ADP1031ACPZ-3-R7 datasheet(32/38 Pages) AD

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ADP1031

Data Sheet

Rev. A | Page 32 of 38

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 clamping

voltage, VCLAMP.

SWPVMAX is the absolute maximum rating of the SWP pin.

VINP (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 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.

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 VOUT2 and the inverting regulator that drives VOUT3.

Determine the maximum output power capability by

PVOUT1 (MAX) = 0.5 × (IPEAK2 − (IPEAK − IAC/2)2) × LPRI × fSW × η

where:

PVOUT1 (MAX) is the maximum output power from VOUT1.

IPEAK is the peak current on the flyback switch.

IAC is the ripple current through the primary side of the

transformer and flyback switch.

LPRI is the primary side inductance of the transformer.

fSW is the switching frequency of the flyback regulator.

η is the expected efficiency of the flyback regulator.

The lower limit of the flyback current-limit threshold, ILIM(FLYBACK),

limits the maximum IPEAK. However, it is not recommended to

operate at this level to avoid unwanted current-limit events due

to variation in transformer inductance, efficiency, flyback

switching frequency, and rectifier diode forward voltage drop. If

the load on the flyback causes the current limit to trip, the

output voltage may not regulate as expected. It is recommended

to choose a peak operating current with built in margin for the

variations mentioned or to calculate the maximum output power

or output load using the worst case transformer inductance,

efficiency, diode forward voltage drop, and flyback switching

frequency.

Calculate the maximum load current on VOUT1 by

IVOUT1 (MAX) = PVOUT1 (MAX)/VOUT1

where:

IVOUT1 (MAX) is the maximum output current from VOUT1.

PVOUT1 (MAX) is the maximum output power from VOUT1.

VOUT1 is the output voltage of the flyback regulator.

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