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PDF AN708 Data sheet ( Hoja de datos )
| Número de pieza | AN708 | |
| Descripción | Low-Power Universal-Input Power Supply Achieves High Efficiency | |
| Fabricantes | Vishay Siliconix | |
| Logotipo |  |
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AN708
Vishay Siliconix
Low-Power Universal-Input Power Supply
Achieves High Efficiency
Expanding global markets have created a demand for what
have become known as universal-input power supplies –– that
is, power supplies that allow devices to be plugged into wall
outlets anywhere in the world. These power supplies must be
able to operate directly from 100-, 110-, and 220-V ac power
lines without the use of selector switches or jumpers. A power
supply with the ability to operate under such conditions while
remaining cost-effective is now becoming a necessity.
In the under 30-W power range, meeting the above
requirements while maintaining high efficiency has been a
challenge. Add to this the need to meet various international
safety standards, and the circuit designer has his hands full.
The demands of low-power universal-input power supplies are
met by the Si9120 pulse width modulation (PWM) controller
from Vishay Siliconix. Using the Si9120, the flyback circuit
presented in this application note demonstrates that designing
universal-input supplies can be a simple task.
CIRCUIT TOPOLOGY
For the low power levels that are of interest here (under 30 W),
the discontinuous-mode (DCM) flyback converter is the
preferred topology. The biggest advantage of this topology is
simplicity. The parts count in the power path cannot get any
lower.
The peak-to-average primary current ratio in a DCM flyback is
high relative to other topologies; however, at low power levels,
this is not a serious drawback. On-state losses are minimal.
Magnetics are small. Also, the transformer reset voltage is set
by the minimum input voltage and remains fairly constant as
the line voltage changes. As a result, a 600-V MOSFET proves
adequate, even with ac inputs up to 300 V RMS.
The DCM flyback converter, when operated under
current-mode control, provides a natural input volt-second
limit, which helps keep the drain voltage from getting out of
control during line or load transient conditions. Also, today’s
power MOSFETs are able to withstand avalanche current
many times greater than a low power circuit can typically
deliver (see appendix A). As such, the MOSFET will serve as
a clamp for the occasional spike which may result from a short
circuit or extreme load transient.
Cross regulation is fairly good, especially if leakage
inductance between windings can be kept low.[1] In a
universal-input application, meeting VDE input-to-output
isolation requirements is essential. Depending on the end
product, this can be as high as 3750-V RMS, primary to
Document Number: 70581
secondary –– a figure that is totally inconsistent with the desire
to achieve low leakage inductance. As a result, cross
regulation between primary and secondary- referenced
windings will be poor. This complicates the regulation of the
primary-side bootstrap winding used to avoid
secondary-to-primary feedback across the isolation boundary.
The addition of a simple spike-blanking circuit solves the
problem (see AN707, “Designing Low-Power Off-Line Flyback
Converters Using the Si9120 Switchmode Controller IC”).
When using the Si9120 for universal-input applications, it is
recommended that a bootstrap winding be employed. While
not strictly necessary, the power dissipation and chip
temperature are higher if bootstrapping is not utilized. As an
example, at VIN = 400 V dc and ICC = 1.5 mA, the power
dissipation on the chip without a bootstrap is 600 mW. If a 10 V
bootstrap supply is used, the dissipation is only 15 mW. This
becomes more of a concern as the gate charge requirements
of the power MOSFET increase, since the value of ICC for the
controller is largely dependent on gate drive demands.
Another advantage of the DCM flyback converter is its
single-pole loop response. This makes compensating the
feedback loop comparatively simple. In addition, transient
response can be quite good in DCM flyback converters. It is
possible (though not practical in a closed-loop system) to slew
the power stage from no load to full load in only one switching
cycle.
DESIGN EXAMPLE
The circuit shown in Figure 1 is an 11.1-W, 3-output off-line
supply. The input voltage is specified from 90- to 260-V ac.
Outputs are +5 V at 1.5 A, +12 V at 150 mA, and –12 V at
150 mA. The design features full VDE isolation, primary side
regulation, and true foldback current limiting. Operating
frequency is 100 kHz.
DCM flyback operating principles are generally well
understood and will not be presented here. Refer to
Vishay Siliconix Application Note AN707 for a detailed design
example. References 2 and 3 are also recommended.
Sizing the input capacitor and rectifiers for universal input
requires more thought than for comparable single-input
converters. Keep in mind that while the maximum input voltage
occurs at high-input line, the maximum current stresses will
occur at low line. This implies that the input capacitor value
must be sized at low line while the voltage rating is dictated by
the high-line condition. The bridge rectifier should be rated at
600-V dc minimum. The RMS current rating is calculated
below.
www.vishay.com S FaxBack 408-970-5600
1
1 page  
AN708
Vishay Siliconix
a)
Short Circuit On 5 V Output
Q1 Drain Voltage (100 V/Div)
Voltage On U1, Pin 4 (Current Sense)
(0.5 V/Div)
NOTE: 0.8 V dc pedestal caused by
the foldback circuit.
b)
Q1 Drain Voltage (100 V/Div)
Q1 Gate Voltage (100 V/Div)
c)
Q1 Gate Drive (5 V/Div)
Voltage On U1, Pin 4 (Current Sense)
(0.5 V/Div)
FIGURE 2. Operating Waveforms (all photos full load, VIN = 150 V dc)
Measured efficiency at VIN = 300 VDC was 73.4%.
During testing, an input capacitor value of as little as 33 mF
proved adequate versus the design value of 68 mF. The
low-value capacitor produces an input ripple voltage of 30 V
pk-pk. Since the primary inductance is slightly lower than the
design maximum value, the circuit is still able to maintain
regulation with the higher input ripple voltage value. This is a
Document Number: 70581
good example of where trade-offs can be made during
development programs. By using the larger input capacitance
and primary inductance, the peak input current could be
reduced slightly, and a slight improvement in efficiency should
result. However, a larger input capacitance will decrease the
conduction angle of the input rectifiers, and consequently will
reduce the input power factor. The priorities of a particular
application will determine the optimal approach.
www.vishay.com S FaxBack 408-970-5600
5
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| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet AN708.PDF ] | |
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