MP6902- FAST TURN-OFF INTELLIGENT CONTROLLER

MP6902 Rev. 1.13

www.MonolithicPower.com

9

1/11/2013

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LL pin, by monitoring the LL pin current (the LL

following:

LL

LL

2.2 s

TR (k )

100k

μ

≈Ω ⋅

Ω

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 MP6902,

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.

Figure 6 shows the typical waveform of QR

flyback. Assume 50% duty cycle and the output

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:

OUT

Vds

Ic Ron

2 I

Ron

Vfwd

=− ×

= − ⋅

×

≤ −

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)

Figure 7 shows the corresponding total delay

driving different Qg Mosfet by MP6902. From

Figure 7, with driving a 120nC Qg Mosfet, the

driver ability of MP6902 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 6—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 7—Total Turn-on Delay vs. Qg

Typical System Implementations

6

1

5

8

3

4

MP6902

R1

R3

C1

C2

C3

VD

VDD

PGND

LL

VSS

VG

EN

R2

2

Figure 8— IC Supply derived directly from

Output Voltage

Figure

8

shows

the

typical

system

implementation for the IC supply derived from

output voltage, which is available in low-side

rectification

and

the

output

voltage

is

(from 8V to 24V).

MP6902 or high-side rectification is used, it is

recommended to use an auxiliary winding from

the power transformer for the IC supply, which is

shown in Figure 9 and Figure.10.