OM1894
Dual sensing precision triac control
1
FEATURES
2
GENERAL DESCRIPTION
•
rfi elimination triac drive circuit
•
Low external component count
•
Low supply current required
•
Variable ON and OFF cycle times
•
Sense circuit outputs available
•
External ON/OFF triac control available
•
No DC component in the AC mains supply
•
The ON period always has an odd number of half cycles,
and the OFF period an even number
•
The sensors are AC powered, minimising DC supply
and filtering needs
•
Hysteresis can be set with user selected components.
•
Independently variable (or fixed) cut-in and cut-out
setting possible with dual sense input circuits
•
Dual or single sensor operation possible
•
Gate pulse width may be externally set
•
Negative triac gate drive (avoids insensitive quadrant
operation)
3
BLOCK DIAGRAM
The OM1894 is a monolithic bipolar circuit for triggering a
triac in applications where accurate control is required
from one or two sensors such as NTC (Negative
Temperature Coefficient) or PTC thermistors. It is suitable
for a broad range of applications, extending from the
zero-crossing control of a compressor cooling motor, a
heating element, or to the control of fan motors and other
complex loads.
It is designed to accept a wide variety of resistive and other
sensors, using two independent balanced current
comparator input circuits in which the signal current
derived from the sensor(s) is compared with currents
derived from a fixed or variable resistor network.
The triac firing circuit uses a unique circuit arrangement by
which load current zero crossing is detected, and a gate
pulse applied during the critical current zero crossing
period. Triac conduction is therefore maintained
throughout the zero crossing time, ensuring that rfi
transients are not generated during this time. Load current
zero-crossing switching is ideal for minimising rfi,
particularly when controlling inductive loads.
220k
LOAD
Triac
BT208X
-600E
22nF
250
Vac
AC
220V
Common
220k
220k
220k
220k
MODE
V
CC
PWR Reset
OM1894
100k SA1
9
100k SB1
8
latch
Power
Supply
Gate
Sensing
2
300Ω
TRG
100uF
16V
15
13
16
12
100k SA2
7
100k SB2
6
5
4
latch
Mode
logic
Zero-crossing
Timing and
Synchronisation
Gate
Drive
10
11
14
1
3
cab
θ
evap
θ
CAP1
CAP2
OP1 OP2
CI
V
EE
PX
470k
100
nF
100
nF
100pF
block1894
Fig.1 Block diagram
© 2006 Integrated Electronic Solutions Pty Ltd. trading as
Hendon Semiconductors, all rights reserved.
Contents are subject to the Disclaimer
2007 Apr 19, Revision 2.0
1
Product Specification
OM1894
Dual sensing precision triac control
4
4.1
PINNING INFORMATION
Pinning layout
4.2
Pin description
PIN
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
DESCRIPTION
Negative supply
Triac gate drive
Gate pulse extender
Sense capacitor 2
Sense capacitor 1
Sense Input B2
Sense Input A2
Sense Input B1
Sense Input A1
Sense output 1
Sense output 2
Reset input/output
Positive Common
Control input, triac ON/OFF
Invert control sense & function
AC line sense and power input
SYMBOL
VEE
TRG
PX
CAP2
CAP1
SB2
SA2
SB1
1
2
3
4
16
15
14
13
PWR
MODE
CI
VCC
ext_Reset
OP2
VEE
TRG
PX
CAP2
CAP1
SB2
SA2
SB1
SA1
OP1
OP2
ext_Reset
pin1894
OM1894
5
6
7
8
12
11
10
9
OP1
SA1
VCC
CI
MODE
PWR
Fig.2 Pin configuration
5
5.1
FUNCTIONAL DESCRIPTION
V
CC
−
Common, positive DC
supply
The positive DC supply rail for the
control IC type OM1894 is used as
the common reference. This is always
connected to the T1 terminal of the
triac, and being the positive supply
rail allows negative gate drive to the
triac in both positive and negative
supply half cycles on T2. By driving
the triac in this way the insensitive
quadrant (negative T2 voltage, and
positive gate triggering signal) of
triacs is avoided.
5.2
V
EE
−
Negative DC supply,
substrate
capacitor needs to be sufficiently
large to maintain the operating
voltage during the half cycle when it is
not being charged, as well as to
provide the energy to drive the triac
gate during the gate pulse.
Internal supply sensing prevents the
commencement of an ON cycle while
the voltage is too low for reliable
circuit operation. If during an ON
cycle the supply voltage falls below
this level the ON cycle will terminate
at the first opportunity consistent with
the logic cycle algorithm.
5.3
PWR
−
Power supply and
synchronisation from the
mains supply line
indicate the phase and magnitude of
the signal on the AC supply. Three
states, positive, zero and negative, of
this signal is recognised for
synchronisation of the triggering
times to the mains.
See Figure 3, OM1894 Power Supply
Circuit.
