Power devices for industrial ac servo drives: A closer look

Industrial motor drives used in robotic applications such as CNC machines, x-y tables, factory automation robots, and actuators often require high performance and robust power devices. The motor of choice today in such applications is the three-phase ac-brushless motor (an ac-servo motor) and the power device of choice today for the inverter is the Trench IGBT. This article reviews some of the key characteristics of Trench IGBTs that are most relevant to the motor drive circuit designer from a high performance and robustness perspective.

The IGBT

So what are the desired qualities of the IGBT to be selected given the nature of this application? At a high level, two key requirements are “robustness” and “high performance.” Often, these requirements place contradicting demands on the power device in terms of its design and characteristics. The challenge is thus to select a device that enables a robust drive solution, and yet achieves the highest possible level of drive performance. These requirements are best satisfied by high conductivity Trench gate IGBTs as discussed in the following sections.
1. Robustness: As the very nature of the word implies, “robustness” means an inherent capability of the device to meet demanding operating conditions, including severe transient events. While there are different aspects to device robustness, in the context of IGBTs, two key datasheet aspects are considered here:

a. Safe operation areas : In servo motor drive applications, the motor has to be able to accelerate and decelerate quickly. This results in high current demands and sudden changes in this current demand, on the inverter. From a device perspective, this implies that the device needs to be able to handle high-pulsed currents (at instants of turn on and turn off). On IGBT datasheets, a useful characteristic that provides guidance in this regard are the safe operating areas (SOAs). As a reference, the SOAs in Fig. 1  are reproduced from a 1,200-V rated Renesas IGBT, RJH1CM7DPQ-E0 datasheet. 

Fig. 1: Forward bias (on left) and reverse bias (on right) SOAs

In essence, the wider (or “squarer”) the SOAs, the more robust is the device’s capability to withstand high peak currents at turn on and turn off. Also, a square turn-off SOA indicates a high level of robustness to withstand high peak voltages resulting due to turn off of inductive load current (such as in case of a motor drive).

b. Short-circuit capability : In the event of a short circuit from one of the motor terminals to the frame (earth ground) or to another terminal, the IGBT affected sees the entire dc-bus voltage and has to withstand the full short-circuit current (limited only by the device transconductance and the circuit impedances). This is an extremely severe condition for the device. Typically, when this happens, the short circuit is sensed by means of current shunt resistors or desaturation detection circuits (that sense the V_CE across the device) and the controller commands the gate drive to the affected IGBT to be disabled (typically at a controlled rate of turn off). In this short time, the device has to be able to withstand several kilowatts of power dissipation until it is turned off. While certain devices have a 5-μs short-circuit time-withstand capability, a device having a ≥10-μs withstand capability to such an event is considered a robust device. This allows that much more time for the controller to respond and shut down the gate drive in case of such an event. For example, Fig. 2 shows the test circuit and actual V_CE , V_GE , and I_C waveforms during a short-circuit test conducted on a 650-V-rated Renesas IGBT, RJH65S04DPQ-A0.

Fig. 2: Short-circuit test circuit and waveforms (RJH65S04DPQ-A0).

2. Performance: A high-performance device would need to have performance characteristics that help make the drive more efficient (reduced losses and operating temperatures, minimized thermal impact), and enable the best possible dynamic performance (response of the drive to sudden load changes for example). Key aspects of the device in relation to achieving a high performance are now reviewed.

a. Reduced losses : For any power device, there are two main loss components, conduction loss and switching loss. A low conduction loss is achieved by means of as low a forward saturation voltage (V_CE(sat) ) as possible for the IGBT, and as low a forward drop (V_F ) (see Fig. 3 ) for the reverse connected diode, as possible. A low switching loss is achieved by as low a turn-on and turn-off loss (E_ON and E_OFF ) as possible for the IGBT, and as low a reverse-recovery loss as possible for the diode (as this loss has to be included in the turn-on loss of the IGBT). So not only are the IGBT’s characteristics very important, but also those of the reverse connected diode. For example, the graphs in Fig. 3 are reproduced from the datasheet of a 1,200-V-rated Renesas IGBT, RJH1CM7DPQ-E0 that highlight the relatively low V_CE(sat) and V_F specifications of the IGBT and reverse connected diode, respectively:

Fig. 3: The V_CE(sat) vs. V_GE , and VF vs. IF characteristics (RJH1CM7DPQ-E0)

b. Dynamic performance : In high-performance ac-servo drives, the dead time (time interval between when the high-side IGBT has turned off and the complementary low-side IGBT is commanded to turn on) must be as short as possible. This helps to minimize nonlinearity in the control loop and helps the current achieve as close to a sinusoidal shape as possible (this is highly desired). To enable a shorter dead time, what is critically important is that the IGBT not exhibit a long turn-off time particularly in terms of its “tail current.” This is the effect of the recombination time of the minority carriers in the IGBT at turn off. This behavior can be better understood by referring to Fig. 4 .

Test conditions; V_cc =300 V,V_GE =15 V,I_c = 50 A,R_G = 10 Ω

Fig. 4: Turn on and turn off waveforms under specified test conditions (RJH60CM7DPQ-E0)

High performance and robustness are both highly desired characteristics in the power device used in industrial ac-motor drives. The device of choice today is the trench-gate IGBT. This article highlights some of the key characteristics of trench-gate IGBTs in the context of high performance and robustness as needed in this application.