The LLC resonant converter is generally configured as a half bridge. In the PowerStar circuit, it is used for stepping down the regulated 400V dc from the PFC circuit to a lower desired voltage, typically in the range from 12V to 48V. The FAN7621 pulse frequency modulation controller simplifies the design as it incorporates such features as fixed dead time, pulse skipping at light load and various protection functions.
Power losses in the output rectification stage of every type of converter have tended to dominate the power loss budget. Even in PowerStar, Schottky diodes could have been used for output rectification because of their low forward voltage drop.
Synchronous rectification, however, is a more efficient topology for the task and its implementation has recently become easier with the release of the FAN6208, a specialised SR controller for LLC converters. FAN6208, combined with the ultra low on resistance of the FDMS86200 fets, provides ideal rectification for the 48V version of the circuit.
Within the overall constraints set for the PowerStar design – European mains input, output voltage of 12 to 48V and loads between 100W and 1kW – there are many potential applications for the board and it has not been developed as a reference design for any specific application.
The board is useful, however, for design engineers looking to implement this combination of high efficiency topologies for the first time, providing practical hands on knowledge of the design issues.
It can also be used as a template for a production design in applications such as brushless dc motor drives and in telecom or audio power supplies. Here, the aim was to enable designers to change components to meet different output voltage and power specifications while reusing most of the circuit design.
The PowerStar board comes with an output voltage of either 24V or 48V. By making small adjustments, ±24V and ±48V can be obtained. A choice of transformers providing 12V or 36V is available on request and this provides for ±12V and ±36V.
The bill of materials and Gerber design files are freely available, which means OEMs can quickly redesign the power supply and customise the layout to suit their application. A further reduction in design time can be achieved by using the spreadsheet software provided with the board to assist in defining component values and optimising the magnetic designs for each application.
A simulation of the LLC circuit using the SIMetrix tool is also provided. The simulation includes the transformer parameters and is supported by a theoretical paper describing a method to derive the parameters from practical measurements.
In conclusion, designers aiming to get more than 90% system efficiency from a 100W to 1kW power supply can use PowerStar in a number of ways: to gain an understanding of the chosen topologies' operation; as a template that provides guidance for a new design; or as the basis of a production design configured to meet specific requirements.
Whichever way the designer uses it, PowerStar helps to reduce the time and risk associated with implementing this highly efficient power supply architecture.
John Stephens is Technical Project Manager in Future Electronics (EMEA)'s System Design Centre.
A design blueprint for a high efficiency ac/dc power supply
4 mins read
The key elements in a topology for achieving very high efficiency ac/dc conversion in circuits rated at up to 1kW have existed for some time, but it is only recently that the last important element – an implementation of synchronous rectification – has been readily available.
Only recently, then, have development teams been able to aim for system efficiencies of more than 90% in designs intended for volume production. And with pressure growing to restrain power consumption in most electronics sectors, any high efficiency topology should merit serious consideration.
It is in this context that Future Electronics' European System Design Centre has developed PowerStar, a new proof of concept development board for advanced ac/dc conversion.
Why should a distributor design and manufacture proprietary hardware? Because customers can implement high efficiency ac/dc converter circuits more easily and more quickly if they can start from a working design. While chip manufacturers offer evaluation boards, these tend to be standalone circuits designed to show off the features of one device. Future's intention was to design multiple devices into a complete circuit that implements every stage of an efficient topology for ac/dc applications rated from 100W to 1kW.
The PowerStar board implements a three stage conversion process (see fig 1):
* Boundary conduction mode (BCM) and interleaving switching in power factor correction (PFC)
* Resonant inductor-inductor-capacitor (LLC) conversion
* Synchronous rectification (SR) of the output stage
With this combination of topologies, PowerStar achieves up to 94% system level efficiency. While each topology offers very high efficiency in its own right, the high efficiency of the circuit is realised through the use of several techniques that mitigate the effects of limitations that exist in even the best components.
PFC circuit
Operating a PFC circuit in BCM results in minimal turn on losses in the power switching fets because switching takes place at 0V and 0A. Because the forward current is zero, the boost diode gives rise to minimal reverse conduction losses. This, combined with the selection of a low forward voltage drop, gives a high efficiency for the topology.
Interleaving in the PFC circuit by means of Fairchild's FAN 9611 PFC control ic reduces the ripple current drawn from the supply and, likewise, the current fed to the bulk capacitors. This reduces the demands made on the input filter and the ripple current rating of the high voltage aluminium electrolytic capacitor.
LLC circuit
LLC resonant conversion is one of the most efficient circuit techniques at high power levels. The topology's advantage comes from the way it uses the parasitic capacitances of active devices and combines the leakage and magnetising inductance of the power transformer. This enables the main power switching fets to switch on at zero voltage and zero current and to switch off at close to zero current.
As a result, switching losses are far lower than in other topologies, leading to a large improvement in overall efficiency. At the same time, transformer design is made easier because a high leakage inductance is required. This has a major benefit in safety isolation.
