DPA424 Power Integrations, Inc., DPA424 Datasheet - Page 18

• Manufacturer Part Number: DPA424
• Description: Highly Integrated DC-DC Converter Ics For Distributed Power Architectures
• Manufacturer: Power Integrations, Inc.

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.
Heat Sinking
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 and
allow the attachment of an external heat sink. If normal PC
board material is used (such as FR4), providing copper areas on
both sides of the board and using thicker copper will improve
heat sinking.
If an aluminum clad board is used then shielding of switching
nodes is recommended. This consists of an area of copper
placed 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.
Quick Design Checklist
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. The following minimum set of tests for
DPA-Switch forward converters is strongly recommended:
1. Maximum drain voltage – Verify that peak V
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DPA423-426
exceed minimum BV
maximum overload output power. It is normal, however, to
have additional margin of approximately 25 V below BV
to allow for other power supply component unit-to-unit
J
5/03
DSS
at highest input voltage and
DS
does not
DSS
2. Transformer reset margin – Drain voltage should also be
3. Maximum drain current – At maximum ambient temperature,
4. Thermal check – At maximum output power, minimum
Design Tools
Up-to-date information on design tools is available at the Power
Integrations Web site: www.powerint.com.
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).
checked at highest input voltage with a severe load step (50-
100%) to verify adequate transformer reset margin. This test
shows the duty cycle at high input voltage, placing the most
demand on the transformer reset circuit.
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 on cycle. Verify that the leading edge current spike does
not extend beyond the blanking period.
input voltage and maximum ambient temperature, verify
that temperature specifications are not exceeded for the
transformer, output diodes, output choke(s) and output
capacitors. The DPA-Switch is fully protected against over-
temperature conditions by its thermal shutdown feature. It is
recommended that sufficient heat sinking is provided to
keep the tab temperature at or below 115 °C under worst
case continuous load conditions (at low input voltage,
maximum ambient and full load). This provides adequate
margin to minimum thermal shutdown temperature
(130 °C) to account for part-to-part R
monitoring tab temperature, note that the junction-to-case
thermal resistance should be accounted for when estimating
die temperature.
DS(ON)
variation. When