BD VFDs Application Note APN140

2. POWER SUPPLY

The power requirements of the BD VFD are: a filament supply (same as ordinary VFDs), logic supply voltage of "VDD1" and display voltage of "VDD2".
The table below indicates the power supplies that are usually required to drive the BD VFDs.

Table 1. Types of Power Supplies

Symbol	Terminal	Item	Function	Notes
Ef	F1 and F2	Filament Voltage	Refer to the specification	2Vac to 10Vac
VDD1	VDD1	Logic Supply Voltage	For driver logic	5Vdc
VDD2	VDD2	Display Supply Voltage	High voltage supply for grid and anode	12Vdc to 72Vdc
EC	G	Grid Voltage	For static drive type only	12Vdc to 72Vdc
EK	-	Filament Bias Voltage	Cut-off voltage	0V to 10V
-	VSS	Ground	Ground of VDD1 and VDD2	Ground 0V

Note: The functions vary from item to item. Please check the specification for details.

2.1 Filament Voltage "Ef"

Fig.6 AC Filament Drive

The BD VFD needs a filament supply voltage because it is a triode vacuum tube with a directly heated cathode like ordinary VFDs. (For details on the structure of ordinary VFDs, please refer to the Vacuum Fluorescent Display Application Note APN102.)

In a standard VFD, the voltage is applied across terminals "F1" and "F2". For the filament to properly function, appropriate power must be fed to heat the filament to a proper temperature. The supply voltage varies from one type of VFD to another, and its optimum value is specified and expressed with the symbol "Ef" in the corresponding specification.
This filament voltage has an important role for the VFD to function properly. If the voltage deviates from the optimum value, the service life of the VFD is likely to be seriously affected. Therefore, the rated voltage MUST BE OBSERVED. Typical values are indicated in the specifications.

2.1.1 AC Drive

A low voltage AC filament drive is the most common method. The Connection diagram is shown in Figure 6.

2.1.2 AC Pulse Drive

Fig.7a AC Pulse Filament Drive

The filament can be also operated with an AC pulse from a push-pull transistor, switching DC voltage (Fig.7a). In this case, the voltage supplied to the filament will be AC. This filament drive method makes the filament bias voltage "Ek" one half of the supply voltage automatically.

Refer to APN201 for detais about filament drive.

2.2 Logic Supply Voltage "VDD1"

Fig. 8 Noise Filtering Capacitor for VDD1

The logic supply voltage is supplied to the logic circuit of the built-in drivers. Normally, +5V voltage is applied across terminals "VDD1" and "VSS". Always insert a noise filtering ceramic capacitor (0.01 to 0.22 uF) between VDD1 and VSS terminals order to avoid false operations due to noise. Some types of VFD come with two pairs of "VDD1" and "VSS", as shown in Figure 8. In this case, apply the power to all the terminals of the two pairs and insert the filtering capacitor separately across each-one of the "VDD1" and "VSS" pairs.

2.3 Display Supply Voltage "VDD2"

This power is supplied to the drivers for the display. Its voltage is applied to the internal grids and anodes through the driver's output.
For this type of VFD, +12V to +72V voltage is applied across the terminals "VDD2" and "VSS". The voltage defined in each individual specification should be applied. The rated value of this display supply voltage has been decided assuming the most standard service conditions for optimized luminance and lifetime. This voltage may be varied within the specified rated voltage range in order to adjust (see note below) the basic brightness of the VFD. Especially when low power consumption is required, the display supply voltage can be held low, with a resulting effect on brightness. The user is requested to contact our office for such specific requirements.

It should be noted that some grid anode independent type VFDs with a multiplex drive come equipped with two "VDD2" terminals, namely, "VDD2G" dedicated to grid drivers, and "VDD2A" indicated to anode drivers. Normally the same voltage to be supplied to "VDD2G" and "VDD2A", but some graphic type BD VFDs may need to be supplied different voltages. Please check the individual specification for detail.

Some of BD VFD requesting to insert a current limiter RD in the VDD2 line in order to avoid false operation. The resistance value of RD is about 20 ohms. Please check the exact recommended value in each specification sheet.

Note: The adjustment of brightness as mentioned herein refers to the setting of the supply voltage. If you plan to control the brightness by software, it is suggested that you adjust it by the blanking control described later, and not by controlling the display supply voltage. For relationships between brightness and drive voltage, refer to the VFD Application Note APN201.

2.4 Grid Voltage "Ec" (for Static Drive Type only)

Fig.9 Grid Voltage

The grid voltage is applied to terminal "G" in a static drive type BD VFD.(Figure 9) The rating of "Ec" on the specification represents the voltage between terminal "G" and the center-tap of the filament transformer (F.C.T.), and does not include the filament bias voltage "Ek" (Para.3.1.5). This means the base level of "Ec" and "VDD1 ","VDD2" are different on the specification.
Normally, the same voltage "VDD2" can be applied to the terminal "G" directly. But in certain cases, the required voltage for "Ec" may be different from "VDD2".
Please refer to the "Block Diagram" on specification which shows how to connect the supply voltages for the "Ec" and other supply voltages.

2.5 Filament Bias Voltage "Ek"

This "Ek" is needed to avoid ghost illumination due to the swing of the AC filament voltage. The "Ek" biases the level of the filament's center(F.C.T.) and ground ("VSS") level.
To create "Ek", insert a zener diode (ZD), or several diodes (forward) between the center-tap of the filament transformer (F.C.T.) and "VSS" level.

In case the "Ek" is close to the logic voltage, the "F.C.T" can be coupled to the +5V side of the logic supply voltage. Depending on the application, some small VFD may not require any filament bias voltage.

Fig.10 Filament Bias Voltages

2.6 Power Supply Sequence

While the display power is being supplied, if the logic supply voltage "VDD1" is floating, or is kept at less than "+4.5V", the driver chips may be PERMENANTLY DESTROYED. Special care must be taken with the power supply SEQUENCE when the system is switched on or off. Please FOLLOW the following sequence in switching the power on and off.

Power On "VDD1" and "VDD2" should be ON at the same time, or "VDD2" should be ON after "VDD1" is ON.

Power Off "VDD1" and "VDD2" should be OFF at the same time, or "VDD1" should be OFF after "VDD2" is OFF.

In other words, this means " NEVER apply VDD2 without VDD1 ". See below on/off sequence.

Fig.11 Power Supply Sequence

There is no specific restriction on the on/off timing of the filament power, it may take one second for the filament to raise its temperature to the optimum value after the filament voltage "Ef" is turned on. Before this time period is reached, no segments can be illuminated.

Back to APN140 INDEX 3. Interface

APN140