DPA426SN Power Integrations, DPA426SN Datasheet - Page 7

• Manufacturer Part Number: DPA426SN
• Description: IC CONV DC-DC DPA SWITCH SPAK
• Manufacturer: Power Integrations
• Series: DPA-Switch®r

Specifications of DPA426SN

• Applications: Converter, Power Over Ethernet and Telecom Applications
• Voltage - Input: 16 ~ 75 V
• Number Of Outputs: 1
• Voltage - Output: 220V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: SPak (5 leads + Tab)
• Mounting Style: SMD/SMT
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

If power dissipation is a problem with the smallest device, select
the next larger device and program the current limit with the
X pin to 10% above the peak primary current. This is done to
limit overload power capability. Refer to the DPA-Switch data
sheet to determine the value of the resistor on the X pin that
corresponds to the desired current limit.
Figure 3 illustrates how the efficiency is related to the selection
of the DPA-Switch. Devices with lower R
power where resistive voltage drop dominates the loss. Thus,
the efficiency is higher for larger devices at low input voltage.
At higher input voltages the RMS current in the DPA-Switch
decreases and the loss from capacitance on the drain increases,
so the lower R
Clamp Circuit
All applications must protect the DPA-Switch from excessive
drain voltage. Figure 1 shows a simple and effective solution.
A Zener diode from the drain to source provides a hard clamp.
The 30 W prototype example (Table 1), uses a 150 V Zener to
guarantee substantial margin from the breakdown voltage of
220 V. A small capacitor across the primary of the transformer
may be necessary in conjunction with the Zener clamp (see
Figure 4).
The designer should put a placeholder for this capacitor on
the initial prototype. In some designs there is sufficient stray
capacitance on the primary of the transformer to remove the
Figure 4. Components of the Transformer Clamp and Reset Circuit.
DC INPUT
DC INPUT
+
VR1
C
CP
DS(ON)
D
S
has virtually no effect on efficiency.
CONTROL
CONTROL
DPA-Switch
U1
C
S
C
R
S
DS(ON)
dissipate less
PI-2875-062204
need for this clamp capacitor. Bench tests will determine
whether the capacitor is required to maintain safe drain-to-
source voltages. In normal steady-state operation, the capacitor
C
leakage inductance to keep the drain-to-source voltage below
the Zener voltage. There is an optimum value for C
typically ranges between 10 pF and 100 pF for converters in
the range of 10 W to 40 W.
The value of C
peak current. The proper value of capacitance will allow most
of the energy in the leakage inductance to be recovered during
the next switching cycle. Too little capacitance will cause the
Zener diode to conduct. Dissipation in the Zener will reduce
efficiency. Too much capacitance will also reduce efficiency
because it will increase turn-on losses in the DPA-Switch and
may also interfere with the reset of the transformer.
The Zener diode does not conduct during normal steady-state
operation, but it is required to limit the drain voltage during
start-up, transient loading and overload conditions.
At higher powers, the clamp capacitor value (C
limiting factor on the efficiency of the power supply. Different
techniques can be used for these higher power applications
(above approximately 40 W). Figures 5 and 6 show a non-
dissipative clamp technique that also resets the transformer.
See references [4] and [5] for a description of this technique.
Transformer Reset Circuit
The flux in the magnetizing inductance of the transformer
must be reset in each switching cycle to maintain volt-seconds
Figure 4. LC (Inductor Capacitor) Reset and Clamp.
CP
across the primary of the transformer absorbs energy from
Voltage
Input
DC
+
D2
D1
CP
L1
depends on the leakage inductance and the
C1
V
1
D
S
CONTROL
CONTROL
DPA-Switch
V
DS
AN-31
CP
), becomes a
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7/04
C
CP
that
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