Download 3 Electronic Switches

Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
3
EL III
Electronic Switches
In the digital circuit-technology are used diodes and transistors as electronic switches.
Switching elements have a nonlinear characteristic, e.g. a nonlinear resistor (EBE-03001).
NLW
I
I
U
I = g(U)
U = f(I)
0
U
Prof. Dr. F. Schubert
University of
Applied Sciences
Hamburg
NONLINEAR RESISTOR
EBE-03001
The characteristic of a nonlinear element can be approximated by straight lines. In this case
by constant resistors. The actual effective resistor is a function of the voltage. So one gets an
equivalent circuit, that consists of resistors , a switch with one position for every resistor and
voltage sources. In this lecture every electronic switch will be reduced to a simple equivalent
circuit, containing only resistors, switches and voltage sources. So a calculation of the
behavior will be very simple.
For the technological realization the following parameters are of interest:
-
Switch-resistor in the ON- and OFF-state (RON and ROFF),
switching time and propagation time,
allowed signal-levels (current and voltage),
control of the switch (powerless or not, potential-free) and
price of the circuit.
These parameters are discussed in the following for the different technically realizable
switching elements.
3.1
PN- and Schottky-diodes
The simplest electronic switch is the semiconductor-diode. For diodes with PN- and Schottkytransitions the behavior is shown. With help of diodes combinatorical logic can be
implemented how AND and OR. These basic-circuits are used frequently at TTL-circuits.
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Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
EL III
3.1.1 Static behavior of diodes
Starting of the ideal characteristic of a diode the courses of voltage shown in the EBE-03101.
U (t)
I (t)
U
D
U (t)
R
2
U (t)
A
0
t
U1
I
U
U (t)
A
U
I
U
Prof. Dr. F. Schubert
University of
Applied Sciences
Hamburg
2
0
IDEAL DIODE AS SWITCH
t
EBE-03101
Through the valve-effect of the diode only the positive parts of the generator-voltage U(t)
appear at the load-resistor R. The negative parts of the generator-voltage with the amplitude
of U1 fall off at the closed diode.
Real semiconductor-diodes originate through diffusion of p- and n-doped areas (PN-diode) or
through a metal/ semiconductor-transition. As semiconductor material today is used Silicium.
In the past Germanium because of the low conducting voltage was used also.
Germanium diodes are manufactured today no longer, because of their high reverse current as
well as low reverse voltage and because of the technology has become obsolete. For superfast circuits today is used doped Galliumarsenid as diode-material. There can be produced
PN-diodes and Schottky-diodes.
Real diodes show final values for the conducting- and reverse-resistors RF and RR (= RON
respectively ROFF) and for the conducting- (forward-) and reverse-voltages UF and UR.
Because of the nonlinear characteristic these values depend on the operating point. The
reverse-resistor and the conducting voltage are depending on the temperature too. The
conducting voltage has a temperature-coefficient of 2 mV/ K, that leads in the allowed
temperature-range (e.g. of 55 C° until +150 C°) to extreme operating-point-displacements.
The figure EBE-03106 shows the equivalent circuit.
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Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
EL III
Prof. Dr. F. Schubert
University of
Applied Sciences
Hamburg
U
R
U
F
F
1
S
EQUIVALENT
CIRCUIT
OF A
DIODE
2
R
R
C
EBE-03106
S
At the equivalent circuit (EBE-03106) the diode for U > UF is in the conducting state (switchposition S = 1) and for U < UF in the reverse state (switch-position S = 2). Because the
conducting resistor (forward resistor) of diodes is very small (RF of 1 until 20 Ω), in many
cases the circuit is calculated with a constant conducting voltage (forward voltage, threshold
voltage) UF (UF >> I*RF). The typical values UF = 0.3 until 0.45 V for Schottky-diodes and UF
= 0.6 until 0.8 V for Silizium-PN-diodes determine therefore the static switching behavior at
the logic-circuits with bipolar transistors.
The reverse current IR of a diode is a function of the temperature. At a temperature of 300 K a
silicum-diode has a reverse current of 0,1 until 10 nA.
