LT3748(RevA) 데이터 시트보기 (PDF) - Linear Technology

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LT3748
(Rev.:RevA)

100V Isolated Flyback Controller

Linear Technology 

LT3748

APPLICATIONS INFORMATION

Saturation Current

As discussed earlier in the Maximum Output Power sec-

tion, because the core of the transformer is being used for

energy storage in a flyback, the current in the transformer

windings should not exceed their rated saturation current

as energy injected once the core is saturated will not be

transferred to the secondary and will instead be dissipated

in the core. Information on saturation current should be

provided by the transformer manufacturers and Table 1

lists the saturation current of the transformers designed

for use with the LT3748.

Leakage Inductance and Snubbers

Transformer leakage inductance (on either the primary

or secondary) causes a voltage spike to appear at the

primary after the MOSFET switch turns off. This spike is

increasingly prominent at higher load currents where more

stored energy must be dissipated. Transformer leakage

inductance should be minimized.

In most cases, proper selection of the external MOSFET

and a well designed transformer will eliminate the need for

snubber circuitry, but in some cases the optimal MOSFET

may require protection from this leakage spike. An RC

(resistor capacitor) snubber may be sufficient in applica-

tions where the MOSFET has significant margin beyond

the predicted DC drain voltage applied in flyback while a

clamp using an RCD (resistor capacitor diode) or a Zener

might be a better option when using a MOSFET with very

little margin for leakage inductance spiking.

The recommended approach for designing an RC snubber

is to measure the period of the ringing at the MOSFET drain

when the MOSFET turns off without the snubber and then

add capacitance—starting with something in the range of

100pF—until the period of the ringing is 1.5 to 2 times

longer. The change in period will determine the value of the

parasitic capacitance, from which the parasitic inductance

can be determined from the initial period, as well. Similarly,

initial values can be estimating using stated switch capaci-

tance and transformer leakage inductance. Once the value

of the drain node capacitance and inductance is known, a

series resistor can be added to the snubber capacitance

to dissipate power and critically dampen the ringing. The

equation for deriving the optimal series resistance using

the observed periods (tPERIOD, and tPERIOD(SNUBBED)) and

snubber capacitance (CSNUBBER) is below, and the resultant

waveforms are shown in Figure 6.

CPAR

=

CSNUBBER

⎛

⎜

tPERIOD(SNUBBED)

⎞2

⎟

–

1

⎝

tPERIOD

⎠

LPAR

=

tPERIOD2

CPAR • 4π2

RSNUBBER =

LPAR

CPAR

90

80

70

60

50

40

30

20

10

0

0

NO SNUBBER

WITH SNUBBER

CAPACITOR

WITH RESISTOR

AND CAPACITOR

0.05 0.10 0.15 0.20 0.25 0.30

TIME (μs)

3748 F06

Figure 6. Observed Waveforms at MOSFET Drain when

Iteratively Implementing an RC Snubber

Note that energy absorbed by a snubber will be converted

to heat and will not be delivered to the load. In high volt-

age or high current applications, the snubber may need to

be sized for thermal dissipation. To determine the power

dissipated in the snubber resistor from capacitive losses,

measure the drain voltage immediately before the MOSFET

turns on and use the following equation relating that volt-

age and the MOSFET switching frequency to determine

the expected power dissipation:

PSNUBBER = fSW • CSNUBBER • VDRAIN2/2

Decreasing the value of the capacitor will reduce the dis-

sipated power in the snubber at the expense of increased

peak voltage on the MOSFET drain, while increasing the

value of the capacitance will decrease the overshoot.

3748fa

15

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