IXAN0071 The SMPD Package and its Mounting

IXAN0071
The SMPD Package and its Mounting Instructions
Abdus Sattar, IXYS Corporation
Introduction:
IXYS has expanded its ISOPLUS package portfolio by offering a new package
called the SMPD package. An illustration of the package shown in Figure 1 shows an
isolated base (Copper) from the semiconductor chip. The electrical isolation between the
semiconductor chip and the base copper is achieved by the Direct-Copper-Bonded (DCB)
structure. The structure has three layers (Copper, Alumina Substrate, Copper) in which
the Alumina substrate is a ceramic layer situated between the two copper layers and
provides the necessary voltage isolation. The rated isolation voltage for this package is
2500V. The DCB is a well known technique for isolation and suitable for high currentcarrying capability, excellent thermal conductivity and high reliability in the application.
In summary, the SMPD package has the following advantages:
High voltage electrical isolation (~2500V)
Lower thermal resistance compared to other standard packages (TO-247, TO-264 etc)
Higher component density (H-bridge circuit)
Higher current carrying capability
The expansion coefficient of the DCB is close to that of the silicon, which results in
high temperature cycle reliability
If this package is compared with the standard package, the current loop is reduced and
the parasitic capacitance is also reduced. Both result in better EMI behavior of the
application.
Figure 1: The SMPD Package Diagram
1
Mounting Instructions:
The SMPD package houses both discrete and module devices. To get the most effective
heat dissipation, it’s required to follow some basic mounting guideline when installing
the device to a heat sink. By following this guideline, it’s possible to enlarge the contact
area, which minimizes the contact thermal resistance.
Use of Thermal Compound/Grease
The use of thermal compound provides a very good contact between the device’s base
and the heat sink surface. It ensures a very low value of contact thermal resistance (caseto-heat sink thermal resistance). The recommendation is to use either Dow Corning 340
(DC340) or silicone free HTCP equivalent thermal compound. Since the devices feature
an internal isolation (DCB), it is not recommended to use an interface material with
isolation (thermal pad) as these materials exhibit a higher thermal resistance compared to
the materials mentioned above.
-
Thermal compound should be applied evenly to the device base plate and the heat
sink surface.
-
Thermal compound layer thickness should be in the range of about 60 to 80 µm.
Heat Sink Surface
In order to ensure a very low value of contact thermal resistance, the contact surface of
the heat sink must be flat and clean and fulfill the following mechanical specifications:
Unevenness: < 50µm over a distance of 100mm
Roughness: < 10µm
When installing the device to a heat sink, excessive uneven fastening force might apply
stress to inside chips, which can lead to damage or degradation of the device. An example
of heat sink assembly is shown in Figure 2.
Mounting Screw
The recommended mounting screw is M3.5 or 6-32 screw with flat and spring washers.
Both flat and spring washers that fit the screw must be used for spreading the pressure
evenly on the heat sink (or the PCB). The minimum length of the screw is 5/8” or 3/4”.
Mounting Force/Torque
The recommended mounting force for the SMPD package is given in the datasheet as
50…200N/11…45lb. To convert it into a mounting torque, we can use the following
formula:
2π . M
(1)
F=
P
Here F is the contact force in N, P is the pitch in m of used screw and M is the torque
applied to the screw in N.m. The above relation states that the contact force is inversely
proportional to the pitch of used screw.
2
http://www.gewinde-normen.de/en/unified-coarse-thread.html
For 6-32 screw from international thread standard
Pitch=0.794mm=0.000794m
Therefore,
50x 0.000794
= 0.0063N.m
F=50N, M =
2π
100x 0.000794
= 0.0126N.m
F=100N, M =
2π
200x 0.000794
F=200N, M =
= 0.0253N.m
2π
In order to convert in the British unit ~lb.inch, the following relations can be used.
One Pound inch to Newton meter, à 1 lb.in=0.113N.m;
One Pound force to Newton, à 1 lb.f.=4.45N; One inch to meter, à 1 inch=0.0254m;
The following Table 1 provides the summary of the mounting requirements for the
SMPD package.
Table 1: SMPD Mounting Torque and Heat Sink Flat Specifications
Parameters
SMPD Device Flatness
Heat Sink Flatness
Mounting Force
Conditions
Limits
Typ.
