ADP1032ACPZ-3-R7 datasheet(30/37 Pages) AD

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ADP1032

Data Sheet

Rev. 0 | Page 30 of 37

Ripple Current vs. Capacitor Value

The output capacitor value must be chosen to minimize the

output voltage ripple while considering the increase in size and

cost of a larger capacitor. Use the following equation to calculate

the output capacitance:

COUT = (LPRI × ISWP2)/(2 × VOUT1 × ΔVOUT1)

where:

COUT is the capacitance of the flyback output capacitor.

LPRI is the primary inductance of the transformer.

ISWP is the peak switch current.

ΔVOUT1 is the allowable flyback regulator output ripple.

Schottky Diode

A Schottky diode with low junction capacitance is recommended

for the rectification diode D1. At higher output voltages and

especially at higher switching frequencies, the junction capacitance

is a significant contributor to efficiency. Choose an output diode

with a forward current rating (IF) that is greater than the maximum

load requirement and with a reverse voltage rating (VR) that is

greater than the summation of the maximum supply voltage

(VINP (MAX)) and the maximum output voltage (VOUT1 (MAX)).

Transformer

The transformer used with the ADP1032 is an important

component within the system, in terms of efficiency and

maximum output power capability. The transformer designs are

listed in Table 17. A number of factors must be taken into account

when designing a transformer for use with the ADP1032.

Turn Ratio

The ADP1032 requires the use of a transformer with a primary

to secondary turn ratio of 1:1 to start up properly.

Primary Inductance

The ADP1032 operates with a transformer with an inductance

in the 80 μH to 560 μH range. However, it is recommended to

choose an inductance value that results in the flyback regulator

output voltage (VOUT1) divided by the transformer primary

inductance being less than or equal to 140,000 to maintain

control loop stability.

VOUT1/LPRI ≤ 140,000

Using a transformer at the lower end of the inductance range can

result in a smaller transformer but also reduces the output power

capabilities due to larger ac ripple current through the transformer.

Conversely, operating at higher inductance can result in higher

output power at the expense of a potentially larger transformer.

Flyback Transformer Saturation Current

Do not exceed the saturation current of the transformer in

operation, or this may lead to much higher losses and overall

lower system efficiency. Choose a transformer with a saturation

current rating that is greater than the expected peak switch

current (ISWP) across line and load conditions.

Series Winding Resistance

In power loss sensitive applications, keep the series resistance of

the primary and secondary windings as low as possible to

improve overall efficiency.

Leakage Inductance and Clamping Circuits

When choosing a transformer to operate with the ADP1032,

minimize transformer leakage inductance. Leakage inductance

causes a voltage spike to appear on the SWP node when the

flyback regulator switch is off due to energy storage in the

leakage inductance that is not transferred to the output. The

voltage spike is more prominent at higher load currents and

increases with higher leakage inductance. It is important to

keep the voltage spikes lower than the voltage rating of the

flyback switch that drives the SWP pin. Margin must be built

into any design to avoid exceeding this limit if no clamp or

snubber circuit is used to protect the flyback switch.

To estimate the leading voltage spike at the SWP pin when the

switch turns off, use the following equation:

VPEAK = IPEAK × (LLEAK/(CP + CSWP))1/2 + VINP + VOUT1 + VD

where:

VPEAK is the voltage spike amplitude.

IPEAK is the peak current on the flyback switch.

LLEAK is the leakage inductance of the transformer.

CP is the parasitic capacitance of the transformer.

CSWP is the capacitance on the flyback switch.

VINP is the input supply voltage.

VD is the forward voltage drop across the rectifier diode.

A snubber or clamp circuit can protect the flyback switch for

cases where the leakage inductance is too high for application

conditions. Two common types of clamping circuits are the

resistor, capacitor, diode clamp shown in Figure 73 and the diode

Zener diode clamp shown in Figure 74. 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 73. Resistor, Capacitor, Diode Clamp

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