LT8302HS8E-3-WPBF Просмотр технического описания (PDF) - Analog Devices

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LT8302HS8E-3-WPBF

42VIN Micropower No-Opto Isolated Flyback Converter with 65V/3.6A Switch

Analog Devices 

LT8302/LT8302-3

APPLICATIONS INFORMATION

Turns Ratio

Note that when choosing an RFB/RREF resistor ratio to set

output voltage, the user has relative freedom in selecting

a transformer turns ratio to suit a given application. In

contrast, the use of simple ratios of small integers, e.g.,

3:1, 2:1, 1:1, etc., provides more freedom in settling total

turns and mutual inductance.

Typically, choose the transformer turns ratio to maximize

available output power. For low output voltages (3.3V

or 5V), a N:1 turns ratio can be used with multiple pri-

mary windings relative to the secondary to maximize the

transformer’s current gain (and output power). However,

remember that the SW pin sees a voltage that is equal

to the maximum input supply voltage plus the output

voltage multiplied by the turns ratio. In addition, leakage

inductance will cause a voltage spike (VLEAKAGE) on top of

this reflected voltage. This total quantity needs to remain

below the 65V absolute maximum rating of the SW pin to

prevent breakdown of the internal power switch. Together

these conditions place an upper limit on the turns ratio,

NPS, for a given application. Choose a turns ratio low

enough to ensure

NPS

<

65V

–

VIN(MAX) – VLEAKAGE

VOUT + VF

For larger N:1 values, choose a transformer with a larger

physical size to deliver additional current. In addition,

choose a large enough inductance value to ensure that

the switch-off time is long enough to accurately sample

the output voltage.

For lower output power levels, choose a 1:1 or 1:N trans-

former for the absolute smallest transformer size. A 1:N

transformer will minimize the magnetizing inductance

(and minimize size), but will also limit the available output

power. A higher 1:N turns ratio makes it possible to have

very high output voltages without exceeding the break-

down voltage of the internal power switch.

The turns ratio is an important element in the isolated

feedback scheme, and directly affects the output voltage

accuracy. Make sure the transformer manufacturer spec-

ifies turns ratio accuracy within ±1%.

Saturation Current

The current in the transformer windings should not exceed

its rated saturation current. Energy injected once the core

is saturated will not be transferred to the secondary and

will instead be dissipated in the core. When designing

custom transformers to be used with the LT8302/LT8302-

3, the saturation current should always be specified by

the transformer manufacturers.

Winding Resistance

Resistance in either the primary or secondary windings

will reduce overall power efficiency. Good output voltage

regulation will be maintained independent of winding

resistance due to the boundary/discontinuous conduction

mode operation of the LT8302/LT8302-3.

Leakage Inductance and Snubbers

Transformer leakage inductance on either the primary

or secondary causes a voltage spike to appear on the

primary after the power switch turns off. This spike is

increasingly prominent at higher load currents where

more stored energy must be dissipated. It is very import-

ant to minimize transformer leakage inductance.

When designing an application, adequate margin should

be kept for the worst-case leakage voltage spikes even

under overload conditions. In most cases shown in

Figure 5, the reflected output voltage on the primary plus

VIN should be kept below 50V. This leaves at least 15V

margin for the leakage spike across line and load condi-

tions. A larger voltage margin will be required for poorly

wound transformers or for excessive leakage inductance.

For more information www.analog.com

Rev. G

15

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