AN-31
90%
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.
88%
86%
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.
84%
82%
DPA424
DPA425
DPA426
80%
With user input, the PI Expert design tool computes the
inductance,theRMScurrentandthepeakstoredenergytoaidin
theselectionorspecificationoftheinductor. Peakstoredenergy
is a useful parameter to select designs that use a closed toroid
core, where magnetic saturation is generally a concern.
35 40 45 50 55 60 65 70 75
DC Input Voltage
Figure 3. Efficiency of the Low Cost EP-21 Prototype with Different
Devices in the DPA-Switch Family (Synchronous
Rectification Would Improve Efficiency).
Additional Winding for Bias Voltage
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.
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.
With the actual number of turns on the transformer, verify that
thedutyratiotoregulatetheoutputattheminimuminputvoltage
is less than the minimum DCMAX 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).
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.
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.
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
systemconstraintspreventgoodthermaldesign. Multiplication
of the RDS(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.
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
C
6
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