As the application of electronic power modules is constantly increasing, new problems are arising beyond the actual discussion about the power factor (cos ) in connection with the Power Factor Correction (PFC).
Why power factor correction? 
Generally, mains AC voltage is transformed into DC voltage via Rectifier Bridge and capacitor. This configuration is used for most power electronics applications.
In this configuration, the DC-link circuit will only be loaded, when the sinus maximum of the mains voltage (VIN) is higher than the voltage at the Dc-link capacitor (VCap) circuit voltage.
This leads to short and very high current pulses (ICap), which may interfere other users in the public power supply network. Actions have to be taken to ensure the functionality of the nationwide power grid systems with a fast growing number of power electronics connected. Applicable international standards demand a sinusoidal current drain.
Thus, the developers of power applications to be connected to the public power supply network are now challenged to care for its realization.
Challenge and chanceFor the development of applications with sinusoidal current consumption more design work will be required than ever before. New national and international standards and laws demand increased activities. And compliance of the applicable standards is a must today.
But an active PFC also generates additional advantages, which does not generally lead to additional costs.
Precondition is a system design that uses the advantages of an active PFC as smaller DC-link capacitor, loss reduction in the application connected to the output achieved by the increased and constant output voltage.
Application examples of active PFC solutions are inverter welding machines, as by application of a PFC, the performance can be increased without affecting the mains fuse. Also converter-controlled fans for clean rooms should be mentioned.
In clean room production facilities typically hundreds or thousands of such ventilators are used and the current drain from the mains have to be controlled in order to keep the system in function.
General requirementsSome general requirements are mandatory for all these applications: compact design, low interference level, and power loss optimization.
To realize an optimally compact PFC design, the switching frequency must be maximized so thatextremely small PFC choke coils can be applied. To control by this caused increased power loss, the coil should have a high performance core with thermal contact to the cooling element. New semiconductors must be suited for higher performance in order to gain smaller space requirements. At the same time it has to be prevented that the advantage is lost by the demand for a largercooling element. Here new technologies of the semiconductor industry are opening doors.
High switching frequencies are leading to considerable cost savings. A compact design leads tocompletely new mechanic concepts for an application, which may also lead to considerable savings in system costs. Also a positiveinterlinking effect can be utilized.
Higher switching frequencies allow the application of smaller components for choke and EMC filter. By its compact design, EMC compatibility will be easier. And this will lead to further savings in system costs and development time.
Fundamentals of active powerfactor correction An active PFC switch is basically an AC/DC converter, as its core is a standard SMPS (Switch Mode Power Supply) structure, which controls the current supplied to the consumer via a ¡°Pulse Width Modulation¡± (PWM). The PWM triggers the power switch, which separates the intermediate DC voltage in constant pulse sequences. This pulse sequence will then be smoothened by the intermediate DC capacitor, which generates DC output voltage.
PFC—boost switch(boost topology)
The boost topology acts as boost converter by converting the input voltage into a higher output voltage. This switching system is mainly used for PFC.
Two different modulation procedures can be applied, the continuous mode and the discontinuous mode.
Discontinuous modeIn the discontinuous mode the transistor is switched on only then, when the energy contained in the choke coil is completely transferred via the diode to the electric DC output circuit. When switching on the transistor the choke contains neither energy nor current. This operation principle has therefore the advantage that switchon losses will not be generated. Another benefit is that the choke can be very small. But this principle generates strongly increased waviness and switch off losses.
Continuous mode
The topology is the same as in the discontinuous mode, but here varies the current in the choke closed to the sinusoidal average. In the continuous mode these large upper waves and the very high switch-off losses (caused by the double as high switchoff current) can be avoided. Therefore, this process will be preferably be used for capacities from about 250W.
Design of a continuous mode PFC
Switch-on losses
In standard PFC switches normally relatively low flow losses are generated compared to the switching losses. Consequently mainly the transistor switching losses are limiting the maximal switching frequency. On the other hand, the reverse current of the diode has a great influence on these switching losses generated in the powerswitch.
Reverse currents of the diode increase the switch-on losses of the transistor and the application of a fast boost diode with a low reverse recovery charge (Qrr) can thus be of great importance. The boost diode has a significant influence into the switch on losses of the power transistor in the boost circuit.
Switch-off losses
Any parasitic inductance in the output causes a voltage-overshot witch endangers the semiconductors and increases the switch off losses. Switching-off the transistor results in a current change. This causes a voltage spike by the current changein the parasitic inductances according to:
VCE(peak) = VCE + L x di/dt To minimize switch-off losses, prevention of parasite induction loops at the PFC output is of great importance. A particularly elegant solution is to short the inductive loop with a capacitor attached as closed as possible. A fast capacitor integrated into the powercomponent would be the optimum.
MOSFET vs. IGBT
To answer this question, the efficiency of a frequency-dependent technology isn't a suitable standard. The reason is the switching of variable currents. In the range of the sinus maximum, only a short pulse will be required to increase the voltage from e.g. 325 (VPeak at 230VAC) to 400 V DC.
