ADP1031ACPZ-3-R7 datasheet(31/38 Pages) AD

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Data Sheet

ADP1031

Rev. A | Page 31 of 38

resistor, capacitor, diode clamp shown in Figure 72 and the diode

Zener diode clamp shown in Figure 73. The resistor, capacitor,

diode clamp quickly dampens the voltage spike and provides

improved EMI performance, and the diode Zener diode clamp

can be used when the clamping level must be consistent and well

defined. The diode Zener diode clamp has slightly higher power

efficiency over the resistor, capacitor, diode clamp. However, the

cost of the diode Zener diode clamp solution is typically higher

than the resistor, capacitor, diode solution.

VOUT1

Tx1

VINP

SWP

1:1

LLEAK

LPRI

RCLAMP CCLAMP

DCLAMP

Figure 72. Resistor, Capacitor, Diode Clamp

VOUT1

Tx1

VINP

SWP

1:1

LLEAK

LPRI

DCLAMP

ZCLAMP

Figure 73. Diode Zener Diode Clamp

Clamping Resistor

To calculate the clamping resistor (RCLAMP) value, the clamping

voltage (VCLAMP) must be determined. The clamping voltage is

the voltage on which any voltage spike that occurs on the

flyback switch is clamped. Choose a clamping voltage (VCLAMP)

that provides sufficient margin between the SWP maximum

voltage rating (SWPVMAX) specified in the Absolute Maximum

Ratings section and that also is greater than the summation of

the maximum input supply (VINP(MAX)) and the maximum flyback

output voltage (VOUT1 (MAX)) of the application as given by

SWPVMAX > VINP(MAX) + VCLAMP > VINP (MAX) + VOUT1 (MAX)

VINP (MAX)

VOUT1 (MAX) + VINP (MAX)

VCLAMP

SWPVMAX

Figure 74. Clamping Waveform

Use the following equation to calculate the value of the

clamping resistor for a given VCLAMP value:

RCLAMP = (2 × VCLAMP × (VCLAMP − VOUT1))/(LLEAK × IPEAK2 × fSW)

where:

RCLAMP is the value of the clamping resistor.

VCLAMP is the clamping voltage.

VOUT1 is the output voltage of the flyback regulator.

LLEAK is the leakage inductance of the transformer.

IPEAK is the peak current on the flyback switch.

fSW is the switching frequency of the flyback regulator.

To calculate the power dissipation across the snubber resistor,

use the following equation:

PRCLAMP = (VCLAMP)2/(RCLAMP)

where PRCLAMP is the power dissipation across RCLAMP. Choose

RCLAMP with power rating of about twice this value to have

margin.

Clamping Capacitor

The clamping capacitor (CCLAMP) is used to minimize the voltage

ripple level (VRIPPLE) superimposed in VCLAMP. Calculate the

clamping capacitor by using the following equation for the

desired VRIPPLE level and the calculated RCLAMP:

CCLAMP = VCLAMP/(VRIPPLE × fSW × RCLAMP)

where:

CCLAMP is the value of the clamping capacitor.

VCLAMP is the clamping voltage.

VRIPPLE is the voltage ripple superimposed in VCLAMP. A VRIPPLE of

about 5% to 10% of VCLAMP is reasonable.

fSW is the switching frequency of the flyback regulator.

RCLAMP is the value of the clamping resistor.

Clamping Diode

Schottky diodes are typically the best choice. However, fast

recovery diodes can also be used. The diode reverse voltage

rating must be higher than the maximum SWP pin voltage

rating.

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