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Electronic low-voltage transformers
The ELVT presents a more difficult problem. Moreover, many makers of MR16 lighting systems have transitioned to ELVTs as a lower-cost, smaller, lighter, and generally more-power-efficient alternative to magnetic transformers. An ELVT switching converter operates over a range from 30 kHz to over 100 kHz. A small magnetic transformer forms part of the circuit to provide galvanic isolation. There are ELVT controller ICs available, but the cost constraints that accompany high-volume products such as MR16 lighting systems mean that most ELVTs used in lighting applications don’t have an IC or, for that matter, a control loop with a feedback circuit like the ones found in DC-DC or AC-DC power supplies. Instead, most ELVTs are self oscillating, using the half-bridge topology shown in Fig. 2.
FIG. 3. Lamp input current (top) and output voltage (bottom) of an ELVT driving a single 35W halogen MR16 bulb.
FIG. 3.
Rather than producing a simple 50- or 60-Hz sine wave with an amplitude of 12VACRMS, the ELVT output consists of an oscillation at the switching frequency that forms an envelope at the desired amplitude and line frequency. Fig. 3 shows a typical waveform from an ELVT driving a single 35W halogen MR16. The top plot is the current into the MR16 lamp, and the bottom plot is the output voltage of the ELVT.
The plots reveal a key problem with ELVTs and LEDs. A typical low-voltage, halogen MR16 lamp draws anywhere from 25W to 50W. From an electrical standpoint the lamp load appears somewhat inductive, but mostly resistive. The ELVT is designed for that load.
In the region of each zero crossing in the signal envelope, even the relatively heavy load of a series of 50W halogen bulbs is not enough to get the converter started until a minimum voltage is reached. You see this in the dead zone following the end of each AC half-cycle in the plot. The ELVT doesn’t operate for some amount of time at the start of the subsequent half cycle.
A typical SSL MR16 uses three 1W LEDs and has an average-quality DC-DC converter LED driver circuit with a power efficiency of 80%. The input power is only 3.75W. To make matters worse, the impedance of a closed-loop DC-DC converter that might be used in the LED lamp is negative. The negative impedance is due to the fact that a closed-loop, switching converter draws less current as the input voltage increases and vice-versa, the opposite of how a resistor, or a halogen lamp, behaves.
A self-oscillating ELVT that was designed for a heavy resistive load, but is instead presented with a light load with negative impedance, can result in flickering light, audible buzzing, or a complete failure to start up. Our experiments show that a typical ELVT failed even when loaded with three 4W LED lamps. With a fourth 4W lamp added, the tested ELVT operated as intended.