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By Robert Kirker, Application Engineer

Advanced Conversion Technology, Inc (ACT) has developed an approach for COTS AC-DC power supplies that takes advantage of the high power densities of standard DC-DC converters. Using standard DC-DC converters, ACT wraps the needed customization around these bricks in order to meet the EMI, housekeeping and rugged mechanical requirements of a military power supply application. These standard brick converters are then combined in a variety of configurations to produce a wide range of output voltages and currents. Using similar proven topologies and physical configurations new single phase and three phase AC-DC military power supplies can be quickly developed that incorporate many of the following features.

• High Power density
• Very low input current harmonic distortion
     -  <3% harmonic distortion over a wide load range, 20% to 100%
• EMI and low input harmonic distortion requirements are maintained for a wide range of changes in the output configuration
• Phase load balancing for three phase power supplies
• Long hold up times during low line input transients, >100 msec.
• A variety of output voltages and currents available within the same form factor without redesigning the unit
     -  Uses standard DC-DC converter bricks
• Shortened development time
• Fully compliant EMI Filtering for MIL-STD-461 and most other EMI standards
• High transient protection for MIL-STD-704, MIL-STD-1399, MIL-STD-1275 and lightening strikes, etc.

Single Phase AC-DC Applications

ACT’s single phase topology is the building block for its AC-DC power supplies and is shown in the following block diagram. The active PFC module converts the AC input voltage into a +375 VDC bus while maintaining very low input current harmonic distortion and a power factor >99%. The +375 VDC bus voltage generated by the PFC module is used to power standard DC-DC converters.

Single Phase AC-DC Block Diagram

Multi-Kilowatt Single Phase Active PFC Module

This proven technology can be easily adapted to a wide range of input voltages and frequencies with minimum changes.

• 108 Vrms to 280 Vrms
• 50 Hz to 800 Hz

Prior to installing standard DC-DC converters into one of its power supplies, ACT wraps the necessary EMI shielding and housekeeping circuitry around the converters in order to prepare them for operation with other DC-DC converter modules. These individual DC-DC converters can then be combined in a variety of configurations to produce a wide range of output voltages and currents. In many cases modules are connected in parallel (often in a N+1 configuration) and load sharing circuitry is employed to make sure the output load is evenly shared among the paralleled bricks. Additionally the DC-DC converters provide the required input to output isolation.

Standard Quarter Brick 150 W DC-DC Converter with Custom Treatment

Three Phase AC-DC Applications

In three phase power supplies ACT places active PFC circuits in each input phase to achieve low input current harmonic distortion and high power factors. To achieve optimum system performance it is important that power load balancing among the phases be implemented. Optimum phase to phase load balancing can be achieved in a number of different methods and can vary from application to application. One common method is to equally distribute the DC-DC converters among the phases such that the total output power is equally shared among the phases.

Three Phase AC-DC Block Diagram
 

In many cases a secondary share buss is often utilized and if phase isolation exists the share buss can be used on the PFC modules.

With the addition of a transformer input module this approach has been successfully used in 440 V multi-kilowatt three phase applications.

Single Phase Active Power Factor Correction (PFC) Front End

ACT’s active PFC front accepts a single phase input voltage and applies this to the Inrush circuit and EMI filter. The filtered input voltage is then full wave rectified and applied to our boost PFC circuitry. A boost topology charging a large output capacitor can generate very large inrush currents during turn-on. In order to minimize this high input current a resistive element is placed in series with the input which limits the input current during turn-on. After the +375 VDC bus voltage is charged up a low loss FET power element is switched on shorting out the resistor. Overall power losses in the EMI filter and Inrush circuits are very low and can achieve efficiencies of 98% or greater.

Single Phase Active PFC Block Diagram
 

The active PFC circuit operates in such a manner as to minimize the input current harmonic distortion by providing a resistive appearing impedance to the input circuit. The result of this is a sinusoidal input current over a wide range of output load conditions. This PFC circuit can maintain input current harmonic distortion of less than 3% over a wide output power range, from 20% through 100%. The following table shows some typical input current harmonic distortion performance that ACT can achieve.

Typical Input Current Harmonic Content

The amount of power that ACT’s PFC circuit can provide is directly related to the size of the boost storage inductor and low line input conditions. For a given size boost inductor ACT’s PFC’s can provide more output power with higher input voltages. Optimizing the boost inductor for size and performance is a key design issue. ACT has developed PFC designs with output power from hundreds of watts to >2 KW and achieved efficiencies of greater than 94%. Therefore, from the AC input to the +375 VDC output bus this approach can achieve efficiencies >91%. If the DC-DC converters have an 85% efficiency than the overall efficiency of the power supply can be >78%.

The +375 VDC bus voltage provides an excellent place to add storage capacitance if the design requires hold time for input voltage transients. Additionally the DC-DC converters typically operate at input voltage levels of <250 VDC providing another level of hold up capability during input transients.

Multi-kilowatt VME AC-DC Power Supply per MIL-STD-1399 Section 300A

In this example we discuss an AC-DC power supply that accepts 115 Vrms, 3 phase delta input voltage at 60 Hz and provides four DC output voltages and 1,278 W of output power;

• +5.1 V @ 180 A; uses 6 half bricks
• +12 V @ 8 A; uses 3 quarter bricks
• -12 V @ 4 A; uses 2 quarter bricks
• +24 V @ 9 A; uses 3 quarter bricks

Based on an overall power supply efficiency of 65%, each of the single phase PFC modules are capable of providing 700 W of power for a total power of 2,100 W of power available to the DC-DC converters. These modules are shown in the picture below.
 

Three 700 W Active PFC Modules

These PFC modules contain enough energy storage to hold up the output voltages under full load conditions for 70 msec. during loss of input voltage.

The +375 VDC power produced by these three active PFC modules are presented to 14 standard DC-DC converters, six half bricks and eight quarter bricks, as shown below.
 

Fourteen Standard DC-DC Converters

Using this existing form factor and staying within the power capabilities of the PFC modules ACT can quickly develop different versions of this COTS AC-DC power supplies.
 

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