Self-Supplied Monolithic
Switcher for Low StandbyPower Offline SMPS
The NCP1015 integrates a fixed−frequency current−mode
controller and a 700 V voltage MOSFET. Housed in a PDIP−7 or
SOT−223 package, the NCP1015 offers everything needed to build a
rugged and low−cost power supply, including soft−start, frequency
jittering, short−circuit protection, skip−cycle, a maximum peak
current set−point and a Dynamic Self−Supply (no need for an auxiliary
winding).
Unlike other monolithic solutions, the NCP1015 is quiet by nature:
during nominal load operation, the part switches at one of the available
frequencies (65−100 kHz). When the current set−point falls below a
given value, e.g. the output power demand diminishes, the IC
automatically enters the so−called skip cycle mode and provides
excellent efficiency at light loads. Because this occurs at typically 0.25
of the maximum peak value, no acoustic noise takes place. As a result,
standby power is reduced to the minimum without acoustic noise
generation.
Short−circuit detection takes place when the feedback signal fades
away e.g. un−true short−circuit or is broken optocoupler cases. Finally
soft−start and frequency jittering further ease the designer task to
quickly develop low−cost and robust offline power supplies.
For improved standby performance, the connection of an auxiliary
winding stops the DSS operation and helps to consume less than
100 mW at high line.
Features
• Built−in 700 V MOSFET with typical R
• Large Creepage Distance between High−voltage Pins
• Current−mode Fixed Frequency Operation: 65 kHz − 100 kHz
• Skip−cycle Operation at Low Peak Currents Only: No Acoustic Noise!
• Dynamic Self−Supply, No Need for an Auxiliary Winding
Powers the Internal CircuitryThis pin is connected to an external capacitor of typically
10 mF. The natural ripple superimposed on the V
participates to the frequency jittering. For improved
standby performance, an auxiliary VCC can be connected
to Pin 1. The VCC also includes an active shunt which
serves as an opto fail−safe protection.
setpoint is adjusted accordingly to the output power
demand.
Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the
Recommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect
device reliability.
Power Supply voltage on all pins, except pin 5 (drain)−0.3 to 10V
Drain peak current during transformer saturation1A
Maximum current into pin 115mA
Thermal Characteristics
°C/W
P Suffix, Case 626A
Junction−to−Lead
9.0
Junction−to−Air, 2.0 oz (70 mm) Printed Circuit Copper Clad
0.36 Sq. Inch (2.32 Sq. Cm)
1.0 Sq. Inch (6.45 Sq. Cm)
77
60
ST Suffix, Plastic Package Case 318E
Junction−to−Lead
14
Junction−to−Air, 2.0 oz (70 mm) Printed Circuit Copper Clad
0.36 Sq. Inch (2.32 Sq. Cm)
1.0 Sq. Inch (6.45 Sq. Cm)
74
55
Maximum Junction Temperature150°C
Storage Temperature Range−60 to +150°C
ESD Capability, Human Body Model (HBM) (All pins except HV)2kV
ESD Capability, Machine Model (MM)200V
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NCP1015
ELECTRICAL CHARACTERISTICS (For typical values T
Symbol
RatingPinMinTypMaxUnit
=25°C, for min/max values TJ=−40°Cto125°C, VCC=8V unless otherwise noted)
J
SUPPLY SECTION AND VCC MANAGEMENT
V
CC(off)
V
CC(on)
V
CCLATCH
DV
VCC increasing level at which the current source turns−off17.98.59.1V
VCC decreasing level at which the current source turns−on16.97.58.1V
Decreasing level at which the Latch−off phase Ends14.44.75.1V
Hysteresis between V
CC
CC(off)
1−1.0−
ICC1Internal IC consumption, MOSFET switching at 65 kHz1−0.921.1mA
ICC1Internal IC consumption, MOSFET switching at 100 kHz1−0.951.15mA
V
clamp
