AN-31
Complexity
and Cost
Synchronous Rectifier Type
Efficiency
Comment
⇑
⇑
⇑
⇓
Winding Driven DC Coupled
Winding Driven AC Coupled
Active Drive
Check Gate Voltage at Power Down
Gate Voltage Controlled at Power Down
High Complexity
⇔
⇑⇑⇑
Table 4. Comparison of Synchronous Rectification Techniques.
winding.CapacitorC17shouldbechosentocapacitivelydivide
the winding voltage between C17 and the CGS of MOSFET
Q2, to provide a voltage on CGS that exceeds the Q2 threshold
voltage. The time constant of C17 and R16 should be about
10 µs for 300 kHz operation. R15 is typically about 10 Ω.
Primary Side Connections
The tab of DPA-Switch is the intended return connection for the
highswitchingcurrents.Therefore, thetabshouldbeconnected
by wide, low impedance traces to the input capacitor. The
SOURCE pin should not be used to return the power currents;
incorrect operation of the device may result. The SOURCE pin
is intended as a signal ground only. The device tab (SOURCE)
is the correct connection for power currents.
Figure 11 shows the DPA-Switch in a single-ended DC-DC
forward converter that uses winding driven AC coupled
synchronous rectification. In this example, the gate of Q1
has enough capacitance to eliminate the need for the discrete
capacitor CS in the transformer reset circuit. Although this
is often the case with synchronous rectifiers that are winding
driven, designers should follow the guidance in the section on
VerificationofTransformerResettoconfirmthatthetransformer
resets properly.
The bypass capacitor on the CONTROL pin should be located
as close as possible to the SOURCE and CONTROL pins. The
circuit trace of its connection to SOURCE should not contain
any switching current from the primary or bias voltages.
All SOURCE pin referenced components connected to the
LINE-SENSE (L) or EXTERNAL CURRENT LIMIT (X)
pins should also be located closely between their respective
pins and SOURCE. Once again, the SOURCE connection
trace of these components should not conduct any of the main
MOSFET switching currents. It is critical that tab (SOURCE)
power switching currents are returned to the negative terminal
of the input capacitor through a separate trace that is not shared
by the components connected to the SOURCE, CONTROL,
L or X pins.
Actively Driven Synchronous Rectifiers
The third category of synchronous rectifier circuits uses
independentactivecomponentsthatmayincludediscretedevices
and integrated circuits to lock onto the switching frequency of
the power supply and to drive the MOSFETs. This solution
relaxes the restriction on the range of input voltage because the
driver can regulate the gate voltage to be independent of the
voltage on the secondary winding. Circuits for actively driven
synchronous rectifiers are much more complex than the other
solutions, and are beyond the scope of this application note.
Table 4 gives a comparison of the techniques for synchronous
rectification.
Any traces to the Lor X pins should be kept as short as possible
and away from the drain trace to prevent noise coupling. Line-
sense resistor (R1 in Figure 11) should be located close to the L
pin to minimize the trace length on the L pin side. In addition
to the CONTROL pin capacitor (C5 in Figure 11), a 220 nF
high frequency bypass capacitor in parallel is recommended
as close as possible between SOURCE and CONTROL pins
for better noise immunity. The feedback optocoupler output
should also be located close to the CONTROL and SOURCE
pins of DPA-Switch.
In general, DPA-Switch with synchronous rectifiers should
operate at the lower switching frequency of 300 kHz. The
synchronous rectifier catch MOSFETs typically have gate-
sourcecapacitancevaluessuchthatthetransformerwouldhave
insufficient time to reset at 400 kHz. Connect the F pin to the
CONTROL pin to select the lower switching frequency.
Heat Sinking
Layout Considerations
To maximize heat sinking of the DPA-Switch and the other
power components, special thermally conductive PC board
material (aluminum clad PC board) is recommended. This
has an aluminum sheet bonded to the PC board during the
manufacturing process to provide heat sinking directly or to
allow the attachment of an external heat sink. If normal PC
Figure 13 shows an example of a proper circuit board layout for
a forward converter with DPA-Switch. Since the DPA-Switch
can operate with large drain current, designers should follow
these guidelines carefully.
C
16 7/04
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