Use silicon carbide MOSFETs to improve the energy efficiency of industrial drives

“As the electric motor falls, it’s worthwhile to squeeze the chaff of the worm and stalk the stalk of the scallion, the scallion, the scallion, the scallion, the scorpion, and the rest. The manufacturer must spend a lot of attention to strengthen the performance of the power components used in the converter stage. . ST’s latest silicon carbide metal oxide semiconductor field effect transistor (SiC MOSFET) technology has set a new performance standard in the field of power switching.

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Authors: STMicroelectronics Carmelo Parisi, Antonino Raciti, Angelo Sciacca

Summary

As the electric motor falls, it’s worthwhile to squeeze the chaff of the worm and stalk the stalk of the scallion, the scallion, the scallion, the scallion, the scorpion, and the rest. The manufacturer must spend a lot of attention to strengthen the performance of the power components used in the converter stage. . ST’s latest silicon carbide metal oxide semiconductor field effect transistor (SiC MOSFET) technology has set a new performance standard in the field of power switching.

This article will highlight the advantages of using silicon carbide MOSFETs instead of silicon-based (Si) insulated gate bipolar transistors (IGBTs) for industrial drives in terms of energy efficiency, heat sink size, or cost savings.

1. Introduction

At present, industrial transmissions are usually talkative about IGBT inverters (inverters), but the recently developed silicon carbide MOSFET components open up new possibilities for this field.

ST’s silicon carbide MOSFET technology not only has very low on-resistance per unit area, and excellent switching performance, but also compared with traditional silicon-based freewheeling diodes (FWD), when the internal diode is turned off The reverse recovery energy of is still in the negligible range.

Considering that industrial drives such as pumps, fans, and servo drives must continue to operate, it is possible to increase energy efficiency and significantly reduce energy consumption by using silicon carbide MOSFETs.

This article will compare the main features of 1200 V Silicon Carbide MOSFET and Si IGBT, both of which are ACEPACK™ packaged, see Table 1.

Table 1: Component analysis

This article will use STMicroelectronics’ PowerStudio software to apply the experimental data and statistical measurement results of the dual pulse test to the simulation. Simulate a 20kW industrial drive and evaluate the annual power consumption of each solution, as well as the requirements of the cooling system.

2. The main technical key drivers and application limitations

In inverter-based transmission applications, the most common topology is to connect three half-bridge bridge arms with six power switches.

Each half-bridge bridge arm is operated by hard-switching commutation on an ohmic inductive load (motor), thereby controlling its speed, position or electromagnetic torque. Because of the off S of the inductive load, 6 anti-parallel diodes are required to perform the freewheeling phase for each commutation. When the lower side flywheel diode shows reverse recovery, the direction of the current will be the same as that of the upper side switch, and vice versa; therefore, the commutation in the on state will overshoot (overshoot), Cause additional power loss. This means that during switching, the reverse recovery of the diode has a great influence on the power loss, and therefore also affects the overall energy efficiency.

Compared with the practice of silicon-based FWD with silicon-based IGBT, silicon carbide MOSFET has much lower values ​​of reverse recovery current and recovery time, so it can greatly reduce the recovery loss and the impact on energy consumption.

Figure 1 and Figure 2 show the commutation situation of silicon carbide MOSFET and silicon-based IGBT in the on-state under the conditions of 50 A-600 VDC, respectively. Look at the blue stripe block, the reverse recovery current and reverse recovery time of the silicon carbide MOSFET are reduced a lot. Increased commutation speed during turn-on and turn-off can reduce power consumption during switching, but there are still some limitations on the speed of switching commutation, because it may cause electromagnetic interference, voltage spikes and vibration problems to worsen.

Figure 1: Silicon carbide MOSFET in the on state

Figure 2: Silicon-based IGBT in the on state

In addition, one of the important parameters affecting industrial transmission is the risk of damage caused by the rapid commutation transient output of the inverter. The ratio of voltage variation (dv/dt) during commutation is high. Longer motor Circuits will indeed increase voltage spikes, making common mode and differential mode parasitic currents more serious, which may cause winding insulation and motor bearing failures in the long run. Therefore, in order to ensure reliability, the voltage variation rate of general industrial drives is usually 5-10 V/ns. Although this condition seems to limit the field application of silicon carbide MOSFETs, because fast commutation is one of its main features, the 1200 V silicon-based IGBTs tailored specifically for motor control can actually fall under these restrictions. Show the speed of exchange. In any case, no matter Figure 1, Figure 2, Figure 3, Figure 4 show, compared with silicon-based IGBT, silicon carbide MOSFET element is guaranteed to reduce energy consumption when it is turned on or off, even at 5 V/ns Under mandatory conditions.

Figure 3: Silicon-based MOSFET in the off state

Figure 4: Silicon-based IGBT in the off state

3. Static and dynamic performance

The following will compare the static and dynamic characteristics of the two technologies. The set conditions are normal operation, and the junction temperature is TJ = 110 ℃.

Figure 5 shows the output quiescent current and voltage characteristic curves of the two components (VI curves). Comparing the two phases, it can be seen that the advantages of the silicon carbide MOSFET are greatly ahead under any conditions, because its voltage is linearly decreasing.

