Positive and Negative Isolated Power Rails for IGBT Gate Drivers

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A new Fly-Buck™ power supply for three phase inverters provides positive and negative isolated power rails for Insulated Gate Bipolar Transistor (IGBT) gate drivers. The reference design from Texas Instruments (TI) uses a single transformer to generate the reference voltages for all 6 IGBTs in a 3-phase bridge connection. The isolation for the outputs is provided by the transformer coupling from input to output. The voltage rails for all the high-side IGBTs are individually isolated, while the voltage rails for the low-side IGBTs are combined.

The input voltage to the module is 24V +/- 20 percent, which supports six IGBT gate drivers for three-phase inverter operation. Bias outputs of +15V and -8V have an output power of 2.3 watts for each IGBT. The TIDA-00199 “Wide-Input Isolated IGBT Gate-Drive Fly-Buck Power Supply for Three-Phase Inverters” reference design is tested to verify that it can support 6 amps of peak gate drive current. The unit can support up to six IGBT gate drivers for 3-phase operation.

The TIDA-00199 reference design employs a TI LM5160A synchronous Fly-Buck converter.  The converter provides the isolated voltage rails needed for the IGBT gate driver to operate (Note: TI has a helpful Fly-Buck FAQ page to answer common design questions about the topology). The isolation is provided by the transformer, and the output voltages are set through the primary side feedback voltage and the transformer turns ratio. Multiple outputs can be provided through multiple transformer windings, and  the number of voltage outputs varies depending on the application. 

The LM5160A buck/Fly-Buck converter has a wide input voltage range, 4.5V to 65V, and can provide up to 1.5 amps maximum load current. An internal feedback loop provides the primary side output voltage regulation to +/- 1 percent over the entire operating temperature range.

The LM5160A is appropriate for applications that require multiple bias voltage outputs, including IGBT drivers for motor control, sensors, and programmable logic controllers (PLC).  

As an example, motor control (the LM5160A and TIDA-00199 reference design combination) specifically addresses the use of AC induction motors (ACIM), an industry standard for both home and factory applications. In the case of IGBT driver operation, the combination provides close control across line/load variation, offering greater system reliability.

With the TIDA-00199 reference design, TI has introduced an easy to use, cost-effective solution for AC motor drive applications. The design can provide reliable bias power for six IGBT drivers in three phase motor drives. Beyond the ACIM applications, the TIDA-00199 reference design may also find application in inverters, servo drives, variable speed drives, or IGBT based high-voltage DC systems.

Three-phase inverters function as variable–frequency drives to control the speed of motors and high power applications such as HVDC power transmission. A typical application is shown in Figure 1.  Each phase uses a high-side and a low-side IGBT to apply DC pulses to the motor coils in an alternating sequence. 

For industrial motor drive, the input low voltage circuit must be isolated from the high voltage output, which can range from hundreds of volts to thousands of volts, due to the motor and application. Depending on the application, the lower IGBTs may have a common power return ground, as shown in Figure 1, or all IGBTs may have independent power supplies for their gate drivers to provide the required separation.