IW1678 Datasheet, PDF (8/11 Pages) Dialog Semiconductor – Low-Power Off-line Digital Green-Mode PWM Controller

iW1678
Low-Power Off-line Digital Green-Mode PWM Controller
9.6 Multi-Mode PWM/PFM Control and
9.9 Voltage Protection Features
Quasi-Resonant Switching
The iW1678 uses a proprietary adaptive multi-mode PWM /
PFM control to dramatically improve the light-load efficiency
and thus the overall average efficiency.
During the constant voltage (CV) operation, the iW1678
normally operates in a pulse-width-modulation (PWM)
mode during heavy load conditions. In the PWM mode, the
switching frequency keeps around constant. As the output
load I_OUT is reduced, the on-time t_ON is decreased, and
the controller adaptively transitions to a pulse-frequency-
modulation (PFM) mode. During the PFM mode, the BJT
is turned on for a set duration under a given instantaneous
rectified AC input voltage, but its off time is modulated by
the load current. With a decreasing load current, the off time
increases and thus the switching frequency decreases.
As the load current is further reduced, the iW1678 transitions
to a deep PFM mode (DPFM) which reduces the switching
frequency to a very low level.
The secondary maximum output DC voltage is limited by the
iW1678. When the V_SENSE signal exceeds the output OVP
threshold at point 1 indicated in Figure 9.3 the iW1678 shuts
down.
The iW1678 protects against input line undervoltage by setting
a maximum T_ON time. Since output power is proportional to
the squared V_INT_ON product then for a given output power as
V_IN decreases the T_ON will increase. Thus by knowing when
the maximum T_ON time occurs the iW1678 detects that the
minimum V_IN is reached, and shuts down. The maximum t_ON
limit is set to 15 μs. Also, the iW1678 monitors the voltage
on the V_CC pin and when the voltage on this pin is below
UVLO threshold the IC shuts down immediately.
When any of these faults are met the IC remains biased
to discharge the V_CC supply. Once V_CC drops below UVLO
threshold, the controller resets itself and then initiates a new
soft-start cycle. The controller continues attempting start-up
until the fault condition is removed.
While operating in the adaptive multi-mode PWM/PFM
control, iW1678 also incorporates a unique proprietary quasi-
resonant switching scheme that achieves valley-mode turn
on for every PWM/PFM switching cycle, in both CV and CC
operations. Due to the nature of quasi-resonant switching,
the switching frequency can vary cycle by cycle, hence the
actual average PWM switching in the power supply can be
lower than the PWM frequency stated in section 6. Together
these innovative digital control architecture and algorithms
enable iW1678 to achieve highest overall efficiency and
lowest EMI.
9.7 Variable Frequency Operation Mode
At each of the switching cycles, the falling edge of V_SENSE
will be checked. If the falling edge of V_SENSE is not detected,
the off-time will be extended until the falling edge of V_SENSE
is detected. The maximum allowed transformer reset time is
75 μs. When the transformer reset time reaches 75 μs, the
iW1678 shuts off.
9.8 Internal Loop Compensation
The iW1678 incorporates an internal Digital Error Amplifier
with no requirement for external loop compensation. For a
typical power supply design, the loop stability is guaranteed
to provide at least 45 degrees of phase margin and -20 dB
of gain margin.
9.10 PCL, OCP and SRS Protection
Peak-current limit (PCL), over-current protection (OCP) and
sense-resistor short protection (SRSP) are features built-in
to the iW1678. With the I_SENSE pin the iW1678 is able to
monitor the peak primary current. This allows for cycle by
cycle peak current control and limit. When the primary peak
current multiplied by the I_SENSE resistor is greater than 1.1 V
over current (OCP) is detected and the IC will immediately
turn off the base driver until the next cycle. The output driver
will send out a switching pulse in the next cycle, and the
switching pulse will continue if the OCP threshold is not
reached; or, the switching pulse will turn off again if the
OCP threshold is reached. If the OCP occurs for several
consecutive switching cycles, the iW1678 shuts down.
If the I_SENSE resistor is shorted there is a potential danger
of the over current condition not being detected. Thus,
the IC is designed to detect this sense-resistor-short fault
after startup and shut down immediately. The V_CC will be
discharged since the IC remains biased. Once V_CC drops
below the UVLO threshold, the controller resets itself and
then initiates a new soft-start cycle. The controller continues
attempting to startup, but does not fully startup until the fault
condition is removed.
Rev. 1.1
iW1678
3/12/12
Page 8