The majority of today’s power suppliestargeting near unity power factoremploy active power factor correction (PFC) in the ac/dc front end. The PFC sectionor ac/dc stageboosts the input ac line voltage to a levelabove the maximum ac line peak voltageusually to the level of 385Vdc.
There are number of important functions and details of the architecture of modern PFC controllers that should be of interest to the designers of off-line power supplies.
Although many companies manufacture various types of PFC controllersthe architecture and structure of these chips remain very similar. Most PFC controllers employ a gain modulatora current error amplifier (IEA)a PFC comparatora voltage error amplifier (VEA) and a PFC pulse output driver.
The inner control loop is a current control loopcomprising the gain modulatorIEA and a PFC comparator. The outer control loop is a voltage control loopwhich uses VEA as an error amplifier. The VEA output (VEAO) is also one of the gain modulator's inputs.
The inner current control loop shapes the line current drawn from the AC mains to be simultaneously proportional to the AC line voltage. The outer voltage loop keeps the PFC DC bus voltage in regulation at about 385V. It is important to note that the current control loop has a much larger bandwidth compared to the voltage control loop.
The PFC comparator-driver section of the PFC controlleris designed around the PFC comparator. The oscillator generates a saw tooth waveform and a CLK signal which is synchronised with the falling edge of the saw tooth waveform. The saw tooth waveform is designated as the ‘PFC_RAMP’ signalalso called RAMP1.
In the beginning of the cyclethe CLK high signal sets the PFC_OUT low. Once the PFC_RAMP exceeds the output of current amplifier (IEAO) level the PFC_OUT becomes higheventually switching the external PFC transistor ON. The PFC_OUT stays high until the end of the cyclewhen the CLK sets it low. Therefore PFC implements the leading-edge of regulation.
Usually the PFC chip ensures that the PFC duty cycle D and the PFC_OUT pulse never reach 100percent. This approach prevents saturation of the boost inductor. The maximum duty cycle occurs at the zero crossing of the ac line voltage.
Thuswhen Vin is zerothe controller IC limits duty cycle to 98percent. For exampleactual waveforms of a PFC controller from Fairchildcalled the FAN4810. This is where Ch4 is the output of current error amplifier.
It is very important for the designer to set a proper value of the current error amplifier output. For examplethe FAN4810 spec defines the RAMP1 valley-to-peak voltage as 2.5V. This means that by regulationthe dynamic span of IEAO voltage should be within the RAMP1 signal swing rang.
The IEA in many ICs is a transconductance amplifier. A Type 2 compensation circuit is connected to the reference voltage pin VREFfor establishing a soft-start function of PFC. At start-up IEAO rises to the reference voltage level; and as the loop comes into regulationIEAO drops from VREF level to the operating level. This guarantees that the PFC duty cycle gradually increases from 0percent to the desired value as required by the loopthus providing a soft start feature.
Gain modulator functionality
The central part of the PFC controller is the gain modulator and voltage error amplifier. The gain modulator generates a program signal for the current control loopwhich shapes the input current. The FAN4810 PFC controller's current control loop ensures that negative voltage from the current sensor is balanced by a positive voltage across an internal Rcp resistor. This positive voltage is generated by gain modulator output current that flows through Rcp. Thusthe drawn line current (I_line) andthereforethe voltage on ISENSE pin is forced to replicate the shape of gain modulator output. In other wordsthe condition: is always met. WhereRs is the line current sense resistor and IGM is the output current of gain modulator.
To shape the drawn line currentthe gain modulator employs three inputs: IACVRMS and VEA output. PinIAC is connected to the output of the ac line rectifier bridge through a 1MOhm resistor. The IAC current is a full wave-rectified sine wave at double the line frequencyand is proportional to the instantaneous ac line voltage. The IAC actually shapes the gain modulator output current to be sinusoidal as described by the following expression.
Voltage on the VRMS pin is a DC voltage proportional to the RMS value of ac line voltage. IGM is designed to be inversely proportional to the square of VRMS.
