DN21

Design Note 21
Issue 2 October 1995
Design Note 21
Issue 2 October 1995
Zetex E-Line transistors are optimised to
possess a very low VCE(sat), and are
therefore much more efficient in high
current switching applications than
conventional power products.
Automotive Alarm Siren Driver
2A DC Rated Super-ß Transistors Provide Reduced Component
Count ’H’ Bridge
Introduction
Background
The majority of modern automotive
alarm sirens employ a moving coil
lou dspe aker , that is driv en by a n
’H’-bridge output stage. This output driver
circuit is commonly effected with
T O - 1 2 6 / T O- 2 2 0 p a c k a g e d p o w e r
transistors, as the currents involved and
the resulting power dissipation are both
in excess of common smaller package
products. Zetex Super E-Line (TO-92
style) transistors however, due to their
superior device geometries and silicone
packaging, can, and have been used in
this application to provide a minimal drive
circuitry topology, and a size, cost and
performance advantage.
The circuit diagrams presented in Figures
1 and 2, show two common methods of
effecting a siren driver circuit. These
circuits include collector-emitter (C-E)
diodes to protect the transistor in the
event of negative rail excursions, due to
i n d u c ti ve lo ad tr ansients, an d
base-emitter (B-E) resistors to provide a
path for any transistor leakage current.
Th is latter point is an impor tant
consideration when using the usual
TO-126/TO-220 power products, as their
high VCE(sat) can lead to appreciable
temperature rises.
important differences to note:
i) As the devices employ an efficient
matrix die geometry, they produce
low VCE(sat) values, this leads to very
low device dissipation, and therefore
low temperature rise. (Viz. ZTX690B:
VCE(sat) = 0.3V at IC=2A, IB =20mA and
ZTX790A: VCE(sat) = 0.4V at IC=2A, IB
=50mA, please consult ZTX688BZTX696B and ZTX788B - ZTX796A
series datasheets). This can be very
important in automotive applications,
where the operating environment can
reach high ambient temperatures. It
follows that in this case, the usual
base-emitter resistors employed to
aid turn-off, and to divert leakage
currents at high junction temperature,
are no longer required.
Some siren manufacturers use TO-220
packaged Darlington power transistors.
While this can reduce the component
count, (though can also increase the
component cost), as the C-E diodes are
sometimes integral to the Darlington, the
intrinsic high on-state voltage of the
Darlington configuration increases
device dissipation. Consequently the
circuit requires B-E resistors, and also
reduces the power delivered to the load.
Circuit description
The circuit shown in Figure 3 has been
de vised using Ze tex Super E-Line
transistors. It is basically a modified
version of figure 2. There are two
+
ZTX
790A
ZTX
790A
ZTX
690B
ZTX
690B
+
+
ii) The Zetex devices chosen, possess a
reasonable hFE, (strictly termed hFC)
when used in the inverted mode, Eg.
with the collector/emitter terminals
reversed. Typical peak values being
125 for the ZTX690B and 100 for the
ZTX790A . This feature means that
they are still capable of operating
when subjected to reverse transients,
such as when driving an inductive
load, and therefore do not always
require protection diodes. For the
siren driver application, the C-E diodes
may be confidently omitted.
O/P
O/P
O/P
_
_
Figure 1.
Conventional Siren Driver Circuit using
Separate Pre-driver Transistors.
_
Figure 2.
Conventional Siren Driver Circuit using
Cross-coupled Darlington Pre-driver
Configuration.
DN21-1
Figure 3.
ZETEX Reduced Component Count
’H’-Bridge Circuit.
DN21-2
Design Note 21
Issue 2 October 1995
Design Note 21
Issue 2 October 1995
Zetex E-Line transistors are optimised to
possess a very low VCE(sat), and are
therefore much more efficient in high
current switching applications than
conventional power products.
Automotive Alarm Siren Driver
2A DC Rated Super-ß Transistors Provide Reduced Component
Count ’H’ Bridge
Introduction
Background
The majority of modern automotive
alarm sirens employ a moving coil
lou dspe aker , that is driv en by a n
’H’-bridge output stage. This output driver
circuit is commonly effected with
T O - 1 2 6 / T O- 2 2 0 p a c k a g e d p o w e r
transistors, as the currents involved and
the resulting power dissipation are both
in excess of common smaller package
products. Zetex Super E-Line (TO-92
style) transistors however, due to their
superior device geometries and silicone
packaging, can, and have been used in
this application to provide a minimal drive
circuitry topology, and a size, cost and
performance advantage.
