AN-9729 Datasheet, PDF (1/17 Pages) Fairchild Semiconductor – LED Application Design Guide Using Half-Bridge LLC Resonant Converter for 100W Street Lighting

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AN-9729
LED Application Design Guide Using Half-Bridge LLC
Resonant Converter for 100W Street Lighting
Introduction
This application note describes the LED driving system
using a half-bridge LLC resonant converter for high
power LED lighting applications, such as outdoor or
street lighting. Due to the existence of the non-isolation
DC-DC converter to control the LED current and the light
intensity, the conventional PWM DC-DC converter has
the problem of low-power conversion efficiency. The
half-bridge LLC converter can perform the LED current
control and the efficiency can be significantly improved.
Moreover, the cost and the volume of the whole LED
driving system can be reduced.
Consideration of LED Drive
LED lighting is rapidly replacing conventional lighting
sources like incandescent bulbs, fluorescent tubes, and
halogens because LED lighting reduces energy
consumption. LED lighting has greater longevity,
contains no toxic materials, and emits no harmful UV
rays, which are 5 ~ 20 times longer than fluorescent tubes
and incandescent bulbs. All metal halide and fluorescent
lamps, including CFLs, n contain mercury.
The amount of current through an LED determines the
light it emits. The LED characteristics determine the
forward voltage necessary to achieve the required level of
current. Due to the variation in LED voltage versus
current characteristics, controlling only the voltage across
the LED leads to variability in light output. Therefore,
most LED drivers use current regulation to support
brightness control. Brightness can be controlled directly
by changing the LED current.
Consideration of LLC Resonant
Converter
The attempt to obtain ever-increasing power density of
switched-mode power supplies has been limited by the
size of passive components. Operation at higher
frequencies considerably reduces the size of passive
components, such as transformers and filters; however,
switching losses have been an obstacle to high-frequency
operation. To reduce switching losses and allow high-
frequency operation, resonant switching techniques have
been developed. These techniques process power in a
sinusoidal manner and the switching devices are softly
commutated. Therefore, the switching losses and noise
can be dramatically reduced^[1-7].
Among various kinds of resonant converters, the simplest
and most popular is the LC series resonant converter, where
the rectifier-load network is placed in series with the L-C
resonant network, as depicted in Figure 1^[2-4]. In this
configuration, the resonant network and the load act as a
voltage divider. By changing the frequency of driving
voltage V_d, the impedance of the resonant network changes.
The input voltage is split between this impedance and the
reflected load. Since it is a voltage divider, the DC gain of a
LC series resonant converter is always <1. At light-load
condition, the impedance of the load is large compared to
the impedance of the resonant network; all the input voltage
is imposed on the load. This makes it difficult to regulate
the output at light load. Theoretically, frequency should be
infinite to regulate the output at no load.
Figure 1. Half-Bridge, LC Series Resonant Converter
To overcome the limitation of series resonant converters,
the LLC resonant converter has been proposed^[8-12]. The
LLC resonant converter is a modified LC series resonant
converter implemented by placing a shunt inductor across
the transformer primary winding, as depicted in Figure 2.
When this topology was first presented, it did not receive
much attention due to the counterintuitive concept that
increasing the circulating current in the primary side with
a shunt inductor can be beneficial to circuit operation.
However, it can be very effective in improving efficiency
for high-input voltage applications where the switching
loss is more dominant than the conduction loss.
In most practical designs, this shunt inductor is realized
using the magnetizing inductance of the transformer. The
circuit diagram of LLC resonant converter looks much the
same as the LC series resonant converter: the only
difference is the value of the magnetizing inductor. While
the series resonant converter has a magnetizing
inductance larger than the LC series resonant inductor
(L_r), the magnetizing inductance in an LLC resonant
converter is just 3~8 times L_r, which is usually
implemented by introducing an air gap in the transformer.
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 11/16/12
www.fairchildsemi.com