Voltage-mode CRM PFC for lighting offers higher efficiency, improves reliability

Critical-conduction mode (CRM) power factor correction (PFC) controllers are available with two different control modes: current-mode and voltage-mode.  While current-mode controllers have been used for quite some time, voltage-mode controllers offer significant advantages particularly for applications such as lighting, where high-efficiency, low total-harmonic distortion (THD) and high power factor are becoming more of a premium. 

The newest voltage-mode controllers offer a few other features and are specifically designed for lighting applications, such as the ability to maintain good power factor, low THD at light loads, zero overshoot at turn on (which reduces the stress on electrolytic capacitors and therefore increases reliability), and a PFC-ready pin for power sequencing of downstream converters. The new voltage-mode devices are designed to be backwards compatibility with older current-mode designs, to allow some of the benefits of this voltage-mode controller to be implemented in older designs without layout changes.

For both the voltage-mode and current-mode CRM PFC controllers, the boost switch is turned on when the boost inductor current reaches zero. This is measured with a zero-current-detect resistor connected to the auxiliary (aux) winding or V_CC winding before rectification, which feeds that signal into the control IC.  Proper optimization of this resistance can result in a level of quasi-resident switching for the power MOSFET in voltage-mode converters.  The main difference between a voltage-mode and current-mode controller is how the controller decides when to turn off the power MOSFET. 

For a current-mode controller, the MOSFET is turned off when the inductor current sensed through the current-sense resistor meets the desired current reference. In this case, a percentage of the rectified AC-line voltage (which is sensed through resistor-divider network) is used to generate the current reference which is input to the control IC via the multiplier pin. This means that there is some power loss in sensing the AC line voltage. 

Additionally, there is also a more significant power loss in sensing the MOSFET drain current. The MOSFET drain current is the inductor current, and that current is being sensed and compared with the reference measured from the AC line input. The value of that signal needs to be high enough to be used for compensation and control. Therefore, the overcurrent protection voltage needs to be above that signal for a current-mode device. This voltage is approximately 1.2 to 1.4 V.

In a voltage-mode control PFC, the MOSFET drain current is also measured, but it is only used for overcurrent or inductor-saturation protection. Therefore, its overcurrent protection voltage is much lower, at approximately 0.8 V. With the voltage-mode device, you will typically have approximately 25-50% less power loss in the current-sense resistor than you would if you were using a current-mode control PFC IC.  This adds to the modest power savings from the input-sense resistors, and the output-sense resistors can provide a better efficiency, particularly at lower-power applications.

Another significant difference between the current-mode and voltage-mode PFC controllers has to do with the placement of its compensation network.  In a current-mode controller, the compensation component is the capacitor C_comp, Figure 1, between the output of the error amplifier and the resistor-divider network sensing the output voltage. When the power supply is first turned on, an overvoltage will typically occur on the output capacitor. 

  Figure 1: CRM PFC current mode (upper) vs. voltage mode (lower).

[Click here and here to enlarge image.]

This is because it takes a finite amount of time and a measurable amount of current to flow through the high-resistance output-sense resistors to affect the charge on the compensation network represented by C_comp in Figure 1. The practical effect of this is that every time the power supply is turned on, the output voltage may ring and the initial peak of that voltage can be quite high, Figure 2. This ringing and overvoltage triggering can result in an audible noise when a lamp is first turned on.

Figure 2: Voltage stress on electrolytic capacitor at turn on.

 [Click here to enlarge image.]

On a voltage-mode controlled PFC, the compensation network is connected from the output of the error amplifier and ground. This has two benefits:

•First, it does not require any current to flow the output voltage at the overvoltage protection limit which is shown in Figure 1. 

•Second is that it reduces the susceptibility to noise induced by layout, since one node of the composition components is connected to GND and the other node is connected to the error amp output.  

The newest generation of voltage-mode PFC controllers, such as the FL7930C, has added a few important features, such as a soft-start feature with zero overshoot, internal THD optimizer, and a PFC-ready pin. The THD optimizer allows for improved total harmonic distortion, while reducing the number of external components required and frees up an external pin which is being used for the PFC ready function. The PFC-ready pin tells the downstream DC/DC converter that the output voltage is at 89% of the set-bus voltage, making power sequencing much easier.

The FL7930C has an improved compensation and control range for very light loads. A  switching-frequency limit of 300kHz, along with internal AC-absent detection, ensures safe operation and restart through a controlled soft-start process should the AC-line voltage be momentarily interrupted, thereby eliminating both high-current surges and high-voltage spikes. These features help to improve PFC efficiency, improve THD performance, and reduce stress, making the FL7930C well suited for long life applications such as Industrial Lighting (277V AC) and dimming applications.

     Careful consideration has been given to the pinout of the FL7930C, in an effort to make the implementation easy when dropping it into an existing design which uses a current-mode controller such as the FAN7527B. Doing this allows the designer to leverage the many  advantages of the FL7930C, such as the ability to maintain good power factor, low THD at light loads,  zero overshoot at turn on, and higher efficiency.

In many cases, you can replace a current-mode CRM PFC IC like FAN7527B (or its alternatives) without layout changes. In this case, the designer will not be able to take advantage of the function of pin 2, or PFC-ready function on the FL7930C. Since the older current-mode controllers did not have this function, it has no effect on the design. The procedure for doing this is outlined in Figure 3 which shows a current-mode PFC design and what changes would need to be done to use the new voltage-mode FL7930C.                                              

Figure 3: Changes made to existing current-mode PFC

 to use the new voltage-mode PFC without layout changes.

[Click here to enlarge image.]

About the author

Alex Craig is a principal engineer for Fairchild Semiconductor Corp. He has worked in the semiconductor field for over fifteen years. He holds BS and MS degrees in electrical engineering from the State University of New York at Binghamton.

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