Selecting a Power Factor Correction IC for AC-DC Conversion

If you’re pulling power off of AC mains, you can use a power factor correction IC to ensure high efficiency AC-DC conversion.

It’s one thing to create a functionally perfect design, but it’s another thing to ensure that design can be legally plugged into mains. Whether the device is safe and functional is not really the issue. Instead, ensuring high conversion efficiency in power supply stages, whether as their own units or integrated onto a PCB, is all about maintaining a high power factor. This is one aspect that can get overlooked by many designers, but it’s critical for standards compliance.

This need to control power factor correction applies at the industrial level, consumer level, and anywhere in between. If you’re running off AC mains and regulating your DC input with a switching regulator, then you should consider adding a power factor correction IC. The benefits include compliance to standards and saving your customers some money on their electric bills. Here’s what to watch for when selecting a power factor correction IC for your next PCB.

What is Power Factor Correction?

The power factor of an electrical system is defined as the actual power drawn by the system to the apparent (or theoretical) power drawn by the system. For simple linear step-up and step-down AC conversion (e.g., with a transformer), or linear DC-DC conversion (e.g., with a voltage divider), the power factor is equal to the system’s efficiency when there is no reactive power. In this case, power factor correction simply involves adding some phase advance or phase lag to the system so that the current draw by the system is perfectly in phase with the input voltage.

For AC-DC conversion and subsequent regulation, the situation is more complicated due to the presence of nonlinear components. Here, nonlinear components, such as the diodes used in a rectifier, will produce harmonic distortion in the current waveform at the input to the regulator stage. In essence, the only time current gets drawn into the rectifier section is when the diodes in the rectifier are conducting, producing a pulse of current into the system.

This is shown in the example waveforms below for a system with a rectifier as measured before the smoothing capacitor. The blue curve shows the rectified AC voltage that is input to the smoothing capacitor, and the red curve shows the current draw whenever the diodes in the rectifier conduct.

Voltage and current waveforms in a rectifier before the smoothing capacitor/regulator stages.

Why should this happen? Note that the diodes in the rectifier, being nonlinear components, effectively change their DC resistance between high and low states once the input voltage exceeds some threshold, so it only draws significant current when the rectified voltage goes high enough. This is why the input current during rectification appears as pulses, rather than a rectified sine wave. This creates harmonic distortion on the AC mains, which must be kept below some specified level as a high THD basically wastes power elsewhere in the grid.

In this example, suppose the system’s power factor is 60% and your regulator’s theoretical efficiency is 95%; the real efficiency will be 60% x 95% = 57%. This should show how, in cascaded power regulation strategies, a low power factor/efficiency in one block will decrease the efficiency in all downstream blocks. By adding a power factor correction circuit, you are smoothing out the current draw into the downstream voltage regulator stage so that it matches closer to the true voltage waveform, which increases the total efficiency of the power supply section.

Selecting a Power Factor Correction IC

By adding a power factor correction IC between the rectifier output and your downstream regulator stage, you’re bringing the overall power factor of the system closer to 1. COTS components can bring the power factor very close to 1. There are other ratings to watch for when selecting a power factor correction IC:

• Max. voltage and current ratings: Power factor correction ICs are not designed for extremely high voltage/high power systems. Pay attention to these ratings to keep the chip from burning up. Note that the power factor of a real IC may be a function

• Topology: PFC circuits can have buck or boost topology. You could build a buck-boost PFC, but this is not commonly used as you normally need to step-up or step-down the AC mains power. The block diagram for these ICs basically looks just like a 

• Modulation frequency: A power factor correction IC uses a switching PWM signal to periodically draw current into the PFC circuit in synchronization with the input voltage waveform. This switching action will smooth out the current drawn from the rectifier stage. Typical values are in the 100 kHz range. Some ICs will provide this PWM signal as an output for use in a downstream switching regulator.

• Conduction mode: There are three conduction modes available: continuous conduction mode (CCM), critical conduction mode (CrCM), and discontinuous conduction mode (DCM). This will relate to the modulation method (either PWM or PFM), of which PWM is quite common.

Here are some of the power factor correction ICs you’ll find on the market:

Analog Devices, LT1509

The LT1509CSW by Analog Devices converts an unregulated high voltage output to an isolated low voltage output using PWM. The duty cycle is internally limited to 47% to prevent transformer saturation. This component will internally synchronize the PWM signal with the PFC controller section to ensure maximum power factor correction (99% rated power factor). The input supply voltage is rated from 11.5 to 25 V with 7.5 V rated output thanks to an integrated reference voltage circuit.

LT1509 power factor correction IC block diagram. From the LT1509 datasheet.

ON Semiconductor, NCL30030B3DR2G

The NCL30030B3DR2G from ON Semiconductor provides integrated power factor correction and regulation (quasi-resonant current-mode flyback topology) in systems requiring kW of power. This particular component was designed for driving banks of LEDs, but it can serve other purposes requiring high power output. The PFC stage provides power factor values near 1 in CrCM with low harmonic distortion. The rated supply voltage ranges from 40 to 700 V with 210V/4A output.

Texas Instruments, UC3854BDW

The UC3854BDW power factor correction IC from Texas Instruments operates in CCM and accepts 10 to 20 V supply voltage (22 V absolute maximum). This IC contains an integrated 200 kHz PWM oscillator with an internal 7.5 V reference for switching and smoothing. The output is stepped down to 5V/1.2A with less than 3% line current distortion. This IC is also available in 16-pin PDIP, wide SOIC, CDIP, and 20-pin PLCC packages.

UC3854BDW power factor correction IC block diagram. From the UC3854BDW datasheet.

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