Power Electronics
Compact Drivers Help LEDs Replace Halogens

Compact Drivers Help LEDs Replace Halogens

Using a driver circuit based on a specialized buck controller, a white LED with integrated heatsink can replace a 10-W halogen bulb in MR16 lamps.

Halogen MR16 lamps are widely used in professional store and home decorative lighting applications. The power dissipation of the most commonly used halogen-based MR16s ranges from 10 W to 50 W, and their light output ranges from 150 lumens (lm) to 800 lm, which equates to an efficacy of about 15 lm/W. The lifetime of a typical halogen bulb is limited to about 2000 hours, and the filament must be protected from high levels of vibration to prevent the bulb from failing prematurely.

LED technology offers a cost-effective alternative that offers greater efficacy, longer lamp life and greater ruggedness. However, LED replacements for halogen bulbs have very different power requirements. Powering LED-based MR16 lamps demands a cost-effective drive circuit that is tailored to meet the LED’s electrical requirements, yet is small enough to fit within the unique form factor of the MR16 lamp assembly.

LED Performance

The latest generation of 5-W (4-mm × 4-mm single-chip package) and 10-W (7-mm × 7-mm four-chip package) high-power LEDs from LedEngin generate typical efficacies of 40 lm/W at 1000 mA when the junction temperature, TJ, equals 120°C. This is equivalent to a luminous output of 140 lm (ILED = 1000 mA, TJ = 120°C) for the 5-W package and 315 lm (ILED = 700 mA, TJ = 120°C) for the 10-W package under typical working conditions. Note that when these LEDs perform at the same brightness level as halogen bulbs, the power dissipation can be reduced by about 50%. In addition, LedEngin predicts a remarkable lumen maintenance of 90% for operation at TJ = 120°C after 100,000 hours, thus eliminating the need for bulb replacement throughout the life of the product.

For the MR16 reference design, Maxim selected the LedEngin 5-W product to demonstrate the 1000-mA drive capabilities of the MAX16820 application circuit. In most MR16 applications, the input voltage supplied to the halogen lamp is 12 Vac ±10%, so this was selected as the input voltage requirement for the drive circuit using the IC.

The MAX16820 has been specifically designed for LED driver applications targeting MR16 LEDs (among others), and is available in a 6-pin TDFN package. A 4.5-V to 28-V input voltage range, along with the ability to drive an external MOSFET, gives MAX16820-based driver circuits a broad range of LED current-drive capabilities. Additionally, its operating temperature range extends to 125°C, allowing the MAX16820 to be safely operated in the high-temperature environment of the MR16 light fixture. While the MAX16820 can control power levels beyond 25 W, a 2-MHz switching frequency results in a small inductor and small capacitors, which allow the driver circuit to be placed in the MR16 fixture.

Fig. 1 shows a 5-W MR16 LED lamp driver circuit, composed of a rectifier bridge (formed by D1, D2, D3 and D4), two 100-µF filter capacitors (C1 and C2) and a buck converter circuit. The buck LED driver is composed of the MAX16820, buck inductor L1, power MOSFET Q1, freewheeling diode D5 and sense resistor RSENSE. The 5-W LEDs require 1 A of drive current, which the LED driver is designed to produce. The hysteretic control method is used to control the buck inductor current (which is also the LED current). The hysteretic control implemented in the MAX16820 results in a simple and robust driver with 5% LED-current accuracy.

The high-side current-sense resistor, R, sets the output current and a dedicated PWM dimming input (DIM) allows independent pulsed dimming over a wide range of light levels. The high-side current-sensing scheme and on-board current-setting circuitry minimize the number of external components, while delivering LED currents with ±5% accuracy, using a 1% sense resistor (Fig. 2).

The MAX16820 features a programmable LED current using a resistor connected between the IN and CSN pins. The following equation can be used to calculate the sense resistor value:

where VSNSHI = 210 mV and VSNSLO = 190 mV.