Active zener voltage positive offset to V
CC(off)
1140200300mV
POWER SWITCH CIRCUIT
R
DS(on)
V
I
DS(off)
t
t
dsb
on
off
Power Switch Circuit on−state resistance (Id = 50 mA)
T
= 25°C
J
T
= 125°C
J
Power Switch Circuit & Startup breakdown voltage
(I
= 100 mA, TJ = 25°C)
DS(off)
Power Switch & Startup breakdown voltage off−state leakage current
T
= −40°C (Vds = 650 V)
J
T
= 25°C (Vds = 700 V)
J
T
= 125°C (Vds = 700 V)
J
Switching characteristics (RL = 50 W, Vds set for Ids = 0.7 x Ids
lim
)
Turn−on time (90% − 10%)
Turn−off time (10% − 90%)
Frequency dithering compared to switching frequency (with active DSS)−±3.3−%
Maximum Duty−cycle626772%
FEEDBACK SECTION
R
up
t
ss
Internal pull−up resistor4−18−
Internal soft−start (guaranteed by design)−−1.0−ms
SKIP CYCLE GENERATION
V
skip
Default skip mode level on FB pin40.5V
TEMPERATURE MANAGEMENT
TSD
Temperature shutdown150°C
Hysteresis in shutdown50°C
2. See characterization curves for temperature evolution
W
mA
ns
mA
kW
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NCP1015
TYPICAL CHARACTERISTICS
−2
−3
−4
−5
−6
−7
IC1 ( mA)
−8
−9
−10
−11
−12
−200204060100120
−4080
TEMPERATURE (°C)
Figure 3. IC1 @ VCC = 8.0 V, FB = 1.5 V
vs. Temperature
0.40
0.38
0.36
0.34
0.32
0.30
0.28
ICC2 (mA)
0.26
0.24
0.22
0.20
−40020406080120
−20100
TEMPERATURE (°C)
Figure 5. ICC2 @ VCC = 6.0 V, FB = Open
vs. Temperature
1.5
1.4
1.3
1.2
1.1
1.0
0.9
ICC1 (mA)
0.8
0.7
0.6
0.5
−200204060100120
−4080
TEMPERATURE (°C)
Figure 4. ICC1 @ VCC = 8.0 V, FB = 1.5 V
vs. Temperature
9.0
8.9
8.8
8.7
8.6
8.5
VCC−OFF ( V )
8.4
8.3
8.2
−400204060100120
−2080
TEMPERATURE (°C)
Figure 6. VCC OFF, FB = 1.5 V vs. Temperature
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NCP1015
TYPICAL CHARACTERISTICS
8.0
7.9
7.8
7.7
7.6
7.5
7.4
VCC−ON ( V)
7.3
7.2
7.1
7.0
−408080−20
−200204060100120
TEMPERATURE (°C)
Figure 7. VCC ON, FB = 3.5 V vs. Temperature
600
550
500
69
68
67
DUTY CYCLE (%)
66
−400204060100120
TEMPERATURE (°C)
Figure 8. Duty Cycle vs. Temperature
450
Ipeak (mA)
400
350
−400204060100120
−2080
TEMPERATURE (°C)
Figure 9. Ipeak−RR, VCC = 8.0 V, FB = 3.5 V
vs. Temperature
110
100
90
(kHz)
80
OSC
f
70
60
50
−200204060100120
−408060−20
100 kHz
(W)
R
65 kHz
TEMPERATURE (°C)
Figure 10. Frequency vs. Temperature
25
20
15
DSon
10
5
0
−400204080100120
TEMPERATURE (°C)
Figure 11. ON Resistance vs. Temperature
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NCP1015
APPLICATION INFORMATION
Introduction
The NCP1015 offers a complete current−mode control
solution (actually an enhanced NCP1200 controller section)
together with a high−voltage power MOSFET in a
monolithic structure. The component integrates everything
needed to build a rugged and low−cost Switch−Mode Power
Supply (SMPS) featuring low standby power. The quick
selection table details the differences in operating
frequency.
• No need for an auxiliary winding: ON Semiconductor
Very High Voltage Integrated Circuit technology lets
you supply the IC directly from the high−voltage dc
rail. We call it Dynamic Self−Supply (DSS). This
solution simplifies the transformer design and ensures a
better control of the SMPS in difficult output
conditions, e.g. constant current operations. However,
for improved standby performance, an auxiliary
winding can be connected to the V
pin to disable the
CC
DSS operation.
• Short−circuit protection: by permanently monitoring
the feedback line activity, the IC is able to detect the
presence short−circuit, immediately reducing the output
power for a total system protection. Once the short has
disappeared, the controller resumes and goes back to
normal operation.
