2.5.2
R
F-driven plasma lighting:
The next revolution in light sources are powered by solid-state RF technology
Recent developments in RF power technology, such as improved cost structure, ruggedness, and power levels
of up to 1200 W per device, have enabled a breakthrough light source technology, called ‘RF plasma lighting’.
All RF plasma lighting sources make use of a small, electrode-less quartz lightbulb that contains argon gas
and metal halide mixtures. The bulb is powered by direct RF radiation, which ignites the gas mixtures to
create and power a bright plasma, the color of which can be tuned by the composition of its constituents.
This technology works without any additional electrodes in
the bulb, unlike standard high-intensity discharge lamps.
No electrodes means very long operating lifetimes, since
the contamination and wire erosion that lead to decreased
efficiency and eventual lamp failure are precluded. The RF
light source lives up to 50,000 hrs when it reaches 50% of its
original light output. Typical high-intensity discharge lamps, by
comparison, achieve 20,000 hrs operating life. Another strong
point of the plasma light is its efficiency: 1 W of RF power is
converted to 130-140 lm of light. This leads to very compact,
very bright lamps that easily emit 10,000 to 20,000 lm of white
light with a close-to-sunlight color rendition.
The key enabler for the RF light source is RF technology,
based on Si LDMOS RF power transistors. LDMOS technology
operating at 28 V is the leading RF power technology for cellular
base stations or broadcast transmitters as final amplifier stages
in the frequency range between a few MHz up to 3.8 GHz.
Recently, another LDMOS format, 50 V LDMOS, has emerged
for use in broadcast, ISM, defense and avionics applications.
It combines high power density to achieve power levels up to
1,200 W per single device and outstanding ruggedness, with
high gain and efficiency at frequencies of up to 1.5 GHz.
Comparison of lighting technologies
The table below summarizes currently available technologies that
generate bright light with varying degrees of efficiency. It lists a
few key parameters, including lifetime, luminous flux, efficacy, color
rendition index, color temperature, start-up time, and re-strike time
(time to start after switch-off from normal operation).
The plasma light source is among the brightest and most
efficient available to date and boasts a very long life time.
Important to note is the high brightness per bulb: much
brighter than LEDs, for example. Consequently, it takes
multiple LEDs to generate the light output of a single plasma
light source. Hence, LED luminaries for street lighting will be
considerably larger than those for plasma light sources.
RF implications
The RF plasma lighting sources can operate at a wide range of RF
frequencies, but initial applications typically focus at frequencies
of around a few hundred megahertz. At these frequencies both
the 28 and 50 V LDMOS technologies can be used, yielding
high efficiency values of 70% to more than 80% and low-heat
dissipation making compact plasma lamp designs possible.
The RF-driven plasma light is a perfect example of novel
applications that can be powered by RF energy in the
industrial, scientific, and medical (ISM) realm. Established
technologies use RF to pump a gas discharge in a laser cavity.
These "gas discharge" applications and, in general, most of
the ISM applications, typically form highly mismatched RF
loads during some part of the usage cycle. In the case of
gas discharges, for example, the gas cavity acts as an "open
circuit" during switch-on. This in turn means that without
protection or other measures, all of the "injected" RF power
reflected back into the final stage of the amplifier needs to
be dissipated in the transistor(s) right there and most likely
destroys the device(s) if this situation lasts too long. After the
discharge strikes, the load impedance reverts to "matched,"
eventually, and the transistor sees an acceptable load.
Obviously, these mismatched conditions occur every time the
plasma is "switched on,” exerting strain on the finals. LDMOS
transistors are designed to be extremely rugged and generally
withstand these mismatch situations without degrading over
time.
This ruggedness, combined with the high power density and
efficiency achievable, make LDMOS the preferred technology
for RF lighting and other equally demanding applications in the
ISM realm.
Color
Start-up Re-strike
Lifetime Luminous Efficacy
Color
flux
temperature
time
time
(lm/W) rendering
(hrs)
(klm)
(K)
(s)
(s)
Incandescent 2,000
1,700
10 to 17
100
3200
0.1
0.1
Fluorescent
10,500
3,000
115
51 to 76 2940 to 6430
0.3
0.1
LED
25,000
130
60 to 100
30
6000
0.1
0.1
Type
HID (high-
intensity
discharge)
RF plasma
20,000
50,000
25,000
25,000
65 to 115
100 to140
40 to 94
70 to 94
4000 to 5400
4000 to 5500
60
30
480
25
Table 1: Comparison of light generation. Note: numbers are only valid for a
qualitative comparison. Source: www.wikipedia.org and references therein.
64
NXP Semiconductors RF Manual 16
th
edition
PHILIPS [ NXP SEMICONDUCTORS ]
