LTC4268IDKD-1#PBF【PDF 29/46 页】IC PD HIGH POWER W/CNTRL 32-DFN

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42681fc

LTC4268-1

applicaTions inForMaTion

In general, better performance is obtained with a lower

turns ratio. A DC of 45.5% yields a 1:8 ratio. 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 coupled longer. For a multiple output

transformer, the turns ratio between output windings is

critical and affects the accuracy of the voltages. The ratio

between two output voltages is set with the formula V

OUT2

= V

OUT1

" N21 where N21 is the turns ratio between the

two windings. Also keep the secondary MOSFET R

DS(ON)

small to improve cross regulation. The feedback winding

usually provides both the feedback voltage and power for

the LTC4268-1. Set the turns ratio between the output and

feedback winding to provide a rectified voltage that under

worst-case conditions is greater than the 11V maximum

turn-off voltage.

OUT

11+ V

where:

=Diode Forward Voltage

For our example: N

11+0.7

2.34

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 BV

DSS

rating. One solution to reducing this

spike is to use a snubber 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 ?0mV 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 snubber 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 snubber

design.

As a rough guide, leakage inductance of several percent

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

exhibit little to no regulation error due to leakage spike

behavior. Inductances from several percent up to 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 current.

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 thinking 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. Once

load current is reduced sufficiently, the system snaps

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

low value, the system has a problem. This is usually evident

by simultaneously viewing the primary side MOSFET drain

voltage to observe firsthand the leakage spike behavior.

A final notethe susceptibility of the system to bistable

behavior is somewhat a function of the load current/

voltage characteristics. A load with resistivei.e., I = V/R

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