MP6920- FAST TURN-OFF INTELLIGENT RECTIFIER

MP6920 Rev. 1.01

www.MonolithicPower.com

6

9/19/2012

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OPERATION

The MP6920 supports operation in discontinuous

conduction mode (DCM) and Quasi-Resonant

Flyback converters. The internal control circuitry

of the MP6920 controls the integrated MOSFET

gate in forward mode and will turn the gate off

when the MOSFET current is fairly low.

Blanking

The control circuitry contains a blanking function.

When it pulls the integrated MOSFET on/off, it

makes sure that the on/off state at least lasts for

some time. The turn-on blanking time is 1.6µs,

which determines the minimum on-time. During

the turn-on blanking period, the turn-off threshold

is not totally blanked, but changes to +50mV

(instead of -30mV). This ensures that the part

can always turn off even during the turn-on

blanking period (albeit slower).

Under-Voltage Lockout

(UVLO) threshold, the part enters sleep mode

and the integrated MOSFET will not turn on.

Basic Operation

The basic operations of flyback converter with

the MP6920 are:

Turn-On Phase

When the switch current flows through the

body diode of the integrated MOSFET, it

turn-on threshold of the control circuitry (-

70mV), which then turns on the integrated

MOSFET after a 200ns turn-on delay

(defined in Figure 2).

Conducting Phase

above the turn-on threshold (-70mV), the

control circuitry stops pulling up the internal

gate driver and the driver voltage of the

integrated MOSFET drops, which makes the

around -70mV even when the switch current

triggering the turn-off threshold (-30mV) of

the internal driver until the current through

the integrated MOSFET has dropped to near

zero.

Figure 3 shows the MP6920 operating in

heavy-load condition. Due to the high current,

the internal driver voltage initially saturates;

around -70mV.

Figure 4 shows the MP6920 operating at

light-load condition. Due to the low current,

the driver voltage never saturates but begins

to decrease as soon as the integrated

Turn-Off Phase

threshold (-30mV), the driver voltage of the

switch goes zero after a 20ns turn-off delay

(shown in Figure 2) by the control circuitry.

Similar to turn-on phase, a 200ns blanking

time after the switch turns off avoid

erroneous trigging.

Light-Load Latch-Off Function

The gate driver of integrated MOSFET in the

MP6920 is latched to save the driver loss at light-

load condition to improve light-load efficiency.

fixed at 2.2μs. During each switching cycle, if the

integrated MOSFET conducting period remains

below 2.2µs, the MP6920 falls into light-load

mode and latches off the integrated MOSFET

After entering light-load mode, the MP6920

monitors the integrated MOSFET’s body diode

integrated MOSFET as a body diode until the

conducting period finishes. If the MOSFET’s

body diode conducting period is longer than

the integrated MOSFET of MP6920 is unlatched

to restart the internal synchronous rectification

(see Figure 6 for details).