Si9123
Vishay Siliconix
Care should be taken to control the operating time using the
internal pre-regulator to prevent excessive power dissipation in
the IC. The use of an external dropping resistor connected in
series with the VIN pin to drop the voltage during start up is
recommended. The value of R EXT is selected to drop the input
voltage to the IC under worst case conditions thereby
dissipating power in the resistor, instead of the IC. If the supply
output is shorted and the auxiliary winding does not provide the
VCC current, then continuous soft-start cycles will occur. The
average power in the IC during start-up where the hiccup
operation would be performed continuously is given by:
Voltage feed-forward is implemented by taking the attenuated
VINEXT signal at VINDET to directly modulate the duty cycle.
This relationship is shown in the Typical Characteristic section,
Duty Cycle vs. VINDET, page 12. The response time to line
transients is very short since the PWM duty cycle is changed
directly without having to go through the error amplifier
feedback loop. At start-up, i.e., once VCC is greater than
VUVLO, switching is initiated under soft-start control which
increases the maximum attainable switch on-time linearly over
the soft-start period. Start-up from a VINDET power down,
over-temperature, or over current is also initiated under
soft-start control.
ꢊt I
ꢇI
ꢈꢌ
CC2 ꢋ t2 CC4 ꢋ ISEC_SYNC
1
Half-Bridge and Synchronous Rectification Timing
Sequence
(
)
Power IC ꢉ VIN
ꢆ
ꢇ
ꢈ
t1 ꢋ t2
The PWM signal generated within the IC controls the low and
high-side bridge drivers on alternate cycles. A period of
inactivity always results after initiation of the soft-start cycle
until the soft-start voltage reaches approximately 2 Vbe and
PWM generated switching begins. The first bridge driver to
switch is always the low-side, DL as this allows charging of the
high-side boost capacitor. The timing and coordination of the
drives to the primary and secondary stages is very important
and the relationships are shown in Figure 3. It is essential to
avoid the situation where both of the secondary MOSFETs are
on when either the high or the low-side switch are active. In this
situation the transformer would effectively be presented with a
short across the output. The SEC_SYNC timing signal is set
to be ahead of the primary drive outputs by 50 − 80 ns.
ꢊt I
ꢇI
ꢈꢌ
CC2 ꢋ t2 CC4 ꢋ ISEC_SYNC
1
ꢇ
ꢈ
ꢇ
ꢈ
Power REXT ꢉ VINEXT ꢍ VIN
ꢆ
ꢇ
ꢈ
t1 ꢋ t2
where ICC2 is the non-switching supply current, ICC4 is the
supply current while switching, ISEC_SYNC is the average
current out of the SEC_SYNC pin, and t1 and t2 are defined in
Figure 4.
After the feedback voltage from the secondary overrides the
internal pre−regulator, no current flows through REXT
example of the feedback circuitry is shown in Figure 15.
. An
The SS pin has a predictable +1.25-mV/_C temperature
coefficient and can be used to continuously monitor the
junction temperature of the IC for a given power dissipation.
Primary High- and Low-Side MOSFET Drivers
The drive voltage for the low-side MOSFET switch is provided
directly from the VCC supply. The high-side MOSFET however
requires the gate voltage to be enhanced above VIN. This is
achieved by bootstrapping the VCC voltage onto the LX voltage
(the high-side MOSFET source). In order to provide the
bootstrapping an external diode and capacitor are required as
shown on the application schematic. The capacitor will charge
up after the low-side driver has turned on. The driver signals
DH and DL are shown in Figure 3. The drive currents for the
primary side MOSFETs is supplied from the VCC supply and
can influence start up conditions.
Reference
The reference voltage of Si9123 is set at 3.3 V. The reference
voltage should be de-coupled externally with a 0.1 ꢁF
capacitor and has 50-mA source capability. The REF pin
voltage is 0 V in shutdown mode.
Voltage Mode PWM Operation
Under normal load conditions, the IC operates in voltage mode
and generates a fixed frequency pulse-width modulated signal
to the drivers. Duty cycle is controlled over a wide range to
maintain the output voltage under line and load variation.
Voltage feed-forward is also included to improve line regulation
and transient response. In the half-bridge topology requiring
isolation between output and input, the reference voltage and
error amplifier are supplied externally, usually on the
secondary side.
Secondary Synchronization Driver
The secondary side MOSFETs are driven by the SEC_SYNC
output via a pulse transformer and gate driver circuits. The
time relationships are shown in Figure 3. Logic circuitry on the
secondary side is required to align the synchronous rectifier
gate drive with the primary drive. The current supplied to the
pulse transformer is drawn from VCC
.
The output error signal is usually passed to the power
converter through an opto-coupling device for isolation. The
error information enters the IC via pin EP and where 0 V results
in the maximum duty cycle, whilst 2 V represents minimum
duty cycle. The EP error signal is gained up by -2.2X via an
inverting amplifier and compared against the internal ramp
generator. The relationship between Duty Cycle and VEP is
shown in the Typical Characteristic section, Duty Cycle vs. VEP
25_C, page 12.
Oscillator
The oscillator is designed to operate at a frequencies up to
500 kHz. The 500-kHz operating frequency allows the
converter to minimize the inductor and capacitor size,
improving the power density of the converter. The oscillator
and therefore the switching frequency is programmable by a
resistor on the ROSC pin. The relationship is shown in the
Typical Characteristics, FOSC vs. ROSC
.
Document Number: 72098
S-40692—Rev. C, 19-Apr-04
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