UK solar firm discloses novel inverter topology

Cambridge-based solar inverter firm Enecsys has revealed details of its novel power architecture, which eliminates electrolytic capacitors to increase reliability.

The firm's designs are intended to be fixed to the back of solar panels, producing 240V 50Hz directly - called micro-inverters by the solar industry to distinguish then from larger remote-mounted inverters that take power from a string of solar panels.

Direct mounting means micro-inverters have to operate at high ambient temperature, and if they contain electrolytic capacitors, said Enecsys, these will cause the inverter to fail well before the panel's predicted 25 year life.

With a patented topology, the firm has eliminated electrolytics and has instead moved to film capacitors, custom made by Epcos, which are rated at 30,000 hours MTBF.

"The various bits of the topology are known. It is the configuration of the three stages that no one has done before," Enecsys founder and chief architect Dr Lesley Chisenga told Electronics Weekly.

The first stage is a boost converter that produces a DC link voltage of up to 600V.

This is followed by second stage: a current output buck converter which is tolerant of up to 120V of ripple on its input. "Another aspect of our intellectual property covers the significant ripple capability of the second stage," added Chisenga.

The main energy reservoir capacitor is connected to the 600V DC link, where the high voltage plus the ripple tolerance of the second stage mean that only 30µF is needed to deliver 240W at the output - a value that can be made from film capacitors without resorting to electrolytics.
A novel topology allows film capacitors to replace electrolytics in Enecsys

Custom capacitors have been sourced simply to keep the profile of the micro-inverter below 30mm.

The third stage is a bridge switch which flips the polarity of every second cosine pulse to produce a 50Hz sine wave.

To remove the need for opto-isolators, which the firm claims is the next most damaging component for MTBF, galvanic safety isolation is provided by a transformer in the first stage where no feedback is required.

"This stage uses a half-bridge with a transformer which has substantial benefits in terms of efficiency, simplicity, and it allows us to use zero-voltage switching for low losses," said Chisenga.

The all-important maximum power point tracking (MPPT) - a technique which extracts the most from a solar panel under varying lighting and temperature conditions - is provided in the second stage.

Usually MPPT would require a measure of voltage and current passing through, but in this case the significant ripple on the capacitor allows rate-of-change-of-voltage across the capacitor to substitute for the current measurement.

"Tracking is done on the DC link component. All the other solar inverters we know about track on the load side," said Chisenga.