EQUATION Generating High Voltage Using PIC16C781/782 Ross Fo

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EQUATION Generating High Voltage Using PIC16C781/782 Ross Fo

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TB053
EQUATION
Generating High Voltage Using PIC16C781/782
Ross Fosler Microchip Technology Inc.
INTRODUCTION
Nixie tube device born middle 20th 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's ability generate high voltage from 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.
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.
EQUATION
VOUT VOUT IPEAK 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 References section more details.
BASIC BOOST TOPOLOGY (DISCONTINUOUS MODE)
FIGURE BOOST CONVERTER TOPOLOGY
VOUT
Controller
VVFB
EQUATION
VOUT 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). RLDT
2002 Microchip Technology Inc.
Preliminary
DS91053A-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 on-time MOSFET, about Using Equation function power terms inductance easily derived. (170 Volts) (P)Watts µS)(9 Volts)2
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 (DAC) changes output voltage, VOUT.
FIGURE
PIC16C781/782 CONTROL LOOP CONFIGURATION
PIC16C781/782
PSMC PWPWM
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 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 170) Volts tfall 0.655 Amps tfall 1.34
VVFB
This feedback method unusual this topology. Energy transferred from inductor when Pulse Width 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%.
DS91053A-page
Preliminary
2002 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 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
VREF VOUT +170
REFERENCES
Ross, Essence Power Electronics, Prentice Hall, York, 1997. Pressman, Abraham Switching Power Supply Design, McGraw-Hill, York, 1998.
2002 Microchip Technology Inc.
Preliminary
DS91053A-page
TB053
APPENDIX
FIGURE A-1:
7805 TC4427 IRF620
SCHEMATICS
HIGH VOLTAGE DRIVER DISPLAY CONTROL
+170 Display) PIC16C781/782 350V
Temperature Sensor LM34 Display bits
FIGURE A-2:
NIXIE TUBE DISPLAY
+170
Nixie Tubes
from Microcontroller
FMMT497TA
CD4028 FMMT497TA CD4028 FMMT497TA
DS91053A-page
Preliminary
2002 Microchip Technology Inc.
Note following details code protection feature PICmicro® MCUs. PICmicro family meets specifications contained Microchip Data Sheet. Microchip believes that family PICmicro microcontrollers most secure products kind market today, when used intended manner under normal conditions. There dishonest possibly illegal methods used breach code protection feature. these methods, knowledge, require using PICmicro microcontroller manner outside operating specifications contained data sheet. person doing engaged theft intellectual property. Microchip willing work with customer concerned about integrity their code. Neither Microchip other semiconductor manufacturer guarantee security their code. Code protection does mean that guaranteeing product "unbreakable". Code protection constantly evolving. Microchip committed continuously improving code protection features product.
have further questions about this matter, please contact local sales office nearest you.
Information contained this publication regarding device applications like intended through suggestion only superseded updates. your responsibility ensure that your application meets with your specifications. representation warranty given liability assumed Microchip Technology Incorporated with respect accuracy such information, infringement patents other intellectual property rights arising from such otherwise. Microchip's products critical components life support systems authorized except with express written approval Microchip. licenses conveyed, implicitly otherwise, under intellectual property rights.
Trademarks Microchip name logo, Microchip logo, FilterLab, KEELOQ, MPLAB, PIC, PICmicro, PICMASTER, PICSTART, MATE, SEEVAL Embedded Control Solutions Company registered trademarks Microchip Technology Incorporated U.S.A. other countries. dsPIC, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB, In-Circuit Serial Programming, ICSP, ICEPIC, microID, microPort, Migratable Memory, MPASM, MPLIB, MPLINK, MPSIM, MXDEV, PICC, PICDEM, PICDEM.net, rfPIC, Select Mode Total Endurance trademarks Microchip Technology Incorporated U.S.A. Serialized Quick Term Programming (SQTP) service mark Microchip Technology Incorporated U.S.A. other trademarks mentioned herein property their respective companies. 2002, Microchip Technology Incorporated, Printed U.S.A., Rights Reserved.
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Microchip received QS-9000 quality system certification worldwide headquarters, design wafer fabrication facilities Chandler Tempe, Arizona July 1999. Company's quality system processes procedures QS-9000 compliant PICmicro® 8-bit MCUs, KEELOQ® code hopping devices, Serial EEPROMs microperipheral products. addition, Microchip's quality system design manufacture development systems 9001 certified.
2002 Microchip Technology Inc.
Preliminary
DS91053A page
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DS91053A-page
Preliminary
2002 Microchip Technology Inc.

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