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DPA426R-TL Datasheet(PDF) 4 Page - Power Integrations, Inc. |
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DPA426R-TL Datasheet(HTML) 4 Page - Power Integrations, Inc. |
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4 / 20 page  AN-31 C 7/04 4 Efficiency Designing a DC-DC converter with DPA-Switch involves several engineering tradeoffs that weigh efficiency against cost and complexity. The circuit configuration in Figure 1 achieves efficiencies greater than 85% over the range of input voltage at medium loads. In typical applications without synchronous rectifcation,approximately25%ofthetotalpowerlosswillbein the DPA-Switch (see DPA-Switch data sheet), 40% in the output rectifiers, and 30% in the magnetic devices. The remainder is distributed among other devices and circuit traces. Higher efficiencies of approximately 91% can be obtained whenSchottkyrectifiersarereplacedbysynchronousrectifiers, allowing lower voltage drops. The efficiency can be raised even higher with the use of the next larger device in the DPA-Switch family that has lower R DS(ON). Further increases in device size may not improve the efficiency due to increased device switching losses. Losses in the magnetic devices can be reduced by using larger cores and by switching at 300 kHz instead of 400 kHz.All these alternatives have compromises in size, cost and complexity that the designer must evaluate. Temperature DC-DC converters usually must operate over an extended range of temperature that goes beyond the limits for ordinary consumer electronics. Designers should be aware that the characteristics of passive components are likely to change significantly with temperature. Attention to these effects to choose suitable components can prevent unexpected and undesirable behavior. Designers must pay particular attention to the selection of the output capacitors and the components in the feedback circuit to guarantee specified performance throughout the temperature range. The details are addressed later in the sections on Output Capacitor Selection and Feedback Design. Bias Voltage There are four ways to generate the bias voltage required for operation of DPA-Switch: (a) DC input derived (b) Transformer bias (unregulated) (c) Output coupled inductor winding (d) Transformer bias (regulated) Figure 2 illustrates the four alternatives. Each one must provide aminimumof8Vatthecollectoroftheoptocouplerunderworst caseoperatingconditions(minimuminputvoltageandminimum load).Thelowestbiasvoltageundertypicalconditionsshouldbe 12V.Theoutputcoupledinductorandtheregulatedtransformer bias techniques give the highest efficiency of the four solutions because the voltage across the optocoupler is controlled. This is countered by increased complexity. Optocoupler dissipation canbesignificantandshouldbeverified.Maximumoptocoupler phototransistor current is equal to the maximum CONTROL pin current (I C(SKIP)) for the selected DPA-Switch. Maximum dissipation therefore occurs at the highest bias voltage (highest input voltage for (a) and (b)) and minimum load. Table 2 provides a comparison of complexity vs performance for all the solutions. a) The DC input derived bias is the simplest of the three solutions.ItusesaZenerdiodebetweenthepositiveDCinput and the collector of the phototransistor of the optocoupler to reducethemaximumcollector-to-emittervoltage,andmore importantly, to limit the dissipation in the optocoupler. The penalty for simplicity is a reduction in efficiency that can be significant at high input voltages. This alternative is best for industrial applications where the input voltage is low (18 V to 36 V). The input voltage in industrial applications is usually low enough to eliminate the Zener diode because the breakdown voltages for standard optocouplers can be as high as 70 V. Designers must check the maximum power dissipation in the optocoupler in either case. b) Thetransformerbias(unregulated)iscreatedfromawinding on the power transformer. The forward bias winding should be connected to the rectifier in a polarity such that it conducts when the DPA-Switch is on. Since the bias voltage is proportional to the input voltage, efficiency is reduced at high input voltages, but the effect is less than with the direct connection to the input. Again, the designer needs to check thepowerdissipationintheoptocoupleratthemaximumbias voltage. For this bias type, worst case is minimum output load and high input voltage. Flyback bias windings are not recommended for DPA-Switch applications since they will affect the transformer reset. c) Output coupled inductor bias uses a winding on the output inductor to develop the bias voltage. This technique provides a well regulated bias voltage when the converter operates in the continuous conduction mode. Regulation is accomplished by phasing the winding such that the bias voltage is proportional to the output voltage by transformer action when the DPA-Switch turns off. The penalty for the higherefficiencyisthecostandcomplexityofacustomoutput inductor.Thebiasvoltagecanbeadjustedbymodifyingturns ratio, bias capacitor size and minimum load on the main output. The designer should verify a minimum bias voltage of 8 V at minimum load and maximum input voltage. d) The transformer bias (regulated) solution peforms the same function as the output coupled inductor bias (c). The bias voltage regulation is not quite as good as with the output coupled inductor bias. However, the solution does provide a reasonably constant bias voltage over a variety |
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