GB2467590A

GB2467590A - Power supply and control apparatus for an environmental conditioning system

Info

Publication number: GB2467590A
Application number: GB0902129A
Authority: GB
Inventors: Kanthimathinathan Thirugnanasambandham; Vignesh Eswara Prasad; Balaji Arcot Sundaravadivelu
Original assignee: Novar ED&S Ltd
Current assignee: Novar ED&S Ltd
Priority date: 2009-02-09
Filing date: 2009-02-09
Publication date: 2010-08-11
Anticipated expiration: 2029-02-09
Also published as: CN101800425A; GB2467590B; GB0902129D0

Abstract

A power supply and control apparatus for an environmental conditioning system comprises a relay R1, a semiconductor switch Q1, an AC load and a DC load, where the relay and semiconductor switch are arranged in series between an AC mains supply and the AC load, and where the DC load is control electronics including a plurality of capacitors C1. The capacitors are arranged such that one of the capacitors continues to supply stored energy even if another capacitor fails. The capacitors are charged using the load current and the stored energy used to run the control electronics. As the charge is used the capacitor is recharged as and when required. The MOSFET Q1 is used as a switch, forcing the load current to charge the capacitors, power stealing (fig. 1(a)), and then to bypass the capacitors and supply the AC load (fig. 1(b)). The AC load is an environmental conditioning load and may be at least one of a lighting load, a heating load and an air conditioning load. The control electronics may be arranged to detect shorting of one or more of the capacitors and operate the relay accordingly, or to sense the load current and operate the relay if it exceeds a predetermined threshold.

Description

An environmental conditioning system

Field of the Invention

The present invention relates to the field of

environmental conditioning systems, that is systems provided in buildings to condition the building's internal environment; such a system may include for example lights, fans, (de-)humidifiers, and other air-conditioning apparatus. More particularly, the invention relates to an apparatus for controlling an environmental conditioning system. Still more particularly, but not exclusively, the invention relates to an apparatus for controlling an environmental conditioning system for installation in a two-wire AC electrical supply installation.

Background of the Invention

A two-wire AC electrical supply installation is a mains-powered electrical installation that does not have a neutral connection. Control of environmental conditioning apparatus installed within a two-wire system is usually achieved simply by providing a switch in series electrical connection between one terminal of the AC mains and the apparatus.

However, an issue arises when more sophisticated control is desired. For example, remote control of an environmental-control installation is desirable, but a remote-control receiving station requires power to operate.

In a three-wire system, power for the control can be obtained from the three wires of the installation, but if power is taken from the wires in a two-wire system, then that power is lost to the environmental conditioning load.

One possibility is to install dedicated additional power supply wires to the control system, but that is undesirable as installation of new wires in a building is disruptive.

Another possibility is to supply the required power from batteries, but that is a relatively expensive solution, and frequent replacement of batteries can be inconvenient for the user.

A known approach to obtaining power for a control apparatus in a two-wire system is known as "power stealing".

In this arrangement, a switch is used to rapidly switch the load current through an energy storage device, with the current being at least in part diverted from the load to the control apparatus. In example prior-art systems, a semiconductor switch (e.g. a field-effect transistor (FET), metal-oxide FET (MOSFET), or a triac) is used to switch the current from the load to the control circuitry at least once every cycle of the AC supply (i.e. at least once every 20 ms for a 50Hz supply) . Diverting the current in that way, for a duration that is short compared with the duration of the mains cycle, produces a more-or-less indiscernible effect on the performance of the load, but can be used to provide sufficient power for control electronics. That can be achieved for example by using the diverted load current to power a capacitor, discharge from which can then be used to power the control electronics after the mains supply has been switched back to the conditioning load.

An example of a prior-art "power-stealing" arrangement can be found in W02008/103491, which describes a load-control device adapted to be disposed in series with an AC voltage source and an electrical load and operable to provide substantially all voltage provided by the AC voltage source to the load. The load-control device comprises a controllably conductive device, a controller, a zero-crossing detector, and a power supply for generating a substantially DC voltage for powering the controller. The power supply is operable to charge an energy storage device (in one described example, a 680 microfarad capacitor) to a predetermined amount of energy each half-cycle. The controller is operable to determine when the power supply has stopped charging from the zero-crossing detector each half-cycle, and to immediately render the controllably conductive device conductive to conduct the full load current. Before the controllably conductive device begins to conduct each half-cycle, only a minimal voltage is said to develop across the power supply to allow the energy storage device to charge.