• Low standby−power: If SMPS naturally exhibit a good
efficiency at nominal load, they begin to be less
efficient when the output power demand diminishes. By
skipping un−needed switching cycles, the NCP1015
drastically reduces the power wasted during light load
conditions. An auxiliary winding can further help
decreasing the standby power to extremely low levels
by invalidating the DSS operation. Typical
measurements show results below 80 mW @ 230 Vac
for a typical 7 W universal power supply.
• No acoustic noise while operating: Instead of skipping
cycles at high peak currents, the NCP1015 waits until
the peak current demand falls below a fixed 0.25 of the
maximum limit. As a result, cycle skipping can take
place without having a singing transformer. You can
thus select cheap magnetic components free of noise
problems.
• SPICE model: a dedicated model to run transient
cycle−by−cycle simulations is available but also an
averaged version to help you closing the loop.
Ready−to−use templates can be downloaded in
OrCAD’s PSpice, and INTUSOFT’s IsSpice4 from ON
Semiconductor web site, NCP1015 related section.
Dynamic Self−Supply
When the power supply is first powered from the mains
outlet, the internal current source (typically 8 mA) is biased
and charges up the V
the voltage on this V
capacitor from the drain pin. Once
CC
capacitor reaches the V
CC
CC(off)
level
(typically 8.5 V), the current source turns off and pulses are
delivered by the output stage: the circuit is awake and
activates the power MOSFET. Figure 12 details the internal
circuitry:
Vref OFF = 8.5 V
Vref ON = 7.5 V
Vref
= 4.7 V
Latch
+
-
Internal Supply
+
Vref
Figure 12. The Current Source Regulates V
V
CC(off)
+200 mV
(8.7 V Typ.)
by Introducing a Ripple
Drain
Startup Source
V
CC
+
CV
CC
CC
Being loaded by the circuit consumption, the voltage on
the V
detects that V
internal current source to bring V
capacitor goes down. When the DSS controller
CC
has reached 7.5 V (V
CC
CC
), it activates the
CC(on)
toward 8.5 V and stops
again: a cycle takes place whose low frequency depends on
the V
takes place on the V
(V
capacitor and the IC consumption. A 1 V ripple
CC
pin whose average value equals
CC
CC(off)
+ V
) / 2. Figure 13 shows a typical operation
CC(on)
of the DSS.
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NCP1015
8.00
Vcc
6.00
4.00
Device
2.00
0
internally
pulses
Startup period
8.5V
Figure 13. The Charge/Discharge Cycle over a 10 mF VCC Capacitor
As one can see, the VCC capacitor shall be dimensioned to
offer an adequate startup time, i.e. ensure regulation is
reached before V
crosses 7.5 V (otherwise the part enters
CC
the fault condition mode). If we know that DV = 1 V and
ICC1 is 1.2 mA (for instance we selected a 11 W device
switching at 65 kHz), then the V
capacitor can be
CC
calculated using:
ICC1 @ t
C w
DV
startup
(eq. 1)
Let’s suppose that the SMPS needs 10 ms to startup, then
we will calculate C to offer a 15 ms period. As a result, C
should be greater than 18 mF thus the selection of a 33 mF /
16 V capacitor is appropriate.
Short Circuit Protection
The internal protection circuitry involves a patented
arrangement that permanently monitors the assertion of an
internal error flag. This error flag is, in fact, a signal that
instructs the controller that the internal maximum peak
current limit is reached. This naturally occurs during the
startup period (V
is not stabilized to the target value) or
out
when the optocoupler LED is no longer biased, e.g in a
short−circuit condition or when the feedback network is
broken. When the DSS normally operates, the logic checks
for the presence of the error flag every time V
V
. If the error flag is low (peak limit not active) then
CC(on)
crosses
CC
the IC works normally. If the error signal is active, then the
NCP1015 immediately stops the output pulses, reduces its
internal current consumption and does not allow the startup
source to activate: V
drops toward ground until it reaches
CC
7.5V
the so−called latch−off level, where the current source
activates again to attempt a new re−start. If the error has
gone, the IC automatically resumes its operation. If the
default is still there, the IC pulses during 8.5 V down to 7.5 V
and enters a new latch−off phase. The resulting burst
operation guarantees a low average power dissipation and
lets the SMPS sustain a permanent short−circuit. Figure 14
presents the corresponding diagram:
Current Sense
Information
V
CC
V
CC(on)
Flag
+
−
Latch
Reset
FB
4 V
Division
Max
Ip
Clamp
Active?
