LT3511MP データシートの表示（PDF） - Linear Technology

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LT3511MP

Monolithic High Voltage Isolated Flyback Converter

Linear Technology 

LT3511

Applications Information

IPEAK

=

VOUT

η•

• IOUT •

VIN • D

2

IPEAK

=

15V

0.75 •

• 0.1A

36V •

•2

0.46

=

0.24A

Ensure that the saturation current covers steady-state

operation, start-up and transient conditions. To satisfy

these conditions, choose a saturation current 50% or more

higher than the steady-state calculation. In this example, a

saturation current between 400mA and 500mA is chosen.

Table 1 presents a list of pre-designed flyback transform-

ers. For this application, the Würth 750311660 transformer

will be used.

Step 4: Choose the correct output diode.

The two main criteria for choosing the output diode include

forward current rating and reverse voltage rating. The

maximum load requirement is a good first-order guess

at the average current requirement for the output diode.

A better metric is RMS current.

IRMS = IPEAK(VIN(MIN)) • NPS •

1– DVIN(MIN)

3

Example:

IRMS = 0.24 • 2 •

1– 0.46 = 0.2A

3

Next calculate reverse voltage requirement using maxi-

mum VIN:

VREVERSE

=

VOUT

+

VIN(MAX )

NPS

Example:

VREVERSE

=

15V +

72V

2

=

51V

A 0.5A, 60V diode from Diodes Inc. (SBR0560S1) will

be used.

Step 5: Choose an output capacitor.

The output capacitor choice should minimize output voltage

ripple and balance the trade-off between size and cost for

a larger capacitor. Use the equation below at nominal VIN:

C = IOUT • D

ΔVOUT • fSW

Example:

Design for ripple levels below 50mV.

C = 0.1A • 0.39 = 3.1µF

0.05V • 256kHz

A 10μF, 25V output capacitor is chosen. Remember ce-

ramic capacitors lose capacitance with applied voltage.

The capacitance can drop to 40% of quoted capacitance

at the max voltage rating.

Step 6: Design clamp circuit.

The clamp circuit protects the switch from leakage induc-

tance spike. A DZ clamp is the preferred clamp circuit. The

Zener and the diode need to be chosen.

The maximum Zener value is set according to the maxi-

mum VIN:

VZENER(MAX) ≤ 150V – VIN(MAX)

Example:

VZENER(MAX) ≤ 150V – 72V

VZENER(MAX) ≤ 78V

In addition, power loss in the clamp circuit is inversely

related to the clamp voltage as shown previously. Higher

clamp voltages lead to lower power loss.

A 68V Zener with a maximum of 72V will provide optimal

protection and minimize power loss. Half-watt Zeners will

satisfy most clamp applications involving the LT3511.

Power loss can be calculated using the equations presented

in the Leakage Inductance and Clamp Circuit section.

The Zener chosen is a 68V 0.5W Zener from On Semicon-

ductor (MMSZ5266BT1G).

3511fa

17

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