LT1737IS(RevA) Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

производитель

LT1737IS
(Rev.:RevA)

High Power Isolated Flyback Controller

Linear Technology 

LT1737

APPLICATIO S I FOR ATIO

which reduces the size of the primary-referred flyback

pulse used for feedback. This will increase the output

voltage target by a similar percentage. Note that unlike

leakage spike behavior, this phenomena is load indepen-

dent. To the extent that the secondary leakage inductance

is a constant percentage of mutual inductance (over

manufacturing variations), this can be accommodated by

adjusting the feedback resistor divider ratio.

Winding Resistance Effects

Resistance in either the primary or secondary will act to

reduce overall efficiency (POUT/PIN). Resistance in the

secondary increases effective output impedance which

degrades load regulation, (at least before load compensa-

tion is employed).

Bifilar Winding

A bifilar or similar winding technique is a good way to

minimize troublesome leakage inductances. However, re-

member that this will increase primary-to-secondary ca-

pacitance and limit the primary-to-secondary breakdown

voltage, so bifilar winding is not always practical.

Finally, the LTC Applications group is available to assist

in the choice and/or design of the transformer. Happy

Winding!

SELECTING FEEDBACK RESISTOR DIVIDER VALUES

The expression for VOUT developed in the Operation sec-

tion can be rearranged to yield the following expression for

the R1/R2 ratio:

( ) ( ) R1+ R2

R2

=

VOUT + VF + ISEC • ESR

VBG

NST

where:

VOUT = desired output voltage

VF = switching diode forward voltage

ISEC • ESR = secondary resistive losses

VBG = data sheet reference voltage value

NST = effective secondary-to-third winding turns ratio

The above equation defines only the ratio of R1 to R2, not

their individual values. However, a “second equation for

two unknowns” is obtained from noting that the Thevenin

impedance of the resistor divider should be roughly 3k for

bias current cancellation and other reasons.

SELECTING ROCMP RESISTOR VALUE

The Operation section previously derived the following

expressions for ROUT, i.e., effective output impedance and

ROCMP, the external resistor value required for its nominal

compensation:

ROUT

=

ESR

⎛

⎝⎜

1⎞

1– DC⎠⎟

( ) ROCMP

=

⎛

K1⎝⎜

RSENSE

ROUT

⎞

⎠⎟

R1|| R2

While the value for ROCMP may therefore be theoretically

determined, it is usually better in practice to employ

empirical methods. This is because several of the required

input variables are difficult to estimate precisely. For

instance, the ESR term above includes that of the trans-

former secondary, but its effective ESR value depends on

high frequency behavior, not simply DC winding resis-

tance. Similarly, K1 appears to be a simple ratio of VIN to

VOUT times (differential) efficiency, but theoretically esti-

mating efficiency is not a simple calculation. The sug-

gested empirical method is as follows:

Build a prototype of the desired supply using the eventual

secondary components. Temporarily ground the RCMPC

pin to disable the load compensation function. Operate the

supply over the expected range of output current loading

while measuring the output voltage deviation. Approxi-

mate this variation as a single value of ROUT (straight line

approximation). Calculate a value for the K1 constant

based on VIN, VOUT and the measured (differential) effi-

ciency. These are then combined with RSENSE as indicated

to yield a value for ROCMP.

Verify this result by connecting a resistor of roughly this

value from the ROCMP pin to ground. (Disconnect the

ground short to RCMPC and connect the requisite 0.1µF

filter capacitor to ground.) Measure the output impedance

with the new compensation in place. Modify the original

ROCMP value if necessary to increase or decrease the

effective compensation.

1737fa

13

Share Link: