Make LED lights compatible with electronic transformers
There are differences between the operation of low-frequency AC transformers and electronic transformers that supply current to MR16 lamps, and there are differences in the current consumption of MR16 halogen lamps and MR16 LED lamps. These contrasts often result in MR16 LED lights not being usable with most electronic transformers. This article describes how a high-brightness (HB) LED driver optimized for MR16 lamps makes the LED lamp compatible with most electronic transformers.
Abstract: There are differences in the operation of the low-frequency AC transformers and electronic transformers that supply current to MR16 lamps, and there are differences in the current consumption of MR16 halogen lamps and MR16 LED lamps. These contrasts often result in MR16 LED lights not being usable with most electronic transformers. This article describes how a high-brightness (HB) LED driver optimized for MR16 lamps makes the LED lamp compatible with most electronic transformers.
This article discusses the operating differences between low frequency AC transformers and electronic transformers that provide current to MR16 lamps. It also explains the difference in current consumption between MR16 halogen lamps and MR16 LED lamps. These differences are important because current draw will typically prevent MR16 LED lights from operating with most electronic transformers. This article will show how a high brightness (HB) LED driver optimized for MR16 lamps makes the LED lamp compatible with most electronic transformers. However, this article does not delve into flicker-free operation of the dimmer and electronic transformer combination of LED MR16 lamps.
Resistive Loads and the Importance of Electronic Transformers
MR16 halogen lamps typically operate from low-voltage AC power, typically generated by a low-frequency AC transformer or a high-frequency electronic transformer. In most MR16 applications, high-voltage alternating current supplied by the power company is converted to low-voltage alternating current by a high-frequency electronic transformer or a low-frequency magnetic transformer. The primary winding of the high frequency electronic transformer is directly connected to 120VAC/230VAC. It uses high switching frequency to provide low voltage (12VAC) for MR16 halogen lamps.
Low frequency AC transformers are large, heavy and take up a lot of space. In comparison, electronic transformers are small, compact and designed to power resistive loads with typical power demands in excess of 20W. When the electronic transformer is powered by 120VAC/230VAC, most electronic transformers will not work if the resistive load on the output is set to less than 20W.
Conventional halogen MR16 lamps draw over 20W from AC power under normal operating conditions, so they work well with electronic transformers. However, LED MR16 lamps require only 7W of power to provide the same light output as 35W MR16 halogen lamps.
Resistive loading and brightness
The MR16 halogen lamp acts as a non-linear resistive load. When the lamp is cold, the resistance is lower and it will draw high current, supporting the operation of the electronic transformer. Once the light comes on, the filament heats up and the resistance increases. When powered by an electronic or magnetic transformer, a typical 35W halogen lamp will consume 35W at 120VAC/230VAC. Since halogen lamps are resistive loads, if the line voltage is lower than the nominal voltage, the brightness will decrease; when the line voltage rises from the nominal value, the brightness will increase.
Schematic of the MAX16840 HB LED driver in boost configuration for the MR16 LED lamp. This configuration provides good compatibility with electronic transformers.
Brightness up, brightness down - this is not a consistent operation required by most applications today. However, it is possible to keep the brightness of the MR16 LED lamp constant when the line changes around the nominal input voltage. But the LED MR16 lamp is not a resistive load, which is what an electronic transformer requires. Therefore, the load behavior of the LED MR16 lamp needs to be adjusted so that it can draw the power required to provide the required light output and keep the electronic transformer running.
Optimizing LED lights for constant current loads
The driver circuit of the MR16 LED lamp can be adjusted so that it draws a constant current load from the output of an electronic transformer. No capacitance can be added to the output of the electronic transformer as this will prevent the MR16 LED lamp from acting as a constant current load. Additionally, the current consumed by the MR16 LED lamp needs to ramp up to the programmed current very quickly. Specifically, it needs to jump to the programmed value within 3µs or 4µs. If it rises slower than this, the electronic transformer may stop switching.
The new design of the HB LED driver will allow most electronic transformers to operate smoothly with MR16 LED lamps. The current drawn by the MR16 lamp is adjusted based on the RMS voltage applied to the lamp. When the voltage is low, the MR16 lamp consumes a certain amount of current. To keep the input power constant, this current will decrease as the RMS input voltage increases.
This HB LED driver integrates a 0.2Ω, 48V MOSFET suitable for most applications. It is configurable for LED string voltages from 6V to 40V. If the number of LEDs in the string is greater than 6, the driver can be used in a boost configuration. For less than 6 LEDs, should be used in SEPIC configuration. In this article we only discuss boost configuration. Figure 1 shows the schematic of the boost configuration.
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