S OLID-STATE LEDs are expected to play a significant
role in lighting systems due to benefits such as improved
color rendering, efficiency, form factor, and operating temperature
range [2]–[6]. Although some trends in commercial highbrightness
LEDs are toward high-power, high-current devices,
most applications still require a large number of LEDs to be used
in a single system [7]–[11]. Typical solutions, especially when
operating from a high-voltage supply or the ac grid, place many
LEDs in a series string and regulate the string current [12]. Such
solutions require the use of high-voltage components operating
at a relatively low switching frequency from tens of kilohertz
to low hundreds of kilohertz in order to limit switching losses.
The high voltage and low frequency result in bulky inductors
designed for large applied volt-seconds. The integration of components,
such as power transistors and gate driver circuits, alsobecomes difficult and expensive. At the same time, these solutions
also risk losing an entire string of LEDs with the failure
of a single element.
As an alternative, a series-input-connected modular structure,
as shown in Fig. 1, enables the use of low-voltage ICs
and components over a scalable range of high dc input voltage
buses [13]. The low-voltage cells can operate efficiently at high
switching frequencies in the megahertz range using low-profile,
light-weight components that match well to the miniature packages,
which are typical of LEDs. It also becomes more feasible
to achieve a high level of monolithic integration. Furthermore,
this structure provides a possible method to respond to individual
LED failure by automatically detecting and shorting the
affected cell from the series system. The dc input line voltage
bus may be the output of a power factor correction (PFC) stage
in an offline ac application or a direct connection in a dc system
(e.g., stand-alone solar, aircraft, naval ships, or potential future
dc wiring in buildings).
One critical issue for the modular structure is distribution
of line voltage, which has been investigated for series-input
parallel-output converters in [14]–[18]. However, most approaches
require an additional control loop for the line voltage
sharing. The common duty cycle approach, introduced
in [19] and inherited in [20]–[23], achieved good line voltage
distribution.
The modular LED driver structure with converters operating
in discontinuous conduction mode (DCM) was reported recently[13]. The approach presented in [13] uses two control loops in
each module, and relies on communications between the cells
to tune the control loops based on relative cell power levels and
achieves proper input voltage sharing.
This paper introduces a series-input-connected modular structure
implemented by converter cells operating in continuous
conduction mode (CCM), with common duty cycle control approach
to automatically distribute the line voltage between the
cells. More efficient operation can be achieved with lower peak
currents by operating in CCM as opposed to DCM. In addition,
only one local feedback loop is necessary in the entire
system using the proposed method to achieve well-regulated
LED output currents. The system loop gain is close to that of
a single converter, and thus, easy to design and compensate.
Two drawbacks of the system include an increase in the number
of components that scales with the number of modules and the
requirement for communications between the series modules.
The effects are partially mitigated by reduced voltage ratings
that scale down with the number of modules and low isolation
requirements since communications occur only between neighboring
modules.
This paper is organized as follows. Section II presents a description
of the overall system and dc operation. Section III
provides the analyses of the small-signal transfer function for
the system and considerations for compensator design. The LED
failure mode is introduced in Section IV. Experimental results
for a 25-W three-cell system with 9 Luxeon K2 high-brightness
LEDs are given in Section V, demonstrating line voltage sharing,
output current copying, and LED failure response.