Configurable LED driver design for MR16 LED lamps

The MR16 lamp is a kind of multi-faceted reflector lamp. It is widely used in commercial retail and home decorative lighting. Because it usually uses halogen filament as the light source, it has many disadvantages such as low efficiency, more heat generation and halogen capsule treatment. . But current LED technology offers an MR16-compatible, highly reliable, cost-effective alternative to halogen lamps. For example, ON Semiconductor's CAT4201, which drives three LEDs in series, offers a new option for engineers designing MR16 LEDs. The following will introduce the energy-efficient MR16 LED lamp driver solution that replaces the MR16 halogen lamp in combination with the characteristics of the MR16 lamp.

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 3.6 V. 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 that does not require input/output (I/O) galvanic isolation with an input voltage of 12 Vac and an output power of 2.7 W. Other specifications include an output voltage of 9.1 V, a nominal input power of 3.4 W, and a nominal average. The current is 300 mA, the maximum average current is 310 mA, the minimum average current is 295 mA, and the typical energy efficiency is 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 1.2 V, 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 25 V. Due to the low forward voltage required, the rectifier bridge uses the MBR0520L surface mount Schottky diode.

The output capacitor C2 is recommended to be at least 4.7 μF to reduce output ripple. The larger capacity of C2 will effectively suppress the output ripple while increasing the output current by a few milliamps. However, because its contribution to total energy efficiency is negligible and the human eye cannot perceive high frequency fluctuations, it is not necessary to use a capacitance greater than 10 μF. The value of L is recommended to be 22 μH to set the switching frequency to approximately 150 kHz. The LED current range is from 0 to 350 mA, so an effective rated current of about 800 mA is sufficient.

The total continuous current flowing through the rectifier is always below 400 mA, even if the output current is 350 mA. Therefore, a continuous rated current of 0.5 A is sufficient for the rectifier. A closer look at the voltage across the freewheeling diode D5 reveals that a voltage spike of more than 20 V occurs despite a 12 Vac input. Higher input voltages (such as 15 V) further increase the stress on the diode and increase the risk of failure. Therefore, the rated voltage on the D5 should be 30 V or 40 V. ON Semiconductor's Schottky rectifier MBR0540 can be used as D5.

The RSET pin is used to configure the output current value. The RSET voltage is regulated at 1.2 V. 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 8.2 kΩ, 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. Performance and characteristics

The basic performance and characteristics of the circuit can be understood from the waveforms shown in Figure 2 to Figure 4 below. Its test equipment includes: Global Specialties 1506 AC isolated voltage variable AC line power supply; Voltech PM1000 power analyzer; Tektronix TDS754D digital phosphor oscilloscope; Tektronix TCP202 current probe; Agilent 34401A multimeter.

Figure 2: Relationship between input voltage, output current and efficiency

In the circuit configured according to the circuit diagram of Fig. 1, Vin = 12 Vac 50 Hz. Due to the AC line input, the LED current exhibits a very low frequency (twice the AC frequency, 2 × 50Hz) fluctuations. Using a larger capacity of C1 or a higher Vin will further smooth the AC-related ripple current.

Figure 3: LED current waveform

In the circuit configured according to the circuit diagram of Fig. 1, the start-up and power-off transients are as shown in Fig. 4.

Figure 4: Startup and Power Down Transients

Compact design

The advantages of the CAT4201 driver are its compact circuit and high energy efficiency, making it ideal for replacing filament-based bulbs such as spotlight applications. This ultra-small design fits into the standard connector (lamp holder) space of the MR16 LED spotlight, separating these temperature-sensitive components from the hot LEDs.

Thanks to its optimization, the design is even better in terms of component count and thermal performance. This design typically uses three 1 W LEDs in the mirror section and can be adjusted to meet the lighting system designer's requirements. In the final solution, LED current and capacitor size must be chosen to achieve the best energy efficiency, accuracy, size and component count.

to sum up

Reducing capacitor and inductor values, or not using CTRL resistors, can further reduce cost and shrink PCB size. The use of small capacitors results in a large drop in average output current, so this is not recommended, but for low brightness (200 to 250 mA) applications. Choosing a high-quality LED with a small forward voltage is critical to achieving a larger output current design.