HL
Application Note
BSP 76, BSP 77, BSP 78,
BTS 118D, BTS 134D, BTS 142 D1
The second generation of the HITFET® family offers low RDS(on)
and restart function in the SOT223 and D-Pak with low-input current drive
Benno Köppl
Introduction
The age of Siemens Smart Power low-side switches began with the introduction of the
®
TEMPFET (TEMperature Protected FET). By integrating further protection functions such
as overvoltage and ESD protection and du/dt limiting and combining these with the familiar
TEMPFET® chip-on-chip technology, the HITFET® family of switches with 18 mΩ through
100 mΩ and 60V VDS clamping voltage represents a logical further development in intelligent single-channel low-side switches.
A distinguishing feature of the second generation of the HITFET® family is the new Ssmart technology that has enabled a 35% reduction in chip surface area while retaining
the RDS(on). The HITFET® range offered until now is being extended in the 28 mΩ 200 mΩ range with 42V VDS clamping voltage. Switches in SOT223 and D-Pak formats
are being offered as package variants.
In view of the fact that the main area of application will be 12V onboard networks in automobiles, the drain source voltage range has been set to 42V. This has enabled low RDS(on)
values to be achieved in small packages.
During the development of the second generation, particular focus was laid on low input
currents in all operating states to minimize the demands made on any µ-controller that
may be used for drive purposes. As a result it has been possible to reduce the input currents from a previous maximum of 6 mA to a maximum of 300 µA on thermal shut down.
The protection functions implemented as latch type in the first generation HITFET® - thermal overload protection and short-circuit protection - have been implemented as an automatic restart type in the second generation.
1
The BSP75/A device is also a member of the HITFET 2 family and has mainly the same functionality. As it
is based on a different technolgy, it shows different behavoir in details. A Appnote for BSP75 is available, too.
APPHIT2.doc
1
HL LH TM 2 - 10/98
B. Köppl
Drain
Overvoltage
prot .
Gate
Gate drive
Power
Input
transistor
Drain
Short
circuit
prot .
ESD
Overtemp.
prot .
Current
limit.
Source/
ground
Figure 1 Block diagram of the new HITFET® family
The advantages of the first generation, such as overvoltage and ESD protection, edge
steepness limiting for the input voltage and current limitation, have been retained.
The analog drive option and the logic level type have also been incorporated.
Figure 1 shows a block diagram to illustrate the structure of the new HITFET® family.
®
®
Table 1 provides an overview of the new HITFET family. The entire HITFET family is
illustrated in the Appendix.
Type
BSP 76
BSP 77
BSP 78
BTS 118
BTS 134
BTS 142
VDS(AZ)
[V]
42
42
42
42
42
42
RDS (ON)
[mΩ]
200
100
50
100
50
28
ID ISO (min)
[A]
1.4
2.2
3
2.4
3.3
4.6
ID (lim) min
[A]
5
10
18
10
18
35
Package
SOT 223
SOT 223
SOT 223
D-Pak
D-Pak
D-Pak
Table 1 Overview of the second generation single-channel HITFET® family
1 Protection functions
The protection functions of the new HITFET® comprise protection against short circuits,
overload, overtemperature and overvoltage. In addition, it has very good ESD characteristics.
1.1 Overload, short-circuit and overtemperature protection
Overload protection, which includes both short-circuit and overtemperature protection, is
implemented as a multi-stage function in the first generation HITFET®. This means that if
the internal current limiting value ID(lim) is exceeded, the Mosfet is not deactivated immedi-
APPHIT2.doc
2
HL LH TM 2 - 10/98
B. Köppl
ately. Instead, the current is limited to ID(lim). The component is operated in the analog
range of the curve, leading to an increase in the drain source voltage. This operating state
results in an increase in chip temperature. To prevent the maximum junction temperature
from being exceeded, the Mosfet is switched off by means of a temperature sensor. This
enables the component to protect itself.
In principle, this behavior also applies to the second generation HITFET®. However, a very
high current limiting value has been selected in relation to the nominal current to avoid
activation of the current limiting function when loads with a capacitive effect, such as filament lamps, are switched on. This makes it possible to switch on the load without delay.
For example, a conventional 21W lamp for automotive applications can be switched with a
BSP 78, without affecting its turn-on characteristics, in order to implement a rapid-reaction
brake light.
VGS
ID (5A/div)
VDS
Figure 2 Switching on a 21W lamp with BSP78 at room temperature
1.2 Short-circuit protection
If the Hitfet is switched on with an existing short circuit, the voltage increases above drain
source. If this exceeds 2.5V, current limiting is activated. This operating mode results in a
rapid temperature increase (dependent on cooling). If a chip temperature of 175°C (typ.) is
reached, the component will deactivate itself. In contrast to the first generation HITFET®,
the new HITFET® is not implemented as a "latch type" but has restart functionality on
cooling. The temperature hysteresis is typically 10 K. Figure 3 illustrates this behavior
when switching on with an existing short circuit.
APPHIT2.doc
3
HL LH TM 2 - 10/98
B. Köppl
Figure 3 also shows that the current limiting function is temperature-dependent. The current increases initially to IL(SCp). This value corresponds to the current limiting value at a
junction temperature (Tj) slightly higher than room temperature. The high power dissipation in the chip causes a rapid increase in chip temperature and the current limiting value
approaches the value for Tj=175°C. If the chip reaches 175°C, it is deactivated. The chip
then begins to cool down. It is reactivated with a typical hysteresis of 10°C. The current
always sets itself to the current limiting value at a junction temperature of approximately
165°C (IL(SCr)). The time between switch-off and restart is mainly dependent on the cooling
of the device.
