Electronic Design

PFC Controller Slashes External Parts Count

Power-supply designers tend to use discontinuous current mode (DCM) for power-factor correction (PFC) for power levels below 200 to 250 W. For higher power levels, they use continuous conduction mode (CCM). Although DCM is simpler to implement, the required electromagnetic-interference (EMI) filtering and other parts make it bulkier than CCM at the higher power levels.

The IR1150 PFC controller from International Rectifier lets designers use CCM from the 75-W level (below which PFC is not presently mandated) to 4 kW. The controller also is simpler than existing PFC controllers. It requires fewer external components and comes in an SO-8 package.

There is a key difference between the IR1150 and earlier chips. The earlier chips use a multiplier circuit in their current-control loop and an external driver. However, the IR1150 uses an integrator and moves the driver on-board.

The multipliers sampled the rectified ac line voltage and created a sinusoidal current reference signal. This in turn controlled the duty cycle of the main pulse-width modulation (PWM) controller, forcing the input current of the converter to follow the sinusoidal shape of the input voltage.

The IR1150's "integrator with reset" (IwR) circuit integrates the output voltage of the voltage error amplifier. This generates a ramp that is then compared to a reference voltage that is the sum of the current and the error voltage. When the integrator ramp reaches this level, the PWM pulse is terminated and the integrator output is recharged to zero until the next clock.

This process eliminates the need for resistors on the ac line to sense the voltage. Instead, the boost inductor creates a current, proportional to the voltage waveform, that's picked up by the RSENSE resistor.

Using an IR1150 controller in CCM mode instead of a DCM design in low-power appications reduces peak currents. It also cuts EMI filter needs by 43%. This yields a 16% reduction in pc-board space and a 10% increase in power density for a 120-W system.

A typical large server supply of 1 kW would need 40% fewer resistors and capacitors and half the number of current transformers. As for either supply's power factor, the IR1150 enables 0.999 PF with 4% total harmonic distortion.

Power-supply designers can choose a clock speed of 50 or 200 kHz, enabling cost/size tradeoffs in magnetics. Maximum gate drive is 1.5 A. An overvoltage protection pin senses if the voltage-feedback line opens. This puts the device to sleep and turns off the gate drive, checking every clock cycle until feedback is restored. Standby current is 150 µA. Removing VCC turns off PFC while keeping the chip alive, which is useful for meeting environmental regulations.

There are four IR1150 devices, differentiated by operating-temperature range and lead or lead-free assembly Pricing is $1.05 each for commercial and $1.38 for industrial-range parts, all in 10,000-unit quantities.

International Rectifier
www.irf.com

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