Power Factor Correction (PFC) maximizes utilized energy is drawn from the ac line. PFC converts input-current pulses from using a bridge rectifier to a sinusoidal current waveform that is in phase with the ac voltage waveform which improves power-line utilization, lowers power losses, and minimizes line-voltage distortion affecting electro-magnetic interference (EMI).

PFC is a common feature in medium and high-power switching power supplies that convert ac to dc and is mandated now by standards & regulations in many countries.

Also known as critical-conduction mode (CRM or CrCM) and transition mode (TM), boundary-conduction mode (BCM) is a variable frequency control method common for PFC boost converters.

BCM is a simple to design, well understood control technique where the PFC MOSFET turns on at zero current and minimum voltage.

Its main benefits higher efficiency than continuous-conduction-mode (CCM) converters (CCM is another popular control method for PFC boost converters).

Unfortunately the peak ripple currents in a BCM converter gets very large in higher power level converters requiring larger line and EMI filters. For this reason, the BCM PFC converters are traditionally limited to around the output power levels of about 300 W.

Interleaving is a special case of paralleling converters and operating each in a special phase relationship where the phase angle is 360° divided by the number of phases.

So an interleaved two-channel BCM converter is a PFC converter where two BCM power stages are paralleled and the switch timing is 180° out of phase from each other.

Note that the idea of interleaving converter is not new. Multi-phase converters for processor power in the dc-dc world have been around for a long time. The implementation of interleaving in the ac-dc world however is relatively recent.

Interleaving extends the higher efficiency benefits of BCM to power levels from 800 W to 1,000 W which is approximately three times the limit of the single BCM operation (about 300 W).

Paralleling itself brings benefits of smaller size stage (easier power stage design), modularity (can reuse power stages), lower component stresses, and easier thermal management (effectively spread the thermal components on the PCB layout).

Interleaving brings additional benefits of EMI reduction (lower peak currents from ripple current cancellation), higher effective frequency (reducing component sizes) without increasing switching losses, and longer component life time from lower stresses (from reduced ripple current).

Running two phases also mean one of them can be shut off (also referred to as phase-shed or phase-drop) to increase efficiency in light load conditions. This is commonly known as phase management.

There is no operational limit on the minimum power level where an interleaved solution can be used. The upper limit is around 800 W to 1,000 W.

While there is no operational lower limit on where an interleaved BCM PFC solution can be used, there are factors such as cost and number of components (board space) where a single BCM solution makes better sense at 200 W or below. However, if higher efficiency and the lower profile design are primary design goals, the interleaved BCM PFC will be the better solution. We had seen a customer use an interleaved BCM PFC solution for a 100-W ac-dc adapter due to the ability to make a very thin profile and high efficiency design.

The upper power limit of an interleaved BCM PFC solution is primarily determined by how large the EMI filter and the line filter (input caps) will be at the maximum power level. In general, the lowest line voltage the converter needs to operate from determines the upper limit since the EMI filter will be the largest there. We have seen designs of an interleaved BCM PFC at 1,200 W but it is a single line operation (not universal) with some limited operating parameters.

For the reasons mentioned above, we say the power range of an interleaved BCM PFC solution is from 100 W to 1,000 W.

Please see the diagram, which depicts Stand-Alone PFC Solutions Selection Guide.

Frequency of a BCM converter is affected by load and line cycle input current (higher frequency at light load and near zero crossings). On the FAN9612, the maximum frequency is clamped to 600 kHz to limit switching losses and the minimum frequency is clamped 18 kHz to avoid audible noise.

The FAN9612 is an interleaved two-channel BCM PFC controller in a 16-pin package. At the heart of the FAN9612 is the proprietary Sync-Lock™ technology which provides accurate interleaving in all operating conditions. In addition, the FAN9612 provides many other important features for (1) maximizing efficiency, (2) protection, (3) ease-of-design, and (4) reducing solution space.

The FAN9612 can be used in any high efficiency ac-dc power supply in the power rage of 100 W to 1,000 W.

Typical application segments include:
Computing power (ac-dc supplies for high-end desktops, entry-level servers, low-profile ac adapters, etc.),
Display power (large LCD, PDP and RPTV power),
Consumer (high power gaming adapters, home audio systems, digital to analog set top boxes),
Fixed-communications (telecom front-end power), and
Industrial power (solar inverters since FAN9612 can operate with DC input).

Fairchild’s Proprietary and patent-pending Sync-Lock™ technology provides interleaving by being:
(a) accurate (keeps the two phases perfectly at 180°out of phase),
(b) fast (responds to change of frequency by in one switching cycle), and
(c) robust (works over all operating conditions)

The challenge of interleaving two variable-frequency converters is not trivial. The frequency will change continuously based on instantaneous operating conditions such as changing line cycle voltage, line voltage transients, changing load power, start-up and shut-down conditions and during phase-management operation. Component tolerances and drifts will also affect frequencies.

A common scheme is to use a master-slave scheme. While this works, it essentially guarantees that one phase will be operating in discontinuous-conduction mode (DCM) which will impact PF (power factor) and THD (total harmonic distortion). Another scheme is the “natural” interleaving method where both channels operate as masters using a method where the on times of each channel is modulated based on the phase and frequency relationship. The issue of the “natural” interleaving is that you can completely lose interleaving under transient conditions such as during start-up and phase-management events. When the interleaving is lost, it will not reduce EMI signature, which is the very benefit of interleaving.

Sync-Lock™ technology is not a master-slave method (any phase can be master or slave) and the interleaving timing is adaptively changed so that interleaving is guaranteed without fail under any transient conditions.