The PWR pin is driven by a current
limiting resistor from the mains
supply. During the positive half cycle
current flows through the upper diode
D1 to the positive common rail, while
on a negative half cycle the current
flows through the lower diode D2, and
charges the V
EE
power supply
capacitor.
This pin connects to the substrate and
the internally generated and
regulated negative DC supply, and
should be bypassed to V
CC
(common)
by a capacitor of typically 100
µF.
The
2007 Apr 19, Revision 2.0
The PWR input provides both a
synchronisation signal for the logic
functions of the OM1894, as well as
the DC current used to provide the
power supply from which the OM1894
is powered. Signals are derived which
2
Product Specification
OM1894
Dual sensing precision triac control
V
CC
-
D1
+
T
p
PWR
-
D2
+
T
n
V
EE
psp1894
Fig.3 OM1894 power supply circuit
The zero crossing is signalled by the
two comparators, the output signals
of which indicate whether the mains
voltage is above the common rail
voltage, or below the negative V
EE
.
There may be additional resistors in a
simple network from the AC supply
and V
EE
to adjust these zero-crossing
signals to provide a symmetrical
response in the positive as well as the
negative going direction.
As the AC signal passes through
zero, comparators provide control
signals Tp (when V
PWR
> V
CC
) and
Tn (when V
PWR
< V
EE
) indicating
whether the voltage on PWR pin is
greater or less than V
CC
or V
EE
respectively. A resistor network
ensures that these switching points
correspond to equal positive or
negative thresholds about the AC
zero thus giving symmetrical
zero-crossing information to the
synchronisation and logic circuit.
Synchronisation is obtained from the
threshold comparators at the levels of
V
CC
and V
EE
on the chip. Adjustment
of the initial switching point, and
2007 Apr 19, Revision 2.0
hence the time of initiation of the first
firing pulse, and its symmetry about
the zero crossing point, is possible by
varying the values of the resistors
connected between PWR and the
active supply, a resistor to V
EE
, and a
resistor to V
CC
.
When the triac has switched on, the
zero-crossing synchronisation
information is derived from the
voltage on the triac gate while it is
conducting, although the polarity
information provided by the PWR
input signal continues to provide
phase information to enable the ON
and OFF transitions at the start and
finish of an ON burst of conducting
half cycles, to be synchronised to
prevent repeated firing in the same
polarity half cycle, and a resultant net
DC load current.
5.4
ext_Reset
−
Reset status
and control
to a sufficient value to sustain full
operation.
The reset signal is active high to
preserve a predictable voltage
relationship with the Common supply
rail (V
CC
) rather than V
EE
. The
ext_Reset pin is pulled high by 50µA
when the internal reset signal is
active. When the power supply
voltage reaches the threshold, the
ext_Reset pin is pulled low (6µA), and
the controller then begins normal
operation.
This ext_Reset signal can be used to
reset counters and other electronic
logic circuits which need to begin
operation in a known state.
Furthermore, when OM1894s are
used in parallel, or with other control
circuits, and external reset signal can
be applied which will override these
internally generated signals.
5.5
CI
−
triac drive control input
As the power supply rises towards its
operating voltage, a reset signal is
generated which holds the logic in an
initial state until the voltage has risen
3
If MODE is connected to V
CC
or V
EE
,
the Control Input (CI) is the input
signal which allows external ON/OFF
Product Specification
OM1894
Dual sensing precision triac control
control of the triac with the exception
of when the MODE pin is floating.
The voltage signal on CI will switch
the triac ON or OFF in a manner
which is synchronised to the mains
zero crossings. This signal may
change at any time, but the triac will
only be switched ON or permitted to
turn OFF, at a time that is consistent
with controlling it at the earliest
available opportunity consistent with
the cycling algorithm which triggers
the triac for an odd number of half
cycles in each ON period, and lets it
remain OFF for an even number of
half cycles. In this way there is no DC
current present in the mains supply
when it is averaged over a large
number of ON cycles, and at the
same time for inductive loads, a new
ON period begins on a half wave of
the opposite polarity to the start and
finish half wave of the previous run
cycle.
The MODE pin provides the facility to
invert the sense of the signal applied
to the CI pin, so that a signal of the
most appropriate polarity can be used
without the need for an external gate
to invert the signal.
5.6
MODE
−
external triac
ON/OFF control sense, or
internal logic
OP2 are combined so that the triac is
not permitted to run until both of the
inputs are agreed that heat is needed.
In a similar manner the triac run cycle
is not terminated until both inputs
have changed state, and both OP1
and OP2 have reached the condition
in which the run cycle can be
terminated.
This is particularly applicable to the
application of controlling a refrigerator
with two sensors, with one sensor in
the food compartment, and another
mounted on the condensor. A cooling
cycle is only permitted to begin if the
food has reached its upper limit, at
which cooling is needed. However
this alone is not sufficient. The
condensor should also have warmed
to a temperature where any ice from
condensed water vapour has been
able to melt, and defrost the
condensor. When both of these
conditions have been satisfied, then a
cooling cycle is permitted to start.
