ST AN2986 Application note
AN2986
Application note
AC switch triggering with 3.3 V power supply
Introduction
This document focuses on the calculation of gate current consumption of various types of
AC switches when using a 3.3 V power supply. Until now, the standard of a 5 V power supply
was used. In home appliance applications there is a bigger focus on the MCU working with
a 3.3 V power supply, and a lower supply voltage brings additional constraints on gate
circuitry. This application note provides a precise calculation process in order to correctly set
the necessary parameters.
This application note concerns Q2 and Q3 operation (i.e. for negative power supply) as this
is the most common mode of operation for AC switches, but advice and comments for
operation in Q1 and Q4 quadrants (i.e. for positive power supply) are also given.
July 2009 Doc ID 15727 Rev 1 1/18
www.st.com
Contents AN2986
Contents
1 Gate resistor choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Variation of gate current with temperature . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Resistor according to environment accuracy and variation . . . . . . . . . . . . 6
1.2.1 Parameters variations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.2 Gate resistor calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2 Gate current variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1 VGT variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Minimum current calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3 Maximum current calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.4 Summary of calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3 Proposed solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.1 MCU current capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2 Average current consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2/18 Doc ID 15727 Rev 1
AN2986 List of tables
List of tables
Table 1. Output low level voltage (V
) for STM8S and STM32 . . . . . . . . . . . . . . . . . . . . . . . . . . 7
OLmax
Table 2. Current consumption of different devices and MCUs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Table 3. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Doc ID 15727 Rev 1 3/18
List of figures AN2986
List of figures
Figure 1. Circuit for direct driving of ACS/TRIAC from MCU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 2. Gate current variation with temperature for Z01 device . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 3. V
Figure 4. Typical V
Figure 5. Typical V
Figure 6. Schematics of using buffer transistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
variation with load current in quadrants 2 and 3 (0.2 A RMS) for Z0103
GT
(T
= 85 °C, IG0 = 7.5 mA). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
j
GTpeak
GTpeak
versus I
versus I
for Z0103 (IG0 = 7.5 mA) . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Tpeak
for ACS108-6T (IG0 = 15 mA) . . . . . . . . . . . . . . . . . . . . . . . 10
Tpeak
4/18 Doc ID 15727 Rev 1
AN2986 Gate resistor choice
1 Gate resistor choice
1.1 Variation of gate current with temperature
The value of the gate resistor (refer to RG in Figure 1, where a negative power supply is
used) is determined to ensure proper turn-on of the ACS™/TRIAC in the temperature range
specified by the application. The minimum gate current required for turn-on of the
ACS/TRIAC increases as the junction temperature decreases. This variation of minimum
gate current is described in the datasheet “Z01 - 1 A Triacs”, see example in Figure 2. The
worst case appears when junction temperature T
Minimum ambient temperature is set to 0 °C for most home appliance applications.
is at minimum ambient temperature.
j
The gate current (I
) level is specified for 25 °C. At temperature 0 °C this level is usually
GT
increased to a range 1.3 - 1.5 times higher for most TRIACs and AC switches.
Figure 1. Circuit for direct driving of ACS/TRIAC from MCU
Load
L
I
T
OUT/A
2
ACS/TRIAC
V
N
V
AC MAINS
T
COM/A
V
S
1
R
G
G
V
G
V
DD
I
G
D
MCU
1
M
1
+
V
D
SS
M
2
2
AM00456
Figure 1 also gives the references for the electrical parameters (VT, IT, VG) which are used in
this application note. Please note that voltage V
is referenced to the G terminal, not the A1
G
or COM terminal in order to get positive values when the current is sunk from the gate.
Doc ID 15727 Rev 1 5/18
Gate resistor choice AN2986
Figure 2. Gate current variation with temperature for Z01 device
1.2 Resistor according to environment accuracy and variation
Temperature is not the only parameter influencing gate resistor choice. Other parameters
have also to be taken into account. Parameters discussed in this section are related to the
accuracy of the components used and to the working environment.
1.2.1 Parameters variations
All the different application parameters that have an impact on gate current level are listed
below.
Power supply voltage (V
taken into account. In appliance applications a capacitive power supply is often used. The
main part of this power supply is the Zener diode. Stability of the Zener voltage is the best at
5.1 V and 5.6 V. Accuracy of this power supply is ±10% at 5 V.
Stability of the diode at 3.3 V and 3.9 V is worse and ±0.6 V (±20%) can be achieved. This
result leads to the use of switch mode power supply (SMPS) for a 3.3 V power supply.
Output voltage levels on the I/O pins of the MCU are important variable parameters. The
most important parameter is low output level (V
when tied to ground. This parameter varies with the current flowing through this pin. The
value is given by the MCU datasheet (for example: STM8S10xxx).
) variation is one of the most important parameters that has to be
DD
) which specifies voltage on the output pin
OL
6/18 Doc ID 15727 Rev 1
Loading...