AN308
Application note
TRIAC analog control circuits for inductive loads
Introduction
The TRIACs of today are well suited to the requirements of switching inductive loads.
TRIAC control circuits must be particularly well tuned to be both economical and applicable
to inductive loads.
The purpo of this document is to prent different methods of TRIAC control with their
applications and to analyze their relative advantages and disadvantages.
A simple circuit offering all the guarantees of reliability is propod for inductive loads.
September 2008 Rev 4 1/16
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Triggering methods AN308
1 Triggering methods
1.1 Triggering with synchronization on the TRIAC voltage
The triggering circuit with synchronization across the TRIAC (See Figure 1 and Figure 2)
turns on the component at an angle 尾 after the current drops to zero, such that
尾 = 蠅 路 Tr.
Time Tr is defined by the time constant (P + Rt)C.
蠅 = 2 路 蟺 路 f with f = mains frequency.
鲁豫有约 米勒Figure 1. Typical circuit - synchronization across the TRIAC
T
D
Diac
1
AC Mains
ZL
C P Rt
2
Figure 2. Synchronization across the TRIAC - waveforms (general ca)
Mains voltage
Gate pul
T
TRIAC voltage
T
尾
TRIAC current T
伪
蠒 : Current lag (full angle)
尾 : Blocking of the component 蠒 full angle
伪 : Conduction angle
This is the simplest possible circuit but in certain cas it can have an important drawback.
For example, consider a highly inductive load (L 蠅 / R > 4) where the TRIAC is turned on
compassionate
with a considerable delay 尾, perhaps 100掳 after the mains voltage zero as inFigure 3.
If the TRIAC is turned on at point A, the conduction (伪) lasts up to about 150掳. The TRIAC
turns off at point B at 伪 + 尾 = 250掳 after the zero voltage point. At that instant a negative
voltage is applied to the triggering circuit which turns on the TRIAC at point C after an angle 菠萝的英文
尾 of 100掳, that is, 350掳 from the starting point.
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AN308 Triggering methods
The cond turn-on occurs at a very low voltage and the angle 伪鈥?is much smaller than 伪.
The following period begins under similar conditions and the unbalance persists. This type
of asymmetrical operation is not only unacceptable but can be dangerous (high current due
to load magnetic saturation due to the dc content of the waveform).
The unbalance is illustrated for a particular ca, starting from zero of the mains voltage.
Other caus also produce this behavior
鈼? variation of the load impedance, for example, with motors, due to torque variation
鈼? modification of the control turn-on angle
This phenomenon is due to the fact that the circuit does not take its time reference from the
mains zero voltage. Rather, the synchronization is taken from the voltage across the TRIAC,
which is dependent on the load current, that is, on the load pha shift.
Figure 3. Synchronization across the TRIAC - waveforms (delayed turn on)
Mains voltage
Gate pul
T
尾
TRIAC voltage B
T
尾
蠒 C 伪'
TRIAC current
A T
corrective
伪
full angle 幼师证
To sum up, this first very simple triggering circuit, synchronized by the voltage across the
TRIAC, has the following characteristics:
鈼? Advantages:
鈥? Simple design and low cost
鈥? Connection by two wires, without polarity issue
鈥? Abnce of a parate power supply
鈥? Little power dissipated in P and Rt
鈼? A rious disadvantage:
鈥? Becau of its principle, this circuit cannot be ud for highly inductive loads with a
narrow conduction angle becau it can result in unacceptable asymmetrical
operation.
This very simple triggering circuit should be rerved for low-cost applications with the
following characteristics:
鈼? Resistive or slightly inductive loads
鈼? No stringent requirements concerning the accuracy of regulation
鈼? Highly inductive loads where the power varies between 85 and 100% of the maximum
power
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Triggering methods AN308
bios是什么意思1.2 Triggering with synchronization by the mains voltage
This triggering circuit of Figure 4 is synchronized by the mains voltage. The puls are treats
always shifted by 180掳 with respect to each other, whatever the type of load.
Figure 4. Typical circuit - synchronization by mains voltage
T
D
1
ZL
AC Mains
C P
2
Rt
Figure 5. Synchronization by the mains voltage - waveforms
stayed怎么读
胃
Mains voltage
Gate pul
T
胃
A
A
TRIAC voltage T
尾
TRIAC current
T
伪 学英文口语
full angle
蠒
蠒 : Current lag (full angle)
尾 : Blocking of the component
伪 : Angle of conduction
胃 : Triggering delay angle
Angle 胃 is the delay between the mains zero voltage and the triggering pul. It can be
宣泄的意思
adjusted by means of potentiometer, P, from 0 to 180掳 to vary the load voltage. The current
in an inductive load (L.R) lags the voltage by an angle 蠒: (tan 蠒 = 蠅 路 L/ R).
For triggering angles, 胃, higher than 蠒, operation is perfectly symmetrical and stable.
This simple circuit can still prent the risk of a fault in the ca where 胃 is smaller than 蠒, as
shown in Figure 6.
As an example, take the ca of a highly inductive load and an angle 胃 = 60掳. The TRIAC is
turned on at point A (60掳). It conducts over more than 180掳, up around 230掳. It is blocked at
point B: (290掳).
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AN308 Triggering methods
The cond triggering pul occurs at point C when 胃 + 伪 = 240掳. It has no action on the
TRIAC which is still conducting. The TRIAC is not turned on for the next half-wave. As in the
previous ca, the operation is totally asymmetrical, and thus unacceptable.
Figure 6. Synchronization by the mains voltage - asymmetrical operation 胃 < 蠒
Mains voltage 胃
Gate pul
T
胃
C B
TRIAC voltage
T
蠒
TRIAC current
T
伪
full angle
To prevent this fault, it is necessary to limit the turn-on angle to maintain 胃 > 蠒. This is
possible for loads who L and R parameters remain strictly constant.
Experience shows that for the majority of inductive loads ud in industrial applications (like
motor controls and transformers) the values of L and R are not constant and vary a great
deal during operation. For the types of applications it is not possible to limit the turn-on
angle without considerably reducing the voltage excursion.
To sum up, this simple triggering circuit, synchronized by the mains voltage, is more
developed than the previous one. It has the following charactersitics:
鈼? Advantages:
鈥? Simple design
鈥? More accurate control than the previous circuit
鈥? No auxiliary power supply or transformer required
鈼? Disadvantages:
鈥? Connection of the circuit by 3 marked wires, instead of 2 without polarity in the
previous circuits
鈥? Higher pow
er dissipated in passive components P and Rt (since the mains voltage
is continuously applied across them)
鈥? Operation becomes completely asymmetrical if the control angle 胃 is less than 蠒.
This triggering circuit can be ud only for applications in which the pha shift of the load
remains constant (air inductor) or if operation is restricted to values of 胃 much higher than 蠒,
that is, for low load voltage operations.
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Triggering methods AN308
1.3 Triggering synchronized by the mains voltage and suitable
for industrial applications
This new circuit is derived from the previous one by improving the triggering pul
generator. The improvement consists of maintaining the triggering signal during each half
wave between values 胃 and 180掳. This is done simply by nding a pul train after the
initial pul so as to maintain the triggering order, as shown in Figure 7.
Figure 7. Triggering by pul train synchronization by the mains voltage
Mains voltage 胃
Gate pul
胃 T
尾
TRIAC voltage
T
蠒
伪 2
TRIAC current T
伪 1
full angle
A B
蠒 : Current lag full angle
伪1 : 1st angle of conduction
伪2 : 2nd angle
尾 : Blocking of triac
胃 : Triggering delay time
For example, suppo that angle 蠒 is equal to 85掳 a
