Short circuit behaviour of the Speed TEMPFET®

Automotive Power Semiconductors
Application
Note
Short circuit behaviour of the Speed TEMPFET®
family
by Benno Koeppl, Jürgen Kositza, Christian Arndt and Jean-Philippe Boeschlin
1. Introduction
The era of the Infineon’s Smart Power Low Side switches began with the introduction of
the TEMPFET®.
TEMPFET® devices are temperature protected FET-switches. They can be controlled as
any discrete FET, but in addition they implement a temperature protection.
Characteristic for TEMPFET® devices is the temperature sensor. This sensor is mounted
on top of a standard MOSFET. Although it is electrically isolated to the discrete FET, it is
thermally well coupled. This concept enables the sensor to measure the real junction
temperature of the MOSFET and hence to protect the FET against overtemperature.
The temperature sensor has the behaviour of a thyristor. It fires at overtemperature.
In the classic version of the TEMPFET®, the temperature sensor is connected already
internally in-between gate and source of the MOSFET. In case of an overtemperature it
simply shorts gate and source and hence switches the MOSFET off. Since this concept
requires a serial gate resistor of about 3kOhm in order to protect the internal thyristor, the
classic TEMPFET® devices are restricted to applications with slower switching times.
With the new Speed-TEMPFET® family the temperature sensor is externally available.
This enables the user of a Speed TEMPFET® on the one hand side to define, how the
Speed TEMPFET® should react at overtemperature, on the other hand it extends the
application area for temperature protected FET switches towards fast-switching PWM
applications.
To ensure short circuit protection for classic TEMPEFT® devices, the maximum gate to
source voltage has to be limited. Individual voltage values for short circuit protection are
given in the TEMPFET® datasheet.
For the Speed-TEMPFET family this information is not given in the data sheets.
Therefore this application note will give a more detailed information, how short circuit
protection of Speed-TEMPFET® devices can be achieved.
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2. The short circuit protection of TEMPFET ® devices
2.1. The structure of a TEMPFET®
Both, TEMPFET® and Speed-TEMPFET® have the same basic superstructure.
In general a standard MOSFET serves as a power stage and is used as a base chip.
Another small chip serves as temperature sensor and is mounted on top of the MOSFET
base chip (Fig. 1).
top chip
base chip
Fig. 1: basic superstructure of the TEMPFET ®
The temperature sensor can be understood as a thyristor. It is non conductive at normal
temperatures but becomes conductive at overtemperatures.
In the event of an overtemperature the sensor simply shorts the gate and the source of a
classic TEMPFET® and therefore switches the TEMPFET® off. In the case of a Speed
TEMPFET® the digital overtemperature signal will be available for further processing at
two output pins. Overtemperature will not explicitly lead to a switch off of the Speed
TEMPFET®, unless the external connection is realised in the necessary way.
A detailed description of this overtemperature functionality is given in the application note
„Temperature protection concept – Speed TEMPFET®“.
2.2. Short circuit protection of classic TEMPFET® devices
In general short circuit protection of the TEMPFET® is based on overtemperature
detection.
In the event of a short circuit, the MOSFET consumes most of the power supply voltage.
The short circuit current that flows is usually limited by the transfer characteristic of the
MOSFET. Therefore the overall power dissipation in the MOSFET is very high. This leads
to a rapid rise of the base chip temperature. The temperature of the sensor follows this
rapid rise with a small delay due the thermal capacitance of the top chip.
In order to limit the maximum power dissipation during a short circuit and hence enable
the top chip temperature to follow the base chip temperature in time, the gate source
voltage of the TEMPFET® has to be limited via an external Zener diode (ZD1 in figure 2).
Vbb
Load
µC
D
RGate
G
ZD1
S
GND
Fig. 2: External wiring of a Classic TEMPFET ®
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If the short circuit current had not been limited via the gate voltage, such high power
losses would have occurred in the base chip that in worst case situations the top chip
would not have responded fast enough. This might eventually result in an already thermally
destroyed base chip before the temperature sensor is even able to sense
overtemperature.
In order to ensure full short circuit protection of the classic TEMPFET® devices the
maximum gate voltage has to be limited. Two diagrams provide the required information
in the data sheets. „Short-circuit protection“ and „Max. gate voltage“.
The „Max. gate voltage“ diagram shows the maximum allowed gate voltage over the
maximum drain source voltage were short circuit protection is ensured. This value is the
voltage rating of the Zener diode ZD1 to be implemented.
Figure 3: Example „Short circuit protection“ and „Max. gate voltage“ diagram
The output characteristic of the MOSFET is specified in the „Short circuit protection“
diagram. The diagram shows the drain current in the event of a short circuit depending on
the drain source voltage and the applied gate voltage.
Since the drain current has its maximum values at low temperatures, the illustrated graph
applies for the minimum temperature (here –55°C).
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2.3. Short circuit detection of Speed-TEMPFET® devices
As with the TEMPFET® devices short circuit detection for Speed TEMPFET® devices is
based on overtemperature detection. To achieve short circuit detection and protection the
temperature sensor has to be monitored.
On account of the Speed TEMPFET® concept it is necessary for the user to switch off the
TEMPFET within 1 ms after receiving the overtemperature signal.
Speed TEMPFET® devices are in general not self-protective. Short circuit protection has
to be realised by the external circuitry and logic. A typical application is shown in fig. 4
+5V
Vbb
Load
µC
in/out
out
D
RGate
G
ZD1
S
GND
Fig. 4: Speed-TEMPFET concept
Speed TEMPFET® devices use for D-Mos power stage the technology S-FET. Since the
RDS(on) is much lower compared to the technology used in classic TEMPFET® devices
much higher current densities will occur in the event of a short circuit.
