Transfer Molded DIP-IPM Package Improves Thermal Performance

The transfer molded DIP-IPM was first introduced in 1998 to address the rapidly growing demand for cost-effective motor control in consumer appliance applications. These devices soon became widely accepted due to their performance, reliability and cost advantages compared to conventional designs based in discrete devices. In the years that followed, continuous improvements in package thermal performance, power chip design, and HVIC (High Voltage Integrated Circuit) technology have enabled the development of a complete line of modules for motors rated from 100W to more than 15KW at line voltages of 100VAC to 480VAC.

In order to cost-effectively provide larger output power ratings, a new transfer molded package structure was developed. Fig. 1. shows the cross section of this new package compared to the previous large DIP-IPM. The DIP-IPMs are fabricated using a transfer molding process like a very large integrated circuit. First, bare power chips and the custom HVIC and LVIC die are assembled on a lead frame. Ultrasonic bonding of large diameter aluminum wires makes electrical connections between the power chips and lead frame. Small diameter gold wires are bonded to make the signal level connections between the IC die and lead frame. This part of the process is basically the same for both devices. Next, they are encapsulated. This is where the packages differ.

A cross section of the previous generation 3 Large DIP-IPM is shown in Fig. 1(a). In this device, a heat spreader made of copper is attached to the lead frame and electrical insulation is provided by a thin layer of the molding compound at the mounting surface. The copper heat spreader gives relatively good thermal performance, but the high thermal resistance of the molding compound limits this construction to devices with ratings of about 50A at elevated case temperatures.

The new generation 4 large DIP-IPM package cross section is shown in Fig. 1(b). This device uses a new low thermal impedance structure based on technology developed for the generation 4 super Mini DIP-IPM [1]. In this novel structure, a partially cured insulating resin sheet is adhered to the rear surface of lead frame after chip bonding. The other surface of the resin sheet is attached to an aluminium heat spreader. The lead frame with the resin and aluminium heat spreader attached is then transfer-molded using epoxy resin. The transfer molding process causes the resin sheet to cure simultaneously with the epoxy resin. The result is a stable high reliability joint with low thermal impedance. The thin insulating resin sheet stays in a fixed form during the process, so it does not need to have the fluidity of the epoxy resin over mold . Thus, it is possible to increase the amount of ceramic fill to improve the thermal conductivity.

In addition, it is possible to achieve a thinner insulating layer because it is not constrained by the limitations of the molding process. The extremely thin layer of high thermal conductivity resin yields a substantial reduction in thermal impedance compared to previous DIP-IPM designs. Fig. 2. shows a photograph of the new large DIP in its final form. The module features a compact 31mm × 79mm footprint.

NEW LARGE DIP-IPM

In addition to the six IGBTs and free wheeling diodes required for a three-phase motor drive, the new large DIP-IPM also contains HVIC and LVIC chips to provide gate drive and protection for the power devices. Fig. 3 shows a complete functional diagram of the DIP-IPM.

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The DIP-IPM includes high voltage level shifting provided by integrated HVICs. The built-in level shift eliminates the need for relatively expensive optocouplers or pulse transformers, and allows direct connection of all six control inputs to the CPU/DSP.

The DIP-IPM is protected from failure of the 15V control power supply by a built-in under-voltage lock out circuit. If the voltage of the control supply falls below the UV level specified on the data sheet, the low side IGBTs are turned off and a fault signal is asserted. In addition, the p-side HVIC gate drive circuits have independent under-voltage lock out circuits that turn off the IGBT to protect against failure if the voltage of the floating power supply becomes too low. This feature is particularly useful considering the sometimes complex dependence of boot-strap power supplies on the switching of the lower devices. If the high side under-voltage lockout protection is activated the respective IGBT will be turned off but a fault signal is not supplied.

The new large DIP-IPM has an integrated short-circuit protection function. The LVIC monitors the voltage across an external current sensing resistor (RSHUNT) to detect excessive current in the emitters of the low side IGBTs. In order to avoid large power dissipation in the shunt resistor, the new DIP-IPM has been equipped with IGBT chips having a current mirror emitter that supplies a low level current that is approximately 1/10,000 of the main emitter current. This low level signal allows use of a small surface mount current sensing resistor. An RC filter (RSF, CSF) with a time constant of 1.5 to 2µs is normally inserted as shown in Fig. 3 to prevent erroneous fault detection due to di/dt induced noise on the shunt resistor and free-wheel diode recovery currents. When the voltage at the CIN pin exceeds the VSC reference level, the lower arm IGBTs are turned off and a fault signal is asserted at the FO output. When an overcurrent condition is detected, the IGBTs remain off until the fault time (tFO) has expired and the input signal has cycled to its off state. The duration of tFO is set by an external timing capacitor CFO.

