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Tiny MCU hosts dual dc/dc-boost converters

23 Mar 2016  | Dhananjay Gadre

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The total period of the PWM wave is T and is a system constant. D is the duty cycle of the PWM wave, and TR is the time during which the diode conducts. At the end of TR, the diode current falls to 0A. The period of the wave is T>D×T+TR for DCM. The difference of the PWM period, T, and (D×T+TR) is the dead time.

The switch that operates the inductor is usually a BJT (bipolar-junction transistor) or a MOSFET. A MOSFET is preferable because of its ability to handle large current, better efficiency, and higher switching speed. However, at low voltages, a suitable MOSFET with low enough gate-to-source threshold voltage is hard to find and can be expensive. So, this design uses a BJT (figure 2).


Figure 2: An Atmel Tiny13 AVR microcontroller regulates two boost-dc/dc-converter outputs using its internal ADCs and PWMs.


Microcontrollers offer PWM frequencies of 10kHz to more than 200kHz. A high PWM frequency is desirable because it leads to a lower inductor value, which translates to a small inductor. The Tiny13 AVR microcontroller from Atmel has a "fast" PWM mode with a frequency of approximately 37.5kHz and a resolution of 8 bits. A higher PWM resolution offers the ability to more closely track the desired output voltage. The maximum inductor current from Equation 1 is 0.81A for a 20-µH inductor. The transistor that switches the inductor should have a maximum collector current greater than this value. A 2SD789 NPN transistor has a 1A collector-current limit, so it is suitable for this dc/dc converter. The maximum load current achievable with these values, from Equation 4, is 54 mA and thus meets the requirement of maximum required load current for an output voltage of 7.5V.

The Tiny13 microcontroller boasts two high-speed PWM channels and four 10bit ADC channels. Another PWM channel and an ADC channel create the second dc/dc converter for an output voltage of 15V and a maximum load current of 15 mA. The inductor for this converter has a value of 100µH. To calculate the output-capacitor value, use Equation 6. For a 5-mV ripple, the value of the capacitor for 7.5V output voltage is 270µF, because the output current is 50 mA and the PWM-time period is 27µsec, so this circuit uses the nearest larger value of 330µF. Similarly, for the 15V output voltage, the required capacitor value is 81µF, so the design uses a 100-µF capacitor.

The programs for the microcontroller are in C and use the open-source AVR GCC compiler. The AVR Tiny13 microcontroller operates at an internal clock frequency of 9.6MHz without an internal-clock-frequency divider, so the PWM frequency is 9.6MHz/256=37.5kHz. The internal reference voltage is 1.1V. The main program alternately reads two channels of ADCs that monitor the output voltages in an interrupt subroutine. The main program executes an endless loop, monitoring the output voltage by reading the ADC values and adjusting the PWM values accordingly.


References
1. "Boost converter," Wikipedia.

2. Pressman, Abraham I, Switching Power Supply Design, Second Edition, McGraw-Hill Professional, Nov 1, 1997, ISBN-10: 0070522367, ISBN-13: 978-0070522367.


About the authors
Dhananjay Gadre contributed this article.


This article is a Design Idea selected for re-publication by the editors. It was first published on May 15, 2008 in EDN.com.


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