3.1.2 Dynamic behavior of diodes
In the equivalent circuit (EBE-03106) the capacitor CS is to see. The depletion layer capacitor
CS is voltage-dependent. It appears in the forward and in the reverse state, also if there is no
current. Parallel is also a constant capacitor of the housing. The dynamic behavior of a diode
is characterized by the switching times at the changes of the state. From blocking to
conducting state only appears a very short time tF. At the transition from conducting to
blocking state appears the longer reverse recovery time trr, that is the sum of the memory time
tS and the discharging time tr of the capacitor CS. At PN-diodes the minimum of trr is 4 ns.
This value is lower at Schottky-diodes.
3.1.3 Schottky-diode
At the Schottky-diode the diode-effect is effected by a metal-semiconductor-transition. It is a
contact between the metal and n-doped silicium. The structure shows figure EBE-03160.
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Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
EL III
metal
SiO2
space charge region
n - epitaxy
A
K
n+ - substrate
Prof. Dr. F. Schubert
STRUCTURE OF A
SCHOTTKY-DIODE
University of
Applied Sciences
Hamburg
EBE-03160
The Schottky-diodes havy much faster memory times as PN-diodes. The metalsemiconductor-transition effects small foreward voltages, but also small reverse voltages.
3.2
Diode-circuits
The valve-effect of diodes makes it very simple to manufacture combinatorical logic. EBE03180 shows the both basic circuits for AND and OR.
UCC
R
D0
D0
D1
D1
I1
I1
Q
I2
Q
I2
R
Prof. Dr. F. Schubert
University of
Applied Sciences
Hamburg
GATE WITH DIODES
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EBE-03180
Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
EL III
The figure shows two different gates:
input level at the anodes and load-resistor grounded
input level at the katodes and load-resistor to supply-voltage UCC
The kind of combinatorical logic will be calculated using the equivalent circuit of a diode. For
example:
Left gate:
I1 = H = 5 V, I2 = L = 0V, UF = 0,7 V, RF = 0 Ω, RR = ∝:
Q = 4,3 V = H
I1 = I2 = L = 0V:
Q=0V=L
I1 = I2 = H = 5V:
Q = 4,3 V = H
OR-circuit
for positive logic
AND-circuit
for positive logic
Inputs
Output
____________________
I2
I1
Q
Q
____________________
L
L
L
0
L
H
H
1
H
L
H
1
H
H
H
1
Inputs
Output
____________________
I2
I1
Q
Q
____________________
L
L
L
0
L
H
L
0
H
L
L
0
H
H
H
1
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Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
3.3
EL III
The transistor as switch
With diodes itself only AND and OR logic can be implemented. The inverter (NOT-function)
can not be realized with diodes. The input-current at diode-circuits are high produced through
the level of the input voltages and the resistor R. An application of transistors let avoid these
disadvantages, because the bipolartransistor is controlled over the base (respectively over the
gate of the MOS-transistor). For the digital circuits the grounded emitter is used mainly. The
transistors mainly are manufactured out of silicium. There are existing two kinds of bipolar
transistors:
Prof. Dr. F. Schubert
University of
Applied Sciences
Hamburg
C
C
B
B
TRANSISTOR
SYMBOLS
E
E
NPN - TRANSISTOR
PNP-TRANSISTOR
EBE-03200
The npn-transistor with the dotation layers n p n and the pnp-transistor with the dotation
layers p n p. The electrodes or contacts are named collector, base and emitter. Collector and
emitter have the same dotation, but a different construction because the collector-base-diode
operates in reverse direction and takes the most power consumption. So it often must be well
cooled. The base-emitter-diode operates in forward direction. The figure EBE-03205 shows
the voltages and currents at the grounded emitter.
It is valid:
and
IB + IC + I E = 0
UCE - UCB – UBE = 0
The most important characteristic is the transistor current gain:
IC = B N * I B
or:
IC = -AN * IE
With the current amplification factor for emitter grounded AN.
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Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
EL III
U
I
B
R
CB
IC
B
R
I
C
U DG
U
CE
I
U
U
R
E
U
U
I
U CC
GS
I
CB
S
N-KANAL
I
B
C
R
I
C
U DG
UCE
D
R
D
UDS
-
-
U BE
I
D
U
DS
+
U CC
NPN
IB
R
+
U BE
U I
D
I
U
E
CC
UI
U
U GS
PNP
I
CC
S
P-KANAL
Prof. Dr. F. Schubert
TRANSISTORS
University of
Applied Sciences
Hamburg
EBE-03205
3.3.1 Current- and voltage-switches
With help of electronic switches voltages or currents are switched at a load-resistor R (ONstate). In the OFF-state no more voltage at the load-resistor R falls off (figure EBE-03210).
