LT3837EFE-TRPBF datasheet(15/28 Pages) LINER

部件名	LT3837EFE-TRPBF

功能描述	Isolated No-Opto Synchronous Flyback Controller

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制造商	LINER [Linear Technology]

网页	http://www.linear.com

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LT3837

15

3837fa

APPLICATIONS INFORMATION

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 note—the susceptibility of the system to bistable

behavior is somewhat a function of the load current/volt-

age characteristics. A load with resistive—i.e., I = V/R

behavior—is the most apt to be bistable. Capacitive loads

that exhibit I = V2/R behavior are less susceptible.

Secondary Leakage Inductance

Leakage inductance on the secondary forms an inductive

divider on the transformer secondary, reducing the size

of the feedback ﬂyback pulse. This increases the output

voltage target by a similar percentage.

Note that unlike leakage spike behavior, this phenomenon

is independent of load. Since the secondary leakage in-

ductance is a constant percentage of mutual inductance

(within manufacturing variations), the solution is to adjust

the feedback resistive divider ratio to compensate.

Winding Resistance Effects

Primary or secondary winding resistance acts to reduce

overall efﬁciency (POUT/PIN).Secondarywindingresistance

increases effective output impedance degrading load regu-

lation. Load compensation can mitigate this to some extent

but a good design keeps parasitic resistances low.

Biﬁlar Winding

A biﬁlar or similar winding is a good way to minimize

troublesome leakage inductances. Biﬁlar windings also

improve coupling coefﬁcients and thus improve cross

regulation in multiple winding transformers. However,

tight coupling usually increases primary-to-secondary

capacitance and limits the primary-to-secondary break-

down voltage, so it isn’t always practical.

Primary Inductance

The transformer primary inductance, LP, is selected based

on the peak-to-peak ripple current ratio (X) in the trans-

former relative to its maximum value. As a general rule,

keep X in the range of 50% to 70% ripple current (i.e., X =

0.5 to 0.7). Higher values of ripple will increase conduction

losses, while lower values will require larger cores.

Ripple current and percentage ripple is largest at minimum

duty cycle; in other words, at the highest input voltage.

LP is calculated from:

L

VDC

fX

P

V

P

IN MAX

MIN

OSC

MAX

IN

IN MAX

= () =

()

•

••

•

2

D

DC

Eff

fX

P

MIN

OSC

MAX

OUT

()2•

••

where:

fOSC is the OSC frequency

DCMIN is the DC at maximum input voltage

XMAX is ripple current ratio at maximum input voltage

Continuing with the 9V to 3.3V example, let us assume a

10A output, 9V to 18V input power with 88% efﬁciency.

Using X = 0.7, and fOSC = 200kHz:

P

A

W

DC

NV

V

IN

MIN

IN MAX

OUT

=

•

=

+

33 10

88

37 5

1

.

%

.

•

()

==

+

=

()

1

3

18

33

35 5

18

0 355

200

0

2

•

.

.%

•.

L

V

kHz

P

7

737 5

78

•.

.

W

H

=μ

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