Reference Circuit 4363

Vacuum Fluorescent Display (VFD) Reference Design for Automotive Applications



Introduction

This reference design shows a solution for obtaining the drive voltage required for a vacuum fluorescent display (VFD) power supply in automotive applications. The design includes the complete schematic, and presents the bill of materials (BOM), load/line regulation measurements, and test results.

VFD Basics

A vacuum fluorescent display (VFD) is a type of display used commonly on consumer-electronics equipment such as video cassette recorders, car radios, and microwave ovens. Unlike liquid crystal displays (LCDs), a VFD emits a very bright light with clear contrast and can easily support display elements of various colors. The technology is related to both the cathode ray tube and the nixie tube. Unlike LCDs, however, most VFDs continue to function normally in subzero temperatures, making them ideal for outdoor devices in cold climates.

The VFD is composed of three basic electrodes—the cathode filaments, anodes (phosphor), and grids—under a high-vacuum condition in a glass envelope. The cathode consists of fine tungsten wires, coated by alkaline earth metal oxides which emit electrons. The grids are a thin metal mesh, which controls and diffuses electrons emitted from the cathode. The anodes are conductive electrodes on which the phosphor is printed to indicate characters, icons, or symbols. Electrons emitted from the cathode are accelerated with positive potential applied to both grid and anode; upon collision with the anode the electrons excite the phosphor to emit light. The desired illuminated patterns can be achieved by controlling the positive or negative potentials on each grid and anode. The anode and grid require a DC-regulated voltage to avoid flickering of the display. For driving large VFDs, the cathode requires AC drive to prevent luminance slant, i.e., the difference in brightness from one side of the display to the other. A frequency range of 20kHz to 200kHz is recommended to avoid audible noise and flicker.

Design Specifications and Setup

This reference design features the MAX15005 power-supply controller optimized for automotive and VFD applications. The application circuit is designed to meet the following specifications:

  • VIN: 9V to 16V continuous, 5.5V to 40V transient
  • VANODE: 77VDC±10% at 18mA (typ), 58mA (max)
  • VGRID: 55VDC±10% at 14mA (typ), 41mA (max)
  • VFILAMENT: 3.1VAC±10% at 350mA (typ), 385mA (max)
  • Output ripple: 77V: 1VP-P; 55V: 0.5VP-P
  • Line regulation, VIN= 9V to 16V:
    VANODE= ±3%
    VGRID= ±3%
    VFILAMENT= ±5%
  • Load regulation: (see Line/Load Regulation Data section below)
  • Switching frequency: 22kHz
  • Temperature: -40°C to 125°C

The schematic for the above specifications is shown in Figure 1. In this design MAX15005B is used in the flyback configuration for obtaining three output voltages.

Figure 1. Schematic of the MAX15005B flyback converter for FSW= 22kHz.

The bill of materials (BOM) for this reference design is given in Table 1.

Introduction

This reference design shows a solution for obtaining the drive voltage required for a vacuum fluorescent display (VFD) power supply in automotive applications. The design includes the complete schematic, and presents the bill of materials (BOM), load/line regulation measurements, and test results.

VFD Basics

A vacuum fluorescent display (VFD) is a type of display used commonly on consumer-electronics equipment such as video cassette recorders, car radios, and microwave ovens. Unlike liquid crystal displays (LCDs), a VFD emits a very bright light with clear contrast and can easily support display elements of various colors. The technology is related to both the cathode ray tube and the nixie tube. Unlike LCDs, however, most VFDs continue to function normally in subzero temperatures, making them ideal for outdoor devices in cold climates.

The VFD is composed of three basic electrodes—the cathode filaments, anodes (phosphor), and grids—under a high-vacuum condition in a glass envelope. The cathode consists of fine tungsten wires, coated by alkaline earth metal oxides which emit electrons. The grids are a thin metal mesh, which controls and diffuses electrons emitted from the cathode. The anodes are conductive electrodes on which the phosphor is printed to indicate characters, icons, or symbols. Electrons emitted from the cathode are accelerated with positive potential applied to both grid and anode; upon collision with the anode the electrons excite the phosphor to emit light. The desired illuminated patterns can be achieved by controlling the positive or negative potentials on each grid and anode. The anode and grid require a DC-regulated voltage to avoid flickering of the display. For driving large VFDs, the cathode requires AC drive to prevent luminance slant, i.e., the difference in brightness from one side of the display to the other. A frequency range of 20kHz to 200kHz is recommended to avoid audible noise and flicker.

Design Specifications and Setup

This reference design features the MAX15005 power-supply controller optimized for automotive and VFD applications. The application circuit is designed to meet the following specifications:

  • VIN: 9V to 16V continuous, 5.5V to 40V transient
  • VANODE: 77VDC±10% at 18mA (typ), 58mA (max)
  • VGRID: 55VDC±10% at 14mA (typ), 41mA (max)
  • VFILAMENT: 3.1VAC±10% at 350mA (typ), 385mA (max)
  • Output ripple: 77V: 1VP-P; 55V: 0.5VP-P
  • Line regulation, VIN= 9V to 16V:
    VANODE= ±3%
    VGRID= ±3%
    VFILAMENT= ±5%
  • Load regulation: (see Line/Load Regulation Data section below)
  • Switching frequency: 22kHz
  • Temperature: -40°C to 125°C

The schematic for the above specifications is shown in Figure 1. In this design MAX15005B is used in the flyback configuration for obtaining three output voltages.

Figure 1. Schematic of the MAX15005B flyback converter for FSW = 22kHz.

The bill of materials (BOM) for this reference design is given in Table 1.