Min
Max.
Unit
--
--
100
µm
Unevenness
--
--
50
µm
Roughness
--
--
10
µm
Screw
6-32
50
11
--
200
45
N
lb
3
Thickness of thermal compound
Weight
60
--
8
80
--
µm
gram
Figure 2: Illustration of a heat sink assembly
Soldering of Terminals:
If the electrical connection is made by soldering, great care must be taken to avoid the
overheating the device. In this case, the following requirements are recommended:
Table 2: Soldering Requirements
Soldering Temperature
<260
o
C
Distance from case
<3mm
Maximum soldering time
3s
4
<260
o
C
>3mm
5s
Mounting to a Heat Sink:
Mounting instructions would be a little different for different heat sinks. In Figure 3, we
use AAVID Thermalloy P/N: 62230. For this heat sink, the dimension and the mounting
process are illustrated in the previous figure 2.
Figure 3: AAVID THERMALLOY Heat Sink P/N: 62230
The size of heat sink is chosen as LxWxH ~ 1.5in (38.1mm) x 1.15in (29.21mm) x1.3in
(33.02mm). The following procedure can be followed to mount the device:
i)
When a PCB is used as shown in Figure 4 then the device should be soldered
on the PCB first. It’s required that the SMPD base and heat sink surfaces are
parallel to each other.
Figure 4: An example for PCB and heat sink assembly
ii)
Create two holes (30.48mm apart) on the surface of the heat sink. For
mounting with screw and nut, the mounting hole should not exceed 0.140inch
(screw 6-32). The recommended depth of thread in aluminum (Al) heat sink is
12mm and in copper (Cu) heat sink is 10 mm depending on the thickness of
the heat sink base. Here, the maximum thickness of the heat sink base is
8.89mm therefore, 5 to 6 mm depth would be sufficient.
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iii)
Heat sink contact surfaces must be flat, clean and free of scratches. It should
have the following specifications:
Unevenness: < 50µm over a distance of 100mm
Roughness: < 10µm
iv)
Place a thin layer of thermal compound on the heat sink surface and on the
device’s base plate just sufficient to cover the entire DCB base to the heat
sink. Best results are achieved when the thermal compound thickness is
between 60 um to 80 um. Thicker layer of thermal compound may increase
the risk of damage to the DCB base plate and internal chip.
The recommended mounting screw is M3.5 or 6-32 UNC with flat and spring
washers. Both flat and spring washers fitting the screw have to be used
between the screw and the heat sink (or the PCB) surface.
v)
vi)
First tighten the screw clockwise then use a torque wrench to apply the
tightening force. The tightening force should not be exceeded the mounting
force given in the datasheet.
vii)
To prevent mechanical stress to leads of the device or on the connections
between the device and the PCB, additional support from the heat sink to the
PCB might be necessary.
6
Heat Sink Size Estimation:
In order to have higher power dissipation capability, the SMPD package should be
mounted on a heat sink. To determine the heat sink size, it’s important to estimate the
maximum power dissipation of the device. The datasheet provides the useful parameters
to determine the power dissipation. The maximum operating switching frequency of the
device can be estimated as,
fs (max) = (PD-PC)/ESW
(2)
Where PD is the maximum device power dissipation, PC is the conduction power loss, fs
(max) are the maximum switching frequency and ESW is the switching energy loss.
The maximum power dissipation by a packaged device is determined by the maximum
junction operating temperature rating, the ambient temperature, and the junction-toambient thermal resistance.
TJ ( Max ) − TA
PD =
= PC ,Total + PSW ,Total
(3)
RthJA
Where PC is the conduction loss and PSW is the switching loss. IXYS datasheet provides
an absolute maximum junction temperature rating called TJM of 150C. Operating in this
temperature can severely impact the device’s reliability. For this reason one important
consideration in equation (3) is that the designer must select a maximum junction
operating temperature, TJ(Max), which should be lower than TJM. IXYS recommends a
value of the maximum operating junction temperature TJ(Max) of between 100 and 125C.
Here are the generic formulas for determining the total power loss in a single Power
MOSFET or IGBT.
