Electroluminescent backlight lamps, (EL lamps) are thin, flexible, and rugged, making them ideal for nighttime backlighting of vehicle instrument clusters. EL lamps have been widely used since their introduction in the early 1960s, but a lack of chip functionality restricted their use in certain markets and applications. Now a Programmable System-on-ChipTM (PSoCTM) mixed-signal array, with configurable analog and digital resources, a Cypress CY8C24xxx for example, allows designers to utilize EL's benefits in ways that were previously impossible.
An EL lamp is, in essence, a capacitor with at least one transparent electrode and a special phosphorescent dielectric. Its capacitance is roughly 0.6 to 1 nf/cm2. Apply an AC voltage and it glows, producing uniform light. Apply a larger voltage and the lamp glows even brighter. Having a thickness as little as 0.12 mm, they can be easily conformed to curved and contoured surfaces desired by today's industrial designers responsible for an instrument cluster's look and feel.
EL lamps provide a flat uniform light source, utilize low power, and generate low heat, making them ideal for instrument clusters. EL lamps require a high AC voltage to turn on. Specifics differ for each application but generally the stimulation voltage is in the range of 40Vrms to 100Vrms at 200 to 500 Hz. For the typical digital-centric engineer, voltage this high must seem like high-energy physics. Because this resource is not readily abundant in an automobile electric system, it has to be generated.
The AC voltage applied to an EL lamp makes the phosphor glow within the EL. Like all light sources, over time the intensity of the emitted light will decrease. A measure of this dimming effect of an EL lamp is Time to Half Luminance (THL). Specifics differ for each application, but generally the THL for an EL lamp can exceed 3,500 hours for an applied voltage of 80Vrms at 200 Hz. It is also worth mentioning that a larger AC voltage also generates greater DC error. This means the THL is reduced when a higher voltage is applied to the EL lamp. It also follows that the THL is extended when a lower voltage is applied.
To turn on a lamp, it is be necessary to generate a large AC stimulus voltage. A popular method is to use a charge pump to generate a large DC voltage and then discharge it at the desired frequency. A charge pump, sometimes known as a boost converter, takes a smaller DC voltage and converts it (by means of an inductor) to a larger voltage. This can be accomplished by using the basic circuit shown in here.
Basic boost-mode charge pump
Neglecting any resistance, when the switch is turned on, a current develops that is proportional to time. At the end of ton, the following current has developed.
When the switch turns off, the magnetic field of the inductor collapses causing the current to be shunted through the diode and stored in the capacitor. For each cycle, a quanta of energy is transferred. With the energy transfer happening at a rate of fh, the following power is developed:
The output voltage will increase until the power used by the EL lamp equals the power generated. This is a unique value for each type and size of EL lamp. Using the manufacturer's data sheet, L, fh, and Vdc can be determined. The output voltage can be then be adjusted with the duty cycle. Care must be taken to prevent the duty cycle from increasing to the point where there is no longer sufficient time left to allow the inductor to totally transfer its energy. This function can be implemented with one configurable digital block within a PSoC device.
An H bridge is used to chop the DC high voltage into an AC signal. The schematic shown below may help to visualize this. For simplicity, the high-side and low-side switches are shown as MOSFETs. It could just as well have been implemented with bipolar transistors, BGJTs, SCRs, or some combination of these switch types.
Basic H-bridge topology
The H-bridge circuit gets its name from the fact that it resembles an uppercase letter H. To generate the AC signal, the following control cycle must be implemented.
Charge the EL lamp is one direction.
Discharge the lamp.
Charge the EL lamp in the opposite direction.
Discharge the lamp.
This technique is known as four phase drive. The control states are shown in the table below.
Four-phase bridge drive
Note that there are eight entries for what should be a four state table. This is because little mini-states have been added to ensure that Phase AHIGH and Phase BLOW (or Phase BHIGH and Phase ALOW) are never, even briefly, both actuated. Simultaneous activation would result in a direct short from the high voltage supply to ground. These mini-states are called dead band states. Hardware to implement this controller is shown below.
Four-phase H-bridge controller with dead band
A pulse width modulator (PWM) with a frequency of fl is fed to a dead band generator to generate under lapped low and high phase signals. It is also fed to a flip flop to generate phase A and phase B signals. These four signals are logically combined to generate the four control signals shown in the table above. This hardware can be implemented with three PSoC configurable digital blocks. The PWM's duty cycle determines how long the voltage (VHV) is applied on the EL lamp. The flip flop insures that the voltage is applied in alternating polarities. This results in the following RMS voltage and switching frequency.