AN-31
C7
1 nF
1.5 kV
C3, C4,
C5, C6
+VIN
36-72 VDC
1 µF,
100 V (x4)
T1
1
L1
J1-1
C13, C14,
C15, C16, C17
100 µF,
1 µH
L3
3.3 µH
20 A
L4
100 nH
20 A
C12
4.7 nF
50 V
C18
100 µF
10 V
C20
100 µF 1 µF
10 V 10 V
C19
2.5 A
D4
42CT030S
10 V (x5)
5 V, 14 A
J2-2
9, 10
R1
619 kΩ
D1
UF4003
2
D5
42CT030S
R5
6.8 Ω
R6
6.8 Ω
J1-1
RTN
3
6, 7
C8
470 pF
200 V
D3
BAV19
WS
C9
4.7 µF
25 V
C1
1 µF
100 V
C2
1 µF
100 V
U2
L2
100 µH
U2
PC357N3T
R8
10 kΩ
R13
DPA-Switch
10.0 kΩ
D
S
L
1%
U1
DPA426R
CONTROL
CONTROL
C23
C
D5
BAV19WS
68 nF
R11
5.1 Ω
R10
220 Ω
X
F
R4
R9
220 Ω
1.0 Ω
VR1
SMBJ
150
C10
220 nF
C21
10 µF
10 V
C22
1 µF
C11
68 µF
10 V
R3
D2
U3
6.8 kΩ
UF4003
R14
10.0 kΩ
1%
J1-2
LM431AIM3
1 %
INPUT RTN
PI-2882-062204
Figure 6. A 70 W DC-DC Converter that uses an Alternative Circuit to Reset the Transformer.
conditions. The resistor in the reset network damps oscillations
from the interaction of the capacitor with parasitic inductance.
The value of the resistor is typically between 1 Ω and 5 Ω.
balance and prevent saturation. Since real transformers have
finite inductance, they store parasitic energy that is represented
as a magnetizing current.
Adifferent reset circuit is required for applications higher than
about 40 W. Figure 6 shows an example of a 70 W converter
that uses the circuit of Figure 5 to reset the transformer and to
limit the voltage on the DPA-Switch.
The magnetizing inductance cannot store very much energy
before it saturates. Since a saturated transformer behaves like
a short circuit, external circuitry must manage the removal
of the energy from the magnetization inductance (reset the
transformer) on each switching cycle.
Verification of Transformer Reset
This transformer reset will require the voltage on the DRAIN
pin to rise above the input voltage. The designer needs to be
sure that the transformer reset does not cause voltage overstress
on the DRAIN pin of the DPA-Switch.
Users should confirm that the transformer resets under worst
case conditions at the lowest and highest input voltages with
measurementsonthebench. Figure7illustratesthreesituations
that show proper transformer reset with the reset circuit in
Figure 4. Three examples of improper transformer reset are
shown in Figure 8.
Figure 4 shows the components for the circuit that resets the
magnetizing energy in the transformer to a safe value at the
end of each switching cycle. The heart of the circuit is the
series RC network (RS and CS) that is connected across the
output rectifier.
The best way to assess the reset characteristics is to observe the
drain-to-source voltage on the DPA-Switch. Figure 7 (a) shows
the voltage on the prototype example when it operates from an
inputof72VDC.Itisoperatingatfullloadwitharesetcapacitor
(CS) of 2.2 nF across the output rectifier. The clamp capacitor
on the primary is 47 pF. See Design Idea DI-24 (available on
www.powerint.com) for a circuit example.
When the DPA-Switch turns off, current in the magnetizing
inductance leaves the transformer through the secondary
winding. The capacitor charges as the magnetizing current
reduces to zero. The capacitor must be small enough to allow
the magnetizing current to go to zero within the minimum
offtime. An additional restriction on the size of the capacitor is
that it must be large enough to keep the drain-to-source voltage
below the voltage of the Zener clamp under normal operating
The figure shows the important intervals of the waveform
within one switching period TS. DPA-Switch is conducting
during the time tON = D TS, where D is the duty ratio. Flux in
C
8
7/04
POWERINT [ Power Integrations ]
