UV radiation consists of a fairly broad spectrum, ranging from 10 nm up to 400 nm, just below our visible spectrum. And while UV lamps and LEDs have a variety of uses such as curing, material inspection, and tanning, the 200-nm to 400-nm range is used for most sanitization applications. Many instruments containing UV technology have been developed specifically for sterilization, so let’s take a look at the design methodologies along with the challenges and considerations and how it’s made such a major impact on minimizing the spread of diseases.
A brief history of UV-C technology
While UV radiation has been recognized since the turn of the 19th century, it wasn’t until about 80 years later that it was discovered to, under certain conditions, kill bacteria due to its short-wavelength light (which turns out to be most effective at about 250 nm). This range is generally known as UV-C and, at certain levels of intensity and exposure, kills micro-organisms by eliminating critical nucleic acids and disabling their DNA and RNA structures.
This method of disinfection has been used for a wide variety of health-related applications, such as wastewater treatment, air filtration and purification, disinfecting food, and for general sterilization of medical equipment. UV-C light kills bacteria such as Clostridium difficile, one of the most common bacteria that are found in hospital environments and contribute to as many as 15,000 deaths a year. Despite its many positive attributes, there are some dangers associated with UV-C, particularly around human skin and eyes.
Most recently, there’s been interest in designing UV-C – based products for personal use; however, these hazards make it difficult to sell an approved device. In addition to human health hazards, this specific wavelength also breaks down some chemical bonds in several materials, including plastics and insulation, and so has many adverse effects that must be mitigated. But that aside, let’s look closely at the LEDs and lamps generating this light and how it can be controlled.
Driving and controlling UV-C technology
For the most part, UV-C LEDs work the same way as a normal LED, except they have a much larger forward voltage due to the high bandgap energy (the minimum energy needed to excite electrons across the conduction/valance bandgap) associated with their much smaller wavelength. Typically, with higher wavelengths come higher amounts of energy needed to emit light.
Getting larger voltages for small, handheld devices can be cumbersome and require boost converters. Fortunately, controlling the LED works the same way, including dimming or adjusting brightness/intensity. There’s a variety of ways to do this, including the use of a MOSFET with a driver, current-limiting resistor, and the voltage source, which needs to be high enough to account for the forward voltage, plus a little headroom.
An example of this can be seen in Figure 1, which consists of a MOSFET driver that allows for a TTL interface, an N-channel MOSFET, connection for the LED, and a current-limiting resistor, which puts a cap on the max current going into the LED to help prevent burnout.
Figure 1: LED driving and dimming circuit for a UV LED with a forward voltage of 6.5 V @ 500 mA
This method will provide you with the bare minimum needed to drive and control an LED. If, however, you require additional protection and power management is needed, using a dedicated LED driving IC (such as the LED6000 IC from STMicroelectronics, shown in Figure 2) can be well worth the cost, especially for higher-power applications.
Figure 2: Block diagram of the LED6000 LED driving IC from STMicroelectronics (Image: Datasheet “LED6000” by STMicroelectronics)
Unlike DC motors, which typically provide a torque/speed proportional to input voltage, an LED’s brightness will be proportional to its current. What both of the aforementioned circuit methodologies (MOSFET versus LED driver IC) have in common is that they provide a means to control the current across the LED in what is known as a “constant current” configuration. As the LED heats up, the forward voltage will drop and the current passing through the LED will increase if not controlled or limited properly. In some cases, this can lead to failure due to thermal runaway.
Understanding differences in cost
Okay, so UV-C LEDs are pretty straightforward and similar to most other LED applications — what about the cost? Aren’t UV lamps a cheaper option?
A quick search may reveal that higher-power UV LEDs with wavelengths specific for sanitization can cost up to 10 × or 20 × more than a standard high-power LED. And while UV lamps are cheaper, this technology’s lifetime is typically only a quarter of an LED. What’s more, it emits a much wider spectrum of frequencies at lower peak intensities and thus requires more time for exposure. And because the exposure is a lot quicker (in some cases, one-tenth the time needed for the same sanitization), the application becomes a lot safer, as there’s less harmful radiation.
UV LEDs have been replacing UV lamps in almost every major application due to the long-term economic and safety benefits, but the driving methodology for UV lamps is quite interesting. Figure 3 shows a typical germicidal UV lamp rated for 1.1-W radiation.
Figure 3: Philips TUV PL-S germicidal UV lamp (Image: Datasheet “TUV PL-S” by Philips)
There are typically two ways to drive a high-voltage DC lamp like this:
- 1. Stepping down from a standard AC voltage input with rectification, filtering, and using a transformer.
- 2. Stepping up from a lower DC voltage to the lamp’s voltage level and controlling with a lamp driver or ballast controller IC. This typically provides additional benefits that come with the controller ICs, such as dimming, enable/disable, and some better output protection and regulation.
An example of a lamp driver IC (UCC2973) used to drive a high-voltage lamp can be seen in Figure 4. Features provided by this IC include lamp current control and dimming, overvoltage protection for the transformer, programmable startup delay, and a feedback-controlled PWM driving stage.
Figure 4: Application circuit of a UCC3972/3 ballast controller IC by Texas Instruments (Image: Datasheet “BiCMOS Cold Cathode Fluorescent Lamp Driver Controller” by Texas Instruments)
Lamps may be better suited for some applications, but it’s been said that UV LEDs will continue to replace lamps in the near future due to the wide range of benefits. One additional benefit with LEDs is that they run a lot less hot and are easier to manage in terms of heat. With higher-power UV LEDs (which normally mount onto a PCB), there are many ways to dissipate the high heat that is generated at its junction. Incorporating things like metal core PCBs (referred to as MCPCBs) and heatsinks greatly reduce the heat seen at the diode itself and are common practice for high-current applications (see Figure 5).
Figure 5: Example of a UV high-power LED star PCBA (Image: LEDSupply)
UV-C applications
Notice that in Figure 5, a small array of three LEDs is used for increased intensity. In fact, some applications consist of large panelized arrangements of LEDs to achieve higher-intensity values, reduce exposure time, and help provide a uniform distribution in terms of disinfection. Violumas, a company that specializes in high-power UV LED solutions, develop chip-on-board (COB) solutions and standard or custom light bar and array options for various applications including disinfection, purification, and horticulture (see Figure 6). Violumas’s 3-pad flip-chip technology helps reduce junction temperature and increase max optical output at higher driving currents.
Figure 6: 12 × 1 UV light bar and a depiction of the 3-pad LED flip-chip technology from Violumas (Image: Violumas)
Aquisense Technologies is an integrator company that incorporates UV LEDs into products and applications primarily used for disinfection, such as the PearlAqua (see Figure 7), one of the world’s first water disinfection systems containing replaceable UV-C LED modules that last a little over a year.
Figure 7: Aquisense’s PearlAqua module (Image: Aquisense)
Atlantic Ultraviolet Corporation has been providing UV-based sterilization products since 1963 and specializes in germicidal UV lamps. The company’s most recent product, the Sanidyne, is a portable area sanitizer containing eight UV-C lamps meant for hospitals, laboratories, and unoccupied rooms or buildings that require extra levels of disinfection. This solution irradiates air and exposed surfaces with UV light at about 254 nm and comes in a few different modules with varying size and output levels (see Figure 8).
Figure 8: Sanidyne UV portable area sanitizers (Image: Atlantic Ultraviolet Corporation)
Progression in LED technology and electronics in general will continue to drive UV technology and give it a “bright” future in the health-care industry while limiting and helping to curb the spread of bacteria and disease.
