Reduced component count, compact reference design for MR16 replacement lamps composed of multiple 1W LEDs

MR16 lamps are a type of multi-faceted reflector lamp that typically uses a halogen filament capsule as the light source. They are suitable for many retail and consumer applications, and their unique size, configurability, focusing ability and aesthetics make them practical and creative. However, low efficiency, heat generation and halogen capsule handling issues are often the disadvantages of this technology. MR16 lamps generally operate with a 12V DC or 12V AC universal electromagnetic transformer.

LEDs are an ideal replacement for halogen lamps because they are more efficient and emit no heat.

This reference design fits into the standard connector space of an MR16 LED spotlight. It has been optimized from parts count to thermal performance. This design is typically used with 3 1W LEDs in the mirror section and can be adjusted to meet the requirements of lighting system designers.

Figure 1 MR16 application using ZXLD1350

Note: For the specifications of ZXLD1350, please visit: http://www.zetex.com/3.0/pdf/ZXLD1350.pdf

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Figure 2 is a system wiring diagram of an MR16 lamp solution using the ZXLD1350 and ZXSBMR16T8, and Table 1 provides the component list.

Figure 2 System wiring diagram of the ZXLD1350 MR16 lamp solution

The ZXLD1350 is designed for LED current drive applications of 350mA or less. This monolithic NMOSFET is moderately sized to provide a cost-effective chip size, with a rated current of 400mA, providing sufficient margin in hysteresis operation mode (the current waveform rises and falls by approximately +/-15% from the nominal current set point). The main features of the ZXLD1350 include:

* Output current up to 380mA

* Wide input voltage range: 7V to 30V

* Internal 30V 400mA NDMOS switch

* High efficiency (more than 90%)

* Switching frequency up to 1MHz

The ZXSBMR16T8 is a space-saving, thermally efficient device designed specifically to meet the key requirements of MR16 applications. It contains a full bridge and a freewheeling diode to achieve nominal 12V AC input operation with low leakage 1A, 40V Schottky diodes. The combination of this Schottky bridge and embedded freewheeling diode helps improve system efficiency compared to standard silicon diodes in an integral design. The reference design has bypass solder points to omit the bridge rectifier and the final lamp design is also suitable for pure DC operation.

Since the ZXLD1350 uses a hysteretic converter circuit topology, the conversion efficiency depends on several factors – input voltage, target current and number of LEDs. An Excel-based calculator is also included for initial evaluation of the system and to determine component selection.

(For details, please visit http://www.zetex.com/3.0/otherdocs/zxld1350calc.xls).

System efficiency and LED current measurements are included in the design, and the ADJ pin is left floating to measure the current rating in the device. The ADJ pin has a high impedance input (200K) and is susceptible to leakage current from other sources. Any current drawn from this pin will reduce the output current. To avoid any electromagnetic coupling, a guard rail is provided around the pin.

Table 1 Components list

From the circuit schematic in Figure 2, it can be seen that only 0Ω resistors are needed to make jumper connections for pure DC operation.

Since the system is not protected against reverse polarity, the user must be particularly careful.

Figure 3 shows the circuit layout design, illustrating the advantages of space saving and compact design. Both the bottom and top layers are clearly shown, and the efficient device placement is clearly visible.

Figure 3 Circuit design

The main wiring design recommendations are as follows:

* All thin devices are placed on the same side

* Use star connection as ground rail

* Protect the ADJ pin with a ground ring

* Check, please:

* Connect R1 to ZXLD1350 as short as possible (sensing line);

* The filter capacitor C3 should be connected as close to the Vin pin;

* The freewheeling current path should be as short as possible to ensure system precision and efficiency.

The top and bottom layers of the board are shown in Figure 4.

Figure 4 Circuit board overview

Inductor selection and switch circuit wiring design

A 100 μH shielded inductor was selected to set the nominal frequency to approximately 250kHz while minimizing radiated EMI. Reducing radiated EMI is important in any switching regulator design. This reference design minimizes critical trace lengths and maximizes grounding areas around critical areas.

Circuit Performance

The performance of the circuit is evaluated based on two main parameters, including system efficiency and current accuracy.

The current of the reference circuit is set at a nominal value of 300mA, but the current can be adjusted to 350mA or lower by simply changing the sense resistor Rsense according to the following formula.

Iref = 0.1 / R1 [A]

When using R1 = 0.33Ω --> Iref = 300mA

Table 2 lists the data for a 12V to 15V DC power supply system, where a Schottky diode bridge was used for the test. The most important parameters are the system efficiency and the error between the rated LED current (300mA) and the actual LED current. In a DC environment, the frequency is between 150kHz and 300kHz, depending on the input voltage. Regardless of the input voltage, the efficiency is above 87% with an error of less than 2%.

Table 2 DC input voltage

Table 3 lists the relevant data for a system powered by SMD tantalum capacitors and AC electromagnetic transformers. The system is designed to save space while avoiding the use of larger and less reliable electrolytic capacitors. There are trade-offs between size, reliability, cost, and average LED current. The output voltage of a transformer rated for 12V AC will vary by ±10%, and the voltage drop across three LEDs is about 10V. As can be seen in Figure 8, if the capacitor value is less than 200 μF, the AC input waveform is distorted. When the rectified voltage is not effectively smoothed, the valley voltage may be lower than the LED voltage. In this case, the switching circuit stops operating, the LED average current drops, and finally the LED output lumen also drops.

Table 3 AC input voltage

Figures 5 to 8 show the input voltage ripple and LX voltage affected by the input capacitor Cin=C1+C2+C3. The larger the input capacitance, the more accurate the output current, which also makes the average output lumens higher. When the input capacitance reaches 300uF, the overall performance, including efficiency and current accuracy, is the best. On the contrary, when the input capacitance is reduced, the output current accuracy decreases by up to 25%, but the efficiency is always higher than 50%.

Figure 5 Cin = 300μF Figure 6 Cin = 200μF

Figure 7 Cin = 150μF Figure 8 Cin = 100μF

Figures 5 to 8: Input ripple and LX voltage (Ch3 is LX pin voltage, Ch4 is input voltage)

Conclusion

ZXLD1350 and ZXSBMR16T8, together with other passive components, present a small, reliable, high-efficiency and low-component solution. This ultra-small design can be placed in the connector housing, separating these temperature-sensitive components from the heat-generating LED as much as possible. In the final solution, the LED current and capacitor size must be traded off to achieve the optimal efficiency, accuracy, size and component count.

This is the first design guide in a series of reference designs for different MR16 solutions and options.