LT8302MPS8E 查看數據表（PDF） - Linear Technology

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LT8302MPS8E

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

Linear Technology 

LT8302

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 breakdown

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 speci-

fies 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, 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 re-

sistance due to the boundary/discontinuous conduction

mode operation of the LT8302.

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 en-

ergy must be dissipated. It is very important 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 Fig-

ure 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 conditions. A

larger voltage margin will be required for poorly wound

transformers or for excessive leakage inductance.

For more information www.linear.com/LT8302

8302fa

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