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LT3511

3511fa

Example:

NPS <

150V – 72V – 40V

15V + 0.5V

NPS < 2.45

NPS = 2

The choice of turns ratio is critical in determining output

power as shown earlier in the Output Power section. At

this point, a third winding can be added to the transformer

to drive the BIAS pin of the LT3511 for higher efficiencies.

Choose a turns ratio that sets the third winding voltage

to regulate between 3.3V and 6V for maximum efficiency.

Choose a third winding ratio to drive BIAS winding with

5V. (Optional)

Example:

NTHIRD

VTHIRD

VOUT

15V

= 0.33

The turns ratio of the transformer chosen is as follows

NPRIMARY: NSECONDARY: NTHIRD = 2:1:0.33.

Step 2: Calculate maximum power output at minimum

VIN.

POUT(VIN(MIN)) = η • VIN(MIN) • IIN = η • VIN(MIN) • D •

IPEAK • 0.5

D =

VOUT + VF

(

) •NPS

VOUT + VF

(

) •NPS + VIN(MIN)

η = Efficiency = ~75%

IPEAK = Peak switch current = 0.26A

Example:

D = 0.46

POUT(VIN(MIN)) = 1.62

IOUT(VIN(MIN)) = POUT(VIN(MIN))/VOUT = 0.11A

The chosen turns ratio satisfies the output current re-

quirement of 100mA. If the output current was too low,

the minimum input voltage could be adjusted higher. The

turns ratio in this example is set to its highest ratio given

switch voltage requirements and margin for leakage in-

ductance voltage spike.

Step 3: Determine primary inductance, switching fre-

quency and saturation current.

Primary inductance for the transformer must be set

above a minimum value to satisfy the minimum off time

requirement.

LPRI ≥

tOFF(MIN) • NPS • VOUT + VF

(

)

IPEAK(MIN)

tOFF(MIN) = 400ns

IPEAK(MIN) = 55mA

Example:

LPRI ≥

400ns • 2 • 15 + 0.5

(

)

0.055

LPRI ≥ 225µH

In addition, primary inductance will determine switching

frequency.

fSW =

tON + tOFF

LPRI •IPEAK

VIN

LPRI •IPEAK

NPS • VOUT + VF

(

)

IPEAK =

VOUT •IOUT • 2

η • VIN • D

Example:

Let’s calculate switching frequency at our nominal VIN

of 48V.

D =

15 + 0.5

(

) • 2

15 + 0.5

(

) • 2+ 48

= 0.39

IPEAK =

15V • 0.1A • 2

0.75 • 48V • 0.39

= 0.21A

Let’s choose LPRI = 350μH. Remember, most transform-

ers specify primary inductance with a tolerance of ±20%.

fSW = 256kHz

Finally, the transformer needs to be rated for the correct

saturation current level across line and load conditions.

In the given example, the worst-case condition for switch

current is at minimum VIN and maximum load.

applicaTions inForMaTion

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