Design Ideas: March 1, 1996

Electroluminescent (EL) panels find wide use in backlighting LCDs in handheld, battery-powered products. EL panels require ac-voltage drive of 50 to 90V at 400 to 1000 Hz. The conventional way to generate such a drive voltage from a low dc-voltage supply is to use a converter built around a step-up transformer. The low operating frequency dictates a relatively large-sized transformer. Alternate designs strive to reduce the size of magnetics by using a step-up converter followed by an H-bridge or chopper. The size of the inductor in these designs depends on the energy the inductor must store. As the size of the EL panel increases, so does the energy demand and the inductor size. Moreover, high-voltage H-bridge ICs are not off-the-shelf parts. The circuit in Figure 1 uses a different topology.

The design uses a high-frequency transformer to step up the voltage and a logic-level, low-frequency signal that controls a switchable-polarity rectifier. The rectifier, using a handful of off-the-shelf parts, provides a positive or negative output, depending on the control signal. This control signal, and not the frequency of the transformer's driving voltage, determines the switching frequency for the EL panel. IC₁ drives the transformer with a push-pull square wave whose frequency is about 200 kHz. The high operating frequency and the absence of dc bias allows you to use a small ferrite core. Because the transformer does not have to store energy, its size does not grow rapidly in proportion to EL-panel size. The turns ratio depends on the desired output voltage; it can range from 1:10 to 1:15.

The switching elements in the rectifier are two logic-level MOSFETs: p-channel Q₁ and n-channel Q₂. To understand the operation, note that D₁ and D₂ form the traditional diode bridge, but in this case it's a floating bridge. Q₁ and Q₂ alternately connect one or the other corners of the bridge to either ground or the 5V rail. Assume that at a certain moment the control voltage at the MOSFET gates is 5V. Q₁ turns off and Q₂ turns on. Assume also that at a certain moment the top end of the secondary is positive and the bottom end is negative. In this case, a secondary current flows in the following direction: ground, EL panel, top half of the secondary, D_2-1, Q₂, ground.

When the polarity of the secondary winding changes, then the current flows through the bottom half of the secondary and D_2-2, but the direction of the current through the EL panel does not change. The center-tap voltage of the secondary looks like the output of a full-wave negative rectifier, with an amplitude equal to the voltage between the center tap and the ends of the secondary winding. Because the EL panel is capacitive in nature, it quickly charges to this amplitude. When the MOSFET gate voltage goes low, then Q₁ turns on and Q₂ turns off. The polarity of the rectified voltage changes to positive because current now flows from the 5V rail to Q₁, D_1-1 or D_1-2, secondary winding, EL panel, ground. The EL panel charges to a positive voltage.

The control-voltage frequency should be in the range of 400 to 1000 Hz. The low-frequency, low-level control voltage causes the high ac voltage having the same frequency and nearly square-wave shape to appear on the EL panel. The circuit can drive an EL panel of about 24 in.² (total capacitance about 0.1 µF), using a 10×8×9-mm transformer. You can either generate the control signal locally or derive it from an external source, such as a µP. IC₁, D₃, and R₁ generate the control signal in the configuration in Figure 1. (DI #1839)