Power MOSFET:
The Conduction loss is
(4)
PC = IRMS2*RDS(on)
Where the duty cycle is included in the rms value of the current
The switching power loss can be defined as
(5)
PSW = ESW*fS(max)
ESW = Turn ON Energy Losses + Turn OFF Energy Losses + Gate Control Energy
Loss+ Output Capacitance loss+ Diode Reverse Recovery loss
(6)
= ESW-ON + ESW-OFF + ESW-Gate +ESW-OutputCAP + ESW-DiodeRR
(7)
ESW-ON = 0.5*IO*VOFF*QSWIG
ESW-OFF = 0.5*IO* VOFF *QSW/IG
(8)
(9)
ESW-Gate = QG*VG
2
ESW-OutputCAP = 0.5*CP* VOFF + O.5*QOSS* Voff
(10)
(11)
ESW-DiodeRR = VOFF*QRR
Where, IO = Output current
VSW= Switching gate voltage
VG = Gate voltage
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QG = Total gate charge
IG = the gate driver current
QOSS = Output charge (if available)
CP = Circuit parasitic capacitance (if available)
VOFF= Off State drain voltage
QRR= Internal diode’s reverse recovery charge
fSW(max)= Maximum switching frequency
Similarly for IGBT application,
IGBT:
The Conduction loss is defined by,
(12)
PC = IC*VCE(sat)
Where the duty cycle is included in the rms value of the current, IC
The switching power loss can be defined as
(13)
PSW = ESW*fS(max)
ESW = Turn ON Energy Losses + Turn OFF Energy Losses + Gate Energy Loss
(14)
= ESW-ON + ESW-OFF + ESW-Gate
ESW-ON or EON is given in the device datasheet
(15)
ESW-OFF or EOFF is given in the device datasheet
ESW-Gate = QG*VG
(16)
(17)
Where,
VG = Applied gate voltage
QG = Total gate charge
Here PC and PSW defined in equations (4), (5), (12) and (13) are for each device. To
obtain PC, Total and PSW, Total, you need to multiply the number of devices used per
application. Now, equation (3) can be used to determine the total power dissipation in the
application, PD.
After determining the maximum power loss/dissipation, PD in watt, we now can estimate
the maximum junction to ambient thermal resistance, RthJA , using equation (3):
RthJA can also be written as,
(18)
RthJA = RthJC + RthC-SH+ RthSH-A
RthJA is the thermal resistance of the entire assembly that has three components: the
thermal resistance of the device, RthJC, the thermal resistance of the interface material
between the device and the heat sink, RthC-SH, and the thermal resistance of the heat sink,
RthSH-A. The data for RthJC and sometimes RthC-SH or RthCS are provided by the device
datasheet. Otherwise, if the thermal compound is used between the heat sink and the base
of the device then we can add a maximum thermal resistance between the case and heat
sink, RthC-SH as 0.2K/W (typical value). After obtaining the values for RthJC and RthC-SH,
we can calculate the value for RthSH-A which is the heat sink thermal resistance.
After obtaining the heat sink thermal resistance value, the next step is to select a heat sink
based on the known thermal resistance and available dimensions that fit the size of the
package.
8
New generation of IXYS Power MOSFETs and IGBTs in the SMPD package are given
in Table 1 & 2.
Table 1: Latest SMPD Packaged Power MOSFET (www.ixyspower.com)
Power
MOSFET
VDS (max)
MMIX1T600N04T2
MMIX1F520N075T2
V
I D 25 (Cont.)
Tc=25C
A
RDS (on ) type
Tc=25C
mΩ
QG (on ) type
Tc=25C
nC
t rr
type
(ns)
40V
75V
600A
500A
1.30
1.60
590
545
100
150
Table 2: Latest SMPD Packaged IGBT
VDS (max)
I D 25 (Cont.)
IGBT
Tc=25C
V
A
MM1X1G320N60B3
600V
400A
I D110 (Cont.)
Tc=110C
A
VCE ( Sat ) type
Tc=25C
V
QG
nC
180A
1.5V
585
RthJC
o
C /W
0.125
21-pin and 9-pin SMPD package illustration:
Isolated Tab
Figure 5: The SMPD Package Illustration
9
Figure 6: The SMPD Package: 21-pin mechanical outline
10
Figure 7: The SMPD Package: 9-pin mechanical outline
11