AN-31
Solder Side
Component Side
TOP VIEW
Output
Diode
V
Transformer
+
D
S
Inductor
(Coupled)
V
DPA-Switch
X
L
C
V
+
DC
Out
-
Opto-
coupler
-
V
Maximize hatched copper area for optimum heat sinking
Via between board layers
V
PI-2883-060602
Figure 13. Layout Considerations for DPA-Switch Using R Package.
board material is used (such as FR4), providing copper areas on
both sides of the board and using thicker copper will improve
heat sinking.
1. Maximum drain voltage – Verify that peak drain-to-source
voltage does not exceed minimum BVDSS at highest input
voltage and maximum overload output power. It is normal,
however, to have additional margin of approximately 25 V
below BVDSS to allow for other power supply component
unit-to-unit variations. Maximum overload output power
occurs when the output is loaded to a level just before the
power supply goes into auto-restart (loss of regulation).
If an aluminum clad board is used, then shielding of switching
nodesisrecommended.Thisconsistsofanareaofcopperplaced
directly underneath switching nodes such as the drain node
and output diode to provide an electrostatic shield to prevent
coupling to the aluminum substrate. These areas are connected
to input negative in the case of the primary and output return
for secondary. This reduces the amount of capacitive coupling
into the insulated aluminum substrate that can then appear on
the output as ripple and high frequency noise.
2. Transformer reset margin – Drain voltage should also be
checked at highest input voltage with a severe load step
(50% to 100%) to verify adequate transformer reset margin.
This test slews the duty cycle at high input voltage, placing
the most demand on the transformer reset circuit.
Quick Design Checklist
3. Maximumdraincurrent–Atmaximumambienttemperature,
maximum input voltage and maximum output load, verify
drain current waveforms at start-up for any signs of
transformer or output inductor saturation and excessive
leading edge current spikes. DPA-Switch has a leading edge
blanking time of 100 ns to prevent premature termination
of the cycle. Verify that the leading edge current spike does
not extend beyond the blanking period.
As with any power supply design, all DPA-Switch designs
should be verified on the bench to make sure that component
specifications limits are not exceeded under worst case
conditions.ThefollowingminimumsetoftestsforDPA-Switch
forward converters is strongly recommended:
C
7/04
17
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