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LTC4278

4278fc

Note the use of the external feedback resistive divider

ratio to set output voltage provides the user additional

freedom in selecting a suitable transformer turns ratio.

Turns ratios that are the simple ratios of small integers;

e.g., 1:1, 2:1, 3:2 help facilitate transformer construction

and improve performance.

When building a supply with multiple outputs derived

through a multiple winding transformer, lower duty cycle

can improve cross regulation by keeping the synchronous

rectifier on longer, and thus, keep secondary windings

coupledlonger.Foramultipleoutputtransformer,theturns

ratio between output windings is critical and affects the

accuracy of the voltages. The ratio between two output

voltages is set with the formula VOUT2=VOUT1•N21where

N21 is the turns ratio between the two windings. Also

keep the secondary MOSFET RDS(ON) small to improve

cross regulation.

The feedback winding usually provides both the feedback

voltage and power for the LTC4278. Set the turns ratio

between the output and feedback winding to provide a

rectifiedvoltagethatunderworst-caseconditionsisgreater

than the the preregulator maximum supply voltage. For

example if the preregulator maximum output were 7V:

NSF >

VOUT

+ VF

where:

VF =Diode Forward Voltage

For our example: NSF >

+ 0.7

1.56

We will choose

Leakage Inductance

Transformer leakage inductance (on either the primary or

secondary) causes a spike after the primary-side switch

turn-off. This is increasingly prominent at higher load

currents, where more stored energy is dissipated. Higher

flyback voltage may break down the MOSFET switch if it

has too low a BVDSS rating.

One solution to reducing this spike is to use a clamp circuit

to suppress the voltage excursion. However, suppressing

the voltage extends the flyback pulse width. If the flyback

pulse extends beyond the enable delay time, output

voltage regulation is affected. The feedback system has a

deliberately limited input range, roughly ±50mV referred

to the FB node. This rejects higher voltage leakage spikes

because once a leakage spike is several volts in amplitude,

a further increase in amplitude has little effect on the

feedback system. Therefore, it is advisable to arrange the

clamp circuit to clamp at as high a voltage as possible,

observing MOSFET breakdown, such that leakage spike

duration is as short as possible. Application Note 19

provides a good reference on clamp design.

As a rough guide, leakage inductance of several percent

(of mutual inductance) or less may require a clamp, but

exhibit little to no regulation error due to leakage spike

behavior.Inductancesfromseveralpercentupto,perhaps,

ten percent, cause increasing regulation error.

Avoid double digit percentage leakage inductances. There

is a potential for abrupt loss of control at high load cur-

rent. This curious condition potentially occurs when the

leakage spike becomes such a large portion of the flyback

waveform that the processing circuitry is fooled into think-

ing that the leakage spike itself is the real flyback signal!

It then reverts to a potentially stable state whereby the

top of the leakage spike is the control point, and the

trailing edge of the leakage spike triggers the collapse

detect circuitry. This typically reduces the output voltage

abruptly to a fraction, roughly one-third to two-thirds of

its correct value.

Onceloadcurrentisreducedsufficiently,thesystemsnaps

back to normal operation. When using transformers with

considerable leakage inductance, exercise this worst-case

check for potential bistability:

1. Operate the prototype supply at maximum expected

load current.

2. Temporarily short-circuit the output.

3. Observe that normal operation is restored.

If the output voltage is found to hang up at an abnormally

lowvalue,thesystemhasaproblem.Thisisusuallyevident

bysimultaneouslyviewingtheprimary-sideMOSFETdrain

voltage to observe firsthand the leakage spike behavior.

APPLICATIONS INFORMATION

LTC4263 Datasheet(PDF) 26 Page - Linear Technology

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