AND8098/D
Low−Cost 100 mA
High−Voltage Buck and
Buck−Boost Using NCP1052
Prepared by: Kahou Wong
ON Semiconductor
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APPLICATION NOTE
INTRODUCTION
This application note presents low-cost high-voltage
100 mA non-isolated power supply using NCP1052 by
buck and buck-boost topology. The NCP1052 is one of the
latest low-cost switching controllers with integrated 700 V/
300 mA power switch from ON Semiconductor. It is
primarily designed for isolated 10 W-range flyback
converter. If isolation is not needed, the IC can also be used
as stepping-down buck and buck-boost converter for
further cost saving by removing optocoupler and replacing
the transformer by an inductor. The output current capability
is 100 mA. The possible operating range is from input range
between 20 Vdc and 700 Vdc to output range of 5.0 V or
above with 100 mA. Typical efficiency around 65% is
obtained in the 12 V buck demo board.
Advantages of the proposed circuits include:
•
Comparing to flyback, buck and buck-boost eliminates
optocoupler and replaces transformer by an inductor for
cost saving.
•
Buck and buck-boost offers smaller voltage stress in
switches comparing to flyback. It minimizes the
switching loss and increases efficiency.
•
NCP105x can power up itself from the high input
voltage with wide range between 20 Vdc and 700 Vdc.
It needs no extra supply circuit.
•
NCP105x operates at 44, 100, or 136 kHz and
accommodates low-cost components such as aluminum
electrolytic capacitors and powered-iron core magnetic.
•
NCP105x offers frequency jittering for reduced
electromagnetic inference (EMI).
•
NCP105x offers thermal and short circuit fault
protection.
•
Simple design as no control-loop compensation is
concerned.
The proposed buck and buck-boost converters are very
similar to each other. Their major difference is that buck
provides a positive output voltage but buck-boost provides
a negative output voltage referring to the input ground.
PRINCIPLE OF OPERATION
Figure 1 shows the proposed buck and buck-boost
converters. The rectifier circuit, which consists of capacitor
C
3
and diode D
3
, is in the front end for AC or DC input
voltage. Then, the NCP1052 is self-powered up from the
rectified input voltage directly with a V
CC
capacitor C
2
.
When the switch inside the IC is opened, there is a voltage
across Drain (D) and Source (S) pins of the IC. If this voltage
is greater than 20 V, an internal current source I
start
= 6.3 mA
(typ.) inside the IC charges up C
2
and a voltage in C
2
is built
up for the operation of the IC. Comparing to the switching
frequency, the V
CC
voltage level is in a lower-frequency
7.5-8.5 V hysteresis loop. This V
CC
hysteresis loop is for
frequency jittering features to minimize EMI and
short-circuit fault timing function.
D
2
Z
2
FB
D
V
CC
C
2
(a) Buck
D
2
Z
2
FB
D
V
CC
C
3
C
2
(b) Buck-boost
S
D
1
C
1
D
L
R
1
C
Z
1
Output
S
D
1
C
1
R
1
L
D
3
Input
C
3
D
C
Z
1
Output
D
3
Input
Figure 1. Proposed Circuit Using NCP1052
In Figure 2a it is noted that in the buck topology the input
voltage powers up the IC through the path across the
inductor L and capacitor C. This charging path passes
©
Semiconductor Components Industries, LLC, 2003
1
June, 2003 - Rev. 1
Publication Order Number:
AND8098/D
AND8098/D
through the output and a low-frequency ripple will be found
in the output voltage. Hence, the value of C
2
is needed to be
small enough to increase this charging frequency f
VCC
in
order to reduce output voltage ripple because some
efficiency is lost due to this low-frequency ripple.
D
2
Z
2
I
start
D
3
FB
D
V
CC
C
2
(a) Buck
D
2
Z
2
I
start
D
3
FB
D
V
CC
C
3
C
2
S
C
1
D
L
R
1
C
Z
1
Output
D
1
S
C
1
L
D
1
R
1
Input
C
3
D
C
Z
1
Output
Input
(b) Buck-boost
Figure 2. Charging Current of C
2
In Figure 2b it is noted that in the buck-boost topology the
charging current path is blocked by diode D and hence the
charging of C
2
does not affect the output voltage directly.
However, it still affects the output voltage indirectly and
slightly by adding some low-frequency noise on the
inductor. Hence, small value of C
2
is also wanted.
D
1
C
1
R
1
V
out
(a) Buck
D
1
C
1
The function of diode D
1
, capacitor C
1
and resistor R
1
are
to transfer the magnitude of output voltage to a voltage
across C
1
so that the IC can regulate the output voltage. In
Figure 3, when the main switch inside the IC is opened and
the diode D is closed. In buck, the potential of the IC
reference ground (pin S) becomes almost 0 V in this
moment. In buck-boost, the potential of the IC reference
ground (pin S) becomes -V
out
in this moment. The voltage
in C
1
will be charged to the output voltage. On the other
hand, when main switch is closed and the diode D is opened,
diode D
1
is reverse biased by a voltage with magnitude V
in
and V
in
+V
out
respectively. Hence, D
1
does not affect the
normal operation of the buck and buck-boost converter.
