LT3752 datasheet(16/48 Pages) LINER

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LT8311

8311f

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The ultimate goal of both reset mechanisms is to raise

the SWP node to a voltage higher than VIN, imposing

appropriate volt seconds on LMAG, and allowing the

magnetizing current to reset. Resetting the magnetic core

every cycle prevents magnetic flux buildup within the

core, and thereby prevents transformer saturation. FSW

tracks the SWP node during transformer reset. CG going

high, allows ILOUT to switch over from being conducted

by MCG’s body diode to MCG itself.

Region 4 (Figure 2)

1. ActiveClampResetCase(redwaveform):AOUTgoing

low causes the gate of M2 to be driven below ground

by the decoupling capacitor, CAOUT. This causes M2,

the active clamp PMOS, to turn on. M2 must be turned

on before ILMAG becomes negative, to allow ILMAG to

sustain conduction through the active clamp capacitor

and get fully reset. Active clamp reset completes by the

end of region 4, and ILMAG is reset to a negative value.

2. ResonantResetCase(bluewaveform):Resonantreset

ultimately completes when SWP’s quasi-sinusoidal

waveform returns to VIN, by which point ILMAG is reset

to a negative value. FSW is eventually clamped by MFG’s

body diode, and conducts ILMAG, through the second-

ary windings, towards the output inductor (similar to

Figure 3, but with ILMAG direction reversed on primary

and secondary sides). With a diode voltage imposed

across the secondary windings, transformer action

causes the primary winding to have a similar voltage

(scaled by turns ratio), resulting in SWP’s voltage

getting clamped to VIN. MCG continues conducting

ILOUT – ILMAG.

Region 5 (Figure 2)

Active Clamp Reset Case: AOUT goes high, turning off

M2. ILMAG, being negative, causes the voltage on SWP

(M1’s drain) to get pulled towards VIN, resulting in the

transformer’s primary winding voltage becoming smaller.

By transformer action, the secondary winding voltage also

becomes smaller. With MCG on (holding CSW at 0V), and

the transformer secondary winding voltage becoming

smaller, FSW collapses towards 0V.

Region 6 (Figure 2)

Eventually, in similar fashion to the resonant reset case,

FSWisclampedtoadiodevoltagebelowGNDbyMFG’sbody

diode, which now conducts ILMAG through the secondary

windings, towards the output inductor. With MFG’s body

diode on, and MCG on, the secondary winding voltage gets

clamped to about a diode voltage. Through transformer

action, SWP gets clamped to approximately VIN. CG goes

low, turning off MCG before M1 can turn on. ILOUT – ILMAG

is conducted through MCG’s body diode. FG goes high,

turningonMFG.Eventually,whenM1turnson,ILOUTwillbe

conducted through the transformer’s secondary winding,

and will flow from the source to drain of MFG.

OPERATION

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