ADP1034ACPZ-1-R7 datasheet(34/41 Pages) AD

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Data Sheet

ADP1034

APPLICATIONS INFORMATION

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Rev. 0 | 34 of 41

Primary Inductance

The ADP1034 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 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

where:

VOUT1 is the flyback regulator output voltage.

LPRI is the primary side inductance of the transformer.

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

result in a smaller transformer but also reduces the output power

capabilities due to larger ac ripple current through the transformer.

Conversely, operating at a 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 much higher losses and overall lower system efficiency may

result. 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 ADP1034, mini-

mize 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 induc-

tance 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 + VVINP + 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.

VVINP is the input supply voltage.

VOUT1 is the output voltage of the flyback regulator.

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 a clamping circuit are the resistor, capaci-

tor, diode clamp shown in Figure 86 and the diode, Zener diode

clamp shown in Figure 87. 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 clamp solution.

Figure 86. Resistor, Capacitor, Diode Clamp

Figure 87. 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 is lower than the SWP

maximum voltage rating (SWPVMAX) specified in the Absolute Max-

imum Ratings section and is greater than the summation of the

maximum input supply (VVINP_MAX) and the maximum flyback output

voltage (VOUT1_MAX) of the application as given by

SWPVMAX > VVINP_MAX + VCLAMP > VVINP_MAX + VOUT1_MAX

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