Download MAXISS: A New Servo Duty IPM With On-Chip

MAXISS: A New Servo Duty IPM With On-Chip Temperature Sensing
E. Motto**, J. Achhammer**, M. Yamamoto*, T. Marumo*, T. Igarashi*
* Power Device Division, Mitsubishi Electric Corporation, Fukuoka, Japan
** Powerex Incorporated, Youngwood, Pennsylvania, USA
ABSTRACT
This paper describes a new family of intelligent power modules optimized for demanding
multi-axis servo drive applications. The new IPMs feature compact packages designed for convenient
installation in the narrow profile (bookshelf) drives that commonly make up multi-axis servo systems.
Fourth generation 1µm planar IGBT chips are utilized to provide low switching and conduction losses.
The new chips are fabricated with an advanced on-chip temperature-sensing feature to provide effective
protection against severe conditions such as locked rotors.
I. INTRODUCTION
Multi-axis servo systems often consist of a group of narrow rack mounted motor drives. In
most applications these drives are required to operate at relatively high modulation frequencies to provide
accurate torque and speed control. Managing the resultant switching losses requires efficient thermal
designs. In addition, these drives are often required to deliver full torque under locked rotor (zero
speed) conditions. This condition further complicates loss management and overtemperature protection
because losses are not evenly distributed within the module. The end result is a demanding requirement
for thermal management and protection in a limited amount of space. In order to simplify and reduce
the cost of these drives, a new series of intelligent power modules called MAXISS (Multi-AXIS Servo)
has been developed.
II. PACKAGE DESIGN
The new MAXISS IPMs feature packages
with reduced size compared to conventional third
generation IPMs. Figure 1 shows a comparison of
the package size of the new IPMs compared to the
conventional devices. There are only two different
mechanical package designs to cover the complete
line-up of MAXISS IPMs as indicated in the figure.
Table 1 summarizes the packages, part numbers and
device ratings for the MAXISS family of IPMs.
The MAXISS family consists of six devices with
current ratings ranging from 50A to 300A. All
devices utilize IGBTs with 600V breakdown ratings
for reliable operation on AC line voltages from
100VAC to 240VAC.
Third generation
Third generation
Figure 1
MAXISS – Package A
MAXISS - Package B
Size Comparison of Third Generation
and MAXISS IPMs
A
B
IC (A)
50
75
100
150
200
300
Part Number
PM50CBS060
PM75CBS060
PM100CBS060
PM150CBS060
PM200CBS060
PM300CBS060
Table 1
sink
VCES (V)
sink
sink
600
380mm
Package
MAXISS Line-up Table
120mm
300mm
heat pipe
In many servo drive applications, the
high losses resulting from the severe operating
60mm
c. Thermal Design
conditions combined with the need for a narrow a. Target Dimensions
b. Thermal Design
for Bookshelf Style
Using MAXISS
Using Third
profile, necessitates the use of a complex and
Servo Drive
IPM
Generation IPM
expensive thermal system. Figure 2a illustrates
Figure 2 Using IPM with Narrow Heat Sink
the size and shape of a typical narrow profile
bookshelf style servo drive. The narrow 60mm wide heat sink does not provide enough mounting area
to accommodate the footprint of a conventional IPM. One solution to this problem is the addition of a
heat pipe as shown in Figure 2b. Unfortunately, this method adds considerable cost and complicates
inverter assembly. The MAXISS IPM’s 50mm wide base plate provides a simple cost effective solution
by allowing direct mounting to the heat sink as shown in Figure 2c.
To achieve the significantly smaller size illustrated in the figures, the MAXISS IPMs utilize a
new package design based on the high reliability, field proven, U-Package technology developed by
Mitsubishi Electric for high power IGBT modules. Figure 3 compares the cross sections of a third
generation IPM to a MAXISS IPM. Like other U-Package devices the power and control electrodes are
molded into the wall of the package to eliminate the need for solder joints and the chip level printed
circuit board material. All connections to the substrate are made using high reliability wire bonding.
This approach simplifies assembly and allows a reduction in the size of the substrate required.
Aluminum Nitride (AlN) substrates with high thermal conductivity are utilized in all MAXISS IPMs to
provide the low thermal impedance required in demanding servo drive applications. The IGBT and
free-wheel diode power chips are soldered to the AlN substrate that is soldered to the copper base plate.
