Detailed interpretation of Toyota bZ4x motor inverter design

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Detailed interpretation of Toyota bZ4x motor inverter design

Posted Date: 2024-01-15

Inverter specifications and structure

The configuration of Toyota's new battery system is shown in the figure below. The new Electric Supply Unit (ESU) integrates a DC/DC converter, on-board charger, DC relay and a distributor.

1. Configuration of four-wheel drive inverter cooling system

The configuration of the inverter cooling system for four-wheel drive is shown in the figure below. A series cooling system is used to ensure sufficient cooling flow to the front and rear inverters.

Since the inlet water temperature of the rear inverter is affected by the front inverter, the required cooling flow rate cannot be guaranteed to match the coolant temperature. This problem is solved by predicting the front-inverter heat loss at the system output. The water pump drive load is varied based on predicted exhaust heat losses from the system output, resulting in a 29% reduction in water pump drive power losses.

2. Inverter installation

The inverter installation adopts a frame structure, in which the components are fixed on the frame and integrated into the electric drive. For this purpose, a series of optimizations are carried out: the shell stiffness is improved; the three-phase copper bars are oil-cooled; and the fastening positions are optimized.

3. Inverter specifications

The table below shows the specifications of the inverter, which shows a 31% reduction in volume, a 40% reduction in weight and a 45.6% increase in maximum output density compared to the previous model. This is mainly due to: the use of new power semiconductors; the reduction of capacitor capacitance through improved inverter control; and the use of coreless current sensors.

4. Inverter structure

The structure of the inverter is shown in the figure below.

Newly developed power semiconductors reduce energy losses, improve cooling performance, and modify inverter control. This significantly suppresses the temperature rise of the power semiconductors and allows the placement of two parallel power semiconductors in the front inverter instead of three in the previous solution.

Other components such as current sensors and capacitor modules have also been reduced in size. As a result, the new inverter is 31% smaller than the previous 150 kW one. Additional Y-capacitors also help improve the vehicle’s electromagnetic compatibility (EMC).

The power module uses the same double-sided cooling structure as the previous model. With this structure, different system output changes can be achieved by increasing or decreasing the number of layers in the power stack.

By changing the number of layers, different power outputs of 150 kW and 80 kW specifications are achieved, as shown in the figure below.

This means that the 150 kW and 80 kW sizes only require changing the shape of the busbars connecting the current sensor and capacitor modules to the power stack.

Each component technology

1. Power module (Si)

The new model of the power card features a newly developed power semiconductor that integrates an IGBT and a FWD (called "RC-IGBT").

By optimizing the specifications of RC-IGBT, downsizing and higher power output are achieved, as shown in the figure below.

By monitoring the temperature of all devices and increasing the temperature limit, the output a single power card can handle is increased. By equipping all six devices with temperature monitors, the new model reduces temperature variations that need to be considered for device temperature protection, compared to the previous model where only one of the six devices was equipped with a temperature monitor.

Thermal protection thresholds compared to the previous model are shown in the figure.

As a measure to increase the upper temperature limit, surface roughening technology is used to form microscopic bumps on the mounting surface of the radiator and improve the adhesion between the resin and the radiator. In addition, a highly heat-resistant resin material is used. The combination of these new technologies has contributed to a 47.3% increase in power output that can be handled by a single power card and shrinking the power module.

2. Power module (SiC)

In addition to silicon power semiconductors, silicon carbide devices are also used in the rear inverter. By increasing the gate voltage to reduce the on-resistance, the switching speed is about 3 times faster than that of Si devices, the power loss is reduced by more than 50%, and the output of each chip is increased by 2.8 times.

3. Motor control (SiC)

The equivalent circuit of the high-voltage system in a four-wheel drive vehicle is shown in the figure.

The circuit board diagram is shown below.

By installing front and rear inverters, LC resonance occurs between the inverters and between the inverter and the high-voltage battery. The overlap of the LC resonant frequency with the inverter carrier frequency and its sidebands amplifies high voltage and current ripples, which has a negative impact on high-voltage equipment such as ESUs.

While this problem can be solved by increasing the capacitance of the inverter capacitor, it can be solved by improving the inverter switching control.

More specifically, the switching pulse pattern is adjusted so that the harmonic components of the inverter do not overlap with the gain peaks shown in the figure. So instead of increasing the capacitance of the capacitor, its capacitance is actually reduced by 37% compared to the previous model.

4. Control ECU

The CB (control board) calculates control commands based on vehicle driving force requests from the host ECU and operating conditions from various sensors inside the inverter, while the GDB (gate drive board) that drives the power module consists of a single board.

The traditional ECU consists of two microcomputers, but by improving the processing power of the microcomputer and replacing the mutual monitoring of the microcomputers with monitoring ICs, a single microcomputer configuration has become possible, contributing to miniaturization.

The CB segment is not a new design for different inverter outputs (150 kW and 80 kW), but a common design, while the GDB segment is changed just to improve efficiency.

The silicon carbide driver power described earlier was also designed by modifying only the GDB part. Thus achieving miniaturization and efficient development.

5. Current sensor

The current sensor is shown in the figure below.

By using coreless sensors, the size of the current sensing system is reduced by 67%. Since the coreless structure is susceptible to magnetic flux from adjacent busbars, a metal plate is added between the top and bottom of the sensor to ensure comparable accuracy to conventional sensors.

Adopts three-phase single-phase system (3 units). In the event of a single element failure, fail-safe operation can be ensured by identifying the failed phase and switching to control using only non-faulty phases.

6. Capacitor module

The purpose of the capacitor module is to stabilize the inverter output voltage to the motor. Reducing the power semiconductor also requires reducing the surge voltage, i.e., a lower equivalent series inductance (ESL). The figures below show the structure of the capacitor module and the capacitor busbar respectively.

The new capacitor module expands the parallel length of the positive (P) and negative (N) pole busbars by improving the layout and structure of the busbars. As a result, ESL was reduced by 50%. In addition, the use of a thinner, newly developed polypropylene film enables further shrinkage and integration into a single module structure.

7. Cooler

The function of the cooler is to cool the power semiconductors coming from both sides. The inner fin structure of previous and newer coolers is shown below.

In previous models, the coolant flow path was partially slow and stagnant, hindering heat exchange efficiency. The new model uses a fin structure that divides the coolant into adjacent flow channels to reduce stagnant areas. This partitioning effect improves cooling performance by 25% compared to the previous model.

Review Editor: Huang Fei

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