iW1707

Low-Power Off-Line Digital Green-Mode PWM Controller

iW1707

the output voltage under most conditions and is used to

regulate the output voltage.

9.4 Constant Voltage Operation

After soft-start has been completed, the digital control block

measures the output conditions. It determines output power

levels and adjusts the control system according to a light

load or heavy load. If this is in the normal range, the device

operates in the Constant Voltage (CV) mode, and changes

the pulse width (T

output voltage regulation requirements.

If no voltage is detected on V

auxiliary winding of the transformer is either open or shorted

and the iW1707 shuts down.

9.5 Current Limit and Constant Current

Operation

At overload condition, the iW1707 enters constant current

(CC) mode to limit the output current on cycle-by-cycle

basis. During this mode of operation the output current is

limited to a constant level regardless of the output voltage,

while avoiding continuous conduction mode operation. In

case of very heavy loading, when the output voltage is low

enough, the iW1707 shuts down.

The iW1707 senses the load current indirectly through the

primary current, which is detected by the pin I

resistor from the BJT emitter to ground.

Output Current

CV mode

Figure 9.4: Power Envelope

9.6 Multi-Mode PWM/PFM Control and

Quasi-Resonant Switching

The iW1707 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 iW1707

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

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.

When the switching frequency approaches to human ear

audio band, the iW1707 transitions to a second level of

PWM mode, namely Deep PWM mode (DPWM). During

the DPWM mode, the switching frequency keeps around

25 kHz in order to avoid audible noise. As the load current

is further reduced, the iW1707 transitions to a second level

of PFM mode, namely Deep PFM mode (DPFM), which

can reduce the switching frequency to a very low level.

Although the switching frequency drops across the audible

frequency range during the DPFM mode, the current in the

power converter has reduced to an insignificant level in

the DPWM mode before transitioning to the DPFM mode.

Therefore, the power converter practically produces no

audible noise, while achieving high efficiency across varying

load conditions.

The iW1707 also incorporates a unique proprietary quasi-

resonant switching scheme that achieves valley-mode turn

on for every PWM/PFM switching cycle, during all PFM

and PWM modes, and in both CV and CC operations.

This unique feature greatly reduces the switching loss

and dv/dt across the entire operating range of the power

supply. Due to the nature of quasi-resonant switching, the

actual switching frequency can vary slightly cycle by cycle,

providing the additional benefit of reducing EMI. Together

these innovative digital control architecture and algorithms

enable the iW1707 to achieve highest overall efficiency

and lowest EMI, without causing audible noise over entire

operating range.