Figure 14. Simplified NCP1015 Short−Circuit
Detection Circuitry
The protection burst duty−cycle can easily be computed
through the various timing events as portrayed by Figure 15:
The rising slope from the latch−off level up to 8.5 V is
expressed by:
P
+ Vin@ ICC1
DSS
start
DV1 @ C
+
IC1
t
The time during which the IC actually pulses is given by:
DV2 @ C
tsw+
ICC1
Finally, the latch−off time can be derived using the same
formula topology:
latch
DV3 @ C
+
ICC2
t
From these three definitions, the burst duty−cycle D can
be computed:
t
start
ǒ
sw
) tsw) t
DV2
DV2
DV1
)
ICC1
IC1
latch
)
DV3
ICC2
(eq. 2)
Ǔ
(eq. 3)
D +
D +
ICC1 @
t
Feeding the equation with values extracted from the
parameter section gives a typical duty−cycle D of 13%,
precluding any lethal thermal runaway while in a fault
condition.
DSS Internal Dissipation
The Dynamic Self−Supplied pulls the energy out from the
drain pin. In the Flyback−based converters, this drain level
can easily go above 600 V peak and thus increase the stress
on the DSS startup source. However, the drain voltage
evolves with time and its period is small compared to that of
the DSS. As a result, the averaged dissipation, excluding
capacitive losses, can be derived by:
P
+ ICC1 @t V
DSS
DS(t)
u
(eq. 4)
Figure 16 shows a typical drain−ground wave−shape
where leakage effects have been removed:
Vds(t)
toff
Vr
Vin
ton
Ts w
dt
Figure 16. A Typical Drain−ground Waveshape
where Leakage Effects are Not Accounted for
By looking at Figure 16 the average result can easily be
derived by additive square area calculation:
t
off
t V
u+ Vin@ (1 * D) ) Vr@
DS(t)
(eq. 5)
t
sw
By developing Equation 5 we obtain:
t V
DS(t)
t
can be expressed by:
off
u+ Vin* Vin@
t
+ Ip@
off
t
on
t
sw
L
p
V
r
) Vr@
t
off
(eq. 6)
t
sw
(eq. 7)
ton can be evaluated by:
L
ton+ Ip@
p
V
in
(eq. 8)
t
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NCP1015
Plugging Equation 7 and Equation 8 into Equation 6 leads
to <V
> = Vin and thus:
ds(t)
P
DSS
+ Vin@ ICC1
(eq. 9)
The worse case occurs at high line, when Vin equals
370 Vdc. With ICC1 = 1.2 mA (65 kHz version), we can
expect a DSS dissipation around 440 mW. If you select a
higher switching frequency version, the ICC1 increases and
it is likely that the DSS consumption exceeds 500 mW. In
that case, we recommend adding an auxiliary winding in
order to offer more dissipation room to the power MOSFET.
Please read application note AND8125/D “Evaluating the
power capability of the NCP101X members” to help
selecting the right part / configuration for your application.
Lowering the Standby Power with an Auxiliary
Winding
The DSS operation can bother the designer when a) its
dissipation is too high b) extremely low standby power is a
must. In both cases, one can connect an auxiliary winding to
disable the self−supply. The current source then ensures the
startup sequence only and stays in the off state as long as
V
does not drop below V
CC
that the insertion of a resistor (R
dc level and the V
pin is mandatory a) not to damage the
CC
or 7.5 V. Figure 17 shows
CC(on)
) between the auxiliary
limit
internal 8.7 V zener diode during an overshoot for instance
(absolute maximum current is 15 mA) b) to implement the
fail−safe optocoupler protection as offered by the active
clamp. Please note that there cannot be bad interaction
between the clamping voltage of the internal zener and
V
of V
since this clamping voltage is actually built on top
CC(off)
with a fixed amount of offset (200 mV typical).
