iW1700

Zero Power No-Load Off-Line Digital PWM Controller

iW1700

Output Current

CV mode

Figure 9.4: Power Envelope

9.6 Multi-Mode PWM/PFM Control and

Quasi-Resonant Switching

The iW1700 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 iW1700

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 iW1700 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 iW1700 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 output 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 iW1700 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 iW1700

to achieve highest overall efficiency and lowest EMI, without

causing audible noise over entire operating range.

9.7 Zero Power No-Load Operation

At the no-load condition, the iW1700 is operating in the DPFM

mode, where the switching frequency can drop as low as

275 Hz and still maintain tight closed-loop control of output

voltage. The distinctive DPFM operation allows the use of

a relatively large pre-load resistor which helps reduce the

no-load power consumption. In the meanwhile, the iW1700

implements an intelligent low-power management technique

that achieves ultra-low chip operating current at the no-

load, typically less than 400 µA. One important feature of

the iW1700 is that it directly drives a low-cost BJT switch.

Unlike a power MOSFET, the BJT is a current-driven device

that does not require a high driving voltage. As a result, the

UVLO threshold of the iW1700 is designed to be as low as

4.0 V (typical). The power supply system design can fully

utilize this low UVLO feature to have a low V

the no-load operation in order to minimize the no-load

power. In addition, the active start-up scheme with depletion

NFET eliminates the startup resistor power consumption

after the ENABLE signal becomes active. All together these

features ensure with the lowest system cost power supplies

built with the iW1700 can achieve less than 5 mW no-load

power consumption at 230 Vac input and maintain very tight

constant voltage and constant current regulation over the

entire operating range including the no-load operation.

While achieving ultra-low no-load power consumption, the

iW1700 implements innovative proprietary digital control

technology to intelligently detect load transient events, and

ensure adaptive fast response.

9.8 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

the off-time will be extended until the falling edge of V

SENSE

is detected. The maximum allowed transformer reset time is

110 μs. When the transformer reset time reaches 110 μs,

the iW1700 shuts off.