An increasing number of light fixtures utilize LEDs as light sources due to their lower energy consumption, smaller size, improved robustness, and/or longer operational lifetime relative to conventional (e.g., incandescent) light sources. An LED is a low-voltage solid state device; it cannot be directly powered by standard high-voltage AC power without controlling the applied voltage and the current flowing therethrough. For example, most LED lamps require a 12 V or 24 V supply voltage; because the LED lamps may act as replacements for traditional lighting, however, they have access only to a 120 V or 240 V source. Therefore, LED lamps usually include a voltage transformer to convert the high input voltage to a lower level usable by the LED and its support circuitry.
The transformer may be a magnetic transformer or an electronic transformer. In a magnetic transformer, there are two coils of wire: the primary coil and the secondary coil. The primary coil carries the high-voltage input and creates a magnetic flow in an iron core, which induces a current in the secondary coil that is also wrapped around the iron core. Because there are more windings in the primary coil than in the secondary, the secondary coil has a lower voltage. The exact output voltage depends on the ratio of windings in the two coils. FIG. 1A depicts an output 100 of a typical magnetic transformer that is approximately 12 V RMS AC at approximately 60 Hz. This frequency allows the LEDs to operate without causing a visible flicker in their outputs. Advantages of the magnetic transformer include the reliability of the output voltage, long lifetime, and/or tolerance of a high operating temperature. The magnetic transformer, however, is usually large, expensive, and heavy.
An electronic transformer, on the other hand, is a complex electrical circuit that produces a high-frequency (i.e., 10 kHz or greater) AC voltage having an “envelope” that approximates the low-frequency output of a magnetic transformer. FIG. 1B illustrates a high-frequency output 102 of an electronic transformer that has a voltage envelope 104 approximating 60 Hz (similar to the frequency produced by the magnetic transformer shown in FIG. 1A). Benefits of electronic transformers include their small size, light weight, and quietness of operation. The electronic transformer, however, may suffer from radio-frequency interference with other components when the transformer is placed close to noise-generating components in the lighting system due to its more complicated circuitry, and may stop working or “stall” when its output current drops too low.
Thus, magnetic and electronic transformers differ greatly in design, behavior, and challenges. A circuit designed to work with one may not work with (or not work well with) the other. A conventional method for identifying the type of the transformer is to analyze the frequency components in the waveforms of its output voltage. The presence of high-frequency components in the waveform indicates that the transformer is electronic, and the absence of high-frequency components indicates the use of a magnetic transformer. It may be difficult, however, to reliably detect the high-frequency components due to (for example) a high noise level, and designing such detection circuitry may be expensive. In addition, filter capacitors are often used to remove high frequency noise, thereby making high frequency detection more difficult. Consequently, there is a need for a circuit that more reliably (and economically) identifies the type of a source transformer.