Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Utilizing three or more electrode solid-state device
Reexamination Certificate
1998-12-10
2001-11-27
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Utilizing three or more electrode solid-state device
C327S439000, C327S441000
Reexamination Certificate
active
06323718
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to bidirectional switches for controlling a load connected to the mains. The present invention more specifically applies to “normally-on” switches, that is, switches which are spontaneously in the on-state (i.e., conducting) and the control of which includes opening the switch to block (i.e., disconnect) the load supply.
2. Discussion of the Related Art
FIG. 1
shows a first example of a normally-on bidirectional switch
1
. Switch
1
is essentially formed of a triac
11
connected between two power terminals
12
,
13
. Gate G of triac
11
is connected, via a controllable switch
14
, to a first power terminal
12
or first anode A
1
of triac
11
. A resistor R
1
of high value connects gate G of triac
11
to a second power terminal
13
or second anode A
2
of the triac, so that the series association of resistor R
1
and of switch
14
is connected in parallel on triac
11
.
Switch
1
is meant to be connected in series with a load
15
(Q) between two terminals
16
,
17
of application of an a.c. supply voltage Vac, for example, the mains voltage.
Control switch
14
can be a manual control switch or receive a control signal CTRL provided by an appropriate control circuit.
The operation of a switch such as shown in
FIG. 1
is the following. It is assumed that, at rest, switch
14
is open. When a voltage is applied across switch
1
, that is, when an a.c. voltage is applied between terminals
16
and
17
, a current flows in gate G of triac
11
through resistor R
1
and triggers the triac which remains on as long as it conducts a current, that is, until the zero crossing of the a.c. voltage. This process is repeated for each halfwave of a.c. supply voltage Vac. For example, at the beginning of a positive halfwave, a current flows from terminal
13
through resistor R
1
, and from gate G to anode A
1
of triac
11
, until this current is sufficient to trigger the triac in the first quadrant (positive gate and anode currents). Once triac
11
has been triggered, the current flows therethrough until the end of the halfwave, where the triac turns off. At the beginning of a negative halfwave, a current flows, from terminal
12
, from anode A
1
to gate G of triac
11
and through resistor R
1
, until this current is sufficient to trigger the triac in the third quadrant (negative anode and gate currents).
Resistor R
1
is sized according to the gate current required to trigger the triac and to the maximum acceptable supply voltage (generally on the order of 20 volts) to turn on switch
1
at the beginning of each halfwave. When switch
14
is closed (by an action exterior to switch
1
), gate G and anode A
1
of the triac are short-circuited and the triac can no longer trigger and remains in the off-state.
A disadvantage of a switch such as shown in
FIG. 1
is that, when the triac is maintained in the off-state (switch
14
being closed), resistor R
1
dissipates a high power. Indeed, the triggering current of a triac is relatively high, which does not allow use of a high resistance R
1
while respecting the imperative of a low voltage triggering which characterizes a normally-on switch. Presently, the triacs which are most sensitive at the triggering require a gate current of several mA. This high triggering current is linked to the structure of a triac. A sufficiently high triggering current to avoid that the residual non-recombined loads in the semiconductor cause a restarting of the triac at halfwave ends must indeed be provided.
For example, among the most sensitive triacs manufactured by SGS-THOMSON Microelectronics, the triacs known under denomination Z0103 and Z0402 require a gate current of 3 mA to be triggered.
With such a minimum gate current value and for a triggering voltage of 20 volts, a resistance R
1
on the order of 7 k&OHgr; has to be provided. This results in a dissipated power on the order of 6 watts when switch
14
is closed and when voltage Vac is the 220 V mains voltage.
The implementation of a normally-on bidirectional switch in which the dissipated power is substantially lower than with the switch of
FIG. 1
has already been provided.
FIG. 2
shows an example of such a normally-on bidirectional switch
2
, connected in series with a load
25
between two terminals
26
,
27
of application of an a.c. supply voltage Vac.
As previously, a triac
21
is connected between two power terminals
22
,
23
of switch
2
to which are respectively connected a terminal (for example,
27
) of application of the supply voltage and a first terminal of load
25
. Gate G of triac
21
is connected to an a.c. input of a diode bridge
28
, the other a.c. input of which is connected to terminal
23
, and thus to an anode (for example, A
2
) of triac
21
. A resistor R
2
, in series with a switch
24
of control of switch
2
, is connected between the (+) and (−) rectified voltage terminals of bridge
28
. A thyristor
29
is connected in parallel to the series association of resistor R
2
and switch
24
, its anode being connected to the positive rectified voltage terminal (+) of bridge
28
and its cathode being connected to the negative terminal (−). The gate of thyristor
29
is connected to the connection node of resistor R
2
and switch
24
.
The operation of switch
2
shown in
FIG. 2
is the following.
It is assumed that switch
24
is open. At the beginning of a positive halfwave, a current flows, from terminal
23
, through a first diode of bridge
28
, resistor R
2
, the gate and the cathode of thyristor
29
, the diode of bridge
28
opposite to the first one, then from gate G to anode A
1
of triac
21
. As soon as the current reaches the value required to trigger thyristor
29
, the latter turns on. Afterwards, as soon as the current flowing through thyristor
29
becomes sufficient to trigger triac
21
, the latter triggers, thus short-circuiting all other components of switch
2
. At the beginning of a negative halfwave, a current flows from terminal
22
, from anode A
1
to the gate of triac
21
, through a third diode of bridge
28
, from the cathode to the gate of thyristor
29
, through resistor R
2
, then through a diode of bridge
28
opposite to the third one. As previously, switch
2
is triggered in two steps, by the turning-on of thyristor
29
short-circuiting resistor R
2
and switch
24
, then by the turning-on of triac
21
.
A thyristor
29
is used in order to provide a component that is more sensitive to triggering than a triac. Indeed, it is known to implement thyristors (with a cathode-gate), having triggering current on the order of some hundred &mgr;A, or even less. Accordingly, resistor R
2
can be sized with a much higher value than in the switch shown in FIG.
1
. As a result, when switch
24
is closed to prevent the triggering of thyristor
29
by short-circuiting its gate and its cathode, the power dissipated in switch
2
is much lower.
As a specific example, assuming that thyristor
29
has a triggering current of 100 &mgr;A and taking, as a triggering voltage, the same value as previously (that is, 20 volts), resistor R
2
can have a value on the order of 200 k&OHgr;. As a result, the dissipated power when switch
24
is on is on the order of 100 mW.
If such a switch overcomes the dissipated power disadvantage of the switch of
FIG. 1
, it however has some disadvantages.
A first disadvantage is that it requires a high number of components due to the presence of diode bridge
28
. Further, all these components have to withstand the high a.c. supply voltage (for example, approximately 220 volts).
Another disadvantage is that switch
24
is not referenced to a.c. supply voltage Vac but to the negative rectified voltage terminal of bridge
28
. This makes the control of switch
24
more complex and limits the applications in which such a switch can be used. Indeed, a control circuit isolated from the mains then has to be used (a power supply with a transformer or using an optocoupler). This prevents the use of the s
Bernier Eric
Rault Pierre
Lam Tuan T.
Morris James H.
Nguyen Hiep
STMicroelectronics S.A.
Wolf Greenfield & Sacks P.C.
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