It is noted that the instantaneous voltage in C
1
can be
possibly greater than the output voltage especially when
output current or output ripple is too large. It directly affects
the load regulation of the circuit since the IC regulates the
output voltage based on the voltage in C
1
. In order to solve
it, larger values of L and R
1
can help to slow down the
charging speed of C
1
. It reduces the maximum instantaneous
voltage in C
1
so that output voltage at high output current
can be pulled up and a good regulation is made.
Larger value of L can help the load regulation but it
usually unwanted because it is bulky. Hence, resistor R
1
is
recommended. Larger value of R
1
makes higher output
voltage. Hence, it is called as a “pull-up resistor” and it can
help to pull up the output voltage slightly.
The voltage in C
1
representing the output voltage is
feedback to the feedback (FB) pin of the NCP1052 through
a diode D
2
and zener diode Z
2
. When output voltage is too
high, there will be a greater-than-50
mA
current inserting
into the feedback pin of the NCP1052. The NCP1052 will
stop switching when it happens. When output voltage is not
high enough, the current inserting into the feedback is
smaller than 50
mA.
The NCP1052 enables switching and
power is delivered to the output until the output voltage is
too high again.
The purpose of the diode D
2
is to ensure the current is
inserting into the feedback pin because the switching of
NCP1052 can also be stopped when there is a
greater-than-50
mA
current sinking from the FB pin. The
purpose of the zener diode Z
2
is to set the output voltage
threshold. The FB pin of NCP1052 with a condition of
50
mA
sourcing current is about 4.3 V. The volt-drop of the
diode D
2
is loosely about 0.7 V at 50
mA.
Hence, the output
voltage can be loosely set as follows:
Vout
+
zener
)
4.3 V
)
0.7 V
+
zener
)
5 V
(eq. 1)
R
1
V
out
(b) Buck-boost
Figure 3. Output Voltage Couples to C
1
with a
Charging Current
According to (1), the possible minimum output voltage of
the circuit is 5.0 V when there is no zener diode Z
2
.
If there is no load, the IC will automatically minimize its
duty cycle to the minimum value but the output voltage is
still possible to be very high because there is no passive
component in the circuit try to absorb the energy. As a result,
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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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AND8098/D
In non-isolated topologies such as buck or buck-boost,
the circuits are mainly designed for CCM. The CCM
burst-mode waveform is different to the PWM waveform in
Figure 5. Because of this characteristic, burst mode requires
a higher peak value of the inductor current in order to have
the same level of averaged inductor current (or output
current).
V
out
V
CC
FB current
time
Output waveforms with big enough V
CC
capacitor
Burst mode
Desired level of V
out
V
CC
PWM
V
out
time
Output waveforms with too small V
CC
capacitor
Figure 5. CCM Inductor Currents in Burst Mode
and traditional PWM Control
As shown in Figure 4 and 5 burst-mode control produces
low-frequency waveform comparing to the switching
frequency. Part of the power loss in this low frequency
becomes audible noise. Therefore, burst-mode control is
not suitable for high power applications such as more than
20 W.
V
CC
Capacitor
Figure 6. Startup Scenarios of the Circuits with
Big Enough or Too Small V
CC
Capacitor
Practically, the NCP1052 consumes approximately 0.5
mA in normal operation. The concerned fault sampling time
for feedback signal is from 8.5 V to 7.5V. Hence,
-3
C
+
I dt
+
0.5 10
· sampling time
1
dV
+
0.5 10- 3 · sampling time
(eq. 4)
The V
CC
capacitor C
2
is the key component to make the
circuit operate in normal mode or fault mode. The device
recognizes a fault condition when there is no feedback
current in the FB pin during the time from V
CC
= 8.5 V to
7.5 V. The V
CC
capacitor directly affects this time duration.
In normal mode, the V
CC
follows a 8.5 V-7.5 V-8.5 V
hysteresis loop. When the circuit is in fault mode, the V
CC
follows a 8.5 V-7.5 V-4.5 V-8.5 V hysteresis loop. The
device keeps its MOSFET opened except for the time from
V
CC
= 8.5 V to 7.5 V and delivers a little amount of power
to the output in fault mode.
A common and extreme case to enter fault condition is the
startup. The MOSFET begins switching at the V
CC
is firstly
charged to 8.5 V and hence output voltage rises. The output
voltage needs some time to build up the output voltage from
0 V to a desired value. When the desired level is reached, a
feedback current flows into the device to stop its switching.
If the feedback current is determined before V
CC
reaches
7.5V, the circuit will remain in normal mode. Otherwise, the
circuit will enter the fault mode and cannot provide the
output voltage at its desired level. Therefore, the V
CC
capacitor is needed to be big enough to ensure sufficient time
for V
CC
going from 8.5 V to 7.5 V to sample feedback
current in startup.