In the zero flow range the pulse frequency is higher, but the current to be switched is then lower. For 230VAC/400VDC applications and switching frequencies of 60kHz and more the MOSFET seems to be the better and cheaper solution.
Applications with focus on a wide input range (e.g. 90VAC .. 240VAC) generate a completely different result. When applying 90VAC at the input and 400VDC at the output, also in the sinus maximum, a relatively long pulse will be required for the transformation from 127V (VPeak at 90VAC) to 400VDC. For such applications the static losses are more decisive for the performancebalance of the PFC level.
For switching frequencies up to nearly 100kHz, application of very fast IGBTs seems to be more attractive. To optimize system costs, switching and flow-through losses shall always be nearly equal. As a rule, PStat/PSwitch = 1 shall apply. Balancing of switching losses with flowthrough losses shall always be the target of product designers in low-cost developments.
Input start switch as shortcircuit
protection and protection against high switch-on currents Output current rates of >500W require a specific input control, which limits the higher start-up current generated by the loading procedure of the DC-link capacitor after activation of the PFC. The switching system above, for which a patent has already been filed, will not only solve this problem— moreover it also provides a shortcircuit protection at the output.
Description of functions:
1. After application of AC voltage at the input, the SCR¡¯s of the semicontrolled input rectifier are not activated. The output capacitor is loaded via a current limiter and auxiliary rectifier. As soon as the controller begins to contact the PFC transistor, the PFC coil will be loaded with current. When switching-off the transistor, the output voltage of the PCF coil will be limited to the initial output voltage level at the PFC capacitor via the PFC diode.
2. When the voltage is high enough, the voltage reduced by the winding ratio N1/N2 via the auxiliary winding at the PFC coils activates the semi-controlled rectifier, in order to limit the losses at the current limiter.
3. In case of a short-circuit at the PFC output, the PFC diode clamps the voltage at the PFC coil down to zero and disables the semi-controlled rectifier. The current limiter reduces the short circuit current.
PFC solution with Tyco¡¯s PFC-IPM
This intelligent power module (IPM) is a complete, universally applicable PFC solution for currents up to 1kW. The PFC-IPM is designed to reduce the input current upper waves and has the required features to realize a performance factor >0.99. The switch is based on a boost topology, provided with a semi-controlled input rectifier.
This solution has an input start switch to limit the input current and a short-circuit protection for consumers connected to the output.
Another characteristic feature is the zero load capability. This means that the PFC-IPM can generate a stable DC output voltage without the necessity that a consumer must be connected to the output— a demanded must, when the application requires a highly stable output voltage.
The following characteristic features have been realized in this flexible, universally applicable PFC solution:
• Nominal input voltage 230VAC
• Output voltage 400VDC
• Output current (zero load) 0.. 1kW
• Efficiency ca. 95% (including the choke)
• Power factor > 0,99
• Starting current limiter < 12A
• Short-circuit protection for static and dynamic failures at the output
PFC solution with Tyco¡¯s flowPFC 0
The following characteristic features have been realized in this flexible, power integrated concept.
Features:
• All semiconductors integrated in one module, no additional efforts for thermal contacting required
• Integration of a temperature sensor for detection of the substrate temperature
• The proven flow concept allows a compact PCB design. Plugs with similar voltage are concentrated to voltage islands
• Symmetric design of the PFC transistor for parallel or alternating operation
• Low-inductive current measurement with a shunt for precise control of the PFC switch
• Capacitor for low-inductive bypassing of the high frequency in the module
PFC-solution with Tyco¡¯s flowPIM+P
Drive solutions with PFC require the additional integration of one fast switching transistor and boost diode. Similar to the standard TYCO flowPIM modules, all power semiconductors are integrated in one module, so that no additional action for the handling of the power dissipation to the heat sink is needed.
Features:
• 1 Phase Input Rectifier
• PFC Transistor + extreme fast Diode
• 3 Phase Inverter IGBT + FRED
• HF-Capacitor in DC Link
• Current sense shunt in the DC—
• Current sense shunt for PFC controlling in the DC—
• NTC temperature sensor
• Approved flow concept for easy routing off the system PCB
• Clip In for mechanical fixing into the PCB
Advantages of PFC solutions with Tyco modules
The Tyco PFC modules offer compact modules and the flow concept offer opportunity to get a compact and easy PCB design. All Tyco PFC solutions short the high frequency with a ceramic capacitor inside the module. The excellent EMC behavior is only possible with module solution and can never achieved with discrete components. The Clip In housing of the modules flowPFC0 and flowPIM0 is adjustable to different PCB and makes the assembly easy and reliable. The integrated temperature sensor protects the module and the application. The UL listing of the module shortens the time to get the application certified. These advantages are a basis for an innovative and cost effective PFC solution.
