Optimized Thermal Management Protects Compact Drive Module

Oct. 1, 2001
Family of IPMs feature integrated temperature and current sensing functions and compact packages that allow installation in the narrow profile motor drives.

Multi-axis servo systems often consist of a group of narrow rack-mounted motor drives. In most applications, these drives must operate at relatively high modulation frequencies to provide accurate torque and speed control. Managing the resultant switching losses requires efficient thermal designs. Often, these drives must deliver full torque under locked rotor (zero speed) conditions; further complicating loss management and overtemperature protection because losses are not evenly distributed within the module. This results in a demanding requirement for thermal management and protection in a limited space. The development of a new series of intelligent power modules (IPMs) simplifies and reduces the cost of these drives.

Package Design

Compared with conventional third-generation modules, MAXISS IPMs feature reduced size packages. IPMs require only two different mechanical package designs to cover the complete lineup. For reliable operation on line voltages from 100Vac to 240Vac, all devices utilize 600V breakdown IGBTs.

In many servo drive applications, the high losses resulting from the severe operating conditions combined with the need for a narrow profile necessitates the use of a complex and expensive thermal system. Fig. 1a illustrates the size and shape of a typical narrow profile bookshelf-style servo drive. The 60-mm-wide heat sink doesn't provide enough mounting area to accommodate the footprint of a conventional IPM. One solution for this is the addition of a heat pipe as shown in Fig. 1b. Unfortunately, this method adds considerable cost and complicates inverter assembly. The new IPM's 50-mm-wide baseplate provides a simple cost-effective solution by allowing direct mounting to the heat sink, as shown in Fig. 1c.

To achieve the smaller size illustrated in the figures, the new IPMs use a new package design based on the high reliability, field proven, U-Package technology developed by Mitsubishi Electric for high power IGBT modules. Fig. 2, on page 40, compares the cross sections of a third-generation IPM to a MAXISS IPM. The U-Package devices mold the power and control electrodes into the wall of the package, eliminating the need for solder joints and the chip level p. c. board material. You can make all connections to the substrate using high-reliability wire bonding. This approach simplifies assembly and allows a reduction in the required substrate size. The new IPMs utilize aluminum nitride (AlN) substrates with high thermal conductivity to provide the low thermal impedance required in demanding servo drive applications. The IGBT and freewheel diode power chips are soldered to the AlN substrate, which is soldered to the copper baseplate. Two smaller p. c. boards contain the gate drive and protection circuits, rather than one larger board. One board provides gate drive and protection for the three low side IGBTs and the other provides gate drive and protection for the three high side IGBTs. Fig. 3, on page 40, shows the internal design of the package “A” IPM.

Chip Design

Low loss is another key requirement for compact servo drives. To meet this requirement, the IPMs utilize a specially designed, fourth-generation IGBT chip (Fig. 4, on page 42). The left side shows one IGBT cell. A typical chip contains hundreds of thousands of these cells connected in parallel. To reduce losses, its 1-μm process uses optimized planar cell geometry. The 1-μm process allowed a significant reduction in cell pitch and increased cell density compared with the 3-μm process used in third-generation devices. The right half of Fig. 4 shows the on-chip temperature-sensing diodes structure. The fabrication of the diodes is in the polysilicon layer on the surface of the IGBT chip.

The trade-off curve in Fig. 5, on page 42, illustrates improvement in VCE(SAT) and ESW of the new fourth-generation chip compared with previous generations. The fine pattern and dense cell structure of the new IGBT chip gives a VCE(sat) at rated current of 1.6V compared with 1.8V for the third generation IPM. Fig 6, on page 42, illustrates the saturation voltage characteristic of the new IGBT chip. The optimized IGBT chip provides minimum switching losses and soft switching characteristics for minimum switching noise and overshoot voltages. Fig. 7 shows the total switching energy characteristic of the new chip compared with the third-generation device. Fig. 8, on page 44, shows the inductive load switching waveform for a 100A, 600V MAXISS IPM, which illustrates the well-behaved, low noise switching of the new IGBT and freewheeling diode.

