Optimization scheme for inverter motor drive power module

Electric motors are used to drive a wide variety of loads - fans used in air conditioning systems, pumps that provide fresh water, and motors used in factories to drive manufacturing equipment are just a few examples. Traditionally, these motors were connected directly to the power supply from the grid. Since the operating frequency of the grid is fixed, the motors run at a constant speed with no direct control of torque. Today's motor drives use variable frequency speed regulation to control the speed and torque of the motor.

The first benefit of using a variable frequency drive is improved efficiency when running at full speed, because the inverter can maximize torque with a given excitation current. The second benefit of variable frequency is further energy savings. In traditional driving methods, the motor is either off or fully on (imagine driving a car when you only allow the accelerator pedal to be fully released, or take your foot off completely). Allowing the motor to run at different speeds saves energy and allows for smoother start-ups and shut-downs.

Intelligent Power Modules (IPMs) are an enabling technology for variable speed drives that contain the inverter and internal driver in a single module. They are the module of choice for single-phase AC input applications. The transfer molding manufacturing method used for these modules provides excellent robustness as well as power cycling and temperature cycling capabilities. These modules may contain a power factor correction (PFC) stage, but they typically do not contain an input rectification stage. The off-the-shelf availability of single-phase AC bridge rectifier components means that this is not a problem. The main benefit of using IPMs is the integration of the driver - adding additional pins for the driver.

For three-phase AC input applications, IPMS become very large due to creepage and clearance requirements to determine the minimum spacing between conducting parts to stop arcing or tracking. Because IPMS provide additional pins for drivers, the minimum spacing requirements make the IPM larger than modules without drivers. Creepage and clearance spacing must be carefully calculated for each application, and these factors include the maximum operating height of the driver, the effective voltage in the system, the isolation used in the system, the contamination level of the module and printed circuit board, and the comparative tracking index (CTI).

Figure 1 shows the schematic of a three-phase AC input module without integrated gate drivers. We will review the required spacing based on a general calculation that covers most three-phase AC input motor drives.

Figure 1: Schematic diagram of the three-phase AC input converter inverter brake (CIB) module

The distance between the NTC terminals and any other terminals must be at least 5.5 mm. This distance includes the distance between the outer edges of the pins. However, if the pins are soldered, or inserted into pads, the relevant distance is between the outer distance of each pad. Wide tolerances on the hole size and the width of the annular pad help improve manufacturability, but will reduce creepage and clearance distances.

A 5mm clearance distance is required between the R, S, T, DBMINUS and DBPLUS pins and any other pins. The required distance between U, V, W depends more on the application, with a minimum of 2.5mm to 3mm being the typical value here.

Add all of these clearance distances, via tolerances, and the size of the annular ring (pad), and the result is a fairly large module - about 70mm minimum. If you add the additional signals required for high-side control on the IPM, the minimum size of the module becomes even larger, making it too large and too expensive to be suitable for low power three-phase input applications.

For low-power industrial three-phase AC input applications, both IPM modules and gel-fill modules are widely used: IPM modules have no rectifier and gel-fill modules have no driver. Gel-fill modules have a pin matrix, while IPMS are usually in a dual-inline package. Gel-fill modules have lower thermal cycling capabilities, but new manufacturing methods have greatly improved their power cycling capabilities. When using gel-fill modules, the flexibility of PCB layout is less than that of IPMS with DIP mounting, because the pins from the gel-fill module pin matrix tend to hinder the routing of the PCB.

Due to the widespread availability of robotic soldering equipment, the trend in new designs is to use solder pins in gel-filled and IPM modules. Certain types of crimp pins are susceptible to corrosive environments, but this problem has not been found in solder pin applications.

Figure 2 shows a cross section of ON Semiconductor’s new TMPIM (Transfer Molded PIM) module. The first part of the manufacturing process is similar to a gel-filled module. The die and thermistor are soldered on the DBC and then wired. In an IPM module, the DBC and some components are soldered on the leadframe. This reduces tool flexibility and requires additional tooling. In contrast, the TMPIM is completely flexible in terms of die layout and structure on the DBC as long as the pinout does not change.

The next stage is soldering the leadframe to the DBC. The final stage is the transfer molding process where the module is encapsulated in epoxy. The bond wire is cut and then bent into shape in a process known as trim and form.

The advantage of this method over modules with the die soldered to the leadframe is that it is easy to change the configuration or die in the module. A different pinout requires a new leadframe and trim-molding tool. Because the tooling costs hundreds of thousands of dollars, this method is used for modules with standard pinouts, such as six-pack, converter-inverter-brake (CIB) modules (Figure 1) and six-pack with interleaved PFC.

Gel filled modules are more flexible to change the way they are customized, but do not have the same thermal cycling capabilities of transfer molded modules. For the same DBC soldering and wire bond connection methods, transfer molded modules will have better power cycling capabilities than gel filled modules.

Figure 2: Cross section of the new TMPIM (Transfer Molded PIM) module.

Figure 2 shows a clear advantage of the TMPIM over existing modules. The scale is stretched for illustration purposes. The total module thickness is 8mm. The gap between the top of the pin and the top of the heat sink is 6mm, which is greater than the required 5.5mm gap. Gel-filled modules also meet this requirement, but they are much thicker (12mm compared to 8mm for the TMPIM); the IPM modules are thinner. Therefore, mechanical designers need to shape the heat sink, which typically adds additional manufacturing cost.

Table 1 shows the spacing between pad edges after accounting for 0.5mm pad ring width, 0.3mm drill tolerance, and lead size. Spacing requirements were extensively considered when designing TMPIM products.

Table 1: Pad to pad spacing for TMPIM DIP-C2 CIB module

Table 1: Pad spacing for TMPIMDIP-C2 CIB module

The IGBTs used in the TMPIMs are Robust Field Resistance II 1200 V IGBTs with a short circuit rating of more than 10 s at 150°C, 900 V bus voltage and 15V gate drive. Prior to release, the modules were extensively tested in motor drive testing, including bench testing. The NCP 57000 isolated gate drivers from ON Semiconductor are ideal for driving the TMPIMs. Each TMPIM uses 6 isolated drivers. The NCP 57000 has a Desat function, which detects overload current and then performs a soft shutdown of the IGBT to prevent excessive voltage spikes from shutting it down too quickly under short circuit conditions.

The TMPIM series can achieve more than 1000 thermal cycles. Standard gel-filled modules without any heat sink can usually only achieve 200 thermal cycles. The power cycling curve of the module shows good power cycling capability depending on the change in junction temperature. For high power modules in TMPIM, a high performance alumina substrate is used. When reading the power cycling curve, the lower thermal resistance leads to reduced thermal changes, resulting in higher power cycling capability.

The current TMPIM series at Semiconductor includes 1200 V CIB modules rated at 25 A, 35 A, 35 A high performance substrate and 50 A high performance substrate. New designs in the series will include 650 V CIB modules, 650 V 6-packs, 1200 V 6-packs and 650 V modules with interleaved PFC and 6-packs.

In summary, the approach taken by the TMPIM series can extend the use of transfer molded modules to higher power levels, while also providing designers of industrial motor drive inverters with a convenient, compact and reliable solution.