues in the renewable energy market is to maximize the use of solar or wind energy, so any losses in the power conversion stage must be kept to a minimum, which is also a basic requirement for shortening the return of capital. For power processing in any application, this is true. For many years, the power conversion topology has continued to develop in the direction of higher efficiency, and now the single-stage efficiency has reached 99% or higher. However, for bidirectional converters, high efficiency must be maintained in both forward and reverse energy flows, which is a more complicated problem. Fortunately, a factor that drives increased efficiency also promotes the bidirectional flow of energy: the use of MOSFETs as synchronous rectifiers in the “third quadrant” operation. As shown in Figure 1, it is a schematic diagram of a typical bidirectional converter that can be used as a battery charger and a feed-in inverter. The MOSFET bridge can be used as a rectifier, inverter or DC-DC converter, depending on the driving device.

Advantages of using CoolSiC TM MOSFETs in bidirectional inverter applications
Figure 1: Bridge-arranged MOSFETs are suitable for bidirectional power converters.

The AC-DC stage must also have a power factor correction (PFC) function, which is best achieved at medium power levels through a two-way “totem pole PFC” topology, where the MOSFET doubles as a line AC Rectifier and boost switch in AC-DC mode. And the inverter switch in DC-AC mode. The feature that the MOSFET can change its function depends on not only being able to conduct from drain to source in a “normal” mode, but also in reverse conduction from source to drain with low loss under the control of the gate drive. MOSFET also has a parasitic body diode from drain to source, which may also be an advantage. Some circuits that require reverse conduction will naturally “commutation” and apply a forward bias to the diode, thereby transferring energy to the output at the appropriate stage of the switching cycle. However, the diode is not ideal and will store a large amount of charge in its junction when it is turned on, which will be released during the reverse bias of each cycle. This leads to “recovery current”, which leads to increased losses, reduced efficiency, and increased EMI. Compared with silicon rectifiers, this diode also has a high forward voltage drop, causing additional losses. If there is a small delay, turn on the MOSFET channel and bypass the diode. After turning off the complementary MOSFET in the bridge arm, the additional dissipation of the diode forward conduction can be minimized.

Phase-shifted full-bridge (PSFB) or “LLC” configuration version of the bidirectional converter can operate with zero voltage switching (ZVS) for maximum efficiency. In this mode, the reverse recovery of the body diode is not important because the applied The reverse voltage will rise resonantly. However, in some cases, the converter may temporarily enter the “hard” switching mode, such as during startup, shutdown, or load step scenarios, during which there is a high voltage when the current is restored, which may cause damage Sexual stress. If the relevant MOSFET channel does not complete recovery during the on-time, it may also cause device failure.

If the MOSFET switch in the bidirectional converter has too high output charge QOSS, There will be some problems. In a hard-switching converter, the current generated during the switching transition will circulate in the primary circuit of the converter, causing losses. The output capacitance Coss also varies greatly with the drain-source voltage, resulting in high Qoss. If these are the main charges to be removed in the soft-switching resonant converter, it is difficult to maintain ZVS and high efficiency in the worst case. The minimum dead time between the high-side and low-side switches must also be increased as a function of Qoss, resulting in significant duty cycle losses at high switching frequencies. In the case of a lower Qoss, the circuit can be fine-tuned to obtain higher efficiency.

Therefore, for all the above reasons, stable and lower output capacitance, lower QOSS And the minimum body diode reverse recovery energy and time are critical to achieving higher efficiency and reliability. In hard-switching topologies such as totem-pole PFC, the current silicon super-junction MOSFET technology is not feasible for specific circuits at all due to the body diode produced by it.

SiC MOSFET is a better solution

Wide Band Gap Silicon Carbide (SiC) MOSFETs are now the mainstream technology. Compared with silicon devices, they have a better figure of merit (FOM), which can improve efficiency under high-frequency operation. They also have a series of other advantages, such as the ability to achieve inherent high temperature operation, low gate charge, lower on-resistance with temperature increase, and higher robustness. For the main topics discussed in this article, it is important that their body diodes have a much lower recovery charge, and the output capacitance changes much less than silicon MOSFETs under drain-source voltage. In addition, for the same RDSON, Q of SiC MOSFETOSS Approximately one-sixth of the silicon super junction MOSFET.

