ADP1034ACPZ-1-R7 datasheet(35/41 Pages) AD

35 / 41 page

Data Sheet

ADP1034

APPLICATIONS INFORMATION

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Rev. 0 | 35 of 41

Figure 88. 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 a power rating of about twice the PRCLAMP 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 clamp-

ing 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 ideal 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 RC network on the resistor, capac-

itor, 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 − VVINP_MAX

where:

VZENER (MAX) is the maximum Zener diode breakdown voltage or

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

VCLAMP.

SWPVMAX is the absolute maximum rating of the SWP pin.

VVINP_MAX is the maximum input supply voltage.

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.

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.

VOUT1 is the output voltage of the flyback regulator.

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.

VOUT1 is the output voltage of the flyback regulator.

VD is the forward voltage drop across the rectifier diode.

VINP is the input supply voltage.

Then, from the duty cycle, calculate 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.

VINP is the input supply voltage.

DCCM is the duty cycle of the flyback switch.

fSW is the switching frequency of the flyback regulator.

LPRI is the primary side inductance of the transformer.

Maximum Output Current Calculation

The maximum output power and current that can be achieved

from the flyback output depends on a number of variables within

the regulator. These variables include the transformer choice, the

operating frequency, and the rectifier diode choice. The flyback

regulator output is the supply to the buck regulator that drives

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