The circuit diagrams presented in Figures
1 and 2, show two common methods of
effecting a siren driver circuit. These
circuits include collector-emitter (C-E)
diodes to protect the transistor in the
event of negative rail excursions, due to
i n d u c tive lo ad tr ansients, an d
base-emitter (B-E) resistors to provide a
path for any transistor leakage current.
Th is latter point is an impor tant
consideration when using the usual
TO-126/TO-220 power products, as their
high VCE(sat) can lead to appreciable
temperature rises.
important differences to note:
i) As the devices employ an efficient
matrix die geometry, they produce
low VCE(sat) values, this leads to very
low device dissipation, and therefore
low temperature rise. (Viz. ZTX690B:
VCE(sat) = 0.3V at IC=2A, IB =20mA and
ZTX790A: VCE(sat) = 0.4V at IC=2A, IB
=50mA, please consult ZTX688BZTX696B and ZTX788B - ZTX796A
series datasheets). This can be very
important in automotive applications,
where the operating environment can
reach high ambient temperatures. It
follows that in this case, the usual
base-emitter resistors employed to
aid turn-off, and to divert leakage
currents at high junction temperature,
are no longer required.
Some siren manufacturers use TO-220
packaged Darlington power transistors.
While this can reduce the component
count, (though can also increase the
component cost), as the C-E diodes are
sometimes integral to the Darlington, the
intrinsic high on-state voltage of the
Darlington configuration increases
device dissipation. Consequently the
circuit requires B-E resistors, and also
reduces the power delivered to the load.
Circuit description
The circuit shown in Figure 3 has been
de vised using Ze tex Super E-Line
transistors. It is basically a modified
version of figure 2. There are two
+
ZTX
790A
ZTX
790A
ZTX
690B
ZTX
690B
+
+
ii) The Zetex devices chosen, possess a
reasonable hFE, (strictly termed hFC)
when used in the inverted mode, Eg.
with the collector/emitter terminals
reversed. Typical peak values being
125 for the ZTX690B and 100 for the
ZTX790A . This feature means that
they are still capable of operating
when subjected to reverse transients,
such as when driving an inductive
load, and therefore do not always
require protection diodes. For the
siren driver application, the C-E diodes
may be confidently omitted.
O/P
O/P
O/P
_
_
Figure 1.
Conventional Siren Driver Circuit using
Separate Pre-driver Transistors.
_
Figure 2.
Conventional Siren Driver Circuit using
Cross-coupled Darlington Pre-driver
Configuration.
DN21-1
Figure 3.
ZETEX Reduced Component Count
’H’-Bridge Circuit.
DN21-2
Design Note 21
Issue 2 October 1995
Design Note 21
Issue 2 October 1995
Example Circuit Specification
+12V
ZTX
790A
Q9
ZTX
790A
C6
470µ
Q10
R7
680
R3
27K
R2
27K
R1
680
R10 R6
560 47K
R5 R4
10K 4K7
R14
330
R13
330
C4
10µ
C3
0.1µ
Q6
Supply Voltage Range
2 – 25V
Output Stage Efficiency
98.5%
6 Ohm moving coil load
Average VCE(sat) =0.1V
R12
3K3
Vsupply =14.2V
C1
0.1u
D4
Output
C2
10µ
Q1
I(dc) =1.2A
Q4
D3
ZTX
690B
Q8
Q5
ZTX
690B
Q2
R9
5K6
D1
R8
5K6
Q3
D2
Q7
R11
3K3
0V
'H' BRIDGE OUTPUT STAGE
Figure 4 .
Example Circuit (Note 1).
Output Stage Efficiency
91.5%
6 Ohm Resistive load
Average VCE(sat) =0.55V
C5
47µ
Vsupply =14.2V
I(dc) =2.12A
SIGNAL GENERATION CIRCUITRY
Q1 - Q6 MPS2222A
Q7 - Q8 ZTX690B
The modified ’H’- bridge siren driver
stage has been incorporated into a
complete siren driver circuit, in order to
demonstrate the capability of Zetex
Super E-Line transistors. The
demonstration circuit is shown in figure
4, and consists of two sections; the
signal generation circuitry, and the ’H’
bridge output stage. The former
comprises a slow running astable
multivibrator to generate a ramp signal,
that is then used to modulate a second
multivibrator, causing it to sweep over a
range of frequencies. This section drives
the ‘H’-bridge output stage described
previously.
