9
AC switch gate control dimensioning
Gate current I
G
is required to switch an AC switch ON. This I
G
current should be higher than the triggering gate
current I
GT
given in the AC switch datasheet. This parameter is temperature dependent, so recalculation is
required if a parameter like ambient temperature changes.
The
evaluation board embeds three AC switches and all gate control circuits have been
calculated to ensure turn-on for ambient temperature from 0 to 60 °C. It also takes into account fluctuation on
VCC_AC voltage (non-regulated auxiliary voltage referenced to mains voltage) from approximatively 13 V to 21 V.
In case of schematic use in another design, you must recalculate the gate control circuit, especially the R
G
resistor (R103 or R106 or R115+R117 pair). Refer to the formulas below to adjust R
G
resistor values.
X101 Triac
X101 is a T1635T-8FP Triac. The figure below gives the I
GT
parameter value and its variation versus the
temperature
Figure 41.
T1635T-8FP datasheet extract
At 0 °C ambient temperature I
GT
* 1.26 = 44 mA is needed to ensure Triac activation. Considering this current and
VCC_AC minimum voltage, the gate resistors calculation is:
R115 + R117 =
V R115 + R117
IG min
=
VCC_AC min + VGT max − VCEsat
IG min
= 13 + 1.3 − 0.5
44 × 10−3
= 313 Ω
The nearest normalized value is 150 Ω.
You can calculate maximum power in the resistors considering VCC_AC maximum voltage:
IG max =
V R115 + R117
R115 + R117 =
VCC_AC min + VGT max − VCEsat
R115 + R117
= 21 + 1.3 − 0.5
300
= 73 mA
PR115 = R115 × IG max
2
= 0.8 W
Therefore, R115 and R117 are 150 Ω/2 W resistors.
Note:
Use the above formulas to recalculate resistor values in case of schematic use in another design with a different
ambient temperature.
X102 AC switch
X102 is an ACST310-8B AC switch. The figure below gives the I
GT
parameter value and its variation against
temperature.
UM2304
AC switch gate control dimensioning
UM2304
-
Rev 3
page 35/46
