Generating High Voltage Using PIC16C781/782 Author: Ross Fosler M

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Generating High Voltage Using PIC16C781/782 Author: Ross Fosler M

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TB053
Generating High Voltage Using PIC16C781/782
Author: Ross Fosler Microchip Technology Inc.
EQUATION
INTRODUCTION
Nixie tube device born middle twentieth century, used display digital information human readable format. Basically, high-voltage numerical display. Today, Nixie tube been replaced more efficient, more durable, longer lasting devices, such displays LCDs. However, this technical brief, Nixie tube serves excellent visual feedback PIC16C782 device's ability generate high voltage from low-voltage source. This technical brief introduces boost converter topology operating Discontinuous mode. example, simple 170V DC-DC converter designed based this topology, used provide power three-digit Nixie tube display. PIC16C782 used control DC-DC converter provides data decoding display. make display useful, PICmicro® samples temperature sensor displays results. peak current achieved moment before turns off. Equation shows peak current, where duty cycle period Pulse-Width Modulation (PWM).
EQUATION
IPEAK current inductor cannot change instantaneously. When switched off, current continues flow through storage capacitor, load, Thus, current inductor decreases linearly time from peak current. discontinuous operation, inductor current actually falls zero. Equation shows this relationship.
BASIC BOOST TOPOLOGY (DISCONTINUOUS MODE)
FIGURE BOOST CONVERTER TOPOLOGY
VOUT
EQUATION
VOUT VOUT IPEAK
Controller
VVFB
During this linear decrease current, energy stored inductor transferred result simple relationship between input output voltage shown Equation This equation derived from simple concept: power equals power out. Refer publications listed under "REFERENCES" more details.
EQUATION
basic boost topology shown Figure input voltage (VIN) always less than output voltage (VOUT). Initially, energy stored inductor when turned From electrical characteristics inductor, current ramps linearly according Equation (assuming inductor series resistance switch resistance negligible). VOUT RLDT
2005 Microchip Technology Inc.
DS91053B-page
TB053
HIGH-VOLTAGE DISPLAY EXAMPLE
Nixie tubes used this design require VDC, peak operating current approximately 0.68 Watts tube. three-digit display, peak operating power slightly over Watts. input supply VDC. Thus, desired power supply design 170V DC-DC converter, with maximum output operating power Watts. programmable functions PIC16C781/782 considered together with design boost power circuit. following options PIC16C781/782 selected which affect DC-DC converter operation: Internal oscillator selected clock FOSC/128 Maximum duty cycle Essentially, this means time MOSFET, about Using Equation function power terms inductance easily derived:
EQUATION
170) Volts tfall 0.655 Amps tfall 1.34
inductor current flowing through total 25.34 period which much larger than total time current flowing. Thus, supply sure stay Discontinuous mode given input load conditions.
CLOSING LOOP WITH PIC16C781/782
control loop closed with PIC16C781/782. Figure shows configuration within PIC16C781/782. Essentially, voltage feedback compared fixed voltage. Digital-to-Analog Converter provides fixed voltage reference. When feedback voltage crosses voltage reference, Programmable Switch Mode Controller (PSMC) output reset. Thus, changing reference voltage provided Digital-to-Analog Converter (DAC) changes output voltage, VOUT.
EQUATION
(170 Volts)2 (P)Watts µS)(9 Volts)2
FIGURE
PIC16C781/782 CONTROL LOOP CONFIGURATION
PIC16C781/782
PSMC
desired peak output power DC-DC converter Watts. achieve this, output power must greater overcome losses voltage conversion. Therefore, inductor size must chosen achieve power output Watts, plus some power loss. inductor chosen this design This means maximum power 2.945 Watts, assuming power loss. With power loss, lowest allowable efficiency chosen inductor 67.9%. Efficiency region reasonable assumption this design should problem. peak current, from Equation inductor
VVFB
EQUATION
IPEAK 0.655 Amps Power inductors, range 0.655A, common readily available. This design intended Discontinuous mode should stay Discontinuous mode throughout load range. Therefore, rise fall time current inductor maximum load compared with switching period. rise time inductor current already known fall time calculated using Equation
This feedback method unusual this topology. Energy transferred from inductor when PulseWidth Modulation (PWM) negative (low output) portion duty cycle. However, PSMC acting feedback control only during positive portion duty cycle. Thus, energy transferred output cycle prior control portion. result pseudo pulse-skip operation, while PIC16C781/782 PSMC mode. Refer "PIC16C781/782 Data Sheet" (DS41171) information about PSMC standard modes operation. smooth output ripple pulse skipping, minimum pulse width PIC16C781/782 25%.
DS91053B-page
2005 Microchip Technology Inc.
TB053
Soft start provided software. slowly increasing voltage reference, output voltage ramps linearly over several hundred milliseconds (Figure Gradually ramping controls current drawn during start-up. This prevents saturating inductor, thus, preventing excessive current through switch. result, smaller used safely.
CONCLUSION
Nixie tubes very much date terms technology have passed into history. However, there some applications that still require high voltage, example, backlighting low-current fluorescent lighting. This application demonstrates ability PIC16C781/782 perform simple DC-DC voltage boost have additional control other system functions.
FIGURE
VOLTAGE OUTPUT REFERENCE
REFERENCES
VREF VOUT +170
Ross, Essence Power Electronics, Prentice Hall, York, 1997. Pressman, Abraham Switching Power Supply Design, McGraw-Hill, York, 1998.
2005 Microchip Technology Inc.
DS91053B-page
TB053
APPENDIX
FIGURE A-1:
7805 TC4427 IRF620
SCHEMATICS
HIGH-VOLTAGE DRIVER DISPLAY CONTROL
+170 Display) PIC16C781/782 350V
Display MCP9700 Temperature Sensor bits
FIGURE A-2:
NIXIE TUBE DISPLAY
+170
Nixie Tubes
from Microcontroller
FMMT497TA
CD4028 FMMT497TA CD4028 FMMT497TA
DS91053B-page
2005 Microchip Technology Inc.
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Microchip received ISO/TS-16949:2002 quality system certification worldwide headquarters, design wafer fabrication facilities Chandler Tempe, Arizona Mountain View, California October 2003. Company's quality system processes procedures PICmicro® 8-bit MCUs, KEELOQ® code hopping devices, Serial EEPROMs, microperipherals, nonvolatile memory analog products. addition, Microchip's quality system design manufacture development systems 9001:2000 certified.
2005 Microchip Technology Inc.
DS91053B-page
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10/31/05
DS91053B-page
2005 Microchip Technology Inc.

Generating High Voltage Using PIC16C781/782 Author: Ross Fosler M

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