MP6908- FAST TURN-OFF INTELLIGENT RECTIFIER

MP6908 Rev. 1.2

www.MonolithicPower.com

10

9/14/2021

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APPLICATION INFORMATION

Slew Rate Detection Function

In DCM operations, the demagnetizing ringing

may bring VDS down below 0V. If Vds reaches

the turn-on threshold during the ringing, SR

controllers without the slew rate detection

function may turn on the MOSFET by mistake.

Figure 3 shows the waveform of this false turn-

on situation. This does not only increase power

loss, but may also lead to shoot through if the

primary side FET is turned on within the

minimum on time.

Figure 3: False Turn-on

(Without slew rate detection)

Considering the slew rate of the ringing is

always much less than that when the primary

MOSFET is really turned off, this false turn-on

situation can be prevented by the slew rate

detection function, as shown in Figure 4. When

the slew rate is less than the threshold set by

the RSLEW, the IC does not turn on the gate even

when VDS reaches the turn-on threshold.

Figure 4: Prevent the False Turn-on

(With slew rate detection)

External resistor on VD and HVC

Over voltage conditions may lead to damage on

the device, so there has to be appropriate

application design to guarantee safe operation,

especially on the high voltage pin.

One of the common over voltage conditions is

when the body diode of the SR MOSFET is

turned on, the forward voltage drop may exceed

the negative rating on the VD pin. In this case,

an external resistor is commended to be placed

between VD and Drain of the MOSFET. In

general, the resistance is recommended to be

no less than 300

Ω.

On the other hand, this resistor cannot be too

large, either, because it compromises the VDD

supply and slow down the slew rate on the VDS

detection. In general, it is not recommended to

use any resistance larger than 1k

Ω, but for

each practical case, it should be checked based

on the condition of VDD supply and the slew

rate.

In the applications where HVC may also suffer

from a negative voltage bias (e.g. in a high-side

set-up without auxiliary winding), there should

also be the same resistance be placed on HVC

externally. If a block diode is used, it is

recommended that the reverse recovery time

be less than 250ns.

Typical System Implementations

Figure

5

shows

the

typical

system

implementation for the IC power supply derived

from

the

output

voltage

(VOUT),

which

is

available in low-side rectification.

VD

VSS

SLEW

VG

VDD

HVC

MP6908

Figure 5: Low-Side Rectification

The MP6908 can support most applications,

even when VOUT is down to 0V for low-side

rectification.

If the MP6908 is used for high-side rectification,

a self-supply can be achieved three ways (see

Figure 6-A, Figure 6-B, and Figure 7).