These higher current densities lead to a faster warm up rate in case of a short circuit.
In order to allow the top chip to follow the base chip temperature in time, the gate voltage
has to be limited again. These limits have to be much smaller compared to the limits of
similar RDS(on) rated classic TEMPFET® devices.
3. Short-circuit protection of Speed-TEMPFET ® devices
In this section the short-circuit behaviour of the Speed-TEMPFET® family is described
and dimensioning hints are given.
3.1. Influence of temperature on the short-circuit behaviour
In general there are two major temperature influences on the short-circuit behaviour.
On the one hand side the output characteristic depends on the temperature. On the other
hand side the start temperature has effects on how fast the sensor will react. A low
starting temperature requires a high thermal energy to be transmitted to the top sensor
than a high starting temperature.
Because of both influences the maximum permissible warm up rate must be determined
at the lowest possible operating temperature. This generates a maximum permissible
gate voltage which is valid for the entire temperature range.
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3.2. Parameter for the Speed-TEMPFET® devices
To realise short circuit protection for Speed TEMPFET® devices the Gate voltage has to
be limited. In this section diagrams are shown, that ensure full short circuit protection.
These diagrams are valid for junction temperatures of Tj = -40 ... 175°C.
If the remaining overall short circuit impedance is that high, that a current limitation due to
the FET does not occur at all, the allowed maximum gate source voltage can be much
higher provided operation in the safe operating area is ensured.
3.2.1. BTS247Z
Max. gate voltage
3.4
Max. Vgs [V]
3.2
3
2.8
2.6
2.4
2.2
2
12
13
14
15
16
17
18
19
20
Vds [V]
Short-circuit protection ISC=f(VDS)
80
VGS= 4.50V
70
VGS= 4.25V
ISC [A]
60
VGS= 4.00V
50
VGS= 3.75V
40
VGS= 3.50V
30
VGS= 3.25V
VGS= 3.00V
20
VGS= 2.75V
10
0
0
1
2
3
4
5
6
7
8
VDS [V]
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3.2.2. BTS244Z
Max. gate voltage
3.2
Max Vgs [V]
3
2.8
2.6
2.4
2.2
2
10
11
12
13
14
15
16
17
18
19
Vds [V]
Short-circuit protection ISC=f(VDS)
100
VGS= 4.25V
90
80
VGS= 4.00V
ISC [A]
70
VGS= 3.75V
60
VGS= 3.50V
50
VGS= 3.25V
40
VGS= 3.00V
30
VGS= 2.75V
20
10
0
0
1
2
3
4
5
6
7
VDS [V]
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3.2.3. BTS282Z
Max. gate voltage
2.75
2.65
Max Vgs [V]
2.55
2.45
2.35
2.25
2.15
2.05
1.95
1.85
1.75
10
12
14
16
18
20
Vds [V]
Short-circuit protection ISC=f(V DS)
120
VGS= 3.75V
100
VGS= 3.50V
I SC [A]
80
VGS= 3.25V
60
VGS= 3.00V
40
VGS= 2.75V
20
0
0
1
2
3
4
5
6
VDS [V]
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3.3. Short-circuit protection exploiting the full RDS(on)
Since limiting of the gate source voltage results in a deterioration of the minimum RDS(on)
of the implemented MOSFET, a solution which exploits the full RDS(on) of the MOSFET is
often demanded.
A possible solution were the optimum RDS(on) will be sustained and full short circuit
protection will be achieved is shown in fig.5.
In this circuit the gate voltage limiting Zener diode D2 is only active as long as the drain
source voltage of the Speed-TEMPFET® exceeds a voltage of about 2V. Since high
currents cause high drain source voltages the Zener diode D2 will be activated in the
event of a short-circuit. By doing so the current will be limited and hence the warm up rate
will be limited to acceptable values.
To explain the function of this circuit in more detail, a switch on procedure of a SpeedTEMPFET® is described next.
Initially the input voltage V1 is equal to 0V. Therefore the drain source voltage V DS of the
Speed TEMPFET® T1 is equal to V bb.
If V1 rises to 5V, the gate of the Speed TEMPFET® T1 is triggered via R2. The Speed
TEMPFET® T1 switches on within a switching time of ton. If R1 and C1 are chosen in that
way, that their time constant R1*C1 is much greater than ton, the Zener diode D2 will not
be active during switching on.
If now a short-circuit occurs via the load, the drain source voltage V DS of T1 rises and the
diode D1 inhibits. As a result C1 will become charged via R1. The transistor T2 switches
on as soon as the threshold voltage of T2 is exceeded. After this automatically the Zener
diode D2 activates and hence limits V GS of the Speed TEMPFET® T1 and thus the shortcircuit current through T1.
The power losses which then occur lead to an activation of the temperature sensor and
the user has to switch off again within 1 ms.
Vbb
V1
Load
D1
R1
D
R2
T1
G
D2
C1
T2
S
GND
Fig. 5: circuit example with Speed-TEMPFET sustaining full R
DS(on)
Remark: special attention has to be paid to the voltage range of the Zener diode and
especially to the maximum Zener voltage at the given current as it represents the
maximum allowed Vgs value in case of a short circuit.
General note:
The information given in this report describes a type of components. It shall not be considered as assured
characteristics.
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