Even the silicon carbide MOSFET must have V_GS = 18 V to reach a very high R_DS(ON), But it can guarantee that the static performance is far better than that of the silicon-based IGBT, which can greatly reduce the conduction loss.

Figure 5: Comparing dynamic characteristics

Both components have been tested using dual pulse waves and analyzed from a dynamic perspective. The comparison between the two is based on the application, for example, the DC voltage of the 600 V bus, the dv/dt for opening and closing are both set to 5 V/ns.

Figure 6 is a summary of the data measured during the experiment. Compared with silicon-based IGBTs, within the current range analyzed in this experiment, the turn-on and turn-off energy consumption of silicon carbide MOSFETs are significantly lower (about 50% reduction), even at 5 V/ns.

Figure 6: Comparison of dynamic features

4. Electric heating simulation

In order to compare the performance of the two components in general industrial transmission applications, we used STMicroelectronics’ PowerStudio software for electrothermal simulation. The simulation sets common input conditions for such applications and uses all temperature-related parameters to estimate the overall energy consumption.

The industrial drive used for comparison has a nominal power of 20 kW and a commutation speed of 5 V/ns (input conditions are listed in Table 2).

Table 2: Simulation conditions

Set two different switching frequencies, 4kHz and 8kHz, to highlight the use of solutions to increase f_swWhat are the benefits of its function.

Because it is considered that over time, all motors usually have to operate at different operating points, we use some basic assumptions to calculate the power loss of the transmission. According to the EN 50598-2 standard that defines the IE-level complete drive module (CDM), and the new IES-level electric drive system (PDS), we apply two operating points in the simulation: one is generated by 50% torque Current, the second is 100%, for our application this means the output current is 24 and 40 A respectively_rms.

In terms of the maximum load point (100% torque current), the thermal resistance of the heat sink of the two components is selected to maintain a junction temperature of about 110 °C.

Figure 7 compares the power loss of silicon carbide MOSFET and silicon-based IGBT solutions under the condition of 50% torque current and switching frequency of 4-8 kHz.

Figure 7: Power loss of each switch at 50% torque current

Figure 8: Power loss of each switch at 100% torque current

Figure 8 compares in the same way at 100% torque current.

Power loss is divided into switching (conduction and switching) and anti-parallel diodes to find the main difference. Compared with silicon-based IGBTs, silicon carbide MOSFET solutions can significantly reduce overall power loss. This result is because no matter static or dynamic conditions, regardless of switch or diode, power consumption will be reduced.

Finally, regardless of the switching frequency of 4 or 8 kHz, the power loss reduction of the two load conditions falls within the range of 50%.

From these results, it can be seen that this can achieve higher energy efficiency, reduce the heat dissipation requirements of the heat sink, and have benefits in terms of weight, volume, and cost.

Table 3 summarizes the simulation results of the related power loss of the entire inverter (operating point 100%), and the related heat sink thermal resistance conditions necessary to maintain the junction temperature of the two components at 110 °C.

Table 3: Overview of simulation results (100% of the operating point)

Under the conditions set by the simulation, Rth will drop from 0.22 °C/W for silicon-based IGBTs to 0.09 °C/W for silicon carbide MOSFETs at 8 kHz. A substantial reduction means that the heat sink can be reduced by 5:1 (in terms of forced convection type products), which has obvious benefits to the system volume, weight and cost. Under the condition of 4 kHz, Rth will drop from 0.35 to 0.17 °C/W, which is equivalent to a 4:1 tolerance reduction.

5. Economic impact on energy costs

When industrial applications have high energy requirements and must be used intensively, energy efficiency becomes one of the key factors.

In order to convert the simulated energy consumption data results into an energy cost comparison profile, it is necessary to set some basic assumptions about the annual load profile and energy cost parameters that will vary with time or location. For the purpose of simplification, we set the situation to a basic load profile with only two power levels (load factor 100 and 50%). The difference between profile 1 and profile 2 lies in the duration of each power level. In order to highlight the reduction in energy costs, we set the situation as a continuously operating industrial application. Task file 1 is set to be at 50% load 60% of the time each year, and 100% load at other times (40%). Mission File 2 is the same.

The economic impact of the annual energy cost of each task file is calculated with 0.14/kWh as the energy cost (Eurostat data, calculated at the price of non-household users).

It can be seen from Table 4 that the silicon carbide MOSFET can save 895.7 to 1415 kWh of energy annually. The corresponding cost saved each year is between 125.4 and 198.1 euros. If the voltage variation ratio limit is not so strict, you can save more.

Table 4: Energy and cost saved by silicon carbide MOSFET for each mission file each year

6 Conclusion

In this paper, performance benchmark tests are carried out for inverters used in industrial transmission of 1200 V silicon-based IGBTs and silicon carbide MOSFETs. The content also specifically discusses the technical limitation of the voltage variation ratio caused by motor winding and bearing protection, and then compares the above-mentioned technology and limitation under the condition of 20 kW industrial transmission. The results show that the use of silicon carbide MOSFETs to replace silicon-based IGBTs can greatly increase the power energy efficiency, even if the commutation speed is limited to 5 V/ns. After comparing the costs, it is also found that under certain assumptions, this approach can reduce the energy costs of general industrial transmission applications.