Vacuum Fluorescent Display (VFD) Reference Design in Automotive

1 Introduction

This reference design provides a solution for obtaining a power supply for a vacuum fluorescent display (VFD), which is very suitable for automotive applications. Test data for load/power regulation and other test results are also provided.

2 VFD Design Basics

Vacuum fluorescent display (VFD) is a common display in consumer electronic devices. It is widely used in video recorders, car radios, microwave ovens, etc. Unlike liquid crystal displays (LCD), VFDs emit strong brightness and have clear 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 below zero degrees, which is very suitable for outdoor equipment in cold climates.

VFD contains three basic electrodes, namely cathode filament, anode (phosphor) and grid, and a high vacuum glass shell. The cathode is composed of tungsten filament, alkaline earth metal oxide coating (electron emission). The grid is a metal wire used to control and transmit the electrons emitted by the cathode. The anode is the electrode, coated with phosphor, used to display characters, charts or symbols. The electrons emitted by the cathode are accelerated by the forward voltage between the grid and the anode. After hitting the anode, the electrons excite the phosphor to emit light. The required brightness template can be obtained by controlling the positive or negative voltage of the grid and the anode. The anode and the grid need to be powered by a DC stable voltage to avoid flickering of the display. In order to drive large VFDs, the cathode needs to be driven by AC to avoid brightness deviation, for example, different brightness levels on both sides of the display. It is recommended to use an operating frequency in the range of 20~200 kHz to avoid audible noise and flickering.

3 Design specifications and circuit design

This reference design optimizes the MAX15005 power-supply controller for automotive and VFD applications.

The application circuit is designed to meet the following specifications:

VIN: 9~16 V continuous change, 5.5~40 V transient; VANODE: 77VDC±10%, 18 mA (typical value), 58 mA (maximum value); VGRID: 55VDc±10%, 14 mA (typical value), 41 mA (maximum value); VFILAMENT: 3.1VAC±10%, 350 mA (typical value), 385 mA (maximum value); Output ripple: 77 V: 1VP-P; 55 V: 0.5VP-P; Power regulation rate VIN is 9~16 V, VANODE=±3%, VCRID=±3%, VFILA-MENT=±5%; Load regulation rate: refer to the power supply/load regulation rate data section; Switching frequency: 22 kHz; Temperature: -40℃～125℃.

The circuit schematic that meets the above specifications is shown in Figure 1. In this design, the MAX15005B is configured in a flyback architecture to obtain three output voltages.

4 Test waveform

Under the test conditions: VIN = 14 V, RANODE = 3.3 kΩ, RGRID = 3_3 kΩ, RFILAMENT = 8Ω, the evaluation board was measured, and the test results are shown in Figures 2 to 5. In Figure 2, Ch1 is the drain voltage (VDRAIN) of MOSFET VQ1; Ch2 is the current detection voltage (VSENSE) across R13. In Figure 3, Ch1 is the anode output voltage ripple, and Ch2 is the gate output voltage ripple. In Figure 4, Ch1 is the positive voltage of the filament (VF1), and Ch2 is the negative voltage of the filament (VF2). In Figure 5, M is the effective voltage of the filament (VF1-VF2).

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5 Power supply and load regulation data

Table 1 shows the power supply and load regulation data obtained from the circuit board test under different input voltage ranges and load conditions.

6 Conclusion

This application note provides a power supply reference design for driving a typical vacuum tube fluorescent display, which is ideal for automotive applications. The circuit was built and tested according to the specifications listed in the article. The circuit schematic and typical test waveforms are given in this article.