LED lamp
circuit: High-PF Flyback Converter with Super-Junction MOSFET
By: Wonseok Kang, Inki Park, Fairchild Semiconductor
As LED becomes popular for indoor lighting solutions, cost
structure has become a critical factor. A simple flyback
converter is one of best candidates for low-cost LED lighting.
However, switching power supplies for LED lighting still
requires high-power factor and high system efficiency. The
latest power devices are essential to overcome this challenge.
This article introduces a new integrated controller and
high-performance high-voltage super-junction MOSFET as an
LED lighting solution. These products enable both simple
structure and high performance.
Primary-side flyback controller:
Fairchild’s FL7732 highly integrated pulse width modulation
(PWM) controller provides several features to enhance the
performance of low-power flyback converters. The patented
topology of FL7732 enables the most simplified circuit design,
especially for LED lighting applications. By using a single-stage
topology with primary-side regulation, an LED lighting board
can be implemented with the fewest external components and
minimized cost without requiring an input bulk capacitor
and feedback circuitry. To implement high power factor and
low total harmonic distortion (THD), constant on-time control
is utilized with an external capacitor. Figure 1 shows typical
application circuit of FL7732 controller.
Fig. 1 Typical application circuit of FL7732
Constant current regulation is also an important feature
for LED lighting. Precise constant-current control function
in the FL7732 regulates accurate output current versus changes
in input voltage and output voltage. The output current
can be estimated by using the peak drain current of MOSFET
and the discharging time of inductor current because the
output current is about the same of the diode current in
steady state. The output current estimator picks up the
peak value of the drain current through a peak detection
circuit and calculates the output current using the inductor
discharging time and switching period. This output current
information is compared to internal precise reference to
generate error voltage which determines the duty cycle of
the MOSFET in constant-current mode. With Fairchild’s innovative
TRUECURRENT® topology, the constant output current can be
precisely controlled as:
Generally, discontinuous conduction mode (DCM) operation
is preferred for primary-side regulation because it allows
better output regulation. The operating frequency is proportionally
changed by the output voltage to guarantee DCM operation
with higher efficiency and simpler design. To maintain DCM
in the wide range of output voltage, frequency is linearly
changed by output voltage in linear frequency control. The
output voltage is detected by auxiliary winding and resistive
divider connected to VS pin. When output voltage decreases,
secondary diode conduction time is increased and the linear
frequency control feature makes switching period longer
to guarantee converter remains DCM operation over wide output
voltage range. The frequency control also lowers primary
rms current that results in better power efficiency in the
full load condition.
The FL7732 also provides the protection functions such as
open LED, short LED and over-temperature protection. One
important feature is that the current limit level is automatically
reduced to minimize the output current and protect external
components in the short LED condition. FL7732 also has frequency
hopping function in the oscillator for better electromagnetic
interference (EMI) performance.
The latest super-junction MOSFET:
In high-voltage MOSFET technologies, the most remarkable
achievement for on-resistance reduction is a charge-balance
technology. This technology, adopted from super-junction
structure, has a deep P-type, pillar-like body structure
as compared to the popular structure of conventional planar
technology. The effect of the pillars is to confine the
electric field in the lightly doped epi region. Due to this
P-type pillar, the resistance of N-type epi can be reduced
dramatically while maintaining the same level of breakdown
voltage. In addition to low on-resistance characteristics,
the second generation SuperFET® technology also achieves
less stored energy in output capacitance as well. This energy
value is more important in the case of small power applications,
like LED lighting, because the energy needs to be dissipated
at every switching-on instance.
A 20W rated LED lighting power board is selected to evaluate
the SuperFET® II technology. The power board was originally
developed with Fairchild’s 60V N-channel MOSFET, the FDD5N60NZ
and the FL7732 device. Among the major components, the FDD5N60NZ
is a planar technology MOSFET with an on-resistance of 2O?.
At the same cost, the SuperFET II technology can provide
0.9O? of on-resistance and less stored energy in output
capacitance together. With these superior electrical characteristics,
the SuperFET II technology can greatly boost system efficiency.
Figure 2 demonstrates the efficiency test results with various
AC inputs.
Fig. 2 System efficiency by MOSFETs
The SuperFET II technology shows the best efficiency results
over the entire input range and there is a large improvement
as compared to planar technology of the FDD5N60NZ. The SuperFET
II technology also recorded better efficiency than the competing
super-junction MOSFET, especially at high-input voltage.
This is a good example of how stored energy in output capacitance
affects system efficiency. Because the competitor super-junction
MOSFET has same on-resistance to the SuperFET II MOSFET,
the gap in efficiency can be considered as coming from the
switching losses. As shown in Figure 3, the competitor MOSFET
holds more energy in output capacitance as drain-source
voltage increases. This means this MOSFET is dissipating
more power during switching-on at higher input voltage.
In Figure 2, device level characteristics are well matched
to board level test results.
Fig. 3 Stored energy in output capacitance
Conclusion:
LED lighting power supply requires high- power factor, high
efficiency, isolated secondary-side for safety standards,
and less components due to limited space. The FL7732 and
SuperFET II MOSFET comprise a complete solution to these
requirements.
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