AND8098/D
output voltage will rise up dramatically and burn the output
capacitor eventually. Hence, a zener diode Z
1
or minimum
“dummy” load resistor is needed to consume the minimum
amount of energy as shown in Figure 1. It is also noted that
when R
1
pulls up the output voltage at a given output current
condition, the output voltages at lower output current
conditions are also pulled up. Hence, the clamping zener
diode Z
1
is needed to be with the breakdown voltage as same
as the output voltage but it will reduce some of the efficiency
at lower output current conditions.
DESIGN CONSIDERATION
Topology
Because of burst-mode control, the effective maximum
duty is lower and said to be 70% roughly. When a buck
converter is in continuous conduction mode (CCM), the
input voltage V
in
and output voltage V
out
are related by the
duty ratio D.
Vout
+
D
t
0.7
Vin
(eq. 2)
The relationship in buck-boost is
Vout
+
D
t
0.7
+
2.33
1
*
0.7
Vin
1
*
D
(eq. 3)
Buck circuit is to step down a voltage. Buck-boost circuit
is to step up or down a voltage. The output voltage is
inverted. The maximum duty of NCP1052 is typically 77%.
Table 1. Summary of Topology Difference Using NCP1052
Buck
Output voltage
Output current
< 0.7 V
in
< 300 mA
Another aspect on topology is the output current. The
maximum output current is always smaller than the
maximum switch current in non-isolated topologies.
However, in isolated topologies such as flyback the
maximum output current can be increased by a transformer.
Buck-boost
Negative & < 2.33 Vin
<< 300 mA, output current is
only a portion of the inductor
current
t
700
*
Vout V
t
700 V
Continuous
Good. The current passes
through inductor only
It is only for standby
improvement or additional
output
No
Flyback
Depending on transformer ratio
< 10 W. It depends on operating
condition and audible noise level
<< 700 V. It depends on
transformer ratio
Discontinuous
Good. The current passes
through primary winding only
It is a must for the main output.
Additional auxiliary winding can
improve standby performance
Yes. Opto coupler can be
eliminated if isolation is not
needed
Input voltage
Operating mode in nominal
condition
Standby ability on V
CC
charging
current
Transformer / Auxiliary winding
< 700 V
Continuous
Bad. The current flows through
output even if there is no load
It is only for standby
improvement or additional
output
No
Isolation
Burst-mode Operation
The NCP1052 is with a burst-mode control method. It
means the MOSFET can be completely off for one or more
switching cycles. The output voltage is regulated by the
overall duration of dead time or non-dead time over a
number of switching cycles. This feature offers advantages
on saving energy in standby condition since it can reduce the
effective duty cycle dramatically. In flyback topology, the
circuit is mainly designed for discontinuous conduction
mode (DCM) in which the inductor current reaches zero in
every switching cycle. The DCM burst-mode waveform can
be represented in Figure 4. It is similar to the pulse-width
modulation (PWM) one.
Burst mode
PWM
Figure 4. DCM Inductor Currents in Burst Mode
and PWM Control
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