AN-31
C7
1 nF
R14
10 Ω
1.5 kV
L1
C11
C12
1 µF
10 V
C10
100 µF 100 µF
10 V 10 V
+VIN
36-75 VDC
1 µH
2.5 A
L2
5 V, 6 A
C17
3300 pF
R1
619 kΩ
1%
R15
10 Ω
R17
R16
10 kΩ
D1
D2
BAV
Q1
10 Ω
19WS
Si4888
DY
RTN
T1
Q2
Si4888
DY
D4
BAV19WS
C4
4.7 µF
20 V
U2
R7
10 kΩ
R10
C1, C2 & C3
1 µF
10.0 kΩ
1%
100 V
DPA-Switch
C16
D3
U2
PC357N1T
100 nF
U1
D
L
BAV19WS
DPA425R
CONTROL
CONTROL
R6
150 Ω
R12
5.1 Ω
C
R9
220 Ω
S
X
F
C13
R4
C14
1 µF
10 µF
1.0 Ω
VR1
SMBJ
150
10 V
R3
C5
220 nF
U3
C6
68 µF
10 V
18.2 kΩ
R11
LM431AIM3
1%
10.0 kΩ
VIN
1%
PI-3472-040903
Figure 11. Example of DPA-Switch in a Single-Ended DC-DC Forward Converter with Synchronous Rectification.
The two modes have different control characteristics. The
converter in discontinuous conduction mode will usually have
a slower response to transients and higher ripple voltage at the
output than in continuous conduction mode. In extreme cases, a
converter that is well-behaved in continuous conduction mode
may actually become unstable at light load or with no load
unlesscorrectlydesigned.ManycommercialDC-DCconverter
modules specify a large minimum load to prevent operation in
discontinuous conduction mode.
ThemagnitudeofthegainatfrequenciesgreaterthanZ1isrelated
directly to the current transfer ratio (CTR) of the optocoupler.
Therefore, the CTR must be controlled to maintain a stable and
well-behaved system. Designers should choose an optocoupler
that has a CTR in the range of 100% to 200% at the maximum
CONTROL pin current of 12 mA. The phototransistor of the
optocoupler must also have a breakdown voltage greater than
the maximum bias voltage.
Figure 10 shows that this example has a desirable phase margin
of60degreesandacomfortablegainmarginof20dB.Sufficient
margin is required in the design of the feedback loop to allow
for tolerances in the CTR of the optocoupler, changes in ESR
of the output capacitor, and the change in gain with operating
voltage. The ESR can change significantly with temperature.
Thisshouldbeaprimaryconsiderationintheselectionofoutput
capacitors. The design must also allow for tolerance variations
in all other components.
A converter that operates deeply in discontinuous conduction
mode requires a very small duty ratio. Operation at very light
loads is not a problem for DPA-Switch because it automatically
reduces the effective switching frequency by skipping cycles
to give duty ratios less than about 5%.
Operation at small duty ratios requires a larger capacitor to
keep the bias voltage above its minimum required value of
8V. Inatrade-offwithsize, costandefficiency, thebestsolution
to a requirement to operate with no load is to include a small
preload in parallel with the output capacitors. The amount of
the load is determined empirically to supplement the natural
loading from the other small-signal circuits that get their power
from the output.
Operation at No Load
Those who design or specify DC-DC converters should
pay particular attention to requirements for minimum load.
The control characteristics are different for operation in the
continuous conduction mode (moderate to heavy loads) and
discontinuous conduction mode (light loads). The boundary
betweenthetwomodesoccursattheloadwhereK∆I =2(without
synchronous rectification).
Synchronous Rectification
The use of synchronous rectification can yield a substantial
increase in efficiency over passive Schottky rectifiers on the
C
14 7/04
POWERINT [ Power Integrations ]
