New 12w LED Driver Transformer for MR16- MR11- G4 LEDLight Bulb Zero Interference with Dab and WiFi Energy Class A++

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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%. 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. 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? 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.

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. 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. The dimming performance (Figures 2 through 11) was tested with an LET75 and a Lutron ® SELV-303P dimmer. 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. 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.

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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. A similar version of this article appeared on Display Plus, July 7, 2012 and in German in Elektronikpraxis, October 1, 2012. Introduction

Conventional halogen MR16 lamps draw more than 20W of power from the AC source under normal operating conditions, so they operate well with electronic transformers. LED MR16 lamps, however, only need 7W of power to provide the same light output as a 35W 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. The correct MR16 LED Driver/Transformer is an investment for your MR16 LED bulb ensuring maximum lifespan and perfect operation. With LED lighting power supplies you are guaranteed our MR16 LED drivers are designed to perform with the latest LED technology. 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 3 µs or 4 µs. If it ramps up slower than this, then the electronic transformer may stop switching.

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. 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. 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 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.

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 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.There is another way to solve the problem: reduce the boost inductor to 10 µH, 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 dimming performance (Figures 2 through 11) was tested with an LET75 and a Lutron® SELV-303P dimmer. 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. 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. But 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. 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.

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 7. LED current waveform when powered by an LET75 with a trailing-edge dimmer at 120VAC. The dimmer is set at maximum light output.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. 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.