R
R
U
I
0
off
0
on
off
voltage switch
Prof. Dr. F. Schubert
University of
Applied Sciences
Hamburg
R
A
on
current switch
PRINCIPLES OF SWITCHING
EBE-03210
At the voltage-switch in the ON-state over the closed switch the voltage U0 falls off at the
resistor R. At the current-switch the current of the current-source over an alteration switch is
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Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
EL III
led to the resistor R. In the position ON at R a voltage UR = I0 * R falls off. In the position
OFF the current is led over a parallel resistor RA. Then at R consequently no voltage falls off.
At electronic switches it is usual, to mark the ON-state with the index "X" or "ON." The OFFstate is marked with the index "Y" or "OFF".
The behavior of the ideal switch can be shown by a characteristic (figure EBE-03212) with
the belonging load-resistor R. On the intersections of the axes and the load-resistor one gets
USX, ISX, USY and ISY.
A mechanical switch shows approximately the ideal behavior. Electronic switches like diodes
and transistors have in the ON-state residual voltages USX. This follows from the ON-resistors
and saturation-voltages. In the OFF-state one gets a terminated blocking resistor and
consequently a leak-current ISY. Therefore one gets noit ideal values for the switch-voltages
and -currents. The advantage of electronic switches is the approximately inertia-free
switching process.
Advantages of electronic switches
maintenance-free
small
bounce-free
high durability
high circuit speed
small requirement of power for the control of the switch
I
U
S
I
R
U
0
U
I
U
S
C
I
U
University of
Applied Sciences
Hamburg
R
0
S
Prof. Dr. F. Schubert
S
S
S
R ON
R
OFF
S
S
ELECTRONIC SWITCH
U
S
EBE-03212
As electronic switch can be used the NPN-transistor shown in figure EBE-03215. The switch
is between collector C and emitter E. Over the control-electrode base B the transistor is
supplied by a voltage UBE or a current IB. The level of the control signal definites the
intersections in the field of the output characteristics with the load-resistor R. For example
one gets the operating points at IB = 0 and at IBÜ respectively IBX.
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Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
EL III
U
I
CC
R
I
I
B
C
U
/R
CC
C
U
CE
U
BE
U
U
CC CE
Prof. Dr. F. Schubert
University of
Applied Sciences
Hamburg
SAT. TRANSISTOR SWITCH
EBE-03215
FAMILY OF CHARACTERISTICS
The broken line is the border for the voltage UCB = 0 V. In this case is UBE equal UCE. This
border-line is named "saturation-border". The intersection of the border-line with the working
characteristic of the resistor R indicates the current at the saturation-border ICÜ. For values
greater than ICÜ the transistor is in the saturation (UCE ≤ UBE). One leaves the linear controlarea, the collector current is no longer the product of base-current and the transistor current
gain BN. An increase of the base-current to IBX only leads to a small increase of the collector
current to ICX, but one gets a small voltage UCEX (LOW-level).
3.4
Inverter with bipolar transistors
In this chapter the inverter with grounded emitter is treated. The saturated voltage-switch is
investigated and calculated. The saturation takes care for small residual voltages at the
blocked transistor-switch. This and consequently the control determine the switching times of
the transistor. To decrease the switching times are used several methods. The dynamic
behavior is very influenced by the load at the output. This is especially valid for capacitive
and inductive loads.
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Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
EL III
3.4.1 Saturated transistor inverter
The figure EBE-03220 shows a simple equivalent circuit for the static transistor-switch.
Prof. Dr. F. Schubert
University of
Applied Sciences
C
Hamburg
I
C
U
CB
I
CB0
A *I
N E
I
B
U
B
R
U
B
BB
U
BE
I
CE
SIMPLE
EQUIVALENT
CIRCUIT
OF THE
STATIC
TRANSISTORSWITCH
BE
E
E
EBE-03220
From IC = ICB0 - AN * IE and BN= AN/ (1 - AN) follows
IC = B N * I B
+ ICB0 * (1 + BN)
The collector leakage current ICB0 of 0.5 until 20 nA generally can be neglected at Silicium
transistors.
