STZW500L1, the innovative IR transducer for Zero Power Standby
Natale Aiello, Roberto La Rosa, Giulio Zoppi
ABSTRACT
The worsening of the economical and energy crisis is putting
more and more in evidence the need to reduce energy consumption
as much as possible. One of the main issues to solve is reduction of
the power consumption of electrical appliances while in standby.
As it is today, despite the huge effort to continuously reduce
standby power consumption, it is still far from being negligible due
to the enormous number of appliances involved. In this paper a
novel technique based on optical to electrical energy transduction
is introduced with the aim to solve this issue. The proposed
solution goes beyond the concept of the standby state itself as it
actually applies to appliances that are turned off. The innovative IC
STZW500L1 will be shown. It is a specifically designed optical to
electrical energy transducer, capable of starting up any
conventional power supply from a distance of up to 8 meters.
Interestingly, this result is achieved by using a conventional IR
remote control as an optical power transmitter. Experimental
results of a complete working prototype will be shown.
INTRODUCTION
A large set of electronic appliances are remotely controlled
and able to stay in standby mode with very low power
consumption. While in standby, most of the circuitry is turned off,
except for the power management components which supply the
MCU and the wireless front end (IR, RF, etc.). For this reason
some energy is wasted in order to keep the system ready to receive
a wake up signal. Although the power wasted in this condition by a
single appliance is normally not very high, (it usually ranges from
around 300mW up to few watts) its consumption becomes relevant
due to the large number of appliances kept in standby. In detail,
since the power management components are permanently on,
their efficiency must be carefully considered. The current trend is
to constantly improve it with careful design at the circuit level. The
main problem is that it is very difficult, if not impossible, to design
a power management architecture with optimal efficiency at both
high and light loads. The state-of-the-art approach is to use two
different power management units (Fig.1): the Main Power Supply
(MPSU) specifically designed to be efficient with high loads, and
the Low Power Supply (LPSU) optimized to be efficient with very
light loads. Although this solution improves overall efficiency, it is
costly in terms of bill of materials (BOM) and it does not really
solve the problem. In this paper, we present an innovative
architecture, which uses the novel high voltage Optical to
Electrical energy transducer STZW500L1 (specifically designed
by STMicroelectronics).
This circuit is used to turn on an electronic appliance in
standby by means of a conventional IR remote controller.
Fig. 1: State of the art solution block diagram
SYSTEM CONCEPT
The very basic idea is to efficiently convert the small optical
energy into sufficient electrical energy to switch on any electrical
appliance [1,2]. One of the main problems is that the optical
energy provided by a conventional IR remote controller is not very
high, as this is not meant to supply energy. For this reason, the
transducer must be high voltage, very efficient, and carefully
designed. Also very important is the transducer leakage current
consumption, which is one of the key parameters to pay attention
to in the design. This performance very much depends on the
environmental light conditions and on the use case of the
appliance. In very typical cases such as an office environment, this
consumption is as low as a few uA, while in dark conditions it is in
the order of few nA. Another issue with this new approach is how
to filter out the infrared light noise always present in any
environment. This is to prevent unwanted starts.
Fig. 2: Zero Power Standby consumption, system concept
The light noise has a static and a dynamic component, thus the
problem is to adequately filter both. While the static infrared noise
is filtered by the dc block capacitor C1 (Fig. 2), the dynamic
component is filtered out by using a digital decoder which, during
start-up, is directly powered by the high voltage transducer (Fig.
2). This digital decoder allows the turn on of an electric appliance
only if the embedded code in the start-up burst is acknowledged. In
this way, a robust and safe turn on is achieved. In case the code is
not acknowledged, the digital decoder will keep the system
completely off with only leakage current consumption from the
mains.
the digital decoder allows the PWM controller to start only if the
embedded code in the startup burst is acknowledged. Only at this
point, the power supply is self-biased through the dedicated
winding Lw of the transformer. When the system is off, though
still plugged in, it does not consume power and can be turned on
remotely by using the optical energy. It is relevant to note that in
this architecture, the auxiliary low power supply is no longer
needed, with the advantage of reducing the BOM and circuit
complexity.
STZW500L1 IN A CONVENTIONAL SMPS
In this section, we show how the proposed architecture can be
integrated in a conventional switching power supply.
