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LT3574

3574f

applications inForMation

Figure 5. Maximum Voltages for SW Pin Flyback Waveform

Figure 4. RCD Clamp

Figure 6. Good Snubber Diode Limits SW Pin Voltage

Figure 7. Bad Snubber Diode Does Not Limit SW Pin Voltage

< 50V

< 55V

< 60V

VSW

tOFF > 350ns

TIME

tSP < 150ns

3574 F05

100ns/DIV

10V/DIV

3574 F06

100ns/DIV

10V/DIV

3574 F07

3574 F04

•

Secondary Leakage Inductance

In addition to the previously described effects of leakage

inductance in general, leakage inductance on the second-

ary in particular exhibits an additional phenomenon. It

forms an inductive divider on the transformer secondary

that effectively reduces the size of the primary-referred

flyback pulse used for feedback. This will increase the

output voltage target by a similar percentage. Note that

unlike leakage spike behavior, this phenomenon is load

independent. To the extent that the secondary leakage

inductance is a constant percentage of mutual inductance

(overmanufacturingvariations),thiscanbeaccommodated

by adjusting the RFB/RREF resistor ratio.

Winding Resistance Effects

Resistance in either the primary or secondary will reduce

overall efficiency (POUT/PIN). Good output voltage regula-

tion will be maintained independent of winding resistance

due to the boundary mode operation of the LT3574.

Bifilar Winding

A bifilar, or similar winding technique, is a good way to

minimize troublesome leakage inductances. However, re-

member that this will also increase primary-to-secondary

capacitanceandlimittheprimary-to-secondarybreakdown

voltage, so, bifilar winding is not always practical. The

Linear Technology applications group is available and

extremely qualified to assist in the selection and/or design

of the transformer.

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