A large value of capacitor is required to power the control electronics in an environmental-conditioning installation. However, electrolytic capacitors, if used in a utility switch circuit, must be rated according to prescribed safety standards. For example, BS EN 60669-2- 1:2004, Clause 102.2: Capacitors says that short-circuiting of a capacitor which would cause a current of 0.5A or more through the terminals of the capacitor must comply with IEC 60384-14 and be in accordance with table 107 which, in turn says that, if a capacitor is connected between live and neutral or live and switched live, and if the capacitor is not in series with an impedance, then the capacitor has to be X rated. The X safety rating for capacitors is defined in IEC 60384-14: X-rated capacitors are specially fabricated capacitors, which are tested for high voltage ratings, ensuring high reliability under extreme transient conditions on the line.

Thus EN60669 calls for an X safety rating, if the capacitor is placed across line-in and line-out, and the capacitance required is of the order of a few hundred microfarads. However, an X-rated capacitor rated in microfarads is very bulky, making it unsuitable for many environmental-conditioning applications (in which it is often desirable for control units to be as small as possible) . Furthermore, in the event of the semiconductor switch failing open, the load current can flow through the capacitor, raising the voltage across the capacitor to dangerously high levels in a few milliseconds.

The present invention seeks to mitigate the above-mentioned problems.

Summary of the Invention

The present invention provides control apparatus for an environmental conditioning system, comprising: a first terminal for connection to an AC mains supply, a second terminal for connection to an environmental-conditioning load, a relay arranged between the first terminal and the second terminal, a first semiconductor switch arranged between the first terminal and the second terminal, and control electronics; wherein the control apparatus further includes a plurality of electrolytic capacitors arranged in parallel with each other; and wherein the relay is operable to switch on and off current from the AC mains supply to the load, and, when the relay is set to supply the current to the load, the first semiconductor switch is operable to divert at least part of the current through the plurality of capacitors; and wherein the capacitors are arranged to store energy from the mains supply when the current is diverted, and to supply the stored energy to the control electronics, when the current is not diverted, the capacitors also being arranged so that at least one of the capacitors continues to supply stored energy to the control electronics even in the event of failure of at least one other of the capacitors.

Thus, provision of a plurality of capacitors, means that, in the event of failure of at least one of the plurality of capacitors, the control electronics is still powered (by at least one of the remaining capacitors); preferably, the control electronics is still able to switch off power to the load, and hence prevent catastrophic failure of the apparatus. The control electronics may for example be arranged to detect shorting of one or more of the capacitors, and may comprise a relay control arranged to operate the relay to switch off current to the load if shorting of one or more of the capacitors is detected.

The capacitors may be arranged in an electrical OR arrangement, in order that at least one of the capacitors continues to supply stored energy to the control electronics even in the event of failure of at least one other of the capacitors.

It may be that the first semiconductor switch is operable to divert part or all of the current through the

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plurality of capacitors. It may be that no or substantially no current is diverted from the load once the capacitors are charged or sufficiently charged.

The first semiconductor switch may be for example a FET (e.g. a MOSFET) or a triac.

The first semiconductor switch may be arranged in series with the relay. The first semiconductor switch and the relay connected between live in and live out.

The current sensor may be a hardware device or circuitry, or it may be an algorithm implemented in a microcontroller.

The control electronics may further include a voltage sensor arranged to sense voltage across the capacitors and a second semiconductor switch arranged to short-circuit the first semiconductor switch (and preferably the capacitors) if the voltage across the capacitors exceeds a predetermined bypass threshold. The second semiconductor switch may be for example a FET (e.g. a MOSFET) or a triac. The control device may further comprise a microprocessor, microcontroller or other suitable hardware arranged to receive data indicative of the sensed voltage and to generate a signal to operate the second semiconductor switch to short-circuit the first semiconductor switch if the voltage across the capacitors exceeds the predetermined bypass threshold. The control electronics may be arranged to turn off the relay when voltage across the capacitors exceeds a predetermined threshold.

The environmental conditioning load may comprise a lighting load. The environmental conditioning load may comprise a heating load. The environmental conditioning load may comprise an air-conditioning load.

The electrolytic capacitors may each have a capacitance of more than 100 microfarads. The electrolytic capacitors may each have a capacitance of more than 200 microfarads.

The electrolytic capacitors may have a capacitance of less than 600 microfarads, preferably less than 500 microfarads, more preferably less than 400 microfarads.

The plurality of capacitors may consist of 2 capacitors. The plurality of capacitors may consist of more than 2 capacitors. There may of course be other capacitors in the control device, but the relevant capacitors are those arranged to store mains energy and then subsequently supply the stored energy to the control electronics, as described above.

