Datasheet LT8309 (Analog Devices) - 9
| Fabricante | Analog Devices |
| Descripción | Secondary-Side Synchronous Rectifier Driver |
| Páginas / Página | 14 / 9 — operaTion. Layout Considerations. Effects of the Body Diode. Figure 3. … |
| Formato / tamaño de archivo | PDF / 232 Kb |
| Idioma del documento | Inglés |
operaTion. Layout Considerations. Effects of the Body Diode. Figure 3. Demo Board Topside Silkscreen
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LT8309
operaTion
to operate during a short, and the current will flow through to a diode drop below ground. When the current reverses the low resistance channel of the MOSFET instead of its direction, the diode does not turn off immediately, and body diode. Make sure to use a resistor in series with the conducts current in the opposite direction for a small period diode to keep VCC below 40V. of time. This is known as the reverse-recovery time. During this time, the magnetizing inductance of the transformer
Layout Considerations
stores energy just like it does when the MOSFET turns The main current loop is the MOSFETs drain-to-source on. When the primary-side body diode finally turns off, current. This should not share the same ground path as the energy is transferred to the secondary side and may the LT8309. The drain resistor needs to sense directly at be enough to turn on the body diode of the secondary the drain of the MOSFET and not have any current of the MOSFET. Then, the LT8309 turns on its MOSFET for a drain current flow through its metal trace. The drain node second time. Since this happens during the discontinuous of the MOSFET is used as the heat sink and will need to ring, the primary side may turn on during this time and be sized according to the power dissipation requirements. cause cross conduction. If this problem does occur, one Figure 3 is an example layout of the LT8309. way to improve the reverse-recovery time of the primary side’s MOSFET is to add a parallel Schottky diode, which
Effects of the Body Diode
will conduct most of the current and turn off much faster When the discontinuous ring voltage goes below ground, than the body diode. Another way of eliminating this issue the body diode of the primary-side MOSFET turns on and is by sizing the turns ratio so that the discontinuous ring begins to conduct current. This diode clamps the voltage never goes below ground.
Figure 3. Demo Board Topside Silkscreen Figure 4. Demo Board Topside Metal
8309fa For more information www.linear.com/LT8309 9
operaTion
to operate during a short, and the current will flow through to a diode drop below ground. When the current reverses the low resistance channel of the MOSFET instead of its direction, the diode does not turn off immediately, and body diode. Make sure to use a resistor in series with the conducts current in the opposite direction for a small period diode to keep VCC below 40V. of time. This is known as the reverse-recovery time. During this time, the magnetizing inductance of the transformer
Layout Considerations
stores energy just like it does when the MOSFET turns The main current loop is the MOSFETs drain-to-source on. When the primary-side body diode finally turns off, current. This should not share the same ground path as the energy is transferred to the secondary side and may the LT8309. The drain resistor needs to sense directly at be enough to turn on the body diode of the secondary the drain of the MOSFET and not have any current of the MOSFET. Then, the LT8309 turns on its MOSFET for a drain current flow through its metal trace. The drain node second time. Since this happens during the discontinuous of the MOSFET is used as the heat sink and will need to ring, the primary side may turn on during this time and be sized according to the power dissipation requirements. cause cross conduction. If this problem does occur, one Figure 3 is an example layout of the LT8309. way to improve the reverse-recovery time of the primary side’s MOSFET is to add a parallel Schottky diode, which
Effects of the Body Diode
will conduct most of the current and turn off much faster When the discontinuous ring voltage goes below ground, than the body diode. Another way of eliminating this issue the body diode of the primary-side MOSFET turns on and is by sizing the turns ratio so that the discontinuous ring begins to conduct current. This diode clamps the voltage never goes below ground.
Figure 3. Demo Board Topside Silkscreen Figure 4. Demo Board Topside Metal
8309fa For more information www.linear.com/LT8309 9