The current amplification factor BN at grounded emitter is valid for the normal operation with
blocked collector diode. BN is not constant, it is depending on the collector current. For our
calculations we take a constant BN. For the control of a transistor-switch are to regard several
intersections of the working characteristic of the resistor with characteristic lines in the field
of output characteristics.
The figure EBE-03255 shows four areas. The blocking state region I, the for digital
applications forbidden linear control area II, the overload area III with the termination of the
load-hyperbola and the saturation-area IV, that is terminated by the saturation border.
The blocking state region is terminated by IB = 0. The intersection of the characteristic for
IB = 0 and a working characteristic R gives the operation point P0 in the blocking state. The
points P1 and P2 bound the overload-area. The operation-point P3 lies on the saturation
border. Here is valid:
BN = ICÜ / IBÜ
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Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
EL III
I
U
CC
R
U
CB
I
I
C
U
/R
CC
C
B
U
CE
U
U
CC CE
Prof. Dr. F. Schubert
University of
Applied Sciences
Hamburg
TRANSISTOR-PARAMETERS
REGION OF SATURATION
EBE-03255
The belonging collector-emitter-residual-voltage UCEÜ is between 0.1 and 1 V.
If the collector current will be increased, so the transistor operates in the saturation area
(above the operation-point P3). One gets a collector current
ICX = (UCC - UCX) / RC > ICÜ
The belonging base current is a m-fold of the base current at the saturation-border IBÜ. This
definites the saturation factor m:
m = IBX / IBÜ = BN * IBX / ICÜ ≈ BN * IBX / ICX
Because of the saturation the residual-voltage between collector and emitter decreases from
UCEÜ to UCEX. For a high reliability one chooses m > 1, so that currents IC > ICÜ are reached
safely also at tolerances of the electronic components.
For a transistor-switch in the ON-state follows for the ON-resistor
RON = UCEX / ICX
In the OFF-state flows the collector leakage current ICB0. One gets an OFF-resistor
ROFF = UCY / ICY = UCC / ICB0
The simplest transistor inverter contains a transistor as switch, a base resistor RB for
impressing of the base current and the load-resistor RC at the collector. In the figure DST02001 is the additional base shunting resistor RA, that improves with the voltage UB the
blocking behaviour of the switch. At a NPN-transistor UB is zero or negative. So one gets in
the blocking state a base-emitter-voltage UBEY lower than UBES = 0,4 Volts.
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Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
EL III
U
CC
R
R
R
G
I
U
U
I
B
I
B
C
I
C
U
I
U
I
Q
U
Q
CE
BE
RA
G
UB
Prof. Dr. F. Schubert
SATURATED
TRANSISTOR-INVERTER
University of
Applied Sciences
Hamburg
DST-02001
Transistor inverter in the ON-condition. It is valid:
m = BN * IBX / ICX
Or for the base-current:
IBX = m * ICX / BN
For the collector current is valid:
ICX = (UCC – UCEX) / RC + IQX
For the base current is valid:
IBX = (UGX – UBEX) / (RGX + RB) - (UBEX – UB) / RA
From these equations one gets for example the minimum value of RA:
RA =
U BEX − U B
U GX − U BEX
R GX + R B
−

m  U CC − U CEX
⋅
+ I QX 

BN 
RC


Transistor inverter in the OFF-state. The value of UBEY must fulfill the following condition:
UBEY < UBES
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Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
EL III
Out of the circuit in figure DST-02001 follows:
U BEY = U GY ⋅
R GY
R GY + R B
RA
+ UB ⋅
+ RB + RA
R GY + R B + R A
Then one gets a maximum value for RA
RA =
(U
BEY
)(
− U B ⋅ R GY + R B
)
U GY − U BEY
3.4.4 Push-pull switch
The until now discussed transistor-switches all operate after the principle of single phase, that
means, that in the ON-state is made by a switch a connection with the supply-voltage (e.g.
GND). In the OFF-state the switch is blocked. This blocking behaviour will not be reached
inertia-less. So this will limit the maximum frequency of the system.
Using two phases or push-pull switches is possible fast switching to all states. Two
complementary driven switches (one is in the ON-state, the other in the OFF-state or viceversa) connect the load either at the positive or at the negative supply-voltage.