The control electronics may be arranged to sense the current to the load and may comprise a relay control arranged to operate the relay to switch off current to the load if said current exceeds a predetermined cut-out threshold. The control device may comprise a microcontroller or microprocessor or other suitable hardware arranged to receive data indicative of the current to the load and to generate a signal to operate the relay to switch off power to the load if said current exceeds a predetermined cut-out threshold.

Example embodiments of another aspect of the invention provide a control apparatus for an environmental conditioning system, comprising: a first terminal for connection to an AC mains supply; a second terminal for connection to an environmental-conditioning load; a relay arranged between the first terminal and the second terminal; a first semiconductor switch arranged between the first terminal and the second terminal; and a control device

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comprising control electronics, wherein the relay is operable to switch on and off power to the load and to the first semiconductor switch, and, when the relay is set to supply power to the first semiconductor switch, the first semiconductor switch is operable to switch power from the AC mains supply away from the load and to the control device.

The control device includes a plurality of electrolytic capacitors arranged in parallel with each other, the capacitors being arranged to store energy when the mains power is supplied to the control device and arranged to supply the energy to the control electronics when the mains power is supplied to the load. Preferably, the control device further includes a current sensor arranged to sense current supplied to the capacitors, and a relay control arranged to operate the relay to switch off power to the load and to the first semiconductor switch if the current sensed by the current sensor exceeds a predetermined cut-out threshold. Preferably, the control device further includes a voltage sensor arranged to sense voltage across the capacitors and a second semiconductor switch arranged to short-circuit the first semiconductor switch and the capacitors if the voltage across the capacitors exceeds a predetermined bypass threshold.

It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention.

Description of the Drawings

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Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 is a schematic circuit diagram of an environmental conditioning installation including a MOSFET switch for power stealing, showing (a) the current path when MOSFET is off, and (b) the current path when MOSFET is on; Figure 2 is schematic diagram illustrating a zero-cross-synched power-stealing device from the installation of Fig. 1; Figure 3 is diagram showing a MOSFET drain current sense circuitry for use in the example embodiment of Fig. 2; Figure 4 is a diagram showing a relay coil driver for use in the example embodiment of Fig. 2; Figure 5 is a diagram showing a triac and driver for use in the example embodiment of Fig. 2; Figure 6 is a diagram showing a capacitor arrangement for use in the example embodiment of Fig. 2; Figure 7 is a diagram showing capacitor short detection circuitry for use in the example embodiment of Fig. 2.

Detailed Description

In a lighting system according to an example embodiment of the invention, a lighting load is controlled by a relay switch. When the relay switch is open, control electronics are powered by a LNK3O2-based power supply unit (PSLJ) However, once the relay is closed, the LNK3O2-based PSU cannot operate, as its input voltage is no longer available.

To sustain operation of the electronics, energy is obtained -10 -from charge stored in a capacitor. The capacitor is charged by "power stealing" from the AC load current.

The concept is illustrated schematically in Fig. 1(a) & (b) . A capacitor Cl is charged using the load current, and the stored energy is used to run the control electronics (DC load) . As charge is used, the capacitor is recharged as and when required. MOSFET Q1 is used as a switch, forcing the load current to charge the capacitor Cl and then to bypass the capacitor after sufficient charge has been stored. Fig. 1(a) and Fig. 1(b) show the current path when the MOSFET Qi is OFF and ON respectively. (In this example, the ground reference is the cathode of Cl, and it is live.) The AC load may for example be an incandescent bulb, a capacitive, inductive or switching load; however, in this example embodiment, the relay switch operates in 2-wire mode (without neutral) only for resistive loads; in this example embodiment, for other load types, neutral is available, and in those cases the LNK3O2-based power supply continually provides the electronics with power, eliminating the need for power stealing.

Fig. 2 shows in more detail a control apparatus for controlling an environmental-conditioning load. The example shown is for a two-gang installation, having two loads (Loadl and Load2) When either Relay 1 or Relay 2 (or both) are open, power for the control electronics is provided (via the 21V power rail) by the Link Power Supply. However, when both Relay 1 and Relay 2 are closed, the Link Power Supply is bypassed, and it can no longer operate. Capacitor Cl then provides power to the electronics, again via the 21V Power Rail, by power stealing, as described above: power is -11 -alternately supplied to the capacitor Cl and the AC load (Loadi and Load2) by the switching action of MOSFET Qi.

