LT8309

9

8309fa

For more information www.linear.com/LT8309

operaTion

Figure 3. Demo Board Topside Silkscreen

Figure 4. Demo Board Topside Metal

to operate during a short, and the current will flow through

the low resistance channel of the MOSFET instead of its

body diode. Make sure to use a resistor in series with the

Layout Considerations

The main current loop is the MOSFETs drain-to-source

current. This should not share the same ground path as

the LT8309. The drain resistor needs to sense directly at

the drain of the MOSFET and not have any current of the

drain current flow through its metal trace. The drain node

of the MOSFET is used as the heat sink and will need to

be sized according to the power dissipation requirements.

Figure 3 is an example layout of the LT8309.

Effects of the Body Diode

When the discontinuous ring voltage goes below ground,

the body diode of the primary-side MOSFET turns on and

begins to conduct current. This diode clamps the voltage

to a diode drop below ground. When the current reverses

direction, the diode does not turn off immediately, and

conductscurrentintheoppositedirectionforasmallperiod

oftime.Thisisknownasthereverse-recoverytime.During

this time, the magnetizing inductance of the transformer

stores energy just like it does when the MOSFET turns

on. When the primary-side body diode finally turns off,

the energy is transferred to the secondary side and may

be enough to turn on the body diode of the secondary

MOSFET. Then, the LT8309 turns on its MOSFET for a

second time. Since this happens during the discontinuous

ring, the primary side may turn on during this time and

cause cross conduction. If this problem does occur, one

way to improve the reverse-recovery time of the primary

side’s MOSFET is to add a parallel Schottky diode, which

will conduct most of the current and turn off much faster

than the body diode. Another way of eliminating this issue

is by sizing the turns ratio so that the discontinuous ring

never goes below ground.