CC(off)
Self−supplying controllers in extremely low standby
applications often puzzles the designer. Actually, if a SMPS
operated at nominal load can deliver an auxiliary voltage of
an arbitrary 16 V (V
10 V (V
) when entering standby. This is because the
stby
), this voltage can drop to below
nom
recurrence of the switching pulses expands so much that the
low frequency re−fueling rate of the V
capacitor is not
CC
enough to keep a proper auxiliary voltage. Figure 18 shows
a typical scope shot of a SMPS entering deep standby
(output un−loaded). So care must be taken when calculating
R
1) to not excess the maximum pin current in normal
limit
operation but 2) not to drop too much voltage over R
limit
when entering standby. Otherwise the DSS could reactivate
and the standby performance would degrade. We are thus
able to bound R
between two equations:
limit
lim
v
V
* V
stby
ICC1
CC(on)
(eq. 10)
V
* V
nom
clamp
I
trip
v R
Where:
V
is the auxiliary voltage at nominal load
nom
V
is the auxiliary voltage when standby is entered
stdby
is the current corresponding to the nominal operation.
I
trip
It thus must be selected to avoid false tripping in overshoot
conditions.
ICC1 is the controller consumption. This number slightly
decreases compared to ICC1 from the spec since the part in
standby does almost not switch.
V
is the level above which V
CC(on)
must be maintained
aux
to keep the DSS in the OFF mode. It is good to shoot around
8 V in order to offer an adequate design margin, e.g. to not
re−activate the startup source (which is not a problem in
itself if low standby power does not matter)
Since R
keep V
select a V
shall not bother the controller in standby, e.g.
limit
to around 8 V (as selected above), we purposely
aux
well above this value. As explained before,
nom
experience shows that a 40% decrease can be seen on
auxiliary windings from nominal operation down to standby
mode. Let’s select a nominal auxiliary winding of 20 V to
offer sufficient margin regarding 8 V when in standby (R
limit
also drops voltage in standby). Plugging the values in
Equation 10 gives the limits within which R
limit
shall be
selected:
20 * 8.7
6.3 m
that is to say: 1.8 kW < R
v R
limit
12 * 8
v
limit
1.1 m
< 3.6 kW.
(eq. 11)
If we are designing a power supply delivering 12 V, then
the ratio auxiliary/power must be: 12 / 20 = 0.6. The I
current has to not exceed 6.4 mA. This will occur when V
CC
aux
grows−up to: 8.7 V + 1.8 k x (6.4 m + 1.1 m) = 22.2 V for
the first boundary or 8.7 V + 3.6 k x (6.4 m +1.1 m) = 35.7 V
for second boundary. On the power output, it will
respectively give 22.6 x 0.6 = 13.3 V and 35.7 x 0.6 = 21.4 V.
As one can see, tweaking the R
value will allow the
limit
selection of a given overvoltage output level. Theoretically
predicting the auxiliary drop from nominal to standby is an
almost impossible exercise since many parameters are
involved, including the converter time constants. Fine
tuning of R
experiments on a breadboard to check V
thus requires a few iterations and
limit
variations but
aux
also output voltage excursion in fault. Once properly
adjusted, the fail−safe protection will preclude any lethal
voltage runaways in case a problem would occur in the
feedback loop.
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NCP1015
V
CC(off)
V
CC(on)
= 8.5 V
= 7.5 V
+
+
-
+
+
−
Drain
Startup Source
V
CC
++
Ground
Rlimit
CVCCCAux
D1
Laux
Figure 17. A Detailed View of the NCP1015 with Properly Connected Auxiliary Winding
u30 ms
Figure 18. The Burst Frequency becomes So Low that it is Difficult to
Keep an Adequate Level on the Auxiliary V
Lowering the Standby Power with Skip−cycle
Skip cycle offers an efficient way to reduce the standby
power by skipping unwanted cycles at light loads. However,
the recurrent frequency in skip often enters the audible range
and a high peak current obviously generates acoustic noise
in the transformer. The noise takes its origins in the
resonance of the transformer mechanical structure which is
CC
excited by the skipping pulses. A possible solution,
successfully implemented in the NCP1200 series, also
authorizes skip cycle but only when the power demand as
dropped below a given level. At this time, the peak current
is reduced and no noise can be heard. Figure 19 shows the
peak current evolution of the NCP1015 entering standby:
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NCP1015
100%
Peak current
at nominal power
Skip−cycle
current limit
25%
Figure 19. Low Peak Current Skip−Cycle Guarantees Noise−Free Operation
Full power operation involves the nominal switching
frequency and thus avoids any noise when running.
