DPA424 Power Integrations, Inc., DPA424 Datasheet - Page 17

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

current limit, a larger DPA-Switch may be used for lower power
applications where higher efficiency is needed or minimal heat
sinking is available. Generally, selecting the next larger device,
than is required for power delivery will give the highest
efficiency. Selecting even larger devices may give little or no
improvement in efficiency due to the improvement in conduction
losses being negated by larger device switching losses.
Figure 48 provides information on switching losses. This together
with conduction loss calculations give an estimate of device
dissipation.
Primary Clamp
A primary clamp network is recommended to keep the peak
DRAIN voltage due to primary leakage inductance to below the
BV
value capacitor connected across the primary winding is a low
cost and low part count implementation.
Output Rectification
Rectification of the secondary is typically performed using
Schottky diodes or synchronous rectification. Schottky diodes
are selected for peak inverse voltage, output current, forward
drop and thermal conditions. Synchronous rectification requires
the additional complication of providing gate drive. The
specified line under-voltage and line overvoltage thresholds of
DPA-Switch simplifies deriving gate drive directly from the
transformer secondary winding for many applications. The
turns ratio of the transformer together with the under/over-
voltage thresholds defines the minimum and maximum gate
voltages, removing the need for Zeners to clamp the gate
voltage.
Soft-Start
Generally a power supply experiences maximum stress at start-
up before the feedback loop achieves regulation. For a period
of 5 ms the on-chip soft-start linearly increases the duty cycle
from zero to the default DC
primary current limit increases from 85% to 100% over the
same period. This causes the output voltage to rise in an orderly
manner allowing time for the feedback loop to take control of
the duty cycle. This integrated soft-start reduces the stress on
the DPA-Switch MOSFET, clamp circuit and output diode(s),
and helps prevent transformer saturation during start-up. Also,
soft-start limits the amount of output voltage overshoot, and in
many applications eliminates the need for a soft-finish capacitor.
If necessary, to remove output overshoot, a soft-finish capacitor
may be added to the secondary reference.
Switching Frequency
The FREQUENCY pin of DPA-Switch offers a switching
frequency option of 400 kHz or 300 kHz. Operating at 300 kHz
will increase the amount of magnetization energy stored in the
transformer. This is ideal for applications using synchronous
rectification driven directly from the transformer secondary
where this energy can be used to drive the catch MOSFET gate.
DSS
specification. A Zener diode combined with a small
MAX
at turn-on. In addition, the
Transformer Design
It is recommended that the forward converter transformer be
designed for maximum operating flux swing of 1500 Gauss and
a peak flux density of 3500 Gauss. When operating at the
maximum current limit of the selected DPA-Switch (during
overload conditions), neither magnetic component (transformer
and output inductor) should be allowed to saturate. When a
larger device than necessary has been selected, reducing the
internal current limit close to the operating peak current limits
overload power and minimizes the size of the secondary
components.
No-load and Standby Consumption
Cycle skipping operation at light or no load can significantly
reduce power loss. In addition this operating mode ensures that
the output maintains regulation even without an external
minimum load. However, if cycle skipping is undesirable in a
particular application, it can be avoided by adding sufficient
pre-load.
DPA-Switch Layout Considerations
The DPA-Switch can operate with large DRAIN current, the
following guidelines should be carefully followed.
Primary Side Connections
The tab of DPA-Switch is the intended return path for the high
switching currents. Therefore, the tab should be connected by
wide, low impedance traces back to the input decoupling
capacitor. The SOURCE pin should not be used to return the
power currents; incorrect operation of the device may result.
The SOURCE is only intended as a signal ground. The device
tab (SOURCE) is the correct connection for power currents.
The CONTROL pin bypass capacitor should be located as
close as possible to the SOURCE and CONTROL pins and its
SOURCE connection trace should not be shared by the main
MOSFET switching currents. All SOURCE pin referenced
components connected to the LINE-SENSE or EXTERNAL
CURRENT LIMIT pins should also be located closely between
their respective pin and SOURCE. Once again, the SOURCE
connection trace of these components should not be shared by
the main MOSFET switching currents. It is critical that
the tab (SOURCE) power switching currents are returned to the
input capacitor through a separate trace that is not
shared by the components connected to CONTROL,
LINE-SENSE or EXTERNAL CURRENT LIMIT pins.
Any traces to the L or 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 25) 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 (C6 in Figure 25),
a high frequency bypass capacitor in parallel is recommended
DPA423-426
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