Halogen and LED characteristics
MR16 halogen lamps are used in creative lighting for many retail and consumer applications due to their unique size, configurability, concentrating power, aesthetics and practicality. Common MR16 halogen lamps have a luminous flux range of 150 lumens (lm) to 800 lm, a luminous efficacy of about 15 lm/W, or a luminous efficiency of 15%. Inefficiencies, heat generation and halogen capsule handling problems are disadvantages of this technique. In addition, typical halogen bulbs have a life of approximately 2,000 hours. It is also possible that the filament is broken due to violent vibrations, causing premature lamp damage.
In comparison, LED has the advantages of energy saving and environmental protection, and the power consumption is quite low. The working voltage of a general LED is 2 to 3.6V. LED is a kind of cold light-emitting technology, the heat is much lower than ordinary lighting source; LED is completely encapsulated in epoxy resin, there is no loose part in the lamp body, it is not easy to damage, and the service life of LED can be reached under proper current and voltage. 5 to 100,000 hours; LEDs are made of non-toxic materials and are free of contamination. The current shortcoming of LED is that the price is relatively expensive, and the processing of heat is difficult, so the driving power is a key factor in the overall life of the LED lamp.
Replace the low-efficiency MR16 halogen lamp with LED MR16 lamp
ON Semiconductor's CAT4201 is a 300 mA LED driver capable of driving three LEDs in series for MR-16 LED lamp design. The device is optimized for both external component count and thermal performance.
· Circuit characteristics
The CAT4201 uses a buck topology and does not require input/output (I/O) electrical isolation. The input voltage is 12Vac and the output power is 2.7 W. Other specifications include: output voltage is 9.1 V, rated input power is 3.4W, rated average current. It is 300 mA with a maximum average current of 310 mA, a minimum average current of 295 mA, and a typical energy efficiency of 83%. Figure 1 shows an MR16 lamp circuit using CAT4201.
Figure 1: MR16 lamp circuit with CAT4201
The CAT4201 is a step-down, easy-to-configure dimmable LED driver. The device is available in a 5-lead SOT-23 package; there are fewer than 10 discrete components (most of which are surface mount components), and the entire board can be small enough to fit into the lamp holder of the MR16 lamp. The VBAT, CTRL, and SW pins are rated at 40 V, so you can drive 12 LEDs with enough voltage. The RSET pin determines the output DC current; CTRL is used for the dimming signal input. SW is the output pin of the internal MOSFET.
·working principle
The application circuit works in a complicated way and is a simple buck converter. In the case of AC applications, the input is rectified via a rectifier bridge. VBAT and GND are directly connected to the two terminals of the bulk capacitor C1, reducing current ripple. During the first switching phase, the internal MOSFET charges the inductor with a linear rising current until the MOSFET is turned off; during the second phase, the MOSFET is turned off, and the current stored in the inductor passes through the Schottky diode (D5) Discharge while the current decays until the next switching cycle. The output capacitor is used to reduce the current ripple in the LED. The voltage at the RSET pin is regulated at 1.2V, so the resistor connected to RSET determines the RSET current. The RSET current is roughly proportional to the constant current output of the CAT4201.
·Circuit configuration
In terms of circuit configuration, VBAT is rated from -0.3 to +40 V, so it is safe to have an input voltage of up to 24 Vac on the rectifier bridge. To maintain a high input voltage level, a large capacity C1 is required. The widely fluctuating rectified AC current will pull the output current to zero at double AC line frequency, thus reducing the output current. A 220 μF capacitor is sufficient for this design. For a 12 Vac input, the C1 rated voltage should be 25V. Due to the low forward voltage required, the rectifier bridge uses the MBR0520L surface mount Schottky diode.
The RSET pin is used to configure the output current value. The RSET voltage is regulated at 1.2V. Adding a resistor between RSET and GND determines the RSET current, which is approximately linear with the output constant voltage:
It is calculated that for a 300 mA output, IRSET = 0.12 mA. The RSET resistance should be 10kΩ. A smaller RSET resistor may increase the output current and a minimum of 8.0 kΩ is recommended for stable operation. If the input voltage is high enough (eg 15 Vac), the RSET can be set to 8.2kΩ, thus guaranteeing an output current of at least 350 mA. CTRL obtains the voltage from the cathode of the LED via R2. The value of R2 is not important.
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