VGS
ID (10A/div)
VDS
Figure 3 Activation of a BSP78 to an existing short circuit;
Temperature behavior of current limitation (dependent on cooling)
If a short circuit occurs during operation, the current increases, starting at the value of the
load current previously flowing, until activation of the current limiting circuit at VDS>2.5V.
Figure 4 illustrates this behavior at a battery voltage of 14V. Due to the extremely high
power dissipation and rapid temperature rise in the chip, this case shows clearly the dependency of the current limiting function on temperature. The second-generation HITFET®
is short-circuit proof across the entire operating voltage range.
To ensure that overtemperature protection works properly in all operating ranges, the
protection logic is supplied via the input voltage VIN.
APPHIT2.doc
4
HL LH TM 2 - 10/98
B. Köppl
The supply threshold for the protection logic is by design lower than the threshold voltage
of the DMOS. This ensures that the protection function will work effectively before the
Mosfet switches on.
VGS
ID
VDS
Figure 4 Short circuit during normal operation
Note: Because the current limiting is not active at V DS < 2.5V, theoretically an operating
mode is possible in which short-circuit currents exceed the specified values. In practice,
however, at a supply voltage of less than 2.5V, the HITFET pure resistance limits the currents to a tolerable level.
Overtemperature protection is also active in these cases and deactivates the device
promptly, ensuring full short-circuit protection in this voltage range as well.
1.3 Switching inductive loads - overvoltage protection
®
For operation of inductive loads such as coils, relays and motors, the HITFET provides
active Zener clamping between drain and gate, in order to protect the component against
overvoltages.
If a voltage of typically 47 V is exceeded when inductive loads are switched off, the Zener
diode chain starts to conduct and activates the gate. In this case the possible inductiv
switch off energy is many times higher than for avalanche breakdown. Figure 5 shows the
switching of a solenoid valve with a clearly visible switch point (change in inductance).
The active Zener clamping with UDS(AZ) > 42 V makes it possible to achieve fast switch-off
times with a high di/dt without risk to the component.
APPHIT2.doc
5
HL LH TM 2 - 10/98
B. Köppl
VGS
ID
VDS
Figure 5 Switching a solenoid valve with a BSP78
Application example:
Inductive overvoltages typically occur
when a relay is switched off. If the
HITFET® is used as a relay driver, it is
not necessary to use a freewheeling
diode, as VDS is limited within the component.
This overvoltage protection requires no
supply voltage.
VBB
D
IN
HITFET
S
Figure 6 HITFET® as a relay driver
APPHIT2.doc
6
HL LH TM 2 - 10/98
B. Köppl
1.4 ESD protection
The HITFET® family offers protection against electrostatic discharge (ESD) greater than
2 kV for every pin combination. This significantly reduces the risk to components during
handling.
APPHIT2.doc
7
HL LH TM 2 - 10/98
B. Köppl
2 Differences between HITFET® generations 1 and 2
The following table describes the differences between the two generations.
HITFET® 1st generation
HITFET® 2nd generation
RDS(on) range
100 - 18 mOhm
200 - 28 mOhm
Package
TO 220-3/-5
Structure
Chip-on-chip
SOT 223
D-Pak
Monolithic
Maximum current consumption at input (protection
mode)
Maximum current consumption at input (normal operation)
Overtemperature protection
< 6.0 mA
< 300 µA
< 6.0 mA
< 300 µA
Latch function
VDS
60 V
Restart function with 10K
hysteresis
42 V
Current limiting ID(lim)
1.4 to 3x ID(ISO)
2.3 to 5.3x ID(ISO)
Current sense
available (BTS 9xx)
not available
Adjustable current limitation
Technology
available (BTS 9xx)
m-Smart
not available
S-Smart
3 Areas of application for the 2nd-generation HITFET®
Its comprehensive protection functions make the HITFET® 2 suitable for many different
areas of application and loads such as relays, signal lamps, motor bridges, valves, etc.
The following characteristics above all qualify it for the rugged automotive world:
• Full functionality up to Tj = 150 °C in normal operation and in overload conditions (operation with current limiting)
• No minimum supply voltage required for the protection functions to work
• Good ESD characteristics
• Small package formats combined with low RDS(on)
The high current limiting capability makes the HITFET2 particularly suitable for delay-free
switching of lamps (e.g. BSP78 for 21 W brake lights).
Where the HITFET® is used as a high-side switch, it is merely necessary to ensure that
the input-source protection circuitry is protected against currents > 2mA if 10V Vin is exceeded. The simplest protection in this case is to include a resistance in the supply to the
input.
APPHIT2.doc
8
HL LH TM 2 - 10/98
B. Köppl
4 Appendix
Ron in mÒ
500
500
BSP 75
200
200
BSP 76
Load
Low Side
Overview of the HITFET® family
100
100
BTS 117
BTS 917
BSP 77
BTS 118
50
50
BTS 133
BTS 933
BSP 78
BTS 134
28
28
BTS 141
BTS 941
18
18
BTS 149
BTS 949
BTS 142
13
13
77
HITFET11
HITFET
TO220
TO220
pins
33pins
SOT223
223
SOT
HITFET22
HITFET
D-Pak
D-Pak
pins
33pins
pins
55pins
Power Semiconductors
HL PS PM 2
08/98
Smart Power Switches
Remark:
This information describes the type of component and shall not be considered as assured
charcteristics.
APPHIT2.doc
9
HL LH TM 2 - 10/98
B. Köppl