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LT8311

8311f

For more information www.linear.com/LT8311

OPERATION

TheLT8311controlsthesynchronousMOSFETsandopto-

coupler on the secondary side of a forward converter.

SynchronouscontroloflowRDS(ON)MOSFETscantypically

lead to lower power dissipation in forward converters. The

lower power dissipation can improve converter efficiency,

resultinginlongtermcostsavingsbyloweringinputpower

requirements to support a certain level of output power.

Improved efficiency can also reduce the size of heat sinks

required to dissipate the heat generated in the rectifiers;

consequentlyincreasingtheoperatingambienttemperature

rangewhichmaybeusefulinmanyindustrialapplications.

The LT8311 also offers opto-coupler control for accurate

output voltage regulation over line and load. The LT8311’s

opto-couplercontrolcircuitrycomeswithahostofstart-up

and steady-state functions to ensure robust transient re-

sponseduringpower-onandoutputshort-circuitrecovery.

FUNDAMENTALS OF FORWARD CONVERTER

OPERATION IN CCM

The timing diagram of a forward converter operating in

continuous conduction mode (CCM) is shown in Figure 2.

Thetimingdiagramisbrokenintosixregionsofoperation.

Please refer to Figures 1 and 2 for the following explana-

tion of each region of operation.

Region 1 (Figure 2)

When OUT goes high, M1 turns on. CG should already be

at 0V before OUT goes high, to ensure that MCG does not

cross conduct with M1. The LT8311’s preactive mode,

which will be explained later, is an innovative scheme to

turn off MCG before M1 turns on. FG must be high during

this period to keep the forward MOSFET, MFG on, thereby

conducting the output inductor current, ILOUT, (via the

transformer’s secondary winding) through a low imped-

ance path. During this phase, magnetizing current, ILMAG,

builds up in the transformer’s magnetic core, and flows

from VIN to GND through M1. Output inductor current,

ILOUT, ramps up at a rate of (VCSW – VOUT)/ LOUT.

Region 2 (Figure 2)

When OUT goes low, and turns off M1, the transformer

becomes high impedance, and stops conducting ILOUT.

Since current in the output inductor cannot go to zero

instantaneously, it pulls the drain of the catch MOSFET,

CSW, towards ground. Ultimately CSW gets clamped at a

diode voltage below ground by MCG’s body diode which

nowsourcestheoutputinductorcurrent(similartoacatch

diode in a traditional buck converter). CSW collapsing

equivalently causes the transformer’s secondary winding

voltage to become smaller. Through transformer action,

Figure 1. Forward Converter with Active Clamp Reset (in Red) or Resonant Reset (in Blue)

8311 F01

VIN

VDRAIN_M2

•

PRIMARY

OUT

VOUT

COUT

LOUT

ILOUT

CSW

SECONDARY

FSW

SWP

ILMAG

LMAG

MFG

RLOAD

MCG

VCL

CCL

AOUT

CAOUT

CRST

ACTIVE CLAMP RESET (RED)

RESONANT RESET (BLUE)

–

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