LT3752 datasheet(33/48 Pages) LINER

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LT8311

8311f

For more information www.linear.com/LT8311

APPLICATIONS INFORMATION

Alternatively, the INTVCC pin can be overdriven externally

up to 16V. For such applications, picking MOSFETs with

a maximum VGS of ±20V should suffice.

4. Calculating MOSFET Losses Due to RDS(ON)

The conduction/ohmic loss associated with the catch and

forward MOSFET is a function of the MOSFET’s RMS cur-

rent and its on-resistance. For the vast majority of forward

converterapplications,whichtypicallyhavehighmaximum

load currents on the output (5A or higher), minimizing

losses associated with the MOSFET’s RDS(ON) will be far

more critical than minimizing losses associated with the

MOSFET’s gate charge.

Catch MOSFET Ohmic Loss = (ICAT_RMS)2 • RCAT

where RCAT is the on-resistance (RDS(ON)) of the catch

MOSFET.

Forward MOSFET Ohmic Loss = (IFWD_RMS)2 • RFWD

where RFWD is the on-resistance (RDS(ON)) of the forward

MOSFET.

5. Calculating Qg Based Loss

There are two aspects to the gate charge (Qg) based loss

associated with the secondary synchronous MOSFETs:

A. Qg Based MOSFET Switching Loss:

The catch MOSFET’s turn-on and turn-off timings,

regardless of preactive or SYNC mode, are ZVS (zero

voltage switching) events. The catch MOSFET turns on

after the inductor current is already flowing through its

body diode. Similarly, when the catch MOSFET turns

off, the inductor current subsequently flows through its

body diode. As a result, the voltage across the drain-

source terminals of the catch MOSFET is small during

switching events, resulting in the catch MOSFET having

insignificant switching loss.

The forward MOSFET’s turn-on and turn-off timings,

regardless of preactive or SYNC mode, are ZVS (zero

voltage switching) and ZCS (zero current switching)

events, respectively. The forward MOSFET turns on

after transformer reset is complete. Transformer reset

completionismarkedbythetransformer’smagnetizing

current flowing through the forward MOSFET’s body

diode,whichallowstheforwardMOSFETtoturnonwith

a small drain-to-source voltage across it. Similarly, the

forward MOSFET typically turns off after the primary-

side MOSFET has turned off. When the primary-side

MOSFET turns off, the only current flowing through

the forward MOSFET is the transformer magnetizing

current, which for all intents and purposes, can be as-

sumed to be zero. Consequently, the forward MOSFET

has insignificant switching losses.

B. Qg Based Converter Power Loss:

As explained earlier in the INTVCC Bias Supply section,

there is a power loss incurred in turning on/off the catch

and forward MOSFETs, associated with supplying gate

charge (Qg) to the gates of these MOSFETs. This charge

is supplied either by the supply voltage connected to

the LT8311’s VIN pin, when using the internal LDO to

regulate INTVCC, or by the supply voltage connected

to the LT8311’s INTVCC pin, when driving the INTVCC

pin externally. In either case, the total loss associated

with supplying the gate charge is:

Power Loss = VSUPP • (QgCAT + QgFWD) • fSW

where VSUPP is the supply voltage connected to the

LT8311’s VIN pin when INTVCC is internally regulated.

Alternatively VSUPP is the supply voltage connected

to the LT8311’s INTVCC pin when INTVCC is externally

driven. QgCAT and QgFWD is the gate charge (Qg) of the

catch and forward MOSFETs, respectively. fSW is the

forward converter’s switching frequency.

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