Why does the output current of the gain modulator need to be inversely proportional to the square of the RMS value of the AC line voltage?
Let’s assume that VEAO is constant. If the input line voltage doublesconsequently to provide the same power; input current andaccordinglyIGM should be reduced in half. When line voltage doublesIAC also doublessee (2). Thus to parry increase in IAC the VRMS signal should be quadruple.
VEA Bandwidth and THD
The third input of gain modulator is output of VEAwhich defines the amplitude of the gain modulator's output current. Inverting the input of VEA is connected to a voltage divider for sensing PFC output voltage. In the case of the FAN4810the FB pin is set to 2.5V at the nominal bus voltage. VEA is a transconductance amplifier with a Type 2 compensation circuit. There are two conflicting requirements for closing a feedback loop around VEA: 1) The loop should have bandwidth low enough to attenuate the 120Hz ripple voltage on the DC bus so as to decrease the total harmonic distortion (THD) of input line currentand 2) The loop should be fast enough to provide acceptable load transient response.
THD is an important factor for designers to consider. The bulk capacitor voltage ripple is displaced by 900as compared to the line voltagedue to the reactive nature of the output capacitor.
Because signal proportional to the displaced voltage ripple feeds directly to the VEAgain and bandwidth of this amplifier should be limited. Usually the voltage control loop crossover frequency is set in an area between 10Hz and 30Hz.
Two interesting features of the gain modulator are rarely discussed. Firstthe transfer characteristic provides a natural brown-out protection. The maximum gain of the gain modulator is designed to be at minimum line voltage. If the input voltage happens to be below the specified minimum (brown-out condition)the gain andconsequentlythe
PFC duty cycle falls sharplythus reducing the bus voltage.
The second interesting feature is an inherited over-current protection. The maximum output voltage of the gain modulator is limited; in the FAN4810 it is 0.8V. Iffor any reasonvoltage on the ISENSE pin falls below -0.8Vdue to overload for examplethe inverting input of IEA becomes more negative with respect to ground. Thereforethe IEAO level will increasereducing PFC duty cycle and bus voltageand thus limiting the output power.
PFC protection circuits
Like other controllersthe FAN4810 includes a number of protection circuits related to PFC operation. These include Over-Voltage Protection (OVP) and Over-Current Protection (OCP). A SR flip-flopset by either PFC OVP or current limitterminates the PFC output pulse. Therefore the OVP and OCP faults are sensed and latched every cycle.
The Fairchild device used in this example also has Tri-Fault protection. This type of protection covers the following faults: (a) FB pin is shorted to ground(b) the line (wire or PCB trace) from PFC DC bus voltage divider to the VFB pin of the IC is brokenand (c) the top or bottom resistor in a voltage divider is open. If FB is short to the ground (a)the voltage on PFC_FB pin is below 0.5V and the Tri-Fault comparator output disables the PFC OUT pulse. In the case of a broken wire (b)an internal current source starts charging the capacitor Ctf . Thenonce the voltage on the FB pin exceeds 2.75Vthe OVP comparator will terminate the PFC output. Capacitor Ctf is optional and defines the time t_F to trigger the protection circuit.
For the fault conditions described in (c)OVP or TRI_FAULT will terminate the PFC_OUT pulse. The OVP protection kicks in iffor any reasonthe dc bus voltage exceeds the nominal level by more than 10 per cent. For exampleif the nominal bus voltage is 385VDC (2.5V on FB pin)OVP triggers if the bus voltage exceeds 423Vdc (2.75V on FB pin).
PFC controllers provide an important role in ac/dc designs by integrating such protection functions as over-voltage protection (OVP)over-current protection (OCP) andin the case of the FAN4810TriFault protection. Converters with active PFC and near unity power factor meet the ICE61000-3-2 specification requirementsalleviating a major source of concern for power supply designers.
Vinit Jayaraj is Principal Design Engineer and Victor Khasiev is Principal Application Engineer at Fairchild Semiconductor. For more informationvisit www.fairchildsemi.com"