Valley switching is a technology that makes sure the MOSFETs turn on at the valley of the resonant waveform to minimize switching losses as well as to lower EMI.

Conventional method for valley switching requires a RC delay circuit to tune for the LC resonant period which is affected by parasitic components and their variation (MOSFET Coss, inductor value, and the delay circuit itself).

The Easy Valley Switching technique in the FAN9612 does not require any RC delay circuit. FAN9612 always detects the valley of the resonant waveform and always guarantee zero-voltage switching or ZVS (when Vin < Vout / 2) or valley switching (when Vin > Vout / 2).

Conventional line voltage sensing uses an external complicated two pole filter which gives an inherent sluggishness from this low pass filter which means line voltage protection functions (such as input voltage feed-forward) can not get fast and accurate updates.

Advanced line voltage sensing method of FAN9612 uses a simple voltage divider with only two resistors that peak detects the line voltage and immediately derives the rms voltage of the line. It provides not only the fast update of the line voltage but there will be no line-current distortion when used for input voltage feed-forward function.

Also known as line-voltage feed-forward, the VFF function uses the input voltage information to modify the PWM action that modulate the output voltage (Vin↑ → D↓).

There are three key benefits of VFF:
(a) It minimizes output voltage (output power) variation against the line voltage variation.
(b) With VFF, the error amplifier output voltage (Vcomp) for a given input power is almost constant regardless of input voltage variations. This means simply clamping Vcomp provides a constant power limit.
(c) Easy feedback loop design since the transfer function is independent of line voltage.

Yes, due the advanced line voltage sensing method used, the FAN9612 can support dc input voltage levels. This means the FAN9612 can be used in boost converters running from 48V battery (telecommunication systems) or output of solar panels (micro-inverters).

In a conventional open- loop soft-start operation, the duty cycle increases progressively at start up. During this operation, the error amplifier is saturated. When the reference point of the output voltage is reached and the feedback loop takes over. However due to the slow voltage loop of a PFC converter, the error amplifier can not reacts fast enough which causes over-shoot of the output voltage. The start-up over-voltage is a condition seen in almost all PFC converters.

In the proprietary and patent-pending Intelligent closed loop soft-start of FAN9612, the reference voltage of the error amplifier is increased adaptively according to the difference between the real output voltage and reference voltage to prevent the error amplifier saturation. The method effectively minimizes any output voltage overshoot.

In any power converter, the switching losses become dominant at light load. For an interleaved converter where there are two or more phases, light-load efficiency can be improved by shutting down one of the phases at light load (also known as phase-shedding or phase-dropping operating). This is commonly known as phase management.

The advanced phase management technique used in FAN9612 causes no visible change in the line current waveforms during phase shedding and adding operations.

The default phase-management thresholds are approximately 12% and 19% of the load. This means when the output power reaches 12%, the FAN9612 will automatically go from a 2-phase to a 1-phase operation (phase shed or phase drop). And when the output power comes back up to 19%, the FAN9612 will automatically go from the 1-phase to the 2-phase operation (phase-add). There is be some upward adjustability of the thresholds. So, the power supply designs needing high efficiency at 20% load (for example to meet the Energy STAR 5.0 or the Climate Savers Computing requirements) can adjust the phase-drop threshold to be 25% by adjusting the maximum on time.

Yes. In some applications, the output voltage of the PFC boost converter is decreased at low power levels in order to increase the light-load efficiency of the downstream dc-dc converter and therefore of the overall power supply.

Implementation of this function is straight forward in the FAN9612 because the error amplifier reference (the positive input) is available (as the soft start (SS) pin) and its voltage can be modulated to affect the output voltage as needed. A simple implementation using four external components is described in the FAN9612 datasheet.

Yes. Boost follower is also known as tracking boost (or follow-booster). In some applications, the output voltage of the PFC boost converter is adjusted based on the input voltage. This again is to increase the efficiency of the downstream dc-dc converter and therefore of the overall power supply.

There are two boost follower implementations. The first one is a discrete two-level boost where the output voltage is around 280 Vdc for a low-line input (110 Vac) and 385 Vdc for a high-line input (220 Vac).

Implementation of either of this function is likewise straight forward in the FAN9612 because the error amplifier reference (the positive input) is available (as the soft start (SS) pin) and its voltage can be modulated to affect the output voltage. Both the implementations are described in the application note AN-8021 - Building Variable Output Voltage Boost PFC Converters.

Adjustable brown-out protection (line under-voltage protection) is provided. FAN9612 also includes a line over-voltage protection function (Line-OVP).

The FAN9612 has two output over-voltage protection (OVP) functions.

The primary OVP is provided by the voltage on the feedback (FB) pin. Due to the error amplifier being a gM type (transconductance amplifier), the FB pin is always proportional to the output voltage and can be used for over-voltage protection as well.

A second-level latching over-voltage protection can be implemented using the OVP pin of the controller. There are two ways to program the secondary OVP. The first option is to connect the OVP pin to the FB pin. In addition to the standard non-latching OVP (set at ~8% over the nominal output voltage), this configuration provides the second OVP protection (set at ~15%) which is latched.

In the case where redundant over-voltage protection is preferred (also called double-OVP protection), a second separate divider from the output voltage can be used. In this case, the latching OVP protection level can be independently established (typically same or above) the non-latching OVP threshold.

For designs using a fixed auxiliary power supply of < 12.5 V, there will be a lower UVLO threshold version called FAN9611. Its UVLO thresholds will be 10 V / 7.5 V. Contact Fairchild sales for additional information.