DPA426 Power Integrations, DPA426 Datasheet - Page 18

• Manufacturer Part Number: DPA426
• Description: (DPA423 - DPA426) Highly Integrated DC-DC Converter ICs for Distributed Power Architectures
• Manufacturer: Power Integrations

Table 6. Flyback Output Power Table for 24 VDC Minimum Input
Notes: 1. Maximum output power is limited by device internal
current limit. 2. See text in this section for a complete description of
assumptions. 3. See Part Ordering Information. 4. Due to higher switching
losses, the DPA425 may not deliver additional power compared to a
smaller device.
2) Designs with lower output voltages and higher currents will
3) Reduced input voltage decreases the available output power
DPA-Switch Selection
Use Tables 1 and 3 through 6 to select the DPA-Switch based on
device dissipation. Selecting the optimum DPA-Switch depends
upon required maximum output power, efficiency, heat sinking
constraints and cost goals. With the option to externally reduce
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 50 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
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
18
Total Device
PRODUCT
Dissipation
DPA423-426
tend to increase the device dissipation listed in the power table.
for the same device dissipation. Tables 3 to 6 are the power
tables for 16 VDC and 24 VDC input voltages. Input voltages
below 16 V are possible, but since the internal start-up current
source is not specified at voltages below 16 V, an external chip
supply current should be fed into the CONTROL pin
approximately equal to but less than I
DPA423
DPA424
DPA425
DSS
specification. A Zener diode combined with a small value
K
1/04
Voltage.
3
OUTPUT POWER TABLE
24-48 VDC RANGE (FLYBACK)
8.5 W 11.5 W 14 W
0.5 W
7 W
-
4
0.75 W
-
-
4
1 W
-
-
4
CD1
.
1.5 W
2
-
-
-
Output
Power
8.5 W
17 W
34 W
Max
1
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
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.
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.
MAX
at turn-on. In addition, the primary current limit