Safety isolation places constraints on the design of conventional pwm transformers because of the need to reduce primary to secondary leakage inductance. The transformer leakage inductance for an LLC circuit can be an order of magnitude higher than in these conventional types and becomes part of the overall transformer design. High leakage inductance is obtained by implementing a split bobbin for the primary and secondary windings: the windings are placed into separate areas in the bobbin, with the result that the creepage and clearance distances can be maintained more easily.
The LLC resonant converter is generally configured as a half bridge. In the PowerStar circuit, it is used for stepping down the regulated 400V dc from the PFC circuit to a lower desired voltage, typically in the range from 12V to 48V. The FAN7621 pulse frequency modulation controller simplifies the design as it incorporates such features as fixed dead time, pulse skipping at light load and various protection functions.
Power losses in the output rectification stage of every type of converter have tended to dominate the power loss budget. Even in PowerStar, Schottky diodes could have been used for output rectification because of their low forward voltage drop.
Synchronous rectification, however, is a more efficient topology for the task and its implementation has recently become easier with the release of the FAN6208, a specialised SR controller for LLC converters. FAN6208, combined with the ultra low on resistance of the FDMS86200 fets, provides ideal rectification for the 48V version of the circuit.
Within the overall constraints set for the PowerStar design – European mains input, output voltage of 12 to 48V and loads between 100W and 1kW – there are many potential applications for the board and it has not been developed as a reference design for any specific application.
The board is useful, however, for design engineers looking to implement this combination of high efficiency topologies for the first time, providing practical hands on knowledge of the design issues.
It can also be used as a template for a production design in applications such as brushless dc motor drives and in telecom or audio power supplies. Here, the aim was to enable designers to change components to meet different output voltage and power specifications while reusing most of the circuit design.
The PowerStar board comes with an output voltage of either 24V or 48V. By making small adjustments, ±24V and ±48V can be obtained. A choice of transformers providing 12V or 36V is available on request and this provides for ±12V and ±36V.
The bill of materials and Gerber design files are freely available, which means OEMs can quickly redesign the power supply and customise the layout to suit their application. A further reduction in design time can be achieved by using the spreadsheet software provided with the board to assist in defining component values and optimising the magnetic designs for each application.
A simulation of the LLC circuit using the SIMetrix tool is also provided. The simulation includes the transformer parameters and is supported by a theoretical paper describing a method to derive the parameters from practical measurements.
In conclusion, designers aiming to get more than 90% system efficiency from a 100W to 1kW power supply can use PowerStar in a number of ways: to gain an understanding of the chosen topologies' operation; as a template that provides guidance for a new design; or as the basis of a production design configured to meet specific requirements.
Whichever way the designer uses it, PowerStar helps to reduce the time and risk associated with implementing this highly efficient power supply architecture.
John Stephens is Technical Project Manager in Future Electronics (EMEA)'s System Design Centre.
The LLC resonant converter is generally configured as a half bridge. In the PowerStar circuit, it is used for stepping down the regulated 400V dc from the PFC circuit to a lower desired voltage, typically in the range from 12V to 48V. The FAN7621 pulse frequency modulation controller simplifies the design as it incorporates such features as fixed dead time, pulse skipping at light load and various protection functions.
Power losses in the output rectification stage of every type of converter have tended to dominate the power loss budget. Even in PowerStar, Schottky diodes could have been used for output rectification because of their low forward voltage drop.
Synchronous rectification, however, is a more efficient topology for the task and its implementation has recently become easier with the release of the FAN6208, a specialised SR controller for LLC converters. FAN6208, combined with the ultra low on resistance of the FDMS86200 fets, provides ideal rectification for the 48V version of the circuit.
Within the overall constraints set for the PowerStar design – European mains input, output voltage of 12 to 48V and loads between 100W and 1kW – there are many potential applications for the board and it has not been developed as a reference design for any specific application.
The board is useful, however, for design engineers looking to implement this combination of high efficiency topologies for the first time, providing practical hands on knowledge of the design issues.
It can also be used as a template for a production design in applications such as brushless dc motor drives and in telecom or audio power supplies. Here, the aim was to enable designers to change components to meet different output voltage and power specifications while reusing most of the circuit design.
The PowerStar board comes with an output voltage of either 24V or 48V. By making small adjustments, ±24V and ±48V can be obtained. A choice of transformers providing 12V or 36V is available on request and this provides for ±12V and ±36V.
The bill of materials and Gerber design files are freely available, which means OEMs can quickly redesign the power supply and customise the layout to suit their application. A further reduction in design time can be achieved by using the spreadsheet software provided with the board to assist in defining component values and optimising the magnetic designs for each application.
A simulation of the LLC circuit using the SIMetrix tool is also provided. The simulation includes the transformer parameters and is supported by a theoretical paper describing a method to derive the parameters from practical measurements.
In conclusion, designers aiming to get more than 90% system efficiency from a 100W to 1kW power supply can use PowerStar in a number of ways: to gain an understanding of the chosen topologies' operation; as a template that provides guidance for a new design; or as the basis of a production design configured to meet specific requirements.
Whichever way the designer uses it, PowerStar helps to reduce the time and risk associated with implementing this highly efficient power supply architecture.
John Stephens is Technical Project Manager in Future Electronics (EMEA)'s System Design Centre.
