Vacuum Fluorescent Display (VFD) Reference Design for Automotive Applications

Abstract: This article describes a vacuum fluorescent display (VFD) and some ideal applications for the technology. The reference design then shows how to use a MAX15005 power-supply controller in a flyback topology to obtain multiple output voltages for a vacuum fluorescent display.

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:

• V_IN: 9V to 16V continuous, 5.5V to 40V transient
• V_ANODE: 77V_DC ±10% at 18mA (typ), 58mA (max)
• V_GRID: 55V_DC ±10% at 14mA (typ), 41mA (max)
• V_FILAMENT: 3.1V_AC ±10% at 350mA (typ), 385mA (max)
• Output ripple: 77V: 1V_P-P; 55V: 0.5V_P-P
• Line regulation, V_IN = 9V to 16V:
  V_ANODE = ±3%
  V_GRID = ±3%
  V_FILAMENT = ±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 MAX15005A is used in the flyback configuration for obtaining three output voltages.


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

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

Table 1. Bill of Materials for VFD Reference Design

Waveform Measurements

The following test results were generated from the board built for evaluating the circuit.

Test conditions: V_IN = 14V; R_ANODE = 3.3kΩ; R_GRID = 3.3kΩ; R_FILAMENT = 8Ω.

Ch1: MOSFET Q1 drain voltage (V_DRAIN); Ch2: current-sense voltage across R13 (V_ISENSE).


Test conditions: V_IN = 14V; R_ANODE = 3.3kΩ; R_GRID = 3.3kΩ; R_FILAMENT = 8Ω.

Ch1: anode output voltage ripple; Ch2: grid output voltage ripple.


Test conditions: V_IN = 14V; R_ANODE = 3.3kΩ; R_GRID = 3.3kΩ; R_FILAMENT = 8Ω.

Ch1: filament positive node voltage (V_F1); Ch2: filament negative node voltage (V_F2).


Test conditions: V_IN = 14V; R_ANODE = 3.3kΩ; R_GRID = 3.3kΩ; R_FILAMENT = 8Ω.

M: effective filament voltage (V_F1 - V_F2).

Line/Load Regulation Data

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

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.