The MAX16820 regulates the LED output current using an input comparator with hysteresis (Fig. 2). As the current through the inductor ramps up and the voltage across the sense resistor reaches the upper threshold, the voltage at DRV goes low, turning off the external MOSFET. The MOSFET turns on again when the inductor current ramps down through the freewheeling diode, D, until the voltage across the sense resistor equals the lower threshold.

The following equation can be used to calculate the operating frequency:

where n = number of LEDs, VLED = forward voltage drop of one LED, and DV = (VSNSHI - VSNSLO). A convenient design tool (available at www.maxim-ic.com/MAX16819-20-Tool) can be used to calculate the inductor value.

LED current ripple is equal to the inductor current ripple (Fig. 3). In cases when a lower LED current ripple is needed, a capacitor can be placed across the LED terminals.

To make a 5-W LED run in constant 1-A current for the entire line frequency period, dc-bus filter capacitors are added to limit the dc bus voltage ripple. The total capacitance should be at least 200 µF. The capacitors can be either two 100-µF tantalums or a single low-cost 220-µF electrolytic, all rated for 25 V.

To maintain the high accuracy of the output current, the maximum di/dt of the inductor current should be limited to less than 0.4 A/µs. The maximum voltage drop permitted across the inductor (L1) in the Fig. 1 circuit is VL1(MAX). The following formula can be used to calculate the resulting value for this inductor:

where d is the fractional variation allowed for VAC(IN), and VOUT is equal to the LED forward voltage. For VAC(IN) = 12 V, d = 10% and VOUT = 3.6 V, L is greater than 37 µH, and the 39-µH standard value is chosen for L1.

Circuit Evaluation

Operation of a 5-W white-LED-based MR16 light fixture from LedEngin that uses the Fig. 1 circuit together with the listed component values (for specific part numbers, see Table 1) was captured with an oscilloscope. The waveforms are shown in Figs. 4, 5 and 6. The output current ripple in these waveforms is about 10%. As shown in the voltage envelopes in Fig. 7, with 200-µF dc-filter capacitance, the dc-bus voltage ripple is about 8.5 V. However, the MAX16820-based hysteretic-mode control is seen to have very good line regulation performance. Thus, the output LED current has minimal variation despite the widely varying dc-bus voltage.

The bench tests also revealed that for the 5-W MR16 LED lamp driver, the input ac ripple and variation can be more that 8.5 V, while the output LED current remains regulated at a 1-A constant current. Electrical specifications for the Fig. 1 driver circuit are summarized in Table 2.

The MR16 pc board consists of two layers. All components are on the top layer. The bottom layer only provides connection to ground. On the board, there are two ac input connection pads and two dc current output connection pads for the LED.

In high-brightness LED applications, it is recommended to limit the junction temperature of the 5-W LedEngin LED to less than 120°C to maintain luminous output within 90% of initial capacity after 100,000 hours of operation. Heat sinking is a low-cost solution to transfer the heat generated in the LED junction to the air. The 5-W MR16 LED lamp has a built-in heatsink, as shown at the left in Fig. 8. The pc board, pictured at the center of Fig. 8 is mounted on the backside of the heatsink of 5-W LED and positioned within the MR16 casing in the relative position indicated at the right of Fig. 8.

Noteworthy is the MR16 heatsink design of the lamp assembly. In halogen-based assemblies, the lamp heat is primarily radiated to the environment. In LED-based designs, however, only the visible light is radiated to the environment, while the heat is transferred to the surrounding air through the heatsink via convection. The heat is thereby directed away from the illuminated object.

When compared to other lower-power LED solutions, such as 1-W and 3-W LEDs), the high-power MR16 solution with a 5-W LedEngin LED in combination with the MAX16820 IC circuit significantly increases the amount of usable light, and therefore eliminates the need for multiple emitters to match the performance levels of a 10-W halogen solution. PETech

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