LT3511HMS-TRPBF データシートの表示（PDF） - Linear Technology

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LT3511HMS-TRPBF

Monolithic High Voltage Isolated Flyback Converter

Linear Technology 

LT3511

Applications Information

VSW

<150V

<100V

VLEAKAGE

VSW

<150V

<140V

<100V

tOFF > 400ns

tSP < 150ns

without Clamp

tOFF > 400ns

TIME

tSP < 150ns

with Clamp

TIME

3511 F05

Figure 5. Maximum Voltages for SW Pin Flyback Waveform

Two circuits that can protect the internal power switch

include the RCD (resistor-capacitor-diode) clamp and the

DZ (diode-Zener) clamp. The clamp circuits dissipate the

stored energy in the leakage inductance. The DZ clamp

is the recommended clamp for the LT3511. Simplicity of

design, high clamp voltages, and low power levels make the

DZ clamp the preferred solution. Additionally, a DZ clamp

ensures well defined and consistent clamping voltages.

Figure 5 shows the clamp effect on the switch waveform

and Figure 6 shows the connection of the DZ clamp.

Proper care must be taken when choosing both the diode

and the Zener diode. Schottky diodes are typically the best

choice, but some PN diodes can be used if they turn on

fast enough to limit the leakage inductance spike. Choose

a diode that has a reverse-voltage rating higher than the

maximum input voltage. The Zener diode breakdown volt-

age should be chosen to balance power loss and switch

voltage protection. The best compromise is to choose the

largest voltage breakdown. Use the following equation to

make the proper choice:

VZENER(MAX) ≤ 150V – VIN(MAX)

For an application with a maximum input voltage of 72V,

choose a 68V VZENER which has VZENER(MAX) at 72V, which

will be below the 78V maximum.

The power loss in the clamp will determine the power rat-

ing of the Zener diode. Power loss in the clamp is highest

at maximum load and minimum input voltage. The switch

LS

Z

D

3511 F06

Figure 6. DZ Clamp

current is highest at this point along with the energy stored

in the leakage inductance. A 0.5W Zener will satisfy most

applications when the highest VZENER is chosen. Choosing

a low value for VZENER will cause excessive power loss as

shown in the following equations:

DZ

Power

Loss =

1

2

•L



• IPK(VIN(MIN))2

•

fSW

•

⎛

⎝⎜⎜1+

NPS • (VOUT + VF

( VZENER – NPS • VOUT

)

+

VF

)

⎞

⎠⎟⎟

L = Leakage Inductance



IPK( VIN(M IN))

=

η

•

VOUT • IOUT • 2

VIN(MIN) • DVIN(MIN)

( ) fSW

=

1

tON + tOFF

=

LPRI • IPK(VIN(MIN))

1

+ LPRI • IPK(VIN(MIN))

VIN(MIN)

NPS • VOUT + VF



3511fa

12

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