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Figure 7. LED current waveform when powered by an LET75 with a trailing-edge dimmer at 120VAC. The dimmer is set at maximum light output. 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.
A low-frequency AC transformer is bulky, heavy, and occupies a lot of space. In comparison, an electronic transformer is small and compact, designed to power a resistive load with a typical power demand exceeding 20 W. When an electronic transformer is powered by 120 VAC/230 VAC, most will not work if the resistive load on the output is set to draw less than 20 W. 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 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%.The driver circuitry of the MR16 LED lamp can be adjusted so that it draws a constant current load from the output of the electronic transformer. No capacitance can be added to the electronic transformer's output, since this can prevent the MR16 LED lamp from acting as a constant current load. Moreover, the current drawn by the MR16 LED lamp needs to ramp up to the programmed current at a very fast rate. Specifically, it needs to jump to the programmed value within 3µs or 4µs. If it ramps up slower than this, then the electronic transformer may stop switching. An MR16 halogen lamp acts as a non-linear 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 35 W halogen lamp will draw 35 W of power at 120 VAC/230 VAC 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.
A similar version of this article appeared on Display Plus, July 7, 2012 and in German in Elektronikpraxis, October 1, 2012. Introduction A new design for an HB LED driver will keep most electronic transformers operating smoothly with MR16 LED lamps. The current drawn by the MR16 lamp is adjusted with the RMS voltage applied to the lamp. When the voltage is low, the MR16 lamp draws a certain amount of current. To keep the input power constant, this current will reduce when the RMS input voltage is increased. The dimming performance (Figures 2 through 11) was tested with an LET75 and a Lutron® SELV-303P dimmer. 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%. 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.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? 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. 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. 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 7: LED current waveform when powered by an LET75 with a trailing-edge dimmer at 120 VAC. The dimmer is set at maximum light output.
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