Download ACS Control with opto-Triac

POWER ELECTRONICS
ACS Control with opto-Triac
Laurent GONTHIER
Application Manager for
Appliance &
Industrial segments,
STMicroelectronics
Jean - Michel SIMONNET
Application Engineer for
Appliance &
Industrial segments,
STMicroelectronics
A
C switches are silicon devices which control
AC loads directly connected to the AC
mains. This means that the driving
reference terminal of the AC switch can be
connected to the Line potential. This paper
discusses the need for an insulation layer for the control
unit and the way to implement it for the specific case of
an AC switch device.
Is insulation required between MCU
and AC switch?
The drive reference terminal of an AC switch can be
connected to the Line potential. If an MCU is used to
directly control this device, it will then also be linked to
the Line potential.
It was thought in the past that connecting an MCU to
the Line has to be avoided as it will lead to a poor
appliance immunity. But it has been demonstrated for
many years that such topology has good immunity.
Connecting an MCU supply to a stable non-floating
reference is even better for immunity.
In terms of safety for people, animals and goods,
appliances sold in Europe have to fulfill the 73/23/CEE
directive, called the "low voltage directive".
Manufacturers must then follow some European
harmonized standards (such as the EN60335-2-X for
home appliances, EN60730-2-X for automatic
electrical controls for household or EN60669-2-for
static or mechanical switches for home applications).
These standards specify how to implement operational
or safety insulation. Operational insulation is required
between two conductors, biased by two different
potentials, to allow proper operation of the appliance.
Such an insulation level doesn’t deal primarily with
safety but the requirements also help to minimize
exposure to ignition or fire due a too high leakage
current between two printed circuit board tracks.
Safety insulation has to be implemented between
Antoine PASSAL
Application engineer for
Appliance &
Industrial segments,
STMicroelectronics
accessible parts and high voltage circuits to protect end
users against electric shocks. Several levels of
insulation are defined according to the equipment class
and the way it is implemented.
It’s not required to ensure safety insulation by insulating
low voltage control circuits (like MCU) from highvoltage parts (like AC switches). Indeed the insulation
could be implemented elsewhere, for example on the
keyboard to which the end user has access, and leave
the MCU connected to the Line. This could be cost
effective as a non-insulated power supply and noninsulated-drivers would then be sufficient.
Operational insulation is required when the control
circuit reference is not the same as the AC switch
reference. This is the case with new appliances using an
inverter for 3-phases motor control, where the MCU is
connected on the DC rail, and the ACS switch is
referenced to Line. Then a level-shifter has to be used to
allow communication between the MCU and the power
switch. A usual way to implement this is to use an optoTriac but such a device will not work properly for whole
AC switches.
Could an opto-Triac be used to drive
an ACS?
Among AC switches available today, different
technologies and designs are proposed. The main
known families are the standard Triacs, the snubberless
Fig. 1: ACS symbol (a) and simplified silicon structure (b)
POWER ELECTRONICS
Triacs and also the ACS devices released in the early
90’s.
To switch-on a Triac or an ACS, a gate current must be
applied between Gate (G) and terminal A1 for Triac, or
between Gate and terminal COM for ACS.
For Triac, the gate current could be positive and
negative thanks to the two back-to-back diodes,
implemented between G and A1.
The silicon structure of an ACS is different from a Triac
(see ‎Figure 1). Here the gate is the emitter of an NPN
bipolar transistor. So there is only one PN junction
(implemented by P1 and N1, see ‎Figure 1). The gate
current can then only be sunk from the gate, and not
sourced to it.
The usual solution to trigger a Triac with an insulated
control is to use an opto-Triac connected in series with
the Triac A2 and G terminals. A resistor is also
connected in series to reduce the current applied to the
gate. The device is then triggered by a positive gate
current when the voltage across the Triac, just before
turn-on, is positive, and by a negative current in the
reverse situation. Such a solution works for all Triacs.
The Triac is then triggered in what is called Q1 and Q3
quadrants.
For ACS devices, as they can only be triggered by a
negative current, an opto-Triac will drive the ACS only
when the Line voltage is negative. This will lead to a
load half-cycle conduction and this is not convenient
for most applications. However there are new
applications where such an operation is requested.
This is, for example, the case for some pumps used in
coffee machines which feature an internal diode, and
also for some electromagnets used for door-lock
function in washing machines.
Then an ACS driven by an opto-Triac could be used for
such applications. The only point to deal with is that a
voltage higher than 10 V has not be applied to G
Fig. 2: Solution for half-cycle ACS control with series diode
Fig. 3: Solution for half-cycle ACS control with parallel diode
referred to COM, not to damage the G-COM junction.
To avoid the positive Line voltage to be applied when
the opto-Triac is ON, there are two different solutions:
- adding a high-voltage diode in series with the optoTriac (see Figure 2), to block the positive voltage.
- adding a low-voltage diode in parallel with the
COM-G junction (see Figure 3) to bypass the high
positive bias.
It should be noted that in the 1st case, the opto-Triac
and the diode could be replaced by a reverse blocking
opto-thyristor, with its Anode terminal connected to
ACS gate.
How to implement a full-wave ACS
control with an opto-Triac?
For home appliances, most loads have to be controlled
in full-cycle mode. The previous schematics are then
not adapted to ensure ACS triggering for each cycle.
The solution is to add a low-voltage capacitor to store
some charges to apply a gate current at the beginning
of the positive conduction. The schematic of this
Fig. 4: Schematic for full-cycle ACS control with an opto-Triac
solution is presented on Figure 4, which shows two lowvoltage diodes.
1: Opto-Triac is turned on and charge capacitor C
up to VGT parameter (~ 0.7 V), then COM-G junction
is forward biased and the ACS is triggered by a negative
gate current.
2: ACS is ON up to the next zero current crossing
point. G-COM voltage is kept down to -0.7 V due to
ACS conduction and C is kept charged.
3: As ACS current increases, VG-COM increases
and so a negative current is applied by C which triggers
the ACS for the next cycle.
In this solution, the ACS is off at each cycle beginning
for the time required to recharge capacitor C. The ACS
then turns on when the voltage across its terminals
equals approximately 10 V. This behavior doesn’t result
in high conducted noise as the Line current is still
approximately sinusoidal thanks to the charge current
that flows through capacitor C at Zero Voltage crossing
point.