At digital logic circuits (e.g. TTL, CMOS) with a positive supply-voltage (e.g. UCC) it is
switched between this and ground (e.g. GND). This has the consequence, that for each state
one gets a low impedance path. This leads with capacitive loads to small time constants.
τ1,2 = (RON1,2 || Rl) * CL
One kind of push-pull switch is the „Totem pole“ circuit.
The principle was used first for TTL-circuits. The name "Totem pole" should imply, that this
outputs are "singular" is and only have the levels of the belonging logic-circuit. It is not
allowed to connect further outputs parallel (at a totem pole is also only a victim!). At
connected push-pull outputs it leads to undefined levels or short cuts, if at two circuits at the
same time the complementary transistors are conducting.
At a "Totem pole" circuit are the two NPN-transistors T2 and T3 in the output driven of a
driver-transistor T1 "Split phase". This transistor T1 has two operating resistors (R1 and R2) at
his collector and his emitter. T1 generates two signals, which have two phases. At the emitter
the signal is in phase and at the collector in opposite phase.
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Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
EL III
U DD
RQH = R ON1
CL
R QL = R ON2 R L
U
UQ
SS
UQ
0
t
Prof. Dr. F. Schubert
University of
Applied Sciences
PUSH-PULL SWITCH
Hamburg
U
R
R
1
Prof. Dr. F. Schubert
University of
Applied Sciences
Hamburg
2
U
1
BE2
U
CE1
U
D
T
R
CC
3
T
T
DST-06028
U
2
3
U
F
CE3
BE3
"TOTEM POLE" - OUTPUT
DST-06033
At the calculation of the voltages at the transistors it is noticed, that the transistor T2 doesn't
block safely, if the transistor T1 is conducting. It is valid
UBE2 = UCEX1 + UBEX3 – UCEX3
It follows with UCEX1 = UCEX3
UBE2 = UBEX3
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Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
EL III
With insertion of a diode D between T2 and T3 one gets
UBE2 = UBEX3 – UF
The transistor T2 now is surely blocked. All transistors exept transistor T2 operate in
conducting state in the saturation area.
The resistor R3 is a protection at a short-cut. At short-cuts against ground by outside
connections or by not allowed parallel switching of TTL-outputs the current into the circuit is
limited (R3 ≈ 30 Ω until 100 Ω) and the output is protected.
Because of the resistor R3 and especially of the operating-point of T2 in the linear area the Hlevel is lower than the supply-voltage (typical for TTL-circuits is UCEY3 = UQH ≈ 3,2 V until
3,8 V, the manufacturers guarantee in the worst case UQHmin = 2,4 V).
3.4.5 Switching times of the transistor
The calculation of the switching times of the bipolar transistor is very difficult and will not be
made in this lecture. The most important part of the switching time is the result of the
operating point in the saturation. Avoiding the saturation decreases the switching times.
3.4.6 Measures to the decrease of switching times
To the decrease of the switching times two different circuits are used. At discrete circuits an
acceleration-capacitor CB ( "Speed up" capacitor) parallel to the base resistor. For integrated
circuits this method cannot be used, because the implementing of capacitors needs wide areas.
In integrated circuits with bipolar transistors the deep saturation is avoided by the use of
Schottky-transistors.
It is valid
UBEX = UCEX + UFSD
or
UCEX = UBEX - UFSD ≈ 0,7 V – 0,3 V = 0,4 V
This is the lowest value for UCEX. That means, that the transistor is in the ON-state in the
saturation area near the saturation-border. The mounting of a Schottky-diode decreases the
switching times about the factor of 10 until 20. The realization of a Schottky-transistor shows
figure EBE-03410.
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Prof. Dr. F. Schubert
Prof. Dr. J. Vollmer
EL III
I
C
U
S
U
B
U
C
CE
BEX
E
U
Prof. Dr. F. Schubert
University of
Applied Sciences
Hamburg
TRANSISTOR-INVERTER
WITH SCHOTTKY-DIODE
E
Prof. Dr. F. Schubert
University of
Applied Sciences
Hamburg
THE SCHOTTKY-TRANSISTOR
CIRCUIT AND DESIGN
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CE
DST-02101
B
C
EBE-03410