Analysis and design of single switch forward-flyback two-channel led driver with resonant-blocking capacitor

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Analysis and Design of Single-Switch
Forward-Flyback Two-Channel LED Driver
with Resonant-Blocking Capacitor
Abstract
Introduction:
In the mid-power applications, a two-stage structure,
which is composed of a boost power factor correction (PFC) and
an isolated DC/DC converter stage, is widely used. The boost
PFC provides a high power factor and a low total harmonic
distortion, and the DC/DC stage regulates the LED current to
control the luminance. Because the current of LED determines
its luminance, a series-connected LED string is widely used in
the mid-power applications. However, too many LEDs in series
result in a high output voltage of the DC/DC converter, which
burdens the output capacitor. Furthermore, the reliability of the
driver also can be reduced because the light will be turned off
when only one module is broken. For these reasons, LED strings
are connected in parallel, and a two-channel structure is widely
used in near 100W applications as shown in Fig 1. In the two-
channel structure, the current in each channel must be balanced
because the LED has the negative temperature coefficient of the

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forward voltage drop. Therefore, it is required for the two
channel LED driver to have not only current balancing
characteristic, also the wide output voltage range capability.
Existing method:
The flyback converter is used in both AC/DC and DC/DC
conversion with galvanic isolation between the input and any
outputs. The flyback converter is a buck-boost converter with
the inductor split to form a transformer, so that the voltage ratios
are multiplied with an additional advantage of isolation. The
flyback converter is an isolated power converter. The two
prevailing control schemes are voltage mode control and current
mode control (in the majority of cases current mode control
needs to be dominant for stability during operation). Both
require a signal related to the output voltage. There are three
common ways to generate this voltage. The first is to use
an optocoupler on the secondary circuitry to send a signal to the
controller. The second is to wind a separate winding on the coil
and rely on the cross regulation of the design. The third consists
on sampling the voltage amplitude on the primary side, during
the discharge, referenced to the standing primary DC voltage.

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Proposed system:
In this paper, a single-switch isolated driver for two-
channel LED strings is proposed. First, by using on-time
controlled forward-flyback inverter in the primary side, the
proposed driver obtains a wide output voltage range. Because
the proposed driver can use a small leakage inductor as the
resonant component, the additional external inductor can be
eliminated. Second, the proposed driver has clamped voltage
stress on the rectifier diodes, so that the low-voltage-rated
rectifier diodes can be used. The proposed driver is controlled
by the on time of the main switch. In order to reduce the
switching loss of the primary side inverter, the proposed driver
operates with the valley switching technique, and the
magnetizing inductor of the main transformer operates in the
boundary conduction mode.

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Advantages:
 Cost-effectiveness.
 Reduced conduction loss.
 Increases the power density with a high efficiency.
 Smaller voltage stress on the secondary rectifier diodes.
Applications:
 Light-emitting-diode (LED).

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Call: 0413-4205444, +91 9566355386, 99625 88976.
Block diagram:
BUFFER circuit
5VDC
PIC controller
circuit
OPTO coupler
circuit
12 V DC
INPUT
DC supply
Single switch
converter
with snubber
circuit
High
frequency
Isolation
transformer
Rectifier
and filter
Rectifier
and filter
LED strip

LeMeniz Infotech
Call: 0413-4205444, +91 9566355386, 99625 88976.
Tools and software used:
 MPLAB – microcontroller programming.
 ORCAD – circuit layout.
 MATLAB/Simulink – Simulation.

Analysis and design of single switch forward-flyback two-channel led driver with resonant-blocking capacitor

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Analysis and design of single switch forward-flyback two-channel led driver with resonant-blocking capacitor