Line-powered control applications need to use every possible power-saving feature and technique if they are to achieve an efficient power supply for low-voltage components. A minimal 50 mW of power to the control section can dissipate over 2 to 3 W in the power supply.
Managing this amount of heat adds to the cost of the unit. However, a few clever design ideas and the latest generation of low-power microcontrollers can significantly reduce heat and improve efficiency in line-powered applications.
Low-power microcontrollers: old vs new
Choosing the right microcontroller is fundamental to reducing power and heat in the design. Whilst older CMOS microcontrollers claim to be low power, only the latest low-power microcontrollers, designed specifically for battery operation, offer the features that can significantly reduce current consumption and provide effective power management. Low-frequency clock options, lower voltages, Sleep modes and low-current optimisation combine to significantly reduce the current consumption of the latest microcontrollers compared to older CMOS versions.
Typically the older versions draw a 1 to 2 mA current, whereas a new 3 V microcontroller with a 32 kHz clock draws less than 18 μA while spending at least 50% of its time asleep. Achieving this reduction in current is not difficult with a new microcontroller if its function is to turn on a Triac every half cycle and to monitor a few pushbuttons.
Triac driving
Triacs are a common choice for switching AC power, due to their latching nature and bi-directional switching capability. However, the latching nature of a Triac has wider implications for the design. Because a Triac is latched-on once it conducts more than its minimal hold current, the bias current to the Triac gate can be discontinued, saving considerable current. Consequently, a 3 mA bias pulse, 300 μs wide, on the gate of a sensitive gate Triac is all that is required to turn on the Triac for the full half cycle of the waveform. This means the 3 mA current pulse, averaged over the 60 Hz half cycle, is actually equivalent to a continuous draw of less than 100 μA. So, a narrow pulse drive on the Triac can save almost 96% of the current traditionally used to control a Triac.
User interface
Low-current LEDs are typically used in older designs as indicators in the user interface. However, LEDs can draw up to 1 to 5 mA per LED, a significant disadvantage in a design trying to conserve microamps. Low-power microcontrollers with on-chip LCD drivers can provide a solution.
A typical LCD driver will draw less than 30 to 40 μA, with an additional 100 to 200 μA to generate the bias voltages for the display. When compared to the 1 to 5 mA for a single LED, the advantages of an LCD become clear. Not only does it draw less current, it also offers designers a flexible and more user-friendly display.
Of course, one of the main drawbacks of LCD displays is their poor readability in low light. The obvious solution would be to add a backlight to increase the display contrast although, if an LED backlight is used, the design would be back to a higher current consumption. An alternative is to use an electroluminescent (EL) backlight with an LCD display which avoids this issue because EL panels can be driven directly from the 110 V a.c. supply with only a small current-limiting resistor. An EL backlight, therefore, would have no impact on the low-voltage current consumption.
Power can also be reduced at the pushbutton inputs of the user interface. Traditionally, this interface consists of one or more pushbuttons with individual resistor pullups that are tied to digital inputs on the microcontroller. While the current draw of the pullup resistors may seem too small an opportunity for improvement, the 'weak pullup' feature of new low-power microcontrollers can still help to reduce power consumption. Weak pullups offer a comparable current source, without the cost of the external resistors. They can also be enabled and disabled in software, which can be used to limit their current consumption to only those times when the microcontroller is actually reading the state of the pushbutton.
Overall savings
The combination of power-savings in the microcontroller, user interface and TRIAC driving stages of the design can be significant compared to a traditional circuit.
The traditional design, using a CMOS microcontroller, a Triac, two pushbuttons, and six LEDs, would draw approximately 10 mA: 3 mA for the Triac, 5 mA per LED (assuming only one is lit) and 2 mA for the microcontroller. To create this current at 5 V, over 2,4 W will have to be dissipated in the power supply:
2,4 W = (110 V a.c. - 5 V d.c.) x (10 mA + 10 mA + 3 mA)
The power-supply current during the positive half of the cycle is 10 mA, with another 10 mA to charge the bulk capacitor which supplies current during the negative half cycle. A further 3 mA is used to bias the zener diode.
Using the current reduction techniques discussed above, the current consumption for an equivalent design with a low-power microcontroller would be less than 400 μA: 100 μA average for the Triac, 240 μA for the LCD, and 18 μA for the microcontroller. To create this current, the power supply will only dissipate 140 mW, achieving a reduction of 2,25 W of power compared to a traditional design based on a CMOS microcontroller.
140 mW = (110 V a.c - 3 V d.c.) x (400 μA + 400 μA + 500 μA)
This lower dissipation means that 0,25 W resistors can be substituted for the 3 to 4 W resistors in the traditional power supply. Also, a lower-power zener diode, with a bias of only 500 μA, can be used. Finally, the bulk capacitance can be reduced to one seventh the size of a traditional design. The end result is cost savings, plus a design with an enhanced display for easier use and improved customer appeal.
Implementing power-savings
All of the power-reducing features that are referenced in this article are integrated into Microchip Technology's PIC family of Flash-based 8-bit microcontrollers with nanoWatt technology. They include the lower operating current, clock-frequency controls and sleep mode with options for weak pullups and an on-chip LCD-display driver. Other peripheral options include EEPROM data memory, Capture/Compare/PWM timer functions, 10-bit ADC modules, comparators and various hardware serial-communications peripherals. This combination of low-power features and a wealth of peripheral options gives the system designer the versatility to reduce system power consumption whilst increasing reliability and performance as well as minimising cost by eliminating external components.
Summary
The latest versions of low-power microcontrollers can be used to significantly reduce current requirements in offline applications. Savings can be made at the microcontroller, user interface and Triac-driving stages of the design by using a microcontroller which combines low-power optimisation, weak pullups and an integrated LCD-driver. The overall savings can reduce power-consumption from the typical 2,4 W of a conventional design, to just 140 mW.
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