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Low-power MCUs in line-powered apps

Posted: 01 Apr 2005 ?? ?Print Version ?Bookmark and Share

Keywords:mcu? embedded? power management? transistors? circuit topology?

It is ironic that small line-powered appliances with an almost limitless power source have so much trouble powering a device as small as a microcontroller (MCU). Although the MCU circuit may only require 10mA to operate, low-cost resistive or capacitive transformerless power supplies can dissipate over a Watt of power just to deliver this tiny amount of energy.

While both transformerless topologies are less expensive than the transformer-based alternatives, they can still be made more cost-effective by reducing the amount of current the MCU circuit consumes. One way to do this is to replace the existing higher-current MCU with a low-power one developed for battery-powered applications.

For example, reducing the current consumption of the MCU circuit from an average of 10mA to less than 1mA will lessen the power dissipated in a resistive transformerless power supply by a factor of at least 100:1. Suddenly, the large and expensive power resistors in a resistive transformerless power supply can be replaced with smaller, cheaper resistors. Or the large and expensive micro-Farad size X2 capacitor, commonly found in capacitive transformerless power supplies, can be traded for a much less expensive nano-Farad size capacitor. Lower current draw can significantly reduce the cost of components in both resistive and capacitive transformerless power-supply designs.

Here are five recommendations on how to reduce current draw by 90 percent:

?Replacing older MCUs with newer, low-power ones.

?Reducing the clock frequency and supply voltage.

?Putting the MCU to sleep when it is not needed significantly reduces current consumption.

?Using the latching nature of triac bidirectional rectifiers allows the discontinuation of the trigger current once conduction is established, further reducing current requirements.

?Substituting LEDs with LCD indicators can save 10s of milliamps in supply current.

To illustrate the potential savings, examine a timer/controller for a hot plate or crockpot cooker. Assume the design is MCU-based with a TRIAC for switching 110VAC to the load and six LEDs and two push buttons to select the time delay. The entire design is powered by a resistive transformerless power supply.

Assuming the TRIAC requires 5mA to trigger, the LEDs (which only light one at a time) require 4mA to light and the MCU requires 1mA to operatehence, the total power budget for the system is 10mA. This means that the power supply will dissipate approximately 1.2W in R1 and R2 to supply this current to the circuit.

2.4W = (110Vac - 5Vdc) (10mA + 10mA + 3mA)

The two 10mA values refer to the 10mA current to run the circuit during the positive half of the cycle and the current to charge C1, which supplies the 10mA supply current during the negative half of the cycle. The additional 3mA is the bias current to hold the zener diode D1 in regulation at 5.1V during the positive half of the cycle.

Applying the above recommendations for reducing current draw:

Replacing the older CMOS microcontroller with a new, low-power one reduces its current requirement from 1mA to less than 800?A.

Using the low-power MCU's ability to run at a lower supply voltage means that reducing the supply from 5Vdc to 3Vdc will shrink the current requirement from 800?A to less than 500?A.

Using the MCU's prescaled internal clock to reduce the clock frequency from 4MHz to 32kHz, the current consumption can be reduced to less than 18?A.

By using a low-power MCU with a 32kHz clock and reducing its supply voltage, we have reduced the current consumption of the MCU by over 98 percent. Continuing on with the recommendations:

If the MCU spends just half its time asleep, the average current drops to less than 10?A.

If the standard TRIAC is replaced with a sensitive-gate TRIAC, the bias current requirement drops from 5mA to less than 3mA.

If the bias drive to the TRIAC is turned off after the holding current for the device is established, the bias current (over the 60Hz cycle) drive drops from 3mA to less than 50?A average. This number is based on the TRIAC holding current being established within 13?S after the zero crossing.

Using an LCD in place of an LED reduces the display current requirement from 5mA per LED to less than 500?A for three to four 7-segment displays.

We have reduced the original 10mA power budget to 560?Aa savings of almost 95 percent. This results in two important changes in the power supply:

A lower-power zener can be used due to the lower magnitude and variation in current consumption.

The power dissipated in R1 and R2 is now less than 170mW.

170mW = (110VAC - 5VDC) * (560?A + 560?A + 500?A)

Lower power requirement means that 2W resistors in R1 and R2 can be substituted with 1/8W resistors; D1 has already been replaced by a lower-power and less expensive zener; and C1 can be reduced to 12F from the original 220F without incurring a higher ripple voltage.

The final results for the design are a better display, the removal of over 1W of heat from the power supply, improved reliability due to a lower operating temperature, and a significant reduction in the cost of the power supply.

- Keith Curtis

Principal Applications Engineer

Security, Microcontroller and Technology Division

Microchip Technology Inc.

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