ADP1032ACPZ-5-R7 datasheet(31/37 Pages) AD

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Data Sheet

ADP1032

Rev. 0 | Page 31 of 37

VOUT1

Tx1

VINP

SWP

1:1

LLEAK

LPRI

DCLAMP

ZCLAMP

Figure 74. 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 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 75. 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 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 a power rating of about twice this value to have a

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.

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

about 5% to 10% of VCLAMP is reasonable.

Clamping Diode

Schottky diodes are typically the best choice for clamping diodes.

However, fast recovery diodes can also be used. The diode reverse

voltage rating must be higher than the maximum SWP pin voltage

rating.

Diode, Zener Diode Clamp

A Zener diode can replace the resistor, capacitor (RC) network

on the resistor, capacitor, diode clamp when the clamping level

must be consistent and well defined. Choose the Zener diode

breakdown voltage to balance power loss and switch voltage

protection. Calculate the Zener voltage by using the following

equation:

VZENER (MAX) ≤ SWPVMAX − VINP(MAX)

where VZENER (MAX) is the maximum Zener diode breakdown

voltage or the Zener voltage, which can be the same as the

VCLAMP.

The power loss in the clamp determines the power requirement

for the Zener diode. Use the following equation to calculate the

Zener diode power dissipation:

PZENER = (VZENER × LLEAK × IPEAK2 × fSW)/(2 × (VZENER − VOUT1))

where:

PZENER is the Zener diode power dissipation. Choose a Zener

diode with power rating higher than the calculated value.

VZENER is the Zener diode breakdown voltage or the Zener voltage.

Ripple Current (IAC) vs. Inductance

Calculate the ripple current by first determining the duty cycle

in continuous conduction mode.

DCCM = (VOUT1 + VD)/(VOUT1 + VD + VINP)

where DCCM is the duty cycle of the flyback switch.

Then, from the duty cycle, calculate the IAC in the flyback switch

and transformer primary.

IAC = (VINP × DCCM)/(fSW × LPRI)

where IAC is the ripple current through the primary side of the

transformer and flyback switch.

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