MOSFET Qi is controlled by a MOSFET driver, which is itself controlled by a comparator having a lower trigger point (LTP) set at 22V and an upper trigger point (UTP) set at 24V. The driver and the comparator are powered by the 21V Power Rail.

Thus, the switching of the MOSFET Qi is controlled by the comparator (which in turn switches on and off the MOSFET gate driver) . The comparator is arranged to ensure that a mean voltage of about 23V ((24V+22V)/2) is provided across the capacitor Cl.

However, potential problems would arise with asynchronous switching of the MOSFET Qi, including thermal problems if the Drain-Source diode of MOSFET Qi conducts during the negative half-cycle, and different charging currents depending on when in the cycle the MOSFET Q1 is switched on. Switching at AC mains peaks also results in significant conducted emissions. To minimise the emissions, to minimise the RMS current through the capacitor, and to ensure sufficient charge in the capacitor, power stealing is carried out in every cycle of the AC mains, and it is synchronized with the voltage zero cross of the AC mains.

During synchronised power-stealing, a micro-controller (MCU) resets the comparator (switching off the MOSFET Q1) just before zero cross of the AC cycle; since the MOSFET is off, the capacitor Cl will be charged in the subsequent positive cycle. As soon as the capacitor voltage reaches the UTP of the comparator, the MOSFET is turned on. The MCU computes the time between two successive events of MOSFET drain voltage going high, and uses the computed period to -12 -reset the comparator just before the next zero cross. That computation-reset-recomputation cycle is repeated whenever MOSFET PSU is in operation. The input to the MCU is derived from the MOSFET drain voltage. Whenever the MCU detects a falling edge, it triggers a 200us-width pulse SOOus before the next anticipated zero-cross time. The pulse pulls down the comparator and makes the capacitor charge for the subsequent positive cycle.

Advantages of zero-cross synchronized power stealing include: reduction of body diode conduction period, in turn reducing body diode loss; reduction of conducted emissions; and reduction of flicker of incandescent bulbs (especially lower wattage bulbs) resulting from to the periodic switching of MOSFET.

Attention will now be turned to the protection mechanisms which, in combination, enable the use of electrolytic capacitors as the power-stealing power supply for the control circuitry.

First, the switch includes overload current protection.

The hardware has to withstand a maximum current of 15A for 1 hour (maximum trip time for a lOA Type C MOB), and protective action is taken when the drain current is higher than 15A. For any current above that (due to over-load or short circuit at load side), the relays are opened. The MCU continuously monitors the drain current, using the circuit elements shown in Fig. 3; when that current is too high, the relays are triggered open. CR24 ensures that drain voltages of 25V (when the MOSFET is off) are blocked from reaching the MCU, and the drain voltage is sensed only when MOSFET Ql is ON. R62 and R34 are selected so that the sensed voltage remains within l.lV (ADC Reference voltage) -13 -In this example, the relays used to switch the load current are of the latching type. Use of latching type relays eliminates the need for continuous coil power. Use of a bi-coil type simplifies the required coil driver. The relay contacts are rated for 1OAX, meaning they can withstand a high inrush current (which is typically seen with fluorescent lamp loads) . (In some other embodiments, non-latching relays are used: use of non-latching relays has the advantage that if a capacitor fails short the relay will open automatically, because of the lack of a rail voltage.

However, even a non-latching relay cannot protect the capacitor if the MOSFET fails open.) The coils are driven by NPN transistors (Fig. 4), with integrated base and emitter resistors. As mentioned above, latching type relays are used, with separate coils to open and close the contacts; hence, a pulse of about 30 -4Oms is sufficient to energize the respective coils. The relay driver is a NPN-transistor-based inverter. Practical tests have shown that even if the coils are energized for about 2Oms with a coil voltage higher than l2V, the coil will latch or unlatch successfully.

Second, if the MOSFET itself fails open (or any circuit such as the comparator, voltage level shifter, or driver fails), there would be no limit on capacitor voltage. The line current would keep charging the capacitor towards AC mains voltage; however, capacitors are rated for just 35V.

That being a catastrophic failure, a mechanism is provided to limit the capacitor voltage. Triac Ql8 and associated circuitry does just that (Fig. 5) . If the capacitor voltage goes beyond 27.6V (decided by the 27V Zener CR36 plus the forward drop of CR37), then the triac would be triggered,

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-14 -thereby bypassing the load current from the capacitors. The MCU, which monitors the capacitor voltage, would sense an abnormally high voltage, and the relays would be turned off.