Similarly, when the food has cooled to
the lower cut out temperature for the
cabinet, the evaporator sensor must
also have reached its cold operating
temperature for the cooling cycle to
end.
5.7
TRG
−
Triac gate drive
the zero crossing of the mains supply.
The leading edge of this obtained
from the signal derived from the PWR
resistor network before the falling
mains voltage reaches zero, The gate
pulse is applied for a time determined
by either an internal time delay circuit,
or if it is required to be for an extended
time, by the addition of an RC network
to the gate pulse extension pin PX.
On this first half cycle current is
flowing in the triac, and subsequent
zero crossings of the triac can be
determined in another way. While a
resistive load may have the zero
crossing determined from the mains
supply, this is not possible when the
load is inductive (for example, when it
is a motor). The current is no longer in
phase with the supply voltage, and
can reach zero at a time significantly
lagging the supply voltage phase.
In the OM1894, the voltage of the
triac gate has been found to provide
an indicator of imminent zero
crossing, and with an appropriate
threshold circuit, the gate drive can be
re-applied before the triac turns fully
off. Again the gate pulse is
determined by the length of the
internal delay circuit, plus any
additional delay from the application
of external resistor and capacitors
applied in parallel from the pin PX to
V
EE
Because the operation of the
OM1894 uses characteristics of triacs
which are common to typical triacs,
but may show some variation in triacs
designed to emphasise specific
uncommon features, it is preferable to
use triacs which have been
characterized and specified as
suitable for use in this application.
5.8
PX
−
triac gate pulse width
external setting
The mode pin is intended to be
permanently wired for each
application, and has three active
states.
When MODE is connected to V
CC
and
held high, then CI is high for the triac
to be ON, and low for OFF.
If MODE is connected to V
EE
, then the
ON/OFF sense is inverted, and for CI
high the triac is OFF, and low for ON.
There is a special function available
when the MODE pin is not connected
which performs a function from the
combined outputs of the two sensing
stages. The signals from OP1 and
2007 Apr 19, Revision 2.0
The triac gate output drives the gate
through an external current setting
resistor. It has in-built protection to
withstand transient voltage signals
which may be induced on the gate of
the triac by mains transients during
firing. The gate drive current should
be set to a value suited to the gate
sensitivity of the triac used. The firing
pulse width will need to be of such a
width that the specified latching
current of the triac when used with the
design load has been reached before
the gate pulse ends.
In the OM1894 the gate drive is first
applied at the start of an ON period at
4
The gate pulse must be wide enough
to be applied from the time the triac is
about to turn off until the increasing
Product Specification
OM1894
Dual sensing precision triac control
current in the triac in the opposite
direction has reached the latching
current of the triac being used.
While there is a time delay circuit
within the OM1894 to provide a
minimum gate pulse width, for lower
powered loads, where it takes longer
for the load current to reach the triac
latching current, then it may be
necessary to extend that gate pulse.
A parallel resistor and capacitor are
connected from pin PX (pulse
extender) to V
EE
giving a pulse
extension time of:
t
pw
≈
1.4
⋅
R
⋅
C
(
s
)
5.9
SA and SB
−
Sensor inputs
The sensor inputs are symmetrical
current inputs designed to accept AC
signals referenced to common. See
figure 4, Sensor current comparator
circuit.
By not using the DC supply rail to
drive the sensing inputs, problems
associated with providing sufficient
DC current to drive the sensor and
associated networks over the full
operating range are avoided. In
addition by providing balanced
differential inputs operating at close to
the V
CC
rail potential, control signals
which either increase or decrease
with the parameter to be regulated
(temperature, pressure, humidity etc.)
can be handled.
The OM1894 has two separate and
independent sensor circuits. This
enables a single sensor to be used
with the voltages dropped across the
sensing elements and the setting
circuit to be able to be independently
applied to each sensing circuit.
Or there may be two sensors,
permitting the temperature to be
measured in different places, and the
outputs of the sensor circuits to be
logically combined from their
independent outputs to give more
useful functions.
While each sensor input circuit has a
single input threshold, a feedback
signal can be derived from the triac to
change these thresholds to another
value depending on whether the triac
is ON or OFF.
Both of the sensor circuits within the
OM1894 are identical.
Thus for a capacitor value of 470pF,
and a resistor of 1 MΩ, the pulse
width would be typically 658µs.
SA
SB
V
CC
V
CC
-0.6 V
-
OP
+
ON/
OFF
latch
-
+
V
EE
+1.2 V
V
EE
CAP
capcct1894
Fig.4 Sensor current comparator circuit
2007 Apr 19, Revision 2.0
5
Product Specification
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