We can use Infineon’s silicon-based 600V CoolMOSTM CFD7 Super Junction MOSFET (IPW60R070CFD7) and 650V CoolSiCTMSiC MOSFET (IMZA65R048M1H) for comparison. They are all TO-247 packaged devices and have similar voltage and on-resistance ratings at 25°C. The overall diode reverse recovery waveform of the two is shown in Figure 2, and the total reverse recovery charge is denoted as QRR. For CoolMOSTM Device, QRRUsually 570 nC, while for CoolSiCTM MOSFET, at twice the forward current and 10 times dIF/dt Q at current rate of changeRROnly 125nC.

Advantages of using CoolSiC TM MOSFETs in bidirectional inverter applications
Figure 2: MOSFET body diode reverse recovery waveform. Q of SiCRR Approximately 20% of the value of Si MOSFET.

Figure 3 shows the output capacitance changes of the two MOSFET technologies, which shows a series of CoolSiCTMDevice and CoolMOSTM Comparison of CFD7 Super Junction MOSFET. Silicon carbide devices exhibit lower Coss at low voltages, and both types are lower at high voltages. But please note that IMZA65R048M1H CoolSiCTM The MOSFET changes about 10 times between the saturation voltage and the complete blocking voltage, while the super junction MOSFET changes about 8000 times. Although low Coss is beneficial to reduce the loss caused by charging and discharging current, it is very helpful for the non-zero value of Coss of SiC at high voltage. It can reduce the need to use gate resistors to reduce the switching speed to reduce the drain-to-source The voltage remains within the recommended maximum derating range. Otherwise, for Si devices, higher value resistors are required to limit the peak drain voltage, which leads to reduced controllability.

Advantages of using CoolSiC TM MOSFETs in bidirectional inverter applications
Figure 3: The output capacitance of a SiC device varies much smaller with drain voltage.

Reference design shows high efficiency

To illustrate the advantages of SiC MOSFETs in bidirectional converters, Infineon demonstrated a 3.3kW totem pole PFC stage (EVAL_3K3W_TP_PFC_SIC) Evaluation board[1], It can achieve 73W/in3 (4.7 W/cm3), the efficiency at peak 230VAC input and 400VDC output is 99.1% (see Figure 4). When the evaluation board runs in inverter mode, it generates 230 VAC at 50 Hz and can also achieve a peak efficiency of over 98.8%. The evaluation board is fully digitally controlled and uses Infineon XMCTM Series microcontrollers.

Advantages of using CoolSiC TM MOSFETs in bidirectional inverter applications
Figure 4: Using Infineon CoolSiCTM High efficiency bidirectional AC-DC/DC-AC converter with MOSFET technology.

in conclusion

Silicon carbide MOSFET is a natural evolution of silicon super junction MOSFET, suitable for medium and high power applications with high switching frequency. Silicon carbide MOSFET can not only significantly improve efficiency, but also reduce the size and cost of related components, especially magnetic components. This in turn can greatly save the cost, size and weight of the final product, and reduce energy costs. In bidirectional converters, SiC devices can perform all high-voltage switching functions with higher efficiency than traditional solutions, and with their excellent body diode characteristics, hard-switching topologies such as totem-pole PFC can become more feasible and more efficient. Cost-effective.

Infineon can provide a series of CoolSiC™ MOSFETs in discrete and module forms, with a rated voltage range from 650V to 1700V and an on-resistance as low as 2mΩ. Using Infineon’s coreless transformer technology, these devices can be further integrated with the series EiceDRIVERTM The gate drivers are used together in non-isolated and isolated variants, and can be used for low-side and high-side drivers. In order to achieve a complete solution, Infineon also provides current sensing ICs and microcontrollers for digital control.

About 650V CoolSiCTM For more information on MOSFET products and related circuit boards, please visit www.infineon.com/coolsic-mosfet-discretes.

references

[1] Adopt 650V CoolSiCTM And XMCTM 3300W CCM two-way totem pole, Infineon application note: AN_1911_PL52_1912_141352

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