Download DPCL Solid State Device Discrete Control Lecture

DPCL
Solid State Device
Discrete Control Lecture
Discrete Input – Output, I/O
All devices/control in this course have been
“analog” measurements; level, flow, temperature
etc.
Discrete control implies two stable “states”,
open-closed, on-off, etc.
Inputs-Outputs
Inputs are push buttons, various switches that
sense physical conditions, such as pressure,
level, temperature, proximity or limit etc.
Outputs are used to control “on-off” devices
such as solenoid valves, motors, relays,
contactors etc.
Terms used to represent discrete behavior
Binary
0
1
Voltage
0 to 1.5 V3.5 to 5 V(or opposite)
Logic
False
Switch
OFF
ON
Level
Low
High
True
Inputs – Switch Configurations
SPST Single Pole Single Throw:
SPDT Single Pole Double Throw:
DPST Double Pole Single Throw:
DPDT Double Pole Double Throw:
Switches
Process Switches are frequently used as safety
interlocking devices.
Interlock – shutdown system
Pressure, level, temperature, flow vibration etc.
The safe way to wire these devices is to assume that
the normal process condition will result in a closed
switch that is there will be current flow to the device
during “normal” operation. This way wiring
failures will “fail safe”, resulting in the alarmed or
interlocked condition.
Discrete I/O Interfacing
The control equipment, either a PLC or a control
system such as DeltaV Distributed Control System,
DCS, requires its internal circuitry to be interfaced
with these industrial electrical or
electromechanical devices.
5V dc TTL (Transistor Transistor Logic) signals
for this interface. Isolate computer wiring with
“field” wiring, via optical or transformer coupling.
Discrete I/O Interfacing
The “field” or machine wiring may be either AC or
DC powered.
Examples of AC and DC interface Input Output
Circuits shown in notes.
Optical I/O Isolation
SSR - Solid State Relays
Solid-state relays (SSRs) control load currents
through solid-state switches such as triacs, SCRs,
or power transistors. These elements are controlled
by input signals coupled to the switched devices
through isolation mechanisms such as
transformers, reed relays, or optoisolators.
Sometimes called thyristors.
The loads or switched devices are electrical power
consuming devices, contactors, transformers, etc.
SSR - Solid State Relays
Silicon controlled rectifiers (SCRs) 3 terminal
device; phase control.
Phase control, continuously variable power is
obtained by controlling the conduction period of a
thyristor or SCR.
SCRs can be used singly for half-wave power
control, or in combination for full-wave control.
Light dimmer control is a common example.
Used to control the amount of voltage and current
to the load from near zero to maximum. This is a
very non-linear relationship and may
cause overshoot if not linearized.
SCR Phase Conduction/Firing
resistive load, ½ wave rectified
Triac
Bidirectional triode thyristor (triac), 2 SCRs in
parallel.One SCR will conduct the positive halfcycle and the other will conduct the negative halfcycle. Discrete control only.
Triac fired by either positive or negative gate pulse.
Gate pulse can be momentary, the triac will remain in
conduction until the conditions for commutation are
satisfied, i.e. reversed polarity. Zero crossing shown.
Inductive Loads
Current and voltage are not in phase. Triac can
conduct current in both directions, it has only a brief
interval during which the sine wave current is
passing through zero to recover and revert to the
blocking state.
Blocking voltage must appear across the triac to
switch it off. If this voltage appears too rapidly, the
triac will resume conduction and control is lost. In
order to achieve control with certain inductive
loads, the rate of rise in voltage (dV/dt) must be
limited by a series RC network placed in parallel
with the power triac. Called a “Snubber”
Switching inductive loads with an SCR
will result in a high dv/dt transient due
to collapsing the stored magnetic filed in
the inductor. This can damage the SCR.
Adding a “Snubber”, a RC series
network in parallel with the SCR can
reduce these transients. The following
SPICE simulated circuits show the
effect. Notice the reduced dv/dt.
With Snubber
Without Snubber
Transformer Loads
Ferromagnetic materials are have a non-linear
magnetization characteristics. At high magnetic
fields, H the magnetic flux, B, will saturate. When
H is reduced to zero, the ferromagnetic material
retains a certain magnetic flux B, called the
residual flux density. This can create a surge when
the SSR is switched. In order to reduce the surge in
the first half-period, a “peak switching” relay is
used. The peak switching relay never performs the
actual switching function until the first peak
voltage is reached. After the first half-period it is
works the same as a zero switching relay.
DC SSR Applications
When DC inductive loads, solenoid valves etc. are
switched off the magnetic field stored in the coil
will collapse. All this energy will be released across
the contacts or circuit if not protected.
Spark will occur
Without protection, device can be damaged.
Solution: Wire diodes across the load or circuit to
protect the surge.
With diode
Without diode
Thermal Design Considerations
A major design concern for SSRs is heat removal.
Semiconductor reliability is inversely proportional
to the operating temperature.
In order to transfer the heat dissipated by the device,
device is mounted to a finned metal plate - heat sink.
The semiconductor thermal ratings are the junction
temperature and the “thermal resistances”.
For silicon devices the junction temperature is < 125
DegC. The thermal resistances are between
Junction and the case
Case and the heat sink
Heat sink and the ambient air
Solid State Device Thermal Network
 jc
 cs
 sa
T
PT
Ta
Tj
Tc
Ts
Solid State Device
Thermal Network
Junction Temperature
0.65 DegC/Watt Junction to Case
0.2 DegC/Watt Case to Sink
Junction T
jc
0.4992 DegC/Watt Sink to Ambient
Case Temperature
1.349 DegC/Watt Total
63 Watts
Power
40 DegC
Ambient Temperature
125 DegC
Junction Temperature
 cs
P
84 DegC
Case Temperature
71 DegC
Sink Temperature
jc
Sink Temperature
P
 sa
Case Te
cs
Ambient Temperature
Sink T
sa
Enclosure Ventilation
Use forced air to remove the heat.
Calculate the volume of air required:
 P 

V  6T2 
 T2  T1 
Where V is the fan capacity in ft^3/minute
P is the Power in Kilowatts
T1 is the inlet temperature in degR
T2 is the outlet temperature in degR; that is the
temperature inside the enclosure
Semiconductor Reliability a function of the
Arrhenius Model
l  l0 e
DE

kT
DE = Activation Energy
l
= Failure Rate
k
T
= Boltzmann’s constant 8.61x10^-5 eV/degK
= Temperature in DegK
MTTF = 1/l
Mean Time To Failure of Semiconductor Components
5
MTTF hours
10
4
10
3
10
0
50
100
150
Temperature DegC
200
250