AN-31
Ifpowerdissipationisaproblemwiththesmallestdevice,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.
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
CCP across the primary of the transformer absorbs energy from
leakage inductance to keep the drain-to-source voltage below
the Zener voltage. There is an optimum value for CCP that
typically ranges between 10 pF and 100 pF for converters in
the range of 10 W to 40 W.
Figure 3 illustrates how the efficiency is related to the selection
of the DPA-Switch. Devices with lower RDS(ON) dissipate less
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 RDS(ON) has virtually no effect on efficiency.
The value of CCP depends on the leakage inductance and the
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.
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 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 (CCP), becomes a
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.
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
Transformer Reset Circuit
The flux in the magnetizing inductance of the transformer
must be reset in each switching cycle to maintain volt-seconds
CS RS
DC INPUT
+
+
CCP
D2
C1
DPA-Switch
DC
U1
D
Input
L1
VDS
Voltage
CONTROL
D
S
C
VR1
CONTROL
V1
S
DPA-Switch
D1
DC INPUT
PI-3474-032603
PI-2875-062204
Figure 4. Components of the Transformer Clamp and Reset Circuit.
Figure 4. LC (Inductor Capacitor) Reset and Clamp.
C
7/04
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