circuit coll

Normally LED control circuits are based on buck boost or flyback principles, where the circuit is configured to produce a constant DC for illuminating an LED series.

The above LED control systems have their respective drawbacks and the positives in which the range of operating voltage and the number of LEDs at the output decide the efficiency of the circuit. Other factors like whether the LEDs are included in parallel or series or whether they need to bedimmed or not, also affects the above topologies. These considerations make these LED control circuits rather dicey and complicated.

The circuit explained here employs a different approach and relies on a resonant mode of application.

Though the circuit does not provide direct isolation from the input AC, it has the features of driving many LEDs with current levels as high as 750 mA. The soft switching process involved in the circuit ensures greater efficiency to the unit.

Basically the mains transformerless LED control circuit is designed around the fluorescent lamp dimmer control IC IRS2530D. The circuit diagram shows how the IC has been wired up and how its output has been modified for controlling LEDs in place of the usual fluorescent lamp.

The usual preheating stage required for a tube light utilized a resonant tank which is now effectively replaced by a LC circuit suitable for driving LEDs.

Because the current at the output is an AC, the need of a bridge rectifier at the output became imperative; this makes sure that current is continuously passing through the LEDs during every switching cycle of the frequency.

The AC current sensing is done by the resistor RCS, placed across the common and the bottom of the rectifier.

This provides an instant AC measurement of the amplitude of the rectified LED current.

The DIM pin of the IC receives the above AC measurement via the resistor RFB and capacitor CFB.

This allows the dimmer control loop of the IC to keep track of the LED current amplitude and regulates it by instantaneously varying the frequency of the half bridge switching circuit, such that the voltage across the LED maintains a correct RMS value.

The dimmer loop also helps to keep the LED current constant irrespective of the line voltage, load current and temperature changes.

Whether a single LED is connected or a group in series, the LED parameters is always maintained correctly by the IC.

Alternatively the configuration may also be used as a high current transformerless power supply circuit.

Courtesy - http://www.irf.com/pressroom/articles/594LEDP0902.pdf