Power Electronics

PFC/PWM Controller Disables PFC to Limit Standby Power

The NCP1603 from ON Semiconductor (Phoenix, Ariz.) combines a power-factor correction (PFC) control chip and a pulse-width modulation (PWM) control chip in a single 16-pin SOIC package. This combination simplifies the task of designing an off-line ac-dc power supply that simultaneously meets requirements for PFC and low standby power consumption. Target applications for this device include consumer products such as notebook adapters, TV monitors and set-top boxes.

In these applications, PFC is typically required when a power supply is operating under normal load conditions. However, under no-load conditions, PFC is not required. Furthermore, if a PFC is operating during no-load conditions, the quiescent current drawn by this circuit may make it impossible to meet the limits on standby power consumption established by the California Energy Commission, Energy Star and other organizations.

Under light-load conditions, the NCP1603 controller disables the PFC function, saving perhaps 100 mW to 200 mW of power. Currently, the most aggressive standards limit standby power consumption to 500 mW with plans to lower this limit to 300 mW in the future. Given those goals, the power saved by turning off the PFC is significant. Moreover, disabling the PFC ensures that the controller can meet even the 300-mW limit for standby power. Combining the PFC and PWM controllers also makes it possible to turn off the PFC during overload conditions.

When separate PFC and PWM controllers are used in a power-supply design, additional circuitry is required to disable the PFC. The simplest approach would use a bipolar transistor to pull down the VCC of the PFC controller. The circuit also would require a comparator with hysteresis and components for sensing the load conditions — perhaps 5 to 10 additional components in all.

The NCP1603's PFC stage is implemented in either critical-conduction or discontinuous-conduction mode (CRM/DCM) boost, providing near-unity power factor. It operates in voltage-mode and features programmable switching frequency for DCM operation, and the ability to synchronize PWM and PFC operation for improved noise immunity. Under normal to heavy loads, the PFC can operate in critical-conduction mode to maintain lower peak currents (though without PWM-PFC synchronization). Then, under light-load conditions, the PFC can switch to fixed-frequency discontinuous mode to save power and lower EMI. In addition, the PFC offers a variety of protective features including programmable overcurrent protection as well as thermal shutdown and undervoltage lockout, both with hysteresis.

The PWM stage is implemented in continuous-conduction or discontinuous-conduction mode, using current-mode control. The PFC-bias voltage is disabled when in standby condition, which yields no-load power consumption of less than 200 mW. This stage features an internal 2.5-ms soft-start and ± 6.4% frequency jitter for improved EMI performance. The protection features provided for the PWM stage are similar to those of the PWM controller. Fault protection is implemented by an internal timer, and independent of the auxiliary transformer winding's coupling quality. Pricing for the NCP1603 is $0.96 per unit in quantities of 10,000. For more information, see www.onsemi.com.

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