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 Transformer 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.

There are differences between the operation of low-frequency AC transformers and electronic transformers that supply current to MR16 lamps, and there are also differences in the current draw for MR16 halogen lamps and MR16 LED lamps. These contrasts typically prevent an MR16 LED lamp from operating with most electronic transformers. This article explains how a high-brightness (HB) LED driver optimized for MR16 lamps will allow LED lamps to be compatible with most electronic transformers. The dimming performance (Figures 2 through 11) was tested with an LET75 and a Lutron ® SELV-303P dimmer. 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.The MAX16840 has an EXT pin for driving an external transistor when the voltages on the IN pin are less than 5V. The internal MOSFET will be in the off state. The EXT pin turns on Q5 and the 5Ω load is added to the output of the bridge rectifier. Once the voltage on the IN pin exceeds 5V, 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 5Ω 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.

The 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 300kHz. The maximum voltage on R3 is clamped at 200mV, 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 = V C7/R8. The voltage on the REFI pin will, therefore, be (50µA - V C7/R8) × R4, where 50µA 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%.

The transformer you need for strip lighting depends on multiple factors, such as the length of strip, watts per chip and total wattage. LED strip lighting comes with either small LED chips or larger LED chips, as well as a variety of sizes ranging from 1m to 5m. Small LED strip light chips are marked “3528” and measure 3.5mm x 2.8mm, while large LED strip light chips are marked “5050” and measure 5mm x 5mm. Larger 5050 chips are brighter per metre than small 3528 chips. To calculate the total wattage of the LED strip you wish to power you have to do one of these simple sums: • 60 small LED chips: calculate 5 watts per metre • 120 small LED chips: calculate 10 watts per metre • 60 large LED chips: calculate 15 watts per metre You then need to add another 20% to avoid potential overload on the transformer. Brightness increases, brightness decreases – this is not the consistent operation demanded of most applications today. It is possible, however, to keep the brightness of an MR16 LED lamp constant when the line varies around nominal input voltage. However, LED MR16 lamps are not resistive loads, which is what the electronic transformers require. Therefore, the loading behavior of the LED MR16 lamp needs to be adjusted so it can draw the power required to provide the desired light output and keep the electronic transformer operational. Different LED drivers are required for different environments for example indoor, outdoor use or kitchen or bathroom. If the driver is to be used outside we recommend that a waterproof driver is used with a minimim IP 65 rating. We would also recomend this for kitchens and bathrooms a waterproof driver with a IP67 minimum rating is used. 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.

By using an HB LED driver optimized for MR16 and other 12 VAC lamps, you can make MR16 LEDs compatible with electronic transformers. The LED driver demonstrated here was the MAX16840. One should note, however, that the performance of each, distinct electronic transformer and dimmer combination must be tested. A low-frequency AC transformer is bulky, heavy, and occupies a lot of space. In comparison, an electronic transformer is small and compact, and is designed to power a resistive load with a typical power demand exceeding 20W. When an electronic transformer is powered by 120VAC/230VAC, most will not work if the resistive load on the output is set to draw less than 20W. To solve this problem, this circuit design has an additional load consisting of R18, D7, C14, Q4, D8, R17, R11, R13, and Q3. This circuit adds 5 Ω to the electronic transformer, but only for approximately 80 µs to 90 µs per half-cycle of the rectified AC sine wave, which typically has a frequency of 100 Hz/120 Hz. The load is removed as soon as the current in the inductor has ramped up sufficiently high to keep the transformer operational. The power dissipated by this additional load is small.

Which driver do I need for my strip lighting?

To solve this problem, this circuit design has an additional load consisting of R18, D7, C14, Q4, D8, R17, R11, R13, and Q3. This circuit adds 5Ω to the electronic transformer, but only for approximately 80µs to 90µs per half-cycle of the rectified AC sine wave, which typically has a frequency of 100Hz/120Hz. The load is removed as soon as the current in the inductor has ramped up sufficiently high to keep the transformer operational. The power dissipated by this additional load is small. By using an HB LED driver optimized for MR16 and other 12VAC lamps, you can make MR16 LEDs compatible with electronic transformers. The LED driver demonstrated here was the MAX16840. One should note, however, that the performance of each, distinct electronic transformer and dimmer combination must be tested. 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. This is personal preference. Some customers like to have one transformer per fitting or light strip so they can deal with any potential faults easily. It may not always be possible to use one transformer per fitting due to constraints such as space and cost, so keep this in mind when buying. However, many prefer to use one transformer to power a series of bulbs or light strips (maximum five metres on one transformer). As explained above, the transformer you require to do this depends on the combined wattage of the bulbs you’re powering.