Smart power management solution greatly reduces energy consumption

Nowadays, the number of electrical products used in private homes and offices is growing at an extremely fast pace. The problem with most of these products, such as Audio/Video products and office equipment, is that they consume considerable amounts of electrical power during standby. This is a waste of electrical power, money, and at the same time, it has a negative impact on the environment. That is why today, for a large number of products, there are regulations that fix stringent limits for the energy consumption during standby.
This paper proposes an ST smart power management solution for electrical powered devices during standby mode, compliant with the new normative on-power saving

Besides the intelligent power management, a great advantage of the system architecture is its general-purpose usage independent of the product. It could be easily adapted and embedded in consumer devices, office equipment, and home appliances.

Standby normative consumption overview
Standby Mode refers to a product’s state when it is switched off but still connected to the mains power while not performing normal activity.
Active mode refers to a product’s state when it is switched on and running normal functions.

Because many devices stay in standby mode for most of the time – a DTV or a printer, for example - an energy cutback during this phase would determine an overall energy savings.

Current specifications from Energy Star fix the amount of power consumed when the device is in standby mode to be less than or equal to1-2 Watt according to the type of equipment; for instance for Audio/Video products this value is fixed at 1 Watt.


ST proposes a smart power management control, necessary to design systems with sustainable principles.
This is done by adding a general purpose low power microcontroller to the main system that has specific firmware responsible for power control.

Figure 1 shows the block diagram of a generic system that could be of an Audio/Video product, office equipment, or home appliance product. Independent from the application, we can distinguish two parts:

· Main system block
· Power management block


The main system block performs the primary purpose activity of the system; its hardware topology is different for different products, but each one has a main IC controller with an I2C bus and at least one wake-up source.

The main IC controller manages each system function, interfacing with a Standby controller, wake-up sources, and the other devices.

Wake-up sources are defined as events that can command the Standby controller to exit the standby mode and return to the active mode. Common wake-up sources are a remote controller receiver, a keyboard, VGA and HDMI inputs, and so on. A wake-up source could be also a timer event, used to record, start a special operation, or for periodical update.

During active mode the wake-up sources interface with the main IC controller to perform the functions related the command.

The power management block is a general purpose block independent from the main system. The brain of this part is an 8-bit low power microcontroller typically used as Standby controller. It receives inputs from the main IC controller and wake-up sources, switching on and off, through the relay, the full on power supply block that supplies most of the main system. During standby mode, the low power microcontroller and the wake-up sources are powered by the auxiliary power supply.

According to this architecture, the system works in two different modes (fig.2):
- The active mode, when the Main IC controller and other devices are powered.
- The standby mode when only the low power microcontroller and the wake-up sources are powered.

The default state after reset is the active mode.

Active Mode
As soon as the system is plugged in to the mains power, each part of the system is powered and the system is running. The main IC controller will manage each function; it detects user inputs from wake-up sources such as remote controller and front panels; it interfaces with other devices and the low power microcontroller using the I2C bus, where the main IC controller is configured as master and the other devices, low power microcontroller included, are configured as slaves.

In the communication between main IC controller and low power microcontroller during active mode, the main IC controller updates low power microcontroller registers, to program, for instance, new timer events, and to send the standby command request.

Once the Standby controller receives the power off command from the Main IC controller, it enters Standby mode.

Standby Mode
When the low power microcontroller receives the standby command from main IC controller, it puts the system in standby mode, switching off the relay connected to the full on power supply. At standby mode, only the low power microcontroller and the wake-up sources are powered on. During this phase, the low power microcontroller enters halt mode, waiting for an external input or timer event. Once one of these events happens, the low power microcontroller exits from halt mode, decodes the signal received, and switches on the whole system through the relay. The kind and function of wake-up event is communicated through the I2C bus to the main IC controller to perform the related operation; now the system is in active mode, and each normal function runs.

Application example
A simple example of how to implement a standby management control for a consumer application is described below. Only the power management block is described (fig.3), due to the fact that it can be adapted to different main system blocks.

It is based on the ST7FOXA0 - an ultra low cost ST 8-bit microcontroller.
The wake-up sources of the system are an IR receiver for remote controller and a front panel key (power ON/OFF command).

The 8-bit microcontroller and the wake-up sources are powered at 3.3V; they are the permanent live part of the system. The main IC controller, on the other hand, is powered by a main power supply that is switched on and off by the 8-bit microcontroller according to the standby command. The microcontroller interfaces with the main IC controller (present in the main system) through the I2C communication where the 8-bit microcontroller is configured as slave and the main IC controller as master.



The main microcontroller features are:

· Emulated low speed I2C slave interface communication
· Remote controller receiver and decoder wake-up event
· Front Panel wake-up event
· Main power supply command with programmable polarity
· The I2C slave interface is emulated just using 2 I/O pins.
· The remote controller receiver wakes up the system when in standby mode. The decoding routine is implemented using an I/O pin configured as an external interrupt and the 12 bit Auto Reload timer peripheral.

The front panel wakes up the system when in standby. In this system, a power ON/OFF push button is connected to a pin configured as external interrupt.
The main power supply is driven thought a relay using an I/O pin configured as output. To switch the relay on and off the output polarity can be programmed accordingly. During standby mode, the microcontroller is placed in halt mode to further reduce power consumption; it will exit from standby mode using the external interrupt.

Conclusion
To contribute significantly to the reduction of energy consumption, a smart power solution for products during standby has been proposed by ST. The ST system architecture is based on a low power microcontroller, which is able to greatly reduce the energy consumption during standby mode by disconnecting most of the system from the main power. This both saves energy and reduces the impact on the environment.


Captions

Fig.1 General System Block diagram
Fig.2 State transition
Fig.3 Hardware connection
Fig.4 Application flow



Click here for the illustrations:

Figure 1, Figure2, Figure 3, Figure 4