MP6908- FAST TURN-OFF INTELLIGENT RECTIFIER

MP6908 Rev. 1.2

www.MonolithicPower.com

9

9/14/2021

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OPERATION

The

MP6908

supports

operation

in

discontinuous

conduction

mode

(DCM),

continuous conduction mode (CCM), and quasi-

resonant

flyback

converters.

The

control

circuitry controls the gate in forward mode and

turns the gate off when the synchronous

rectification (SR) MOSFET current drops to

zero.

VDD Generation

The capacitor at VDD supplies power for the IC.

It can be charged up by both HVC and VD.

When VHVC<4.7V, the VD charges up the

external capacitor at VDD via a current source

with 40mA and regulates it at 5V.

When 4.7V<VHVC<9.7V, VD stops charging, and

HVC charges VDD via a current source with

70mA and regulates it at VHVC -0.7V.

When VHVC>9.7V, the HVC charges VDD via a

current source with 70mA and clamps it at 9V.

Start-Up and Under-Voltage Lockout (UVLO)

When VDD rises above 3.75V, the MP6908

exits under-voltage lockout (UVLO) and is

enabled. The MP6908 enters sleep mode, and

VGS is kept low once VDD drops below 3.55V.

Turn-On Phase

When VDS drops to ~2V, a turn-on timer begins

to count. This turn-on timer can be programmed

by an external resistor on SLEW. If VDS reaches

the -86mV turn-on threshold from 2V within the

time(TSLEW) set by the timer, the MOSFET is

turned on after a turn-on delay (around 30ns)

(see Figure 2). If VDS crosses -86mV after the

timer ends, the gate voltage (VG) remains off.

This turn-on timer prevents the MP6908 from

turning on falsely due to ringing from DCM and

quasi-resonant operations.

TSLEW can be programmed with Equation (1):







k

ns

R

T

SLEW

SLEW

400

90

(1)

Turn-On Blanking

The control circuitry contains a blanking function.

When the MOSFET turns on, the control circuit

ensures that the on state lasts for a specific

period of time. The turn-on blanking time is ~1.1µs

to prevent an accidental turn-off due to ringing.

However, if VDS reaches 2 - 3V within the turn-on

blanking time, VGS is pulled low immediately.

Conduction Phase

When VDS rises above the forward voltage drop

(-40mV) according to the decrease of the

switching current, the MP6908 lowers the gate

voltage level to enlarge the on resistance of the

synchronous MOSFET.

-3mV

-86mV

2V

VDS

VGS

Turn On

Delay

Turn Off

Delay

Turn On

Blanking

Driver begin to

be pulled down

~1.1us

Driver turn off

2V

Turn Off

Blanking

-40mV

Figure 2: Turn-On/Turn-Off Timing Diagram

With this control scheme, VDS is adjusted to be

around -40mV even when the current through

the MOSFET is fairly low. This function keeps

the driver voltage at a very low level when the

synchronous MOSFET is turned off, which

boosts the turn-off speed and is especially

important to CCM operation.

Turn-Off Phase

When VDS rises to trigger the turn off threshold

(-3mV), the gate voltage is pulled to zero after a

very short turn-off delay of 25ns (see Figure 2).

Turn-Off Blanking

After the gate driver (VGS) is pulled to zero by

VDS reaching the turn-off threshold (-3mV), a

turn-off blanking time is applied, during which

the gate driver signal is latched off. The turn-off

blanking is removed when VDS rises above 2V

(see Figure 2).