Temperature Sensing

For overtemperature protection, conventional IPMs utilize a thermistor attached to the ceramic substrate near the power chips (Fig. 9a, on page 47). The thermistor detects the module's baseplate temperature, but the baseplate's thermal mass limits its response time. This type of sensor can provide effective protection against events that involve a relatively slow increase in temperature such as overloads, excessive ambient temperatures, or inadequate cooling airflow. However, to provide rapid acceleration and high start-up torque, servo drives often produce high output currents for short periods. This high current condition can produce a rapid increase in junction temperature that may be too fast for conventional baseplate temperature sensors. This problem becomes even worse when delivering the high output current at zero speed (locked rotor). Depending on the position of the rotor this condition may produce the highest losses in a single IGBT chip, as illustrated in Fig. 10, on page 47. If this chip is located far from the baseplate temperature sensor it will take even longer to detect an overtemperature condition.

The new IPMs use an advanced on-chip temperature sensor (Fig. 9b, on page 47) to avoid overtemperature problems. Each IGBT chip has a string of diodes fabricated into the polysilicon on the chip's surface. The voltage drop across the forward biased diode string detects the chip temperature. Like most silicon diodes, the forward voltage drop decreases with increasing junction temperature. Fig. 11, on page 47, shows the typical Vf vs. Tj characteristic of the temperature-sensing diodes. A string of diodes is used to provide a high enough sensing voltage to avoid noise problems.

Looking at Fig. 12, on page 48, you can see the implementation of this overtemperature-sensing circuit. You can compare the Vf of the on-chip sensing diodes to a reference voltage, which sets the overtemperature trip level (OT). If the Vf of the sensing diodes drops below the reference voltage, it indicates an overtemperature condition. If the IGBT chip involved is on the low side, the control IC turns off all three lower IGBTs and generates a fault signal. If the IGBT involved is on the high side, the control IC turns off only that device and does not produce a fault signal. The detection circuit provides hysteresis so that the chip must cool below the overtemperature-reset level (OTr) before normal operation can resume.

Protection Functions

Besides overtemperature protection, the IPM's internal gate control circuits also provide control supply undervoltage lockout, overcurrent, and short-circuit protection. The IPM's internal control circuits operate from a 15Vdc supply. If for any reason this supply voltage drops below the specified undervoltage trip level (UV), the control IC turns off the power devices and generates a fault signal. Normal operation will automatically resume when the supply voltage exceeds the undervoltage-reset level (UVR).

This IPM utilizes a two-level overcurrent protection scheme. It monitors current through each IGBT using the output from a current mirror emitter on the chip. A resistor network provides voltage signals for the gate control IC using current from the mirror emitter. The gate control IC has comparator circuits that determine if an overcurrent or short-circuit condition is present.

In the case of a severe short-circuit condition that causes the current to exceed the data sheet specified short-circuit trip level (SC) the involved IGBT shuts down immediately. For a less severe overcurrent condition that causes the current to exceed the data sheet specified overcurrent trip level (OC) the IPM will wait for a delay of tOFF(OC) that is typically 10 μs before shutting down. The tOFF(OC) delay avoids false tripping of the protection on short current pulses that are not dangerous for the IGBT. Looking at Fig. 13, on page 48, you can see a timing diagram for the short circuit and overcurrent protection. A fault on one of the low side IGBTs turns off all three of the lower IGBTs and generates a fault signal. A fault detected on one of the high side IGBTs shuts down the device and inhibits the gate drive — but does not generate a fault signal. Abruptly interrupting the high currents that cause short-circuit faults will cause a high di/dt. This high di/dt can cause dangerous transient voltages. This IPM avoids this condition by using controlled di/dt (soft) shutdown techniques.

The MAXISS IPM has built-in protection circuits that prevent the power devices from being damaged, should the system malfunction or overstress. Fault detection and shutdown schemes allow maximum utilization of power device capability without compromising reliability.


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