DPA422-426
both sides of the board and using thicker copper will improve
heat sinking.
3. Maximum drain current – At maximum ambient temperature,
maximum input voltage and maximum output load, verify
drain current waveforms at start-up for any signs of trans-
former 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.
4. Thermal check – At maximum output power, minimum 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 (R package),
SOURCE pins at or below 100 °C (P/G packages) 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 RDS(ON) variation. When
monitoring device temperatures, note that the junction-to-
case thermal resistance should be accounted for when
estimating die temperature.
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 VDS does not
exceed minimum BVDSS at highest input voltage and maxi-
mum 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).
Design Tools
Up-to-date information on design tools is available at the Power
Integrations website: www.powerint.com.
2. Transformer reset margin – Drain voltage should also be
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.
19
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