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Use High-Current IGBT Drivers with Built-In Protection for Reliable Industrial Motor Control

September 13, 2019

In an ongoing effort to reduce costs and lower energy consumption in industrial control applications, designers are turning to high-frequency, high-current brushless direct current (BLDC) motors. These increasingly rely upon faster insulated-gate bipolar transistors (IGBTs) instead of metal-oxide-semiconductor field-effect transistors (MOSFETs) for faster switching in order to boost power density. However, to operate efficiently and safely, designers need to include buffer circuitry between the BLDC motor controller’s output and the IGBT power transistors.

Discrete circuitry comprising bipolar junction transistor (BJT) “totem-pole” circuits can perform this buffering role, but such solutions typically lack protection against high-voltage and high-current transients. They also aren’t able to level shift the digital controller’s low-voltage output to the higher voltages and currents needed to properly drive IGBTs. Adding this circuitry also complicates and slows down the design process, consumes space, and adds to the bill of materials (BOM).

To address these issues, a new generation of integrated high-frequency gate drivers for BLDC motor applications combines the buffer and boost circuitry needed to drive IGBTs, while also incorporating protection circuitry. Along with features to boost efficiency, these devices require fewer peripheral devices and have lower operating temperatures. Their smaller footprint further increases a high-frequency motor’s power density and saves space.

This article will briefly cover more driver basics, describing the role of high-current IGBT drivers in modern industrial electric motor applications. It then explains what to look for in an industrial grade device in order to maximize protection and efficiency while minimizing cost and complexity. Along the way, the article introduces sample drivers from ROHM SemiconductorTexas Instruments, and ON Semiconductor, and discusses how to effectively incorporate them into a motor design.

BLDC motor driver basics

A common type of electric motor is the three-phase DC type in which rotor movement is induced by the rotating magnetic field generated by energizing the windings in a controlled sequence (commutation). The rotor speed is proportional to the motor’s operating frequency. Pulse-width modulation (PWM) is superimposed on the base operating frequency to control start-up current, torque, and power.

High-frequency operation offers some inherent advantages. For example, current ripple—an artifact of the alternating current (AC) input after rectification—is reduced, which in turn reduces the size and cost of the passive components needed for filtering. High-frequency operation also reduces the uneven electromotive force (EMF) that can result from a less-than-perfect sinusoidal input to the motor coils—decreasing motor vibration and wear. In general, higher frequency switching increases power density, allowing the use of physically smaller motors for a given output power.

Although there are variations, a typical closed loop control system for high-frequency operation comprises:

  • A speed control input, a controller that supervises motor commutation by generating the appropriate PWM to the driver
  • A driver that switches the low- and high-side power transistors
  • Power transistors, in a half H-bridge topology, which energize the motor coils

In a sensor-controlled BLDC motor, the control loop is closed via feedback from Hall effect sensors monitoring the motor’s rotating shaft (Figure 1). Sensorless models calculate motor position from back EMF (BEMF). (For more on designing a complete closed-loop control system for sensor and sensorless three-phase BLDC motors, see Digi-Key articles How to Power and Control Brushless DC MotorsWhy and How to Sinusoidally Control Three-Phase Brushless DC Motors, and Controlling Sensorless BLDC Motors via Back EMF.)

Diagram of Texas Instruments MSP430 closed loop control system for a three-phase BLDC motorFigure 1: Typical closed loop control system for a three-phase BLDC motor comprising a controller, driver, and power transistor half H-bridge. This control system uses Hall effect sensors for the feedback circuit, though sensorless systems are also popular. (Image source: Texas Instruments)

The driver is a key component in a BLDC motor controller design. It is essentially a power amplifier that accepts a low power input from the BLDC motor controller and in turn produces a high-current drive input for the gates of the high- and low-side power IGBTs in the half H-bridge. That said, the latest versions of drivers for high-frequency operation are highly integrated and can do much more.

Advantages of integrated IGBT drivers

It is possible to build an IGBT driver from discrete components. Shown is a bipolar junction transistor (BJT) “totem-pole” circuit designed for driving a power transistor (Figure 2). In this case a more traditional MOSFET is used, but the configuration is applicable to an IGBT.

Diagram of Texas Instruments BJT totem-pole MOSFET driverFigure 2: A discrete BJT totem-pole MOSFET driver works well but it inverts voltage, suffers from shoot through, and lacks protection. (Image source: Texas Instruments)

Two key disadvantages of this circuit are an inverted voltage at the output and some shoot through during gate voltage transients. In addition, at power on and power off (before the BJT drive supply reaches full operating voltage), the IGBT can be subject to the combination of high voltage and high current. This increases power dissipation and can cause overheating and permanent damage. While it is possible for the designer to add protection circuits required to meet the safety standards demanded of industrial BLDC motors, the design is challenging, and the additional components increase cost, complexity, and size.

Another issue with discrete BJT totem-pole circuits is a lack of level shifting. Digital power control now dominates BLDC motor control but offers only a low current/voltage output. For example, the PWM signal from the digital controller is often a 3.3 volt logic signal, which is not capable of effectively turning on an IGBT. Level shifting is required to raise the low current/voltage PWM signals from the controller to the high current/voltage PWM signals (typically 9 to 12 volts) required to activate IGBTs.

Apart from obvious advantages such as reduced design complexity, compressed development time, and smaller size, integrated high-current IGBT drivers address all the problems of a discrete solution. The devices also minimize the effect of high-frequency switching noise by locating the high-current driver physically close to the power switch while reducing power dissipation and thermal stress in the controllers.

For example, a solution such as ROHM Semiconductor’s BM60212FV-CE2 integrated gate driver is ideal for driving a pair of high- and low-side IGBTs. The device is compatible with either 3.3 or 5 volt controller logic signals while providing a high-side floating supply voltage of up to 1200 volts and a maximum gate drive voltage of 24 volts. Maximum turn on/off time is 75 nanoseconds (ns). The maximum output current is 4.5 amperes (A) (with a peak of 5 A for 1 microsecond (µs)).

Built-in protection

The new generation of IGBT drivers such as the BM60212FV-CE2 include built-in protection circuits, primarily undervoltage lockout (UVLO) and desaturation protection (DESAT).

UVLO is useful for avoiding overheating and damage during switch on. When turning on, if the gate voltage (VGS for a MOSFET or VGE for an IGBT) is too low, there is a danger of the transistor quickly entering its saturation region where conduction losses and power dissipation escalate. An illustration of this effect shows how values of VGS impact a power transistor (Figure 3). Again, a MOSFET is used for illustration purposes, but similar characteristics apply to the IGBT. The right side of the red curve is the saturation region, defined by a constant drain-to-source current (or collector-to-emitter current for the IGBT), dependent on VGS and independent of drain-source voltage (VDS).

Graph of drain current vs. drain-source voltage for VGSFigure 3: Losses escalate if the MOSFET or IGBT enters the saturation region (on the right of the red line) before fully switching on. (Image source: Texas Instruments)

The solution is to incorporate UVLO such that no voltage is applied to the gate until the power supply has reached a sufficient voltage level to ensure that the MOSFET or IGBT can be quickly turned on, and thus avoid excessive power dissipation. Texas Instruments’ UCC27512MDRSTEP IGBT (and MOSFET) gate driver, for example, includes a UVLO mechanism that grounds the output of the driver when the power supply has not reached a UVLO threshold determined by the designer (Figure 4).  The UCC27512MDRSTEP is a low-side gate driver offering a peak sink current of 8 A.

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