LT3957A 查看數據表（PDF） - Linear Technology

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LT3957A

Boost, Flyback, SEPIC and Inverting Converter with 5A, 40V Switch

Linear Technology 

LT3957A

APPLICATIONS INFORMATION

The primary winding peak current is the switch current

limit (typical 5.9A). The primary and secondary maximum

RMS currents are:

ILP(RMS)

≈

PO U T(M AX )

DMAX • VIN(MIN) • η

ILS(RMS)

≈

IO U T(M AX )

1− DM AX

where η is the converter efficiency.

Based on the preceding equations, the user should design/

choose the transformer having sufficient saturation and

RMS current ratings.

Flyback Converter: Snubber Design

Transformer leakage inductance (on either the primary

or secondary) causes a voltage spike to occur after the

MOSFET turn-off. This is increasingly prominent at higher

load currents, where more stored energy must be dissi-

pated. In some cases a snubber circuit will be required to

avoid overvoltage breakdown at the MOSFET’s drain node.

There are different snubber circuits (such as RC snubber,

RCD snubber, Zener clamp, etc.), and Application Note 19

is a good reference on snubber design. An RC snubber

circuit can be connected between SW and GND to damp

the ringing on SW pins. The snubber resistor values should

be close to the impedance of the parasitic resonance. The

snubber capacitor value should be larger than the circuit

parasitic capacitance, but be small enough to keep the

snubber resistor power dissipation low.

If the RC snubber is insufficient to prevent SW pins over-

voltage, the RCD snubber can be used to limit the peak

voltage on the SW pins, which is shown in Figure 6.

The snubber resistor value (RSN) can be calculated by the

following equation:

RSN

=

2

•

VS2N

−

VSN

•

VOUT

•

NP

NS

IS2W(PEAK) • LLK • ƒ

LLK is the leakage inductance of the primary winding, which

is usually specified in the transformer characteristics. LLK

can be obtained by measuring the primary inductance with

the secondary windings shorted. The snubber capacitor

value (CSN) can be determined using the following equation:

CCN

=

VSN

ΔVSN • RSN

•

ƒ

where ΔVSN is the voltage ripple across CSN. A reasonable

ΔVSN is 5% to 10% of VSN. The reverse voltage rating of

DSN should be higher than the sum of VSN and VIN(MAX).

A Zener clamp can also be connected between SW and

GND to ensure SW voltage does not exceed 40V.

Flyback Converter: Output Diode Selection

The output diode in a flyback converter is subject to large

RMS current and peak reverse voltage stresses. A fast

switching diode with a low forward drop and a low reverse

leakage is desired. Schottky diodes are recommended if

the output voltage is below 100V.

Approximate the required peak repetitive reverse voltage

rating VRRM using:

VRRM

>

NS

NP

• VIN(MAX) + VOUT

The power dissipated by the diode is:

PD = IO(MAX) • VD

and the diode junction temperature is:

TJ = TA + PD • RθJA

The RθJA to be used in this equation normally includes the

RθJC for the device, plus the thermal resistance from the

board to the ambient temperature in the enclosure. TJ must

not exceed the diode maximum junction temperature rating.

Flyback Converter: Output Capacitor Selection

The output capacitor of the flyback converter has a similar

operation condition as that of the boost converter. Refer

to the Boost Converter: Output Capacitor Selection section

for the calculation of COUT and ESRCOUT.

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17

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