外文文献4.txt

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 purpose of this document is to present 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 proposed for inductive loads.
September 2008 Rev 4 1/16
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Triggering methods AN308
Triggering methods
Triggering with synchronization on the TRIAC voltage
The triggering circuit with synchronization across the TRIAC (See Figure 1 and Figure 2)
turns on ponent at an angle β after the current drops to zero, such that
β = ω· Tr.
Time Tr is defined by the time constant (P + Rt)C.
ω= 2 · π· f with f = mains frequency.
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 case)
Mains voltage
Gate pulse
T
TRIAC voltage
T
β
TRIAC current T
α
Ï•: Current lag (full angle)
β : Blocking of ponent ϕ full angle
α : Conduction angle
This is the simplest possible circuit but in certain cases it can have an important drawback.
For example, consider a highly inductive load (L ω/ R > 4) where the TRIAC is turned on
with a considerable delay β, perhaps 100° after the mains voltage zero as inFigure 3.
If the TRIAC is turned on at point A, the conduction (α) lasts up to about 150°. The TRIAC
turns off at point B at α + β = 250° 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
β of 100°, that is, 350° from the starting point.
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