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PDF ( 数据手册 , 数据表 ) IW2202

零件编号 IW2202
描述 Digital SMPS Controller
制造商 iWatt Corporation
LOGO iWatt Corporation LOGO 


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IW2202 数据手册, 描述, 功能
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iW2202
Digital SMPS Controller
Preliminary Data
1 Application
4 Description
Blue-Angel-compliant PFC-controlled switch-mode
power supplies up to 150 watts.
2 Features
§ PulseTrainregulation allows voltage, current
and PFC to be controlled independently
§ Primary-only feedback eliminates optoisolators
and simplifies design
§ No loop compensation components required
§ ±1% regulation over a 100:1 load variation
§ Built-in soft-start
§ Adaptive pulseTrain regulation keeps the bulk
capacitor voltage below 400V
§ Operates in critical discontinuous conduction
mode (CDCM)
§ Low start-up and supply current
§ Reduced EMI noise
§ SO-8 package
3 Benefits
§ Ideal for single-stage, single-switch power factor
correction (PFC)
§ Enables 97% power factor correction resulting in
EN6100-3-2 compliance
§ SmartSkip mode provides low standby dissipation
of the power supply enabling Blue Angel
Compliance
§ Efficiency greater than 85% across line and load
variation
§ Universal input (85-270V, 50-60 Hz)
§ Low parts count
§ Reduced design time due to the elimination of
loop compensation design
The iW2202 is a digital switching mode power supply
controller for PFC applications. Its is typically used
with the PFC-corrected BIFRED (Boost Integrated with
Flyback Rectifier/Energy storage DC/DC) topology,
shown in Figure 1. The BIFRED topology is a single-
stage, single-switch topology that combines a boost
converter with an isolated flyback converter, achieving
power-factor correction with a low parts count.
An iW2202-based power supply looks like a resistor to
the AC line. Unlike attempts to control the BIFRED
topology with analog controllers, the all-digital
iW2202 provides a near-unity power factor without
placing high voltage stresses on the bulk capacitor.
The iW2202 uses a proprietary new digital control
technology called pulseTrainto achieve efficiencies
in excess of 85% across a wide load range, and across
the universal input range of 85-270VAC, 50-60 Hz.
Internally, the iW2202 uses real-time waveform analysis
to determine crucial circuit parameters. The reflected
secondary voltage of the flyback transformer is sensed
at precisely calculated times to determine the
secondary voltage, the transformer reset time, and the
ideal zero-voltage switching point Measurements are
performed during the OFF time of every cycle, and the
results determine what is done on the next cycle. The
dynamic response time of the circuit is less than half a
cycle.
85-270 V,
50-60 Hz
AC
Input
Boost Inductor
Auxiliary
Winding
Switch
iW2202
Flyback
Winding
Output
+ Vout
Current
Sense
Boost
(Bulk)
Capacitor
GND
Figure 1. iW2202 system concept
Revision 1.1
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IW2202 pdf, 数据表
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iW2202 Data Sheet
11.1 Zero-Voltage Switching
PulseTrain achieves zero-voltage switching (ZVS) by
using the resonance (ringing) that occurs in
discontinuous-mode flyback circuits. This resonance
occurs after the secondary current falls to zero,
indicating the transition from power transfer to
open-circuit conditions.
Vaux = Vout*Nsec/Naux
Post-Conduction
Resonance
Vaux=0
Vaux = -Vin*Npri/Naux
t1 t2
ZVS Algorithm (Power Cycles):
1. Wait for Vaux < 0
2. Wait an additional tZVS
3. Turn on switch
tZVS = (t2-t1)/2
tZVS is measured on
sense cycles
Figure 6. Auxiliary voltage and zero-voltage switching
As shown in Figure 5, post-conduction resonance is
a damped oscillation that falls very close to zero
volts on its first cycle. Zero-voltage switching can be
achieved very easily, simply by measuring the
resonant period on a sense cycle, and switching the
output transistor when the voltage is closest to zero
on subsequent power cycles.
The algorithm for ZVS is shown in Figure 6. On each
power cycle, pulseTrain waits for the primary
voltage to drop below VIN, (on the auxiliary
winding, this occurs when the voltage goes
negative). This indicates that we are in the post-
conduction resonance. After this event, the
controller waits an additional T that will take us to
the minimum voltage, then turns on the switch for
the next power or sense cycle. The time between the
zero-crossing on the auxiliary winding and the
minimum primary voltage is estimated as being
one-half the time between the negative-going zero
crossing and the positive-going zero-crossing, as
shown in Figure 6. Given the geometry of the
resonant signal, this estimate is extremely accurate.
By achieving zero-voltage switching, we also
achieve critically discontinuous conduction mode,
because we have turned the transistor back on
immediately after the transformer’s magnetic field
has reset. This eliminates dead time between cycles,
fully utilizing the output transformer. As a result,
the transformer operates at lower flux levels than
Revision 1.1
Preliminary Data
traditional technologies, resulting in lower core
losses and thus higher efficiencies.
Because the waveform is being monitored in real
time, critically discontinuous conduction mode is
maintained across all variations in line and load
conditions. In addition, this method of extracting
maximum performance from the inductor is
insensitive to component variations, since the circuit
behavior is measured, not assumed.
11.2 Primary-Only Feedback
PulseTrain uses primary-only feedback, measuring
the secondary voltage by analyzing its reflected
voltage as seen by an auxiliary winding. This
reflection reveals what is happening at the
transformer secondary. However, the voltage at the
load differs from the secondary voltage by a diode
drop and IR losses. The diode drop is a function of
current, as are IR losses. Thus, if the secondary
voltage is always read at a constant secondary
current, the difference between the output voltage
and the secondary voltage will be a fixed V.
Furthermore, if the voltage can be read when the
secondary current is small, the V will also be small.
As shown in Figure 5, secondary current has a linear
ramp down to zero. Zero secondary current is
signaled (on the reflected voltage waveform) by the
beginning of post-conduction resonance. Using
resonance as a marker, is a simple matter to
calculate a fixed T that will pinpoint a time where
secondary diode current is still flowing, but is very
small. The exact value of T is not crucial, so long as
it places the measurement at a point where current
is still nonzero. T is recalculated on sense cycles.
Measuring voltage on sense cycles uses the same T
as on power cycles. Because the slope of the
secondary current ramp is independent of ON time,
the current at a fixed T is the same, regardless of
whether the cycle is a sense cycle or a power cycle.
11.3 Constant Peak Current
Like the decision to turn the transistor on, the
decision to turn the transistor off is controlled by
real-time waveform analysis -- this time in the
current domain. The maximum desirable primary
current, Ipeak, is set with external resistors.
On every power cycle, the power transistor is kept
on until the primary current ramps up to Ipeak.
When this level is reached, the transistor is turned
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