Unlike switch mode power supplies, three-phase motor-driven inverters typically use a low switching frequency; only tens of thousands of hertz. High-power motors are large in size and have high-inductance windings; therefore, current ripple is acceptable even at low switching frequencies. As motor technology advances, power density increases; the motor's external dimensions become smaller, faster, and require higher electrical frequencies.
Low-voltage brushless DC or AC induction motors with low stator inductance are increasingly used or specialized in precision applications such as servo drives, CNC (computer numerical control) machines, robots and utility drones. In order to keep the current ripple within a reasonable range, these motors – due to their low inductance – require switching frequencies up to 100 kHz; phase current ripple is inversely proportional to the PWM (pulse width modulation) switching frequency and is converted to mechanical Torque ripples, generate vibration, reduce drive accuracy and efficiency.
So why don't engineers increase the switching frequency? As a consistent principle in engineering, this is a compromise. The power loss of the inverter mainly includes conduction loss and switching loss. You can reduce the switching losses at a given operating frequency by reducing the size of the switching components (usually MOSFETs), but this can result in increased conduction losses.
In an ideal design, the highest achievable efficiency is limited by the technology of the semiconductor switch. Using conventional low-voltage 48V silicon MOSFET-based inverters, the switching losses at 40kHz PWM may have been significantly higher than the conduction losses, making up the bulk of the overall power loss. In order to dissipate excess heat, a larger heat sink is required. Unfortunately, this increases system cost, weight, and overall solution size, which is undesirable or unacceptable in space-constrained applications.
Gallium nitride (GaN) high electron mobility transistors (HEMTs) offer many advantages over silicon MOSFETs, opening up new possibilities. GaN transistors can achieve much higher dV/dt slew rates and therefore can switch faster than silicon MOSFETs, significantly reducing switching losses. Another advantage of GaN transistors is that there is no reverse recovery charge, and the reverse recovery charge of a conventional silicon MOSFET design causes the switching node to ring. Table 1 compares silicon FETs and GaN FETs.
parameter
Si-FET
TI's GaN (HEMT)
Remarks
Component structure
Vertical
Landscape
Specific RDS(ON), area
>10mW-cm2
5-8mW-cm2
Lower conduction loss.
Gate charge QG
~4nC-W
~1-1.5nC-W
Reduce gate driver losses for faster switching speeds, reduced switching losses and deadband distortion.
Output charge QOSS
~25nC-W
~5nC-W
Lower output capacitance for faster switching speeds and reduced switching charge losses
Reverse recovery QRR
~2-15mC-W
no
Zero reverse recovery enables efficient half-bridge inverters and reduces/eliminates ringing in hard switches.
Table 1: Comparison of Silicon Power MOSFETs and TI's GaN FETs (HEMT)
If you replace the existing silicon MOSFET with a new GaN FET, you can enjoy the benefits and the world will be easy. For example, achieving high slew rates in gate drive circuits and printed circuit board (PCB) layouts is uniquely challenging. A higher dV/dt means increased electromagnetic interference (EMI) if not handled properly. Propagation delay mismatch between channels will limit the optimal achievable dead time, preventing GaN FETs from achieving their best performance.
TI's LMG5200 GaN power stage overcomes these difficulties by integrating two 80V/10A 18-mΩ GaN FETs with gate drivers in the same unbonded 6mm x 8mm quad flat no-lead (QFN) package. The package leads are designed for low power loop impedance and a simple PCB layout. The inputs are 5V TTL and 3.3V CMOS logic compatible and have a typical propagation delay mismatch of 2ns. This enables very short dead times and reduces losses and output current distortion.
The TI Design 48V / 10A High Frequency PWM 3-Phase GaN Inverter Reference Design for High Speed ​​Drives implements a B6 inverter topology with three LMG5200 half-bridge GaN power modules. Figure 1 is a simplified block diagram. This reference design provides a TI BoosterPackTM module compatible interface for connection to the C2000TM microcontroller (MCU) LaunchPadTM kit for performance evaluation.
Figure 1: Reference Design for High Frequency Three Phase GaN Inverters
With so many theories in mind, are you curious about how fast switching can be achieved in practice? Figure 2 shows a switch node with a slew rate of approximately 40V/ns. Despite the ultra-fast switching speed, the switching node overshoot is less than 10V. Unlike conventional silicon FET designs, this requires a small margin between the VDS breakdown voltage of the FET and the maximum allowable Vbus supply voltage.
Heating system control cabinets are designed to coordinate the work of electrical heating systems. At the request of the consumer heating systems Control Cabinet can be equipped with manual or automatic control system. Additionally, optional equipment related to both control systems (manual and automatic control) and the ability to switch between them.
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Functionality of the heating systems control cabinet
Functions of heating control system may differ depending on the requirements and specifications of the customer. Main functions are:
the possibility to switch control modes (manual or automatic);
maintain the set temperature of the heated object;
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a complete and safe shutdown of heating equipment from the power supply in case of accidents.
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