power converter design review
high power converter design is presented and reviewed. This practical 200 kHz full bridge converter operates from a 385 Volt dc input bus, typical of a Power Factor Correction circuit, and outputs a regulated 48 Volt dc bus for distributed power and Telecommunications applications. Low profile, planar magnetics are featured with full 3750 Vac safety isolation. Over 93% efficiency is achieved. Power density approaching 40 Watts per cubic inch (2.5 W/c1r?) is featured, suitable for load-shared, modular power converter applications. Operating waveforms, printed circuit board layout and measured efficiency of the actual breadboard are also presented. Design Overview and Considerations The fixed frequency, phase shifted ZVT conversion technique was implemented using the Unitrode UC3875 IC for control. Power to this unit including the IC bias supply is delivered by a universal Ac (85-265 Vac) input ZVT Power Factor Correction module. The phase shifted converter s output voltage feedback path will incorporate an optocoupler to maintain the safety isolation voltage. Wherever possible, standard, commercially available products have been incorporated to simplify development efforts. A goal to achieve a standar ssembly, suitable for modular power applications has been set. The trend toward half-inch total physical height, while recognized, was not currently possible to achieve for this 500 Watt design. Topology A mastery of the basic full bridge topology is needed before initiating the design of this phase shifted derivative. This knowledge allows for some fIrst round approximations regarding the active semiconductor components, passive components and isolation levels required throughout the various sections of the converter. Only items unique to the phase shifted version of the full bridge converter will be covered in significant detail. Based upon conservative design practices, the author has taken the liberty to make a few initial selections which are outlined in the next section. Semiconductors The MOSFET switches selected for this 500 Watt, 400 Volt application are readily available IRF840 types. These 500 V devices are conservatively derated by twenty-five percent during normal use. Although the onresistance, hence conduction power losses could be lowered by using heftier devices as in a conventional design, large switches are not used in this design. Several factors and tradeoffs have been weighed to arrive at this decision. They are: longer transition times, reduced maximum duty cycle, higher primary circulating currents and potentially lower efficiency. All factors focused on the use of the840 switches as the optimal choice which was confIrmed by a computer spreadsheet program and presented elsewhere in this paper. The output rectifiers selected are 200 Volt, 16 Amp ultrafast recovery devices to keep power losses low. A series resistor/capacitor (R/C) snubber is used across the transformer secondary windings to minimize voltage excursions. While switching waveforms on the primary are virtually noiseless, the leakage inductance between the secondary windings can lead to ringing and the need to passively snub. The snubber circuit used limits the output rectifier transient voltage overshoot to approximately 150 Volts, leaving an adequate safety margin. The 16 Amp diodes were selected to keep conduction losses reasonable while amply handling the rms current. Recovery time is not critical due to the series ZVT inductance on the primary , but fast recovery assists in keeping losses low. A series inductor is used on the primary to make the Zero Voltage switching Transitions possible under various output loads. Energy is stored each time power is transferred from the prim 4CY to the secondary , and circulated in the primary only during the freewheeling periods. This stored energy is necessary only to accomplish the left leg resonant transitions, and is transparent during thelinear right leg transition. Additionally, this inductor limits the rate of change in primary current, further softening the power transfer edges. A dc blocking capacitor is used in series with the primary for this example, primarily because voltage mode operation (duty cycle control) is incorporated. No net voltage was expected to be measured across the capacitor since the applied volt-second products in consecutive switching cycles should be identical. Experiments with voltage feedforward, current mode control and average current mode control were not conducted. Overcurrent protection has been incorporated by means of a current sense transformer also in series with the primary winding. A standard Pulse Engineering toroidal transformer, model #PE-64977 was used with a single turn primary winding and an isolated 20 turn secondary .This part features low leakage inductance, 1250 VRMS isolation and 20 A peak current sense capability. A full bridge diode array reconstructs the signal referenced to the UC3875 controllergroundreturn. A small R/C filter cleans up the waveform which is input to the IC s current sense pin. Resistor R is selected such that a 2.5 V peak signal corresponds to a 15% overload and will
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