Table 1. Bill of Materials for VFD Reference Design

Designator Value Description Part Number Footprint Manufacturer Quantity
C1, C11, C12 10nF, 100V Capacitor C2012X7R2A103K 0805 TDK® 3
C2, C7 270pF, 100V Capacitor GRM188R72A271KA01D 0805 Murata® 2
C3, C5 100nF, 100V Capacitor C2012X7R2A104K 0805 TDK 2
C4 3.3nF, 25V Capacitor 08053A332FAT2A 0805 AVX® Corporation 1
C6, C8 1µF, 50V Capacitor C3216X7R1H105K 1206 TDK 2
C9 100pF, 100V Capacitor GRM2165C2A101JA01D 0805 Murata 1
C10 330µF, 35V Capacitor SMD TDK 1
C13, C14, C15, C16, C17, C18 2.2µF, 100V Capacitor GRM32ER72A225KA35L 1210 Murata 6
D1 3A, 400V Diode S3G SMC Vishay® 1
D2, D3, D4 1A, 200V Diode ES1D SMA Vishay 3
Q1 11A, 55V n-FET BUK92150-55A NXP® 1
R1 32.4kΩ Resistor SMD, 5%, 0.125W 0805 KOA 1
R2, R9, R17 100kΩ Resistor SMD, 5%, 0.125W 0805 KOA 3
R3 21kΩ Resistor SMD, 1%, 0.125W 0805 KOA 1
R4, R6 100kΩ Resistor SMD, 1%, 0.250W 1206 KOA 2
R5 1.62kΩ Resistor SMD, 1%, 0.125W 0805 KOA 1
R7 1.43kΩ Resistor SMD, 1%, 0.125W 0805 KOA 1
R8 10kΩ Resistor SMD, 5%, 0.125W 0805 KOA 1
R10 499Ω Resistor SMD, 1%, 0.125W 0805 KOA 1
R11 100Ω Resistor SMD, 5%, 0.125W 0805 KOA 1
R12 1kΩ Resistor SMD, 1%, 0.333W 1210 KOA 1
R13 0.06Ω Resistor SMD, 1%, SL1 SL1 KOA 1
R14 33kΩ Resistor SMD, 5%, 0.125W 0805 KOA 1
R15, R16 1.0Ω Resistor SMD, 1%, 0.250W 1206 KOA 2
T1 54µH Transformer DCT20EFD-UxxSOA5 SMD TDK 1
Z1 9.1V Zener diode 1SMB5924BT SMB Vishay 1
IC1 MAX15005B Boost controller MAX15005BAUE+ 16TSSOP MAXIM® 1

Waveform Measurements

The following test results were generated from the board built for evaluating the circuit.
Test conditions: VIN = 14V; RANODE = 3.3kΩ; RGRID = 3.3kΩ; RFILAMENT = 8Ω.
Figure 02.
Ch1: MOSFET Q1 drain voltage (VDRAIN); Ch2: current-sense voltage across R13 (VISENSE).

Test conditions: VIN = 14V; RANODE = 3.3kΩ; RGRID = 3.3kΩ; RFILAMENT = 8Ω.
Figure 03.
Ch1: anode output voltage ripple; Ch2: grid output voltage ripple.

Test conditions: VIN = 14V; RANODE = 3.3kΩ; RGRID = 3.3kΩ; RFILAMENT = 8Ω.
Figure 04.
Ch1: filament positive node voltage (VF1); Ch2: filament negative node voltage (VF2).

Test conditions: VIN = 14V; RANODE = 3.3kΩ; RGRID = 3.3kΩ; RFILAMENT = 8Ω.
Figure 05.
M: effective filament voltage (VF1 - VF2).

Line/Load Regulation Data

The following line/load regulation data was taken from the test board over the input voltage range and load.

VIN I77 (mA) I55 (mA) V77 (VDC) V55 (VDC) VF (VRMS)
9.0 7.7 5.5 77.0 55.2 2.41
  7.7 16.7 77.0 55.0 2.64
  7.7 44.0 77.0 54.8 3.03
  23.0 5.5 77.0 55.4 2.82
  23.0 16.7 77.0 55.2 2.97
  23.0 44.0 77.0 55.0 3.24
  61.6 5.5 77.0 55.8 3.35
  61.6 16.7 77.0 55.6 3.43
  61.6 44.0 77.0 55.4 3.62
14.0 7.7 5.5 77.0 55.2 2.52
  7.7 16.7 77.0 55.0 2.75
  7.7 44.0 77.0 54.8 3.14
  23.0 5.5 77.0 55.4 2.80
  23.0 16.7 77.0 55.2 3.08
  23.0 44.0 77.0 55.0 3.36
  61.6 5.5 77.0 55.8 3.50
  61.6 16.7 77.0 55.7 3.59
  61.6 44.0 77.0 55.4 3.79
16.0 7.7 5.5 77.0 55.2 2.63
  7.7 16.7 77.0 55.0 2.86
  7.7 44.0 77.0 54.8 3.25
  23.0 5.5 77.0 55.4 3.04
  23.0 16.7 77.0 55.2 3.20
  23.0 44.0 77.0 55.0 3.49
  61.6 5.5 77.0 54.8 3.25
  61.6 16.7 77.0 55.0 3.49
  61.6 44.0 77.0 55.4 3.92

Conclusion

This application note presents a power-supply reference design for driving a typical vacuum fluorescent display in an automotive application. The design was built to the specifications presented here. The design was then tested. The circuit schematic, BOM, and typical waveforms have been presented.

 
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