TEMPERATURE SENSOR

The LVIC in the new module includes a circuit that produces a voltage proportional to the modules temperature. This circuit eliminates the need for an external heat sink mounted thermistor to provide over temperature protection. In addition, unlike an external sensor, the integrated sensor is capable of detecting problems with the module to heat sink interface for improved manufacturing quality control. The circuit provides a buffered analog voltage feedback signal that is suitable for direct connection to the controller thereby eliminating the need for the associated buffering and amplifying components that would have been required with an external sensor. Fig. 4 shows the temperature sensor characteristic. The module itself will not shut down in the case of excessive temperature allowing the user to implement the most appropriate remedial action based on the measured temperature and operating conditions.

The new large DIP-IPM has eight microprocessor compatible input and output signals. The built-in HVIC level shifters allow all signals to be referenced to the common ground of the 15V control power supply. The signals are compatible with 3.3V to 5V CMOS logic in order to permit direct connection to a PWM controller. Fig. 5 shows the equivalent internal circuit of the DIP-IPMs control signals and a simplified schematic of a typical external interface circuit. The components shown in dashed blue lines are optional noise filtering that may be required depending on the circuit layout and its proximity to noise sources. On and off operations for all six of the DIP-IPMís IGBTs are controlled by the active high control inputs UP, VP, WP, UN, VN, WN. These inputs are pulled low internally by a 3.3kΩ resistor. The controller commands the respective IGBT to turn on by pulling the input high. Approximately 1.8V of hysteresis is provided on all control inputs to help prevent oscillations and enhance noise immunity.

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The fault signal output (FO) is in an open collector configuration. Normally, the fault signal line is pulled high to the 5V logic supply with a 10kΩ resistor as shown in Fig. 5. When an over-current, over temperature condition or improper control power supply voltage is detected the DIP-IPM turns on the internal open collector device and pulls the fault line low.

DIP-IPM SYSTEM ADVANTAGES

Inverters for small AC motors used in appliance applications are required to meet stringent efficiency, reliability, size and cost constraints. Historically, many of these small inverters have utilized discrete IGBTs (Insulated Gate Bipolar Transistors) and free-wheel diodes in TO-247 or similar packages along with separately packaged HVICs (High Voltage Integrated Circuits). There are, however, several problems with this approach. One drawback is the high manufacturing cost associated with mounting and isolating multiple high voltage discrete components. Each of the discrete devices must be individually mounted using special hardware and insulating materials which typically results in a complex assembly and significant manufacturing time. In addition, relatively large and complex printed circuit designs are required to meet all of the spacing and layout requirements for the HVIC and discrete power device combination. Another equally perplexing problem is maintaining consistent performance and reliability when the characteristics of the HVIC drivers and IGBTs are not properly matched.

A better approach, realized in the DIP-IPM, is to assemble bare power chips and HVICs using a transfer molded lead frame design to maintain low cost and consistent, reliable performance. With the fully isolated DIP-IPM, mounting is accomplished with only two screws and no additional isolation material is required. The reduced manufacturing time and simplified assembly provided by the DIP-IPM will allow improvements in both cost and reliability of the finished system. Another advantage of the DIP-IPM is that the integrated HVIC and LVIC gate drive and protection functions are factory tested with the IGBTs as a subsystem. This eliminates uncertainty about the critical coordination of the electrical characteristics of these components. The end result is more consistent system performance and reliability.

The table shows the new gen. 4 large DIP-IPM line-up. Modules are available with blocking voltage ratings of 600V and 1200V which are appropriate for 100VAC to 480VAC applications. Devices with nominal current ratings of 50A to 75A at 600V and 5A to 35A at 1200V are all available in the same compact package outline. The table also shows the usable sinusoidal RMS motor current per phase for some typical application conditions. These values are calculated using the loss simulation software available from the Powerex website.

Powerex also offers devices in smaller transfer molded packages with nominal current ratings of 3A to 30A at 600V.

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