For example, if sampling time or startup transient is
designed to be 20 ms, 10
µF
V
CC
capacitor is needed.
Inductor
The 300 mA current limit in the NCP1052 is measured
with a condition that the di/dt reaches 300 mA in 4
µs.
When
the buck or buck-boost circuit is designed for universal ac
input voltage (85 to 265 Vac), the rectified input voltage will
be possibly as high as 375 Vdc. In order to keep the 4
µs
condition, the inductance value will be 5 mH by (5) and (6).
For buck,
di
+
Vin
*
Vout
[
Vin
dt
L
L
(eq. 5)
For buck-boost,
di
+
Vin
dt
L
(eq. 6)
The 5 mH is practically too high and hence not very
practical. Therefore, the inductor is basically selected by
market available inductor models which is with a normally
smaller inductance (but not too small). It must have enough
saturation current level (>300 mA). If inductance is too
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AND8098/D
small, the di/dt becomes too high and the NCP1052 will
have a very high current limit effectively because there is a
propagation delay (typically 135 ns) to turn off the switch.
The current flowing through the inductor L includes three
parts. First, there is a V
CC
charging current I
start
in Figure 2.
It happens when V
CC
needs charging. Its magnitude is 6.3
mA. It is noted that the V
CC
discharging current does not
flow through the inductor. Second, it is the main inductor
current to deliver the output current. It is noted that the peak
of burst-mode inductor current is higher than PWM one as
in Figure 5 for the same level of averaged inductor current
(or output current). Finally, there is a current flowing
through diode D
1
to charge up C
1
. It also flows through the
inductor as shown in Figure 3. Its magnitude is a
greater-than-50
µA
current and practically it is about 1 mA.
Hence, the saturation current of the inductor L is needed to
be bigger than their sum.
Another consideration on the inductor is the low-pass
filtering capability for the V
CC
hysteresis low frequency
(and the 50/ 60 Hz rectified AC line voltage ripple). As
shown in Figure 2, there is a low-frequency charging current
with magnitude 6.3 mA flowing through the inductor and
causes low-frequency ripple in the output voltage. A higher
value of the inductance can help to reduce the output ripple.
It is noted that when the output power is higher, the startup
time becomes longer. It needs bigger V
CC
capacitor and
makes lower V
CC
charging frequency. As a result, a bigger
inductance is needed.
The last consideration is the effect of load regulation.
Large inductor can limit the inrush current flowing into
capacitor C
1
as shown in Figure 3. High inrush current is not
desirable because it can make the C
1
voltage higher than the
output voltage. It makes load regulation poor. If there is no
pull-up resistor R
1
, inductor value L is chosen to be as large
as possible, say 2 mH.
Output Capacitor
Buffering Capacitor
Buffering capacitor C
2
is to provide a greater-than-50
µA
to the feedback pin of NCP1052. It is relatively much
smaller than the output capacitor because the current
consumption in this capacitor is much smaller and the output
voltage cannot copy to this buffering capacitor if the
buffering capacitor voltage is higher than the output voltage.
Diodes
D and D
1
are recommended to be the same part for
compatibility in speed and voltage drop. It helps the voltage
in the capacitor C
1
to be similar to the output voltage. The
reverse blocking voltage of D and D
1
is needed to be large
enough to withstand the input voltage in buck and input
voltage plus output voltage in buck-boost respectively.
D
2
is not a critical component. Its function is to make sure
that feedback current is only in one direction. The accuracy
of its voltage drop used in (1) is not important since the 4.3V
reference voltage in the NCP1052 is loosely set.
Zener Diodes
Z
1
is to clamp the output voltage when there is light load
or no load. Hence, the accuracy of Z
1
helps the regulation
accuracy in the light load or no load condition. It is also the
main component to consume energy when the circuit is in no
load condition. The output voltage is clamped and hence the
output capacitor is protected.
Z
2
and R
1
are to set the output voltage at the nominal load
current. Hence, their accuracy affects the regulation
accuracy at the nominal load condition. The relationship
between zener voltage and output voltage is shown in (1).
Higher value of R
1
helps to pull up the output voltage higher
by reducing the charging rate of the buffering capacitor C
1
.
Standby Condition
Because of the burst-mode characteristic and the
low-frequency V
CC
charging current, the output ripple is
larger than those in PWM. Hence, a relatively bigger output
capacitor is needed to keep output ripple small. However,
big output capacitor needs a long time to build up the output
voltage initially and hence the circuit may enter into fault
mode in the startup in Figure 6.
The standby ability of the proposed buck converter is not
good. It is because there is a V
CC
charging current I
start
flows
through the output capacitor in Figure 2(a). This charging
current is a low-frequency pulsating signal. As a result, the
voltage in the output capacitor continuously rises up by the
charging current pulses. In order to prevent over voltage in
the output capacitor, the zener Z
1
absorbs the charging
current. It consumes main portion of energy in standby.
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