While the multivibrator method is valid,
(some manufacturers use a similar
circuit, while others favour op-amp
derived circuits, or even small
micro-controllers - see Note 2), there
can be a problem with cross-conduction,
Eg. Simultaneous conduction by both
Load dependent; 14.2V nominal
Q9 - Q10 ZTX790A
D1 - D4 1N4148
NPN and PNP devices during
switch-over, thus leading to excessive
power dissipation. To prevent this, the
output stage on the demonstration
circuit has two additional signal diodes,
D3 and D4. These components inhibit
the driver transistors Q5 and Q6, until
the opposing half of the ‘H’-bridge has
started to switch off. More elaborate
signal generators incorporate delays
between changes of output state, (using
an extra gate or software) so would not
require these extra diodes.
The circuit has been designed assuming
a load impedance of 6 Ohms; a common
value for the drive units used. Lower
loads can be driven but may require
either a change to the bias resistors,
R13 /14, to provide sufficient base
current, or for very low impedance
loads, a change of output transistors:the ZTX851/ZTX951 with a continuous
current rating of 5 Amps and 4 Amps
respectively are eminently suitable.
DN21-3
Junction Temperature Rise
19°C
6 Ohm moving coil load
Control ramp
1 to 9V, 4Hz
Vsupply =14.2V
Swept frequency range
870Hz – 1.9kHz
Vsupply =14.2V
Vsupply =14.2V
Note 1:
D3 and D4 are not required for signal generation circuits possessing output/output
deadtime.
Note 2:
ZETEX now offer an IC solution for the signal generation circuitry - namely the ZSD100.
DN21-4
Design Note 21
Issue 2 October 1995
Design Note 21
Issue 2 October 1995
Example Circuit Specification
+12V
ZTX
790A
Q9
ZTX
790A
C6
470µ
Q10
R7
680
R3
27K
R2
27K
R1
680
R10 R6
560 47K
R5 R4
10K 4K7
R14
330
R13
330
C4
10µ
C3
0.1µ
Q6
Supply Voltage Range
2 – 25V
Output Stage Efficiency
98.5%
6 Ohm moving coil load
Average VCE(sat) =0.1V
R12
3K3
Vsupply =14.2V
C1
0.1u
D4
Output
C2
10µ
Q1
I(dc) =1.2A
Q4
D3
ZTX
690B
Q8
Q5
ZTX
690B
Q2
R9
5K6
D1
R8
5K6
Q3
D2
Q7
R11
3K3
0V
'H' BRIDGE OUTPUT STAGE
Figure 4 .
Example Circuit (Note 1).
Output Stage Efficiency
91.5%
6 Ohm Resistive load
Average VCE(sat) =0.55V
C5
47µ
Vsupply =14.2V
I(dc) =2.12A
SIGNAL GENERATION CIRCUITRY
Q1 - Q6 MPS2222A
Q7 - Q8 ZTX690B
The modified ’H’- bridge siren driver
stage has been incorporated into a
complete siren driver circuit, in order to
demonstrate the capability of Zetex
Super E-Line transistors. The
demonstration circuit is shown in figure
4, and consists of two sections; the
signal generation circuitry, and the ’H’
bridge output stage. The former
comprises a slow running astable
multivibrator to generate a ramp signal,
that is then used to modulate a second
multivibrator, causing it to sweep over a
range of frequencies. This section drives
the ‘H’-bridge output stage described
previously.
While the multivibrator method is valid,
(some manufacturers use a similar
circuit, while others favour op-amp
derived circuits, or even small
micro-controllers - see Note 2), there
can be a problem with cross-conduction,
Eg. Simultaneous conduction by both
Load dependent; 14.2V nominal
Q9 - Q10 ZTX790A
D1 - D4 1N4148
NPN and PNP devices during
switch-over, thus leading to excessive
power dissipation. To prevent this, the
output stage on the demonstration
circuit has two additional signal diodes,
D3 and D4. These components inhibit
the driver transistors Q5 and Q6, until
the opposing half of the ‘H’-bridge has
started to switch off. More elaborate
signal generators incorporate delays
between changes of output state, (using
an extra gate or software) so would not
require these extra diodes.
The circuit has been designed assuming
a load impedance of 6 Ohms; a common
value for the drive units used. Lower
loads can be driven but may require
either a change to the bias resistors,
R13 /14, to provide sufficient base
current, or for very low impedance
loads, a change of output transistors:the ZTX851/ZTX951 with a continuous
current rating of 5 Amps and 4 Amps
respectively are eminently suitable.
DN21-3
Junction Temperature Rise
19°C
6 Ohm moving coil load
Control ramp
1 to 9V, 4Hz
Vsupply =14.2V
Swept frequency range
870Hz – 1.9kHz
Vsupply =14.2V
Vsupply =14.2V
Note 1:
D3 and D4 are not required for signal generation circuits possessing output/output
deadtime.
Note 2:
ZETEX now offer an IC solution for the signal generation circuitry - namely the ZSD100.
DN21-4
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