Calculated Datasheet Parameter: ISO-Nominal Load Current and Total Power Dissipation in HITFET® and PROFET® Datasheet

Application Note ANPS073E, V1.0, July 2004
Calculated Datasheet Parameter
ISO-Nominal Load Current and Total Power
Dissipation in HITFET® and PROFET® Datasheet
Automotive Power
N e v e r
s t o p
t h i n k i n g .
ISO-Nominal Load Current and Power Dissipation
Edition 2004-07-07
Published by Infineon Technologies AG,
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D-81541 München, Germany
© Infineon Technologies AG 7/19/04.
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Application Note ANPS073E
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V1.0, 2004-07-07
ISO-Nominal Load Current and Power Dissipation
Introduction
1
Introduction
The Hitfet and Profet are power-MOSFET devices with integrated protection functions.
In order to ensure a long lifetime the application should be designed in a way that
overload conditions does not occur or are at least minimized. Therefore the power
MOSFET devices Hitfet and Profet offer integrated protection functions. Under fault
conditions, for example over temperature, overload or short circuit, the integrated
protection will turn off the device. Of course this unwanted turn off can cause a
malfunction of the application. The unwanted turn off due to over temperature or
overload can be avoided by choosing a lower ohmic device.
In order to simplify this selection process the data sheets show the values ISO load
current ID(ISO) and Nominal load current ID(Nom).
One other parameter for simplifying the device selection is the value of power loss Ptot.
2
Definition
The ISO current approach considers the thermal resistance between Junction and Lead
frame RthJC as shown in Figure 1. This assumes the lead frame as an isothermal area
with the temperature tambient=85°C. Under this conditions the ISO current is calculated in
a way that the voltage drop between Drain and Source becomes maximal VDS=0.5V. If
this current leads to a junction temperature of TJ>150°C then the ISO current is reduced
to become TJ=150°C. The ISO current is interesting for devices directly mounted to the
heatsink, but not for surface mounted devices (SMD) on a printed circuit board (PCB),
because the thermal resistance between Junction and Case RthJC is used.
The difference of the nominal load current ID(Nom) is that this approach considers the
thermal resistor between Junction and Ambient RthJA. Under this conditions the nominal
load current is the current which meets both following conditions. First this current will
heat up the junction to TJmax=150°C and second the voltage drop will be limited to
VDS≤0.5V.
TJ
T C=Tambient = const
Chip
Case
TA
Chip
RthJC
Case
RthJA
PCB
ISO_vs_Nom.emf
Figure 1
Device with constant Case temperature versus device on PCB
related to ambient temperature.
Application Note ANPS073E
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ISO-Nominal Load Current and Power Dissipation
Computation
3
Computation
Since the data sheet parameters Nominal load current and ISO load current are
calculated values which can not be tested directly during production this application-note
will describe how this parameters are calculated.
The computation process for the ISO load current is shown in Figure 2. Starting with the
ambient temperature the RDS(on) is calculated. The ISO definition allows a voltage drop
of 0.5V, which gives the current. Via the on-state-losses the junction temperature TJ
comes out. This process is repeated until there is no more change in the junction
temperature. If now the calculated junction temperature is below the maximum allowed
value TJmax the ISO current is the value from the last iterative step. If the junction
temperature is above the maximum allowed value then the ISO current is calculated from
the maximum allowed power loss at TJmax.
The nominal load current is calculated in a similar way, as shown in Figure 3. After the
temperature dependent RDS(on) is calculated two currents will be taken into account. The
first current is the value coming from 0.5V voltage drop. The second value is given by
the maximal power loss at the given Junction temperature. The lowest of this two values
is chosen as nominal load current ID(Nom). Via the on-state-losses the new junction
temperature TJ is calculated. This process is repeated until there is no more change in
the junction temperature.
The parameter Ptot is also a calculated value. It shows the maximum allowed on state
loss for a constant Case temperature TC. Therefore the thermal resistance RTHJC is used.
The formula is
T JCmax – T C
P tot = ----------------------------R thJC
Application Note ANPS073E
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V1.0, 2004-07-07
ISO-Nominal Load Current and Power Dissipation
Computation
TJ (n ) = Tambient
TJ ( n −1) = TJ (n )
RDS ( on ) = f (TJ (n ) )
I D ( ISO ) =
0.5V
RDS ( on)
2
Pon = I DISO ⋅ RDS ( on )
TJ (n ) = Tambient + Pon ⋅ RthJC
No
TJ (n ) = TJ ( n −1) ?
Yes
Yes
TJ (n ) < TJ max ?
No
I D ( ISO )
Figure 2
I D ( ISO ) =
TJ max − Tambient
RthJC ⋅ RDS (on ) (TJ max )
IDiso.emf
Iterative computation process for ID(ISO) current
Application Note ANPS073E
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V1.0, 2004-07-07
ISO-Nominal Load Current and Power Dissipation
Computation
TJ ( n ) = Tambient
TJ ( n −1) = TJ (n )
RDS ( on ) = f (TJ (n ) )
I1 =
I2 =
0.5V
RDS ( on )
TJ max − Tambient
RthJA ⋅ RDS (on )
I DNom = Min(I1 , I 2 )
2
Pon = I DNom ⋅ RDS ( on )
TJ (n ) = Tambient + Pon ⋅ RthJA
No
TJ ( n ) = TJ ( n −1) ?
Yes
I DNom
IDnom.emf
Figure 3
Iterative computation process for Nominal load current ID(Nom)
Application Note ANPS073E
6
V1.0, 2004-07-07
ISO-Nominal Load Current and Power Dissipation
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2004-07-07
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Application Note ANPS073E
7
V1.0, 2004-07-07
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