The triac would handle the load current only for one half of the AC mains cycle, and would be ON only until the relays are turned off. That means that the Triac can be rated just enough to handle the maximum rated load current of bA for about 5Oms (the time required for the MCU to conclude that capacitor voltage is abnormal, and the relays' contacts should be triggered open) . CR37 is provided because the triac need not be on during the negative cycle of the AC mains -because the capacitors themselves cannot be charged during negative cycle. R98 limits the gate current of the triac, while R92 and C24 can be adjusted to avoid false triggering of the triac.

In addition, if the relay(s) is closed and AC power resumes, then the MOSFET would be off (because 017 and Cl8 have no charge), and the inrush current would try to charge 017 and C18. That would be dangerous to the circuit, since the capacitors would be charged by inrush currents which could be hundreds of amps in amplitude. Should the capacitors be charged above 27.6V, then the triac would bypass the current, protecting the circuit. By then, power supplies would be up and running and the MOSFET would be turned ON. Practical tests show that the MOSFET is controllable within 2ms from 2lV net reaching minimum of 7.5V, considering all delays involved in control path of MOSFET.

Third, the component selection rules of EN60669-2-1, require that, if any electrolytic capacitor is shorted, then no more than 500mA should be able to flow through the short.

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However, if the capacitors across the MOSFET (C17 and C18) are shorted, then the full load current would flow through the short, violating the standard. Hence, the capacitors are not directly paralleled; rather they are electrically ORed through diodes. CR14, CR17 are charging paths by MOSFET PSU, CR19, CR22 are charging paths by Link PSU, and CR20, and CR21 are discharging paths by onboard electronics (21V rail) . Since the diodes are equivalent, voltages across both the capacitors track each other closely. When any one of the capacitors is shorted, following chain of events would happen (Figs. 6 & 7): Q15 would be forward biased; that would in turn trigger the off coils of both relays through R95, thereby causing the relays to act like a fuse, disconnecting the load. If the switch is configured in 2-wire mode, then the Link PSU would try to start; however, because of a shorted output, LNK3O2 would go to over-current shutdown mode. Next, the MCU (AVR MOO is interrupted through MCU Abnormality Indicate signal) is simultaneously informed about a catastrophe through R91 and R96.

The combination of those three capacitor protection features enables use of electrolytic capacitors that are sufficiently small to be incorporated in compact switch installations for a building.

Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described.

Where in the foregoing description, integers or

elements are mentioned which have known, obvious or -16 -foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims

I-17 -Claims 1. Control apparatus for an environmental conditioning system, comprising: a first terminal for connection to an AC mains supply, a second terminal for connection to an environmental-conditioning load, a relay arranged between the first terminal and the second terminal, a first semiconductor switch arranged between the first terminal and the second terminal, and control electronics; wherein the control apparatus further includes a plurality of electrolytic capacitors arranged in parallel with each other; and wherein the relay is operable to switch on and off current from the AC mains supply to the load, and, when the relay is set to supply the current to the load, the first semiconductor switch is operable to divert at least part of the current through the plurality of capacitors; and wherein the capacitors are arranged to store energy from the mains supply when the current is diverted, and to supply the stored energy to the control electronics, when the current is not diverted, the capacitors also being arranged so that at least one of the capacitors continues to supply stored energy to the control electronics even in the event of failure of at least one other of the capacitors.
2. Apparatus as claimed in claim 1, in which the control electronics is arranged to detect shorting of one or more of the capacitors, and comprises a relay control arranged to operate the relay to switch off current to the load if shorting of one or more of the capacitors is detected.S

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3. Apparatus as claimed in claim 1 or claim 2, in which the capacitors are arranged in an electrical OR arrangement.
4. Apparatus as claimed in any preceding claim, wherein the control electronics further includes a voltage sensor arranged to sense voltage across the capacitors and a second semiconductor switch arranged to short-circuit the first semiconductor switch if the voltage across the capacitors exceeds a predetermined bypass threshold.
5. Apparatus as claimed in any preceding claim, in which the environmental conditioning load comprises at least one load selected from the following: a lighting load, a heating load, and an air-conditioning load.
6. An apparatus as claimed in any preceding claim, in which the plurality of capacitors consist of 2 or more capacitors.
7. An apparatus as claimed in any preceding claim, in which the control electronics is arranged to sense the current to the load and comprises a relay control arranged to operate the relay to switch off current to the load if said current exceeds a predetermined cut-out threshold.
8. A method of protecting electrolytic capacitors in a control apparatus for an environmental conditioning system substantially as herein described with reference to the accompanying drawings.-19 -
9. A control apparatus for an environmental conditioning system substantially as herein described with reference to the accompanying drawings.