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LT3511

3511fa

applicaTions inForMaTion

Turns Ratio

Note that when using an RFB/RREF resistor ratio to set

output voltage, the user has relative freedom in selecting

a transformer turns ratio to suit a given application. In

contrast, the use of simple ratios of small integers, e.g.,

1:1, 2:1, 3:2, provides more freedom in setting total turns

and mutual inductance.

Typically, choose the transformer turns to maximize avail-

able output power. For low output voltages (3.3V or 5V), a

N:1 turns ratio can be used with multiple primary windings

relative to the secondary to maximize the transformer’s

current gain (and output power). However, remember that

the SW pin sees a voltage that is equal to the maximum

input supply voltage plus the output voltage multiplied by

the turns ratio. In addition, leakage inductance will cause

a voltage spike (VLEAKAGE) on top of this reflected voltage.

This total quantity needs to remain below the absolute

maximum rating of the SW pin to prevent breakdown of

the internal power switch. Together these conditions place

an upper limit on the turns ratio, N, for a given application.

Choose a turns ratio low enough to ensure:

N <

150V – VIN(MAX) – VLEAKAGE

VOUT + VF

For larger N:1 values, a transformer with a larger physical

size is needed to deliver additional current and provide a

large enough inductance value to ensure that the off-time

is long enough to accurately measure the output voltage.

For larger N:1 values, choose a transformer with a larger

physical size to deliver additional current. In addition,

choose a large enough inductance value to ensure that

the off-time is long enough to measure the output voltage.

For lower output power levels, choose a 1:1 or 1:N trans-

former for the absolute smallest transformer size. A 1:N

transformer will minimize the magnetizing inductance

(and minimize size), but will also limit the available output

power. A higher 1:N turns ratio makes it possible to have

veryhighoutputvoltageswithoutexceedingthebreakdown

voltage of the internal power switch.

The turns ratio is an important element in the isolated

feedbackscheme.Makesurethetransformermanufacturer

guarantees turns ratio accuracy within ±1%.

Saturation Current

The current in the transformer windings should not ex-

ceed its rated saturation current. Energy injected once the

core is saturated will not be transferred to the secondary

and will instead be dissipated in the core. Information on

saturation current should be provided by the transformer

manufacturers. Table 1 lists the saturation current of the

transformers designed for use with the LT3511.

Primary Inductance Requirements

The LT3511 obtains output voltage information from the

reflected output voltage on the switch pin. The conduction

of secondary winding current reflects the output voltage

on the primary. The sampling circuitry needs a minimum

of 400ns to settle and sample the reflected output voltage.

Inordertoensurepropersampling,thesecondarywinding

needs to conduct current for a minimum of 400ns. The fol-

lowing equation gives the minimum value for primaryside

magnetizing inductance:

LPRI ≥

tOFF(MIN) • NPS • VOUT + VF

(

)

IPEAK(MIN)

tOFF(MIN) = 400ns

IPEAK(MIN) = 55mA

Leakage Inductance and Clamp Circuits

Transformer leakage inductance (on either the primary or

secondary) causes a voltage spike to appear at the primary

after the output switch turns off. This spike is increasingly

prominent at higher load currents where more stored en-

ergy must be dissipated. When designing an application,

adequate margin should be kept for the effect of leakage

voltage spikes. In most cases the reflected output voltage

on the primary plus VIN should be kept below 100V. This

leaves at least 50V of margin for the leakage spike across

line and load conditions. A larger voltage margin will be

needed for poorly wound transformers or for excessive

leakageinductance.Figure5illustratesthispoint.Minimize

transformer leakage inductance.

A clamp circuit is recommended for most applications.

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