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DPA424PN Datasheet(PDF) 6 Page - Power Integrations, Inc. |
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DPA424PN Datasheet(HTML) 6 Page - Power Integrations, Inc. |
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6 / 20 page  AN-31 C 7/04 6 If the transformer has a winding for the bias voltage, be sure that it has enough turns to maintain a minimum of 8 V bias at the lowest input voltage. Perform bench verification to confirm that the converter shuts off at low input voltages by virtue of the under-voltage lockout circuit, and not because the bias voltage is too low. With the actual number of turns on the transformer, verify that thedutyratiotoregulatetheoutputattheminimuminputvoltage is less than the minimum DC MAX specified for DPA-Switch. The AC flux density contributes to the core losses. For this reason the AC flux density should be maintained in the range between 1000 and 1500 gauss (0.1 to 0.15 tesla). Output Inductor For a single output application with no bias winding, the inductor can be a standard off-the-shelf component. Inductors with multiple windings are typically custom designs. Theinductorvalueisdeterminedchieflybytheamountofcurrent ripple that the designer is willing to tolerate. Higher ripple current will allow an inductor that is smaller both electrically and physically. The consequence of higher ripple current is the requirement for more output capacitance with lower equivalent seriesresistance(ESR)tomeetthespecificationforoutputripple. Higher current ripple in the inductor also translates to higher peak current in the DPA-Switch for a given output power. It also leads to generally greater loss and lower efficiency because the RMS value of all the currents will be higher. A convenient design parameter for selection of the inductor is K ∆I, defined as the ratio of the peak-to-peak ripple current to the average current in the inductor. Smaller K ∆I corresponds to 90% 86% 88% 35 40 45 50 55 60 65 70 75 DC Input Voltage 80% 82% 84% DPA424 DPA425 DPA426 Figure 3. Efficiency of the Low Cost EP-21 Prototype with Different Devices in the DPA-Switch Family (Synchronous Rectification Would Improve Efficiency). lower ripple and a larger inductor. Recommended values for K ∆Iare between 15 and 20 percent. The choice of K∆I involves a trade-off between the size of the inductor, the number and type of output capacitors, efficiency, and cost. Higher values of K ∆I are not recommended, as these higher ripple currents increase both the stress and the ripple voltage on the output capacitor. Whether the inductor is standard off-the-shelf or custom, the design should minimize the number of turns to reduce the resistive loss. The construction should also use a low loss core material. With user input, the PI Expert design tool computes the inductance,theRMScurrentandthepeakstoredenergytoaidin the selection or specification of the inductor. Peak stored energy is a useful parameter to select designs that use a closed toroid core, where magnetic saturation is generally a concern. Additional Winding for Bias Voltage IftheconfigurationinFigure2(c)ischosenforgenerationofthe bias voltage, choose the number of turns on the bias winding to give12Vattheoptocouplerundernominalconditions.Compute the required number of turns from the lowest regulated output voltage and the highest forward voltage drops for the output rectifierandthebiasrectifier.Checkthebiasvoltageatminimum load, maximum line, and add a preload if necessary to maintain the bias voltage at 8 V minimum. It may also be necessary to increase the bias winding turns to meet the minimum voltage requirement with a reasonably small pre-load. DPA-Switch Selection The first criterion for the selection of the DPA-Switch is peak current capability. From the turns ratio of the transformer and thepeakcurrentintheoutputinductor,estimatethepeakcurrent in the primary of the transformer. The magnetization current of thetransformershouldbenegligibleforthisestimate.Forlowest cost,selectthesmallestDPA-Switchthathasaminimumcurrent limit that is at least 10% greater than the maximum primary current. The allowance of 10% greater current gives design margin with the ability to respond to transient loading. The second criterion for the selection is power dissipation. The smallest DPA-Switch that will handle the current may dissipate too much power to meet the efficiency requirements. Even if efficiencyisnotaconcern,thesmallestdevicemaygettoohotif system constraints prevent good thermal design. Multiplication of the R DS(ON) by the square of the RMS current in the primary gives a reasonable estimate of the power dissipation in the DPA-Switch . The DPA-Switch dissipates approximately 25% of the total system loss in designs without synchronous rectifiers. |
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