UK company Verdi Semiconductor has designed two power technologies specifically for the LED lighting industry.
One is RAIS (pronounced 'raise'), a novel single-stage mains power supply for one or more LEDs, which Verdi claims is inherently dimmer-compatible with a naturally high power factor: the second is an equally-novel low-loss active rectifier called RUB.
RAIS is based around an inductor-capacitor-inductor (LCL) resonant circuit operated in a particular mode that allows it to act as a current multiplier and voltage divider (see circuit).
It is not a resonant converter in the traditional sense, where switching edges are timed to zero current or zero voltage points.
Instead, it is an impedance converter spending much of its time in free-resonance, briefly kicked once on every half cycle by a fixed frequency, fixed mark/space ratio switching waveform from a proprietary chip.
"It is a continuous drive signal and a huge proportion of the time both transistors are off so you see a sine wave on output of chip," Verdi managing director, David Summerland told Electronics Weekly.
"The chip [briefly] clamps the input of L1 [see fig 2] to the supply rail. L1 is not used as storage device, but as an impedance. Voltage and current are in phase," said Summerland. That is how energy gets into the LCL network.
Careful choice of input frequency, component ratios, and resonant frequencies means the energy leaves in a controlled way.
"Where the clever trick comes in, is that you don't have to switch current in L2," he said. "If you have current in L1 and at the right frequency, you could have up to 20x that current in L2. It is not normal LC resonance, it is LCL resonance - three resonances with very soft edges, current magnification and flat input current draw."
It is this flat input current draw (see fig 1) that Summerland said is fixed by the forward voltage of the LED load, that makes the circuit dimmer-compatible.
Domestic lighting wall dimmers designed to work with incandescent bulbs have minimum loads - 50W for example - to keep them working properly.
Particularly in the case of triac-based dimmers, if the load is too low at some point in the cycle, current drops below the triac's minimum hold value and conduction abruptly stops.
LEDs are ten times more power efficient and can easily draw insufficient current to keep dimmers operating properly.
To get over this, most dimmer-compatible off-line switching chips for LEDs include an internal transistor switch connected to a large off-chip resistor.
If LED current is too low, the transistor is turned on and the resistor wastes enough power to keep the triac conducting.
With its square input current waveform of typically 20mA, Summerland argues that RAIS keeps triac conducting without needing a bleed resistor.
Domestic dimmers are also designed to give a pleasing dimming characteristic when connected to normal light bulbs, with apparent brightness following the movement of the control knob.
LEDs connected directly to these dimmers have a terrible dimming characteristic, appearing to change little for most of the knob's rotation, with all control one end.
Almost all off-line LED chips include algorithms to solve this issue.
How does RAIS fare compared with incandescent dimming?
"The characteristic's not completely identical, but close," said Summerland.
Flat input current means a high power actor, and added to soft switching reduced the need for input EMC filtering.
"You do not need common mode filter or an X2 cap," said Summerland. The output side of the converter depends the input voltage waveform.
According to Summerland, the overall effect of the RAIS impedance converter is that:
output current is dependent on input voltage
input current is dependent output voltage
there is no coupling between output voltage and current.
"Output current is dependent on input voltage but independent of output voltage, so any number of LEDs can be connected in series," he said.
So, within reason, any number of LEDs can be connected in series and get the same current as one LED would, with and the amplitude of the rectangular current input waveform varying accordingly.
At the low end, with only one output LED, efficiency can be cut considerably by the forward voltage of the necessary rectifier bridge.
And this is where RUB, the second power technology, comes in. Anyone who has constructed an active bridge from four cross-coupled mosfets or bipolar transistors will know they can easily and cheaply offer forward voltage drops below 0.1V.
However, they do not work if any energy storage capacitor or battery is connected to the DC side as they short it out when the input voltage falls between cycles.
RAIS, with its inherent rectified 50Hz sine wave output current, can need an output capacitor to smooth LED current in low-flicker applications.
"You always have a cap on the diodes, so you can't have a self-operating active bridge," said Summerland.
The first chip
Verdi has integrated its RUB active rectification, plus a bandgap reference, into a chip, and will not reveal much about how it works.
"It has four active switches controlled by two little microprocessors, one for each half cycle," said Summerland. "There is no current sensing, it is all in the timing. It gets rid of most of the loss and we never get reverse current."
Both the RAIS and RUB chips are tiny: packaged in 1.6x2.3mm and 1.6x2.2mm respectively.
"We don't need to have low on-resistance from the mains switches so the RAIS chip can be small," said Summerland. "The whole circuit with the inductors and capacitors could fit on a 10p coin and be about 8mm high for 7-8W output."
The RAIS chip can be set to work anywhere between 20 and 70kHz.
With no control over frequency or mark-space ratio on the primary side, and fully resonant operation, control of output brightness might have had to be left to separate dimmers, but Verdi claims to have another trick up its sleeve: control by shorting the output through a control pin on the RUB rectifier chip.
"If output voltage is zero and output current is 200mA, that is 0W, and the output rectifier drops 0V, so that is also 0W," said Summerland. "You can shut out any half cycle at 20-70kHz. So if you shut out every second you get half output power and half input power. You are can regulating a resonant circuit without altering resonant frequency or using feedback."
There are actually three control pins on the RUB chip, nominally called: half power enable, inverted half power enable, and chip enable.
Different modes can be activated, including in-rush current limiting.
For other applications, there is a mode that chops energy stored on the output capacitor back into AC on what was once the input.
And has the firm got customers?
Summerland said that it is working with at least two "big OEM customers that have been with us for about a year".
"Chips have been going through qualification testing with them. We believe customers want to get product on market this year," he added.
