LT3752 datasheet(32/48 Pages) LINER

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LT8311

8311f

For more information www.linear.com/LT8311

APPLICATIONS INFORMATION

In reality, the drain voltage of the forward MOSFET will

havesome“bowing”overandabovethevoltagecalculated

here, associated with the energy shuttled between LMAG

and CCL during the reset process. For most applications,

this bowing can be accounted for by adding a 20% safety

margin on the forward MOSFET’s VDS(MAX) equation.

When using resonant reset:

Forward MOSFET VDS(MAX) ≈

VOUT

fSW • 2 • LMAG •CRST

where fSW is the forward converter’s switching frequency,

LMAG is the magnetizing inductance of the transformer’s

primary winding, and CRST is the resonant reset capacitor

used on the primary side.

Unlike the catch MOSFET, the VDS(MAX) equation of the

forward MOSFET typically does not need to account for

leakageinductancevoltagespikes.Thisisbecausetheturn-

on and turn-off events of the forward MOSFET, typically,

do not involve the forward MOSFET’s drain having to dis-

sipate large amounts of stored leakage inductance energy.

2. Maximum IDS Rating

Most power MOSFET data sheets have a rating for

continuous-drain current, and pulse-drain current.

Continuous-drain current is the RMS drain current of the

catch and forward MOSFET, which is a function of the

inductor current, and the duty cycle at which the forward

converter is operating. Pulse-drain current is the

instantaneousmaximumdraincurrentseenbytheMOSFETs,

and is typically the peak of the inductor current waveform.

Prior to calculating the maximum continuous-drain cur-

rent, it is useful to calculate the minimum, maximum and

average duty cycles of the forward converter:

DMIN =

VOUT

VIN(MAX) •









DMAX =

VOUT

VIN(MIN) •









DAVG =

DMAX +DMIN

where VIN(MAX) and VIN(MIN) are the maximum and mini-

mum input voltages of the forward converter.

The catch MOSFET’s maximum continuous drain current,

ICAT_RMS, can be calculated as:

ICAT_RMS = 1−DMIN

(

)• ILOAD(MAX) 2 +

IRIPP(P−P)













where DMIN is the minimum duty cycle of the forward

converter, ILOAD(MAX) is the maximum output load current

of the forward converter, and IRIPP(P-P) is the peak-to-

peak ripple current in the output inductor. IRIPP(P-P) is

calculated as follows:

IRIPP(P−P) = VOUT •

−DAVG

fSW •LOUT

where DAVG is the average duty cycle of the forward con-

verter, fSW is the converter’s switching frequency, and

LOUT is the output inductance value.

TheforwardMOSFET’smaximumcontinuous-draincurrent

(IFWD_RMS) is:

IFWD_RMS = DMAX • ILOAD(MAX) 2 +

IRIPP(P−P)













Both, the forward and catch MOSFET should have a peak

pulse current rating that is higher than the highest pos-

sible peak of the inductor current. This highest possible

peak occurs at the maximum load current, and is equal to:

ILOAD(MAX) +

IRIPP(P−P)

3. Maximum VGS Rating

As explained earlier in the INTVCC Bias Supply section,

INTVCC is regulated internally to 7V by the LT8311. By

extension, the catch and forward MOSFET gates can be

drivenashighas7VwhenusingtheLT8311’sinternalLDO

to regulate INTVCC. For applications using the LT8311’s

internal LDO, picking a maximum VGS greater than 10V

should suffice.

LT3752 Datasheet(PDF) 32 Page - Linear Technology

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