The gate drive and protection circuits are constructed on two smaller printed circuit boards, rather than 1
Interconnect
Terminal
Control Board
Case
Al Wire
Cu Base plate
IGBT FWDi
PCB
Material
DBC
Ceramic
Substrate
PCB
Material
Control Boards Interconnect
Terminal
Case
Al Wire
IGBT FWDi
DBC
Ceramic
Substrate
a. Third Generation
Figure 3
Cu Base plate
b. MAXISS
Cross Sections of 150A, 600V IPMs
Emitter Electrode
Cathode
Anode
SiO2
Gate
Electrode
Polysilicon
n+
p
Polysilicon
Diode
Collector
Electrode
IGBT Cell
Tj sense
Figure 5 Structure of Fourth Generation
Chip with Tj Sensing Function
Emitter
Cathode
Anode
Gate
Current
Mirror
Figure 4
Internal View of MAXISS IPM
Figure 6 Photo of Fourth Generation
Chip with Tj Sensing Function
larger board, located above the power devices. 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.
Figure 4 is a photograph showing the internal design of the package “A” MAXISS IPM.
III. CHIP DESIGN
Another key requirement for compact servo drive applications is low loss. To meet this
requirement the MAXISS IPMs utilize a specially designed fourth generation IGBT chip. The structure
of the new chip is illustrated in Figure 5. The left side of Figure 5 shows one IGBT cell. A typical
chip contains hundreds of thousands of these cells connected in parallel. To reduce losses a 1µm
process and optimized planar cell geometry were utilized. The 1µm process allowed a significant
reduction in cell pitch and increased cell density compared to the 3µm process used in third generation
devices. The right half of Figure 5 shows the structure of the on-chip temperature sensing diodes.
These diodes are fabricated in the polysilicon layer on the surface of the IGBT chip. Figure 6 is a
photograph showing the features of the new IGBT chip. The connections for the main emitter, gate,
on-chip temperature sensor and current mirror are on the top surface of the chip. The current mirror
emitter provides current sensing for the IPMs short-circuit protection functions.
The trade-off curve in Figure 7 illustrates the improvement in VCE(SAT) and ESW of the new
fourth generation chip compared to 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 to 1.8V for the third generation IPM.
The saturation voltage characteristic of the new IGBT chip is illustrated in Figure 8. The new IGBT
chip has been optimized to provide minimum switching losses and soft switching characteristics for
minimum switching noise and overshoot voltages. Figure 9 shows the total switching energy
characteristic of the new chip compared to the third generation device. Figure 10 shows the inductive
2.8
VCE(sat) (V)
125°C, Jc=130A/cm2
2nd Generation (5µm planar)
2.4
3rd Generation (3µm planar)
2.0
1.6
4th Generation (1µ
µm planar)
1.2
～
～
0.1
1.0
Esw(off)
10.0
(mJ)
125°C, Inductive Load
Figure 7
Saturation Voltage versus Turn-off switching Energy
MAXISS
T hird Generation
Vce(sat) (V)
2
1.5
1
0.5
Tj = 125°C
0
0
25
50
75
100
125
Ic (A)
Figure 8
Saturation Voltage Characteristics for 100A, 600V IPMs
MAXISS
T hird Generation
12
Esw(off) (mJ/pulse)
10
8
6
4
Tj=125ºC
Vcc=300V
Inductive Load
2
0
0
20
40
60
80
100
120
Ic (A)
Figure 9
Total Switching Energy for 100A, 600V IPMs
load switching waveform for a 100A,
600V MAXISS IPM.
This
waveform illustrates the well-behaved,
low noise, switching of the new IGBT
and free-wheeling diode utilized in
the MAXISS IPM.
IV. ON-CHIP TEMPERATURE
SENSING
Conventional IPMs utilize a
thermistor attached to the ceramic
substrate near the power chips, as
shown in Figure 11.a, to provide
overtemperature protection.
The
thermistor detects the base plate
temperature of the module and its
response time is limited by the
thermal mass of the base plate. This
type of sensor can provide effective
protection against events that cause a
relatively
slow
increase
in
temperature such as overloads,
excessive ambient temperatures or
inadequate cooling airflow. Servo
drives are often designed to produce
high output currents for short periods
of time to provide rapid acceleration
and high start-up torque. This high
current condition can produce a rapid
increase in junction temperature that
may be too fast for a conventional
base plate temperature sensor. This
problem becomes even worse if the
high output current is delivered 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 Figure 12. If this
chip is located far from the base plate
temperature sensor it will take even
longer to detect an overtemperature
condition.
