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Conventional halogen MR16 lamps draw more than 20 W of power from the AC source under normal operating conditions, so they operate well with electronic transformers. MR16 LED lamps, however, only need 7 W of power to provide the same light output as a 35 W MR16 halogen lamp. The driver you need depends on how many bulbs you want it to power multiplied by the wattage. It’s also advised the total wattage of all bulbs combined doesn’t exceed 80% of the transformer’s wattage. So if you choose to power 4 x 4W MR16 bulbs (16W total) from one driver, you’ll need a 20W driver. If you want to power 4 x 6W MR16 bulbs, you’ll need a 30W transformer, and so on. Figure 3: Input current when powered by an LET75 at 120 VAC without a dimmer. This waveform is taken with a 40 µs time base and clearly shows the extra load added by the circuit comprised of Q3 and Q4. This load is removed after the first 80µs on every half-cycle of the AC waveform. MR16 halogen lamps normally operate from a low-voltage AC source typically generated by a low-frequency AC transformer or by a high-frequency electronic transformer. In most MR16 applications, the high-voltage AC provided by electric power companies is converted to a low-voltage AC by a high-frequency electronic transformer or by a low-frequency magnetic transformer. A high-frequency electronic transformer has a primary winding that connects directly to the 120 VAC/230 VAC. It uses high switching frequencies to provide the low voltage (12 VAC) which is applied to the MR16 halogen lamp.
There is another way to solve the problem: reduce the boost inductor to 10µH, operate at high switching frequencies, and remove the additional load. The high switching frequencies will cause higher switching losses, but would not need the additional load. Both the above mentioned methods are proprietary to Maxim Integrated. Performance data are shown here for a 6 W boost, MR16 LED lamp when powered by different electronic transformers. The boost MR16 was tested with seven LEDs on the output. Tables 1 and 2 summarize performance with different transformers. MR16 halogen lamps normally operate from a low-voltage AC source typically generated by a low-frequency AC transformer or by a high-frequency electronic transformer. In most MR16 applications, the high-voltage AC provided by electric power companies is converted to a low-voltage AC by a high-frequency electronic transformer or by a low-frequency magnetic transformer. A high-frequency electronic transformer has a primary winding that connects directly to the 120VAC/230VAC. It uses high switching frequencies to provide the low voltage (12VAC) which is applied to the MR16 halogen lamp.Figure 3. Input current when powered by an LET75 at 120VAC without a dimmer. This waveform is taken with a 40µs time base and clearly shows the extra load added by the circuit comprised of Q3 and Q4. This load is removed after the first 80µs on every half-cycle of the AC waveform.
Figure 1. Schematic of MAX16840 HB LED driver in a boost configuration for MR16 LED lamps. This configuration provides good compatibility with electronic transformers. Making the LED MR16 Lamp Compatible with the Electronic TransformerThe MAX16840 uses average current-mode control to control the input current. The voltage on the current-sense resistor, R3, is controlled by the voltage on the REFI pin; the average voltage on the resistor R3 is regulated for each switching cycle by the voltage on the REFI pin. The switching frequency is set internally at 300 kHz. The maximum voltage on R3 is clamped at 200 mV, so the current cannot exceed 0.2/R3. The bridge rectifier provides the rectified input voltage on pin 3 of the diode bridge D2. This rectified voltage is now averaged by R7 and C7. The DC voltage on C7 is converted to a current by resistor R8. The current mirror circuit consisting of Q2, R10, and R19 will create a current sink on the REFI pin, where the current drawn by Q2B = VC7/R8. The voltage on the REFI pin will, therefore, be (50 µA - VC7/R8) × R4, where 50 µA is the internal current source present on the REFI pin. The values of R8 and R4 are adjusted so that the input power varies within ±5% for the input voltage which, in turn, varies within ±10% of nominal. This design keeps the input power almost constant for line-voltage variations of ±10%.
An MR16 halogen lamp acts as a nonlinear resistive load. When the lamp is cold, the resistance is low and it will draw high currents which support the operation of the electronic transformers. Once the lamp lights up, the filament gets hot and its resistance increases. A typical 35W halogen lamp will draw 35W of power at 120VAC/230VAC when it is powered by an electronic or magnetic transformer. Since the halogen lamp is a resistive load, the brightness will decrease if the line voltage drops from nominal; brightness will increase when the line voltage rises from nominal.Figure 1: Schematic of MAX16840 HB LED driver in a boost configuration for MR16 LED lamps. This configuration provides good compatibility with electronic transformers. The MAX16840 has an EXT pin for driving an external transistor when the voltages on the IN pin are less than 5 V. The internal MOSFET will be in the off state. The EXT pin turns on Q5 and the 5 Ω load is added to the output of the bridge rectifier. Once the voltage on the IN pin exceeds 5 V, this load is turned off. This approach is useful when the electronic transformer is operated with trailing-edge dimmers. With some electronic transformer and dimmer combinations, the transformer does not switch properly when the dimmer is set at the minimum light output. This occurs when AC power is applied to the transformer with the dimmer set at minimum. The circuit of Q5, R20, and the EXT pin of the MAX16840 overcomes this problem by adding the 5 Ω load to the electronic transformer. This load is removed as soon as the LEDs turn on and provide light because the IN pin is directly connected to the boost output voltage. You may have bought or are thinking about buying some LED strip lighting or MR16 fittings or spotlights for your home. Either way you’re here to find out the whats and whys about LED drivers (or transformers). This handy guide will help you make an informed decision and give you the fundamentals you need when buying a driver. Why do I need an LED driver?
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