U2102B Datasheet, PDF (8/19 Pages) TEMIC Semiconductors – Multifunction Timer

6. Reverse Phase Control
In the case of normal phase controls, e.g., with a triac, the load current will only be switched on
at a certain phase angle after the zero crossing of the mains voltage. In the following zero cross-
ing of the current, the triac gets extinguished (switched-off) automatically. Reverse phase control
differs from this in that the load current is always switched on by a semiconductor switch (for
example, IBGT) at the zero crossing of the mains voltage and then switched back off again after
a certain phase angle α. This has the advantage that the load current always rises with the
mains voltage in a defined manner and thus keeps the required interference suppression to a
minimum.
The charging current for the capacitor C₃ at pin 2 is set with the resistor R₃ at pin 3. When the
synchronization circuit recognizes a zero crossing, an increased charging current of I₂ ≈ 4 × I₃
is enabled which then charges C₃ up to ≈ 0.45 V. The output stage is switched on at this value
and the charging current for C₃ is reduced to I₂ = I₃. Since the actual zero crossing of the supply
voltage occurs later than recognized by the circuit, the load current starts to flow quite close to
the exact zero crossing of the supply voltage. While the output stage is switched on, C₃ is
charged until the control voltage, set externally at pin 4, is reached. When this condition is
reached, the output stage is switched off and C₃ is charged again with the increased current (I₂
≈ 4 × I₃) to V₂ ≈ 5.5 V. The charging current is switched off at this point and C₃ is discharged
internally. The whole process then starts again when the circuit recognizes another zero cross-
ing (Figure 3-3 on page 7).
Figure 6-1. Signal Characteristics of Reverse Phase Control
V_mains
t
V₂
1.1 V × V_Ref
V₄
0.09 V × V_Ref
t
V₁₄
t
8 U2102B
4767B–INDCO–10/05