The MAXISS IPM uses an
advanced on-chip temperature sensor,
Turn-on
Turn-off
(VCE:100V/div, VCIN:10V/div, IC:50A/div, t:400ns/div)
Tj = 125ºC, VCC = 300V, IC = 100A
Figure10
Inductive Load Switching Waveforms
as shown in Figure 11.b, to avoid these problems. Each IGBT chip in the MAXISS IPM has a string of
diodes fabricated in the polysilicon on the surface of the IGBT chip. Chip temperature is detected by
forward biasing the diode string and measuring the voltage drop. Like most silicon diodes the forward
voltage drop decreases with increasing junction temperature. Figure 13 shows the typical Vf versus Tj
characteristic of the temperature sensing diodes. A string of diodes is used to provide a high enough
sensing voltage to avoid problems with noise. The MAXISS overtemperature protection function is
implemented as shown in Figure 14. The Vf of the on-chip sensing diodes is compared to a reference
voltage, which sets the overtemperature trip level (OT). If the Vf of the sensing diodes drops below the
reference voltage an overtemperature condition is indicated. If the IGBT chip involved is on the low
side, all three lower IGBTs will be turned off and a fault signal is generated. If the IGBT involved is on
Thermistor
IGBT
a. Conventional IPM
Gate - Emitter
Sensor
b. MAXISS IPM
Figure 11
Diode to
detect chip
temperature
IGBT
Temperature Sensor Location Comparison
Motor
the high side, only that device will be
turned off and a fault signal is not
provided.
The detection circuit
provides hysteresis so that the chip
must cool below the overtemperature
reset level (OTr) before normal
operation can resume.
V. PROTECTION FUNCTIONS
Figure 12
Locked Rotor
Vf (V)
In
addition
to
overtemperature protection, the IPM’s
4
internal gate control circuits also
provide control supply undervoltage
3.5
If = 3mA
lockout, overcurrent, and short-circuit
protection.
The IPM's internal
3
control circuits operate from a 15V
DC supply. If for any reason, this
2.5
supply voltage drops below the
specified undervoltage trip level (UV),
2
the power devices are turned off and a
0
25
50
75
100
125
150
175
fault signal is generated. Normal
T j (°C)
operation will automatically resume
Figure 13 Temperature Sensor Characteristics
when the supply voltage exceeds the
undervoltage reset level (UVR).
The MAXISS IPM utilizes a
Part of control IC
two level overcurrent protection
scheme. Current through each IGBT
is monitored using the output from the
On control
board
current mirror emitter on the IGBT
chip. The current from the mirror
emitter is diverted through a resistor
network that provides voltage signals
Diodes on
for the gate control IC. The gate
IGBT Chip
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
Figure 14 Overtemperature Sensing Circuit
exceed the data sheet specified
short-circuit trip level (SC) the IGBT involved will be immediately shut down. In the case of 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 10µs before shutting down. The
tOFF(OC) delay is provided to avoid false tripping of the protection on short pulses of current that are not
dangerous for the IGBT. Figure 15 shows a timing diagram for the short circuit and over current
protection. If the fault occurs on one of the low side IGBTs all three lower IGBTs will be turned off
and a fault signal will be generated. If the fault is detected on one of the high side IGBTs the device
will be shut down and gate drive is inhibited but a fault signal will not be generated. If the high currents
that cause short-circuit faults are abruptly interrupted, a high di/dt will occur. This high di/dt can cause
dangerous transient voltages. The MAXISS IPM avoids this condition by using controlled di/dt (soft)
shutdown techniques.
Figure 15
Short-Circuit and Overcurrent Protection Timing Diagram
The MAXISS IPM has built-in protection circuits that prevent the power devices from being
damaged should the system malfunction or be over-stressed. Fault detection and shutdown schemes
allow maximum utilization of power device capability without compromising reliability.
VI. CONCLUSION
A new IPM designed for multi-axis servo drive applications has been presented. The new
MAXISS IPM utilizes a specially designed compact package to allow optimized thermal management for
narrow profile bookshelf style servo drives. A new IGBT chip with a 1µm planar structure is utilized to
provide low losses with integrated temperature and current sensing features. Built in protection
functions allow maximum power device utilization while maintaining high reliability.
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June 1990
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1992
[5] Powerex "IGBTMOD and Intellimod Application and Technical Data Book" Second Edition,
PUB#9DB-200, 1998
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