Fig. 4: SMPS architecture with Zero Power Standby consumption
STZW500L1 WITH MECHANICAL RELAYS
Fig. 2: Conventional SMPS architecture
Figure 3 shows a conventional switching power supply [3]. In
this circuit the PWM controller is biased through the VCC input.
The Rstup resistor and the zener diode DZ1, provide the proper
voltage (about 10-15 volt) to start the controller. The startup
circuitry is needed at start up and is no longer used when the
system is running. The controller is permanently supplied through
the dedicated winding Lw of the transformer. Without the resistor
Rstup, the power supply would not start up unless it was replaced
with a (mechanical or optical) switch either which enables the
system to be remotely controllable.
Figure 4 shows an innovative circuit architecture that uses the
STZW500L1 device for the start up. By using the ST transducer, if
the system is turned off while plugged in, there is basically no
power consumption (only leakage). The energy sent by the remote
controller is converted into electrical energy, and a well-defined
current charges the capacitor Cvcc in response to the IR burst until
the startup threshold of the PWM controller is reached.
In order to obtain a robust circuit able to withstand any
accidental noise, spikes, etc., the IC is designed so that the voltage
across Cvcc reaches the threshold voltage of the PWM controller
in several steps, so it appears as a ladder (Fig.4 and Fig.7). Further,
In this application (Fig.5) the idea is to directly drive a relay
(RLY1). When the system is off, there is no power consumption
from the mains, as the relay, which supplies the appliance, is
normally open. To turn on the system, the optical energy converted
into electrical energy supplies the digital decoder. This, upon
recognition of the embedded code, turns on the transistor which
drives the coil of the relay. At the end of the IR burst, the transistor
no longer drives the relay, which switches back to open. For this
reason, some extra circuitry must be added to keep the relay
shorted. Two different solutions are possible: either use a step or a
latch relay, or provide a proper self-bias voltage to the digital
decoder from the mains. In the latter case, the energy to switch the
relay is still provided by the optical burst, while the energy to keep
the relay shorted is provided by the mains. In the case of the step
or latch relay, no energy is needed to keep it shorted.
Fig. 4: Remote relay driving circuit with Zero Power Standby consumption
EXPERIMENTAL DATA
A board prototype (Fig.6) which includes a SMPS with the
STZW500L1 high voltage optical startup has been developed.
the desired value by design, with the only side effect being an early
saturation of the optical transducer. In the best case scenario of
dark conditions, the proposed system is four orders of magnitude
better performing than an advanced conventional system. In the
typical case, such as an office environment, it is two orders of
magnitude better performing, while in the worst case, direct
sunlight, the power consumption is comparable.
Figure 8 shows the leakage losses versus time of a board that
has been placed near a window facing south in summer (June
2014) for 24 hours of light exposure. The maximum power
consumption due to leakage is 70mW at noon (direct sunlight) and
goes to 1.4mW at night with an average “standby” power
consumption of 18mW.
Fig.6: Board prototype
The circuitry of the board is based on the architecture
described in Fig. 4. As shown in Fig. 7, without the IR burst, the
circuit is not supplied. After several IR bursts, the VCC node gets
energized, and its voltage increases enough to properly supply the
digital decoder. After the code has been detected, the
PWM_ENAB goes up, and the power management is self-supplied
from a secondary winding of the transformer. After the IR burst
(start-up sequence) has ended, the system continues to work.
Fig. 8: Power consumption in laboratory during 24 hrs
CONCLUSIONS
In recent years, research and a focus on low power design
have led to a gradual decrease in standby power consumption.
However, total standby power will likely grow in the future as the
number of remotely controlled home appliances increases. In this
work, we have proposed, implemented, and tested a new remote
control system that decreases by up two orders of magnitude the
standby power in typical use case conditions, reaching “zero”
(100nA) in deep dark condition.
Fig. 7: Experimental results
REFERENCE
Experimental results on the board prototype have shown that
a conventional IR remote controller is able to turn on the board
from a maximum distance of 2.5 meters, without optical aid, in any
environmental lighting condition. By using a Fresnel lens (1
centimeter diameter), which improves the optical energy transfer,
the maximum distance has been improved up to 8 meters. While
the system is off, power consumption is limited to the leakage
current of STZW500L1. This current consumption is very much
dependent on the environmental lighting condition, as shown in
Fig. 8, being as low as 100nA in very dark conditions, about 4uA
in an office environment rising up to 100 mA in direct sunlight
(worst case scenario, unlikely to occur). In the latter case the
current consumption is no longer negligible and can be limited to
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