Many power electronic applications, including automotive traction drives, PV inverters, railway traction drives, and servo drives, necessitate the use of a three-phase inverter power stage to convert DC to three phase AC voltage. Gate drivers are required to power semiconductor devices. Kevin Lenz and Vikneswaran Thayumanasamy investigate topology options.
Isolated gate drivers that provide isolation between the low voltage and high voltage sides are required in most cases due to safety requirements. Isolated gate drivers typically require a 5.0V power supply to power the logic circuit and an isolated supply voltage (8.0 to around 25V) to power the power semiconductor device.
The flyback converter is one of the most common topologies for generating the isolated power supply for the gate driver, and there are several types of isolated power supply architectures for gate drivers.
The isolated power supply architectures for the gate drivers in a three-phase inverter system are classified as distributed power supply architecture and centralised power supply architecture.
Centralised power supply architecture:
The implementation of a centralised power supply architecture in a three-phase motor drive is depicted in Figure 1. The three high-side switches are labelled HS1, HS2, and HS3, while the three low-side switches are labelled LS1, LS2, and LS3. GD HS and GD LS are the gate drivers for high and low side switches, respectively. A 5.0V supply is typically required to drive the logic block of isolated gate drivers, and isolated power supplies are required for the driver’s output side.
A single transformer with multiple secondary windings is used to generate the isolated power supplies required for the output stage. A flyback converter is the topology depicted in Figure 1. The transformer has a single primary winding and is powered by a low voltage battery (typically 12V) in a hybrid electric vehicle (HEV) or plug-in hybrid electric vehicle (PHEV) (PHEV). Three of the secondary windings are responsible for powering the three high-side switches (isolated supply HS1, HS2 and HS3).
The three low side switches can be powered by a single winding (isolated supply, LS). A centralised power supply uses a single transformer with multiple secondary side windings, a single MOSFET switch, and a controller IC.
Figure 1 depicts the architecture of a centralised isolated power supply for a gate driver.
Distributed Power Supply Architecture
Figures 2 and 3 depict two different types of distributed power supply architecture realisations for isolated gate drivers in a three-phase motor drive. Figure 2 depicts a distributed power supply architecture with six separate flyback converters that generate the isolated power supply needed by the six switches.
Figure 3 depicts a distributed power supply architecture that employs a different method of realising a distributed power supply. A single transformer with two separate windings is used to generate the isolated power supply for the two gate drivers in a phase leg in this example.
Six transformers, MOSFETs, and controller ICs are required for the complete three-phase inverter system in the first example (Figure 2). The distributed supply architecture in the second example (Figure 3) necessitates three transformers (with two secondary windings), MOSFETs, and controller ICs.
Figure 2: A distributed power architecture with six independent flyback converters.
Distributed Vs Centralised Power Supplies
In the case of the centralised power supply architecture depicted in Figure 1, a single transformer is required to house the four separate windings required to supply the high and low side switches. The average gate current for all six switches in a three-phase inverter is supplied by the same transformer.
In the case of a distributed power supply architecture, one transformer is assigned to supply one isolated gate driver, and the average gate current for only one switch must be considered in the end. As a result, the power rating of the transformer in a centralised power supply architecture is higher than in a distributed power supply architecture.
Due to the increased power requirement and the ability to house four separate windings on the secondary side, the flyback transformer in a centralised architecture is typically large and bulky.
As a result of the increased power requirement in a centralised setup, the primary switch and filtering capacitor sizes increase. Because of the overall need for bulky components, the centralised power supply architecture takes up more space than the distributed power supply architecture.
On the other hand, distributed power supply architecture necessitates smaller discrete components, resulting in a more compact design. Despite the fact that six transformers and switches are required, the dimensions of each component are typically small.
Another disadvantage of a centralised architecture is the increased layout complexity. Because the isolated power supply rails emerge from the same transformer, it is difficult to locate all of the gate drivers close together. To connect the transformer’s secondary windings to the gate driver power supply pins, longer tracks are required. Such complexities in PCB routing increase design time.
More importantly, the unnecessary longer track lengths and multiple windings within a single transformer result in poor EMI performance and noise coupling between channels.
The transformers and primary switch can be placed close to the individual gate drivers, which is one advantage of a distributed power supply architecture. This reduces overall track lengths and improves PCB routing ease. Because of the distributed architecture, the EMI characteristics are improved and the design is more compact.
Figure 3: Isolated power supply generated by a single transformer with two separate windings in a distributed power architecture.
Gate Drivers with Built-In Flyback Controllers
Gate drivers with built-in flyback controllers are commercially available. With coreless transformer technology, all of the gate drivers in this series are isolated at 2.5kVrms. These gate drivers are also tough, with a common mode transient immunity of 100V/ns and are appropriate for platforms containing IGBTs and SiC MOSFETs.
A simple distributed power supply can be built with only a few extra components and a small transformer placed near the gate driver IC. External controller ICs are not needed because the flyback controller is integrated within the gate driver.
Figure 4 depicts a simple block diagram incorporating an isolated gate driver and an integrated flyback controller. A simple transformer, primary switch, and filters are required to generate an isolated power supply using gate drivers and flyback controllers. The extra feedback and compensation network is required for the in-built controller’s current mode control.
The extra feedback and compensation network is required for the in-built controller’s current mode control.
An auxiliary winding from a transformer and resistive dividers can be used to easily implement the feedback circuit. The flyback controller, which is integrated into the gate driver IC, also includes several built-in protection functions to ensure the safe operation of both the flyback converter and the gate driver. Under-voltage lock out, over-voltage protection, over-current protection, a soft-start up function, and adjustable switching frequency are some of the protection features.
Figure 4: Gate drivers with integrated flyback controller are shown in a distributed power supply architecture.
In this example, the gate drivers with built-in flyback controllers have a wide input logic supply voltage range. The logic block can be driven by a maximum supply voltage of 5.5V in conventional isolated gate drivers. The gate drivers with integrated flyback controller can accept voltages ranging from 4.0 to approximately 32V. (and 8.0 to around 24V in some cases). The low voltage battery (typically 12V) used in a battery EV or PHEV, for example, can be connected directly to the gate driver IC, eliminating the need for additional circuits to generate a 5.0V supply.
Summary
A distributed isolated power supply architecture based on gate drivers with an integrated flyback function provides numerous design level advantages, such as compact design, ease of PCB layout/routing, and improved EMI characteristics. Gate driver with multiple integrated protection functions can ensure safe system level operation in power management systems for e-powertrain and other emerging applications.
