26 Sep 2018
Influence of Converter Topology on the System Interactions
“Electronic converters are widely used in order to supply a LED module. The electrical interface between both has not been standardized over many years.
At first glance this interface appears to be trivial, however a professional matching requires considerations of medium complexity. Inappropriate combinations can impact the reliability of the luminaire or degrade the quality of light. In partnership to ZHAGA a Module-Driver Interface Special Interest Group, short MD-SIG, has been founded.
MD-SIG provides a free download of interface specifications: http://md-sig.org/specifications/
The power interface specification describes which electrical information shall be provided in the data sheets of converters and LED modules, how to interpret this information and practical tests for verification. The aim is to provide the data and a method which allows luminaire manufacturers a simple, but reliable matching.
The development of LED follows the main goal to design a light source. Strong focus is on nonelectrical properties like efficiency, optical properties and cost. MD-SIG avoids to put additional burden in terms of electrical requirements and foresees wide parameter ranges in order to cover also future LED generations.
Deep knowledge about the system interactions is required during the development of the converter. The considerations have to cover basic matching needs, fundamental characteristics of topologies including their operating modes, control theory and system interactions. The specification of MD-SIG is advanced beyond the common focus on a pure DC operating point. During converter design it allows an improved prediction of the following key interactions.
1. The LED module is part of the output filter:
In many converter topologies the module parameters directly influence the roll off frequency of the filter. This is especially important for low cost designs which use single stage power converters. In these designs the output filter has to suppress not just high frequency ripple but also the 100Hz or 120Hz modulation caused by the supply from standard AC mains. The resulting current ripple and the modulation of the light is not only depending on the converter, but can change when a luminaire manufacturer changes just the module.
A second consequence of the filter transfer function is the resulting modulation of the module voltage. Large voltage modulations have the potential to trigger an overvoltage shut down or voltage limiter even when the average voltage is below the protection threshold.
2. The LED module is part of the current control loop:
Depending on the converter topology and operating mode, the module parameters influence the loop gain of the control loop by multiple mechanisms. The module parameters can directly lift the loop gain, decrease gain margin and phase margin and cause instabilities. Such instabilities can cause flicker, stroboscopic effects, flashing or in worst case damage to the hardware.
LED module, electronic converter topology, MD-SIG power interface specification, differential resistance, instantaneous power, PFC, flicker, current source characteristic, voltage source characteristic, continuous mode, transition mode, discontinuous mode, filter, control theory, loop gain, phase margin, gain margin”