MP6900- FAST TURN-OFF INTELLIGENT CONTROLLER

MP6900 Rev. 1.12

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8

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© 2012 MPS. All Rights Reserved.

-70mV

-30mV

Vds

Isd

Vgs

t0

t1

t2

Figure 4—Synchronous Rectification

Operation at light load

SR Mosfet Selection and Driver ability

The Power Mosfet selection proved to be a trade

off between Ron and Qg. In order to achieve high

efficiency, the Mosfet with smaller Ron is always

preferred, while the Qg is usually larger with

smaller Ron, which makes the turn-on/off speed

lower and lead to larger power loss. For MP6900,

because Vds is regulated at ~-70mV during the

driving period, the Mosfet with too small Ron is

not recommend, because the gate driver may be

pulled down to a fairly low level with too small

Ron when the Mosfet current is still fairly high,

which make the advantage of the low Ron

inconspicuous.

Fig.5 shows the typical waveform of QR flyback.

Assume 50% duty cycle and the output current is

To achieve fairly high usage of the Mosfet’s Ron,

it is expected that the Mosfet be fully turned on at

least 50% of the SR conduction period:

Vfwd

Ron

I

Ron

Ic

Vds

OUT

−

≤

×

⋅

−

=

×

−

=

2

MP6902, which is ~70mV.

So the Mosfet’s Ron is recommended to be no

application,

the

Ron

of

the

Mosfet

is

recommended to be no lower than 7mΩ)

Fig.6 shows the corresponding total delay during

different Qg Mosfet by MP6902. From Fig.6, with

driving a 120nC Qg Mosfet, the driver ability of

MP6900 is able to pull up the gate driver voltage

of the Mosfet to ~5V in 300ns as soon as the

body diode of the Mosfet is conducting, which

greatly save the turn-on power loss in the

Mosfet’s body diode.

Id

Ipeak

Vg

SR Conduction Period

50% SR Conduction Period

Ic

Figure 5—Synchronous Rectification typical

waveforms in QR Flyback

Turn-on Delay vs . Qg

0

50

100

150

200

250

300

350

0

20

40

60

80

100

120

140

Qg (nC)

Figure 6—Total Turn-on Delay vs. Q