Vacuum Tube Fluorescent Display (VFD) Reference Design in Automotive

introduction

This reference design provides a solution for obtaining power from vacuum tube fluorescent displays (VFDs), making it ideal for automotive applications . The design includes a complete schematic and bill of materials (BOM), and provides load/power regulation test data and other test results.

VFD design basics

Vacuum tube fluorescent display (VFD) is a common display in consumer electronic equipment and is widely used in video recorders, car radios, microwave ovens, etc. Unlike liquid crystal displays (LCDs), VFDs have stronger luminous brightness and clearer contrast, and can easily support display units of different colors. This technology is related to cathode rays and cathode tubes. Unlike LCDs, most VFDs can maintain effective operation at sub-zero temperatures, making them ideal for outdoor equipment in cold climates.

A VFD contains three basic electrodes - a cathode filament, an anode (phosphorescent substance) and a grid - a high-vacuum glass envelope. The cathode is composed of tungsten wire and alkaline earth metal oxide coating (which emits electrons). The grid is a metal wire used to control and transmit electrons emitted by the cathode. The anode is the terminal electrode, coated with phosphorescent material and used to display characters, diagrams or symbols. The electrons emitted from the cathode are accelerated by the forward voltage between the gate and the anode. After impacting the anode, the electrons excite the phosphorus and emit light. The required brightness template can be obtained by controlling the positive or negative voltage of the grid and anode. The anode and grid require a stable DC voltage supply to avoid display flickering. In order to drive a large VFD, the cathode needs to be AC ​​driven to avoid brightness deviations, such as different brightness levels on both sides of the display. An operating frequency range of 20kHz to 200kHz is recommended to avoid audible noise and flicker.

Design specifications and circuits

This reference design optimizes the MAX15005 power supply controller and is ideal for automotive and VFD equipment. The application circuit design meets the following specifications:

• V _IN : 9V to 16V continuous change, 5.5V to 40V transient

• V _ANODE : 77V _DC ±10%, 18mA (typ.), 58mA (max.)

• V _GRID : 55V _DC ±10%, 14mA (typ.), 41mA (max.)

• V _FILAMENT : 3.1V _AC ±10%, 350mA (typ.), 385mA (max.)

• Output ripple: 77V: 1V _P-P ; 55V: 0.5V _P-P

• Power Regulation, V _IN = 9V to 16V:
  V _ANODE = ±3%
  V _GRID = ±3%
  V _FILAMENT = ±5%

• Load Regulation: (Please refer to the Line/Load Regulation Data section below)

• Switching frequency: 22kHz

• Temperature: -40°C to 125°C