Experiments carried on a 5 W universal mains board
unveiled a standby power of 300 mW @ 230 Vac with the
DSS activated and dropped to less than 100 mW when an
auxiliary winding is connected.
Frequency Jittering for Improved EMI Signature
By sweeping the switching frequency around its nominal
value, it spreads the energy content on adjacent frequencies
rather than keeping it centered in one single ray. This offers
VCC Ripple
VCC
65 kHz
OFF
the benefit to artificially reduce the measurement noise on
a standard EMI receiver and pass the tests more easily. The
EMI sweep is implemented by routing the V
CC
ripple
(induced by the DSS activity) to the internal oscillator. As a
result, the switching frequency moves up and down to the
DSS rhythm. Typical deviation is
±4% of the nominal
frequency. With a 1 V peak−to−peak ripple, the frequency
will equal 65 kHz in the middle of the ripple and will
increase as V
rises or decrease as VCC ramps down.
CC
Figure 20 shows the behavior we have adopted:
67.6 kHz
62.4 kHz
VCC
ON
Figure 20. The VCC Ripple Causes the Frequency Jittering on the Internal Oscillator Saw−tooth
(65 kHz version)
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Internal Sawtooth
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NCP1015
Soft−Start
The NCP1015 features an internal 1 ms soft−start
activated during the power on sequence (P
V
reaches V
CC
, the peak current is gradually
CC(off)
). As soon as
ON
increased from nearly zero up to the maximum internal
clamping level (e.g. 350 mA). This situation lasts 1 ms and
further to that time period, the peak current limit is blocked
to the maximum until the supply enters regulation. The
V
CC
0 V (Fresh PON)
or
4.7 V (Overload)
Current
Sense
soft−start is also activated during the over current burst
(OCP) sequence. Every re−start attempt is followed by a
soft−start activation. Generally speaking, the soft−start will
be activated when V
ramps up either from zero (fresh
CC
power−on sequence) or 4.5 V, the latch−off voltage
occurring during OCP. Figure 21 shows the soft−start
behavior. The time scales are purposely shifted to offer a
better zoom portion.
8.5 V
Max Ip
Figure 21. Soft−Start is Activated During a Start−up Sequence or an OCP Condition
Non−latching Shutdown
In some cases, it might be desirable to shut off the part
temporarily and authorize its re−start once the default has
disappeared. This option can easily be accomplished
through a single NPN bipolar transistor wired between FB
ON/OFF
Figure 22. A Non−latching Shutdown where Pulses are Stopped as long as the NPN is Biased
1.0 ms
and ground. By pulling FB below the internal skip level
(V
), the output pulses are disabled. As soon as FB is
skip
relaxed, the IC resumes its operation. Figure 22 shows the
application example:
18
27
3
45
+
CV
CC
Transformer
Full Latching Shutdown
Other applications require a full latching shutdown, e.g.
when an abnormal situation is detected (over temp or
overvoltage). This feature can easily be implemented
through two external transistors wired as a discrete SCR.
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When the OVP level exceeds the zener breakdown voltage,
the NPN biases the PNP and fires the equivalent SCR,
permanently bringing down the FB pin. The switching
pulses are disabled until the user un−plugs the power supply.
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NCP1015
Rhold
DSS
12 k
BAT54
+ P
MOSFET
Clamping Elements
OVP
10 k
0.1 mF
10 k
Figure 23. Two Bipolar Transistors Ensures a Total Latch−off of the SMPS in Presence of an OVP
R
ensures that the SCR stays on when fired. The bias
hold
current flowing through R
the V
ramp up (8.5 V) and down (7.5 V) when the SCR
CC
should be small enough to let
hold
is fired. The NPN base can also receive a signal from a
temperature sensor. Typical bipolar can be MMBT2222 and
MMBT2907 for the discrete latch. The NST3946 features
two bipolar NPN + PNP in the same package and could also
be used.
Power Dissipation and Heatsinking
The power dissipation of NCP1015 consists of the
dissipation DSS current−source (when active) and the
dissipation of MOSFET. Thus P
tot
= P
When the PDIP7 package is surrounded by copper, it
becomes possible to drop its thermal resistance
junction−to−ambient, R
down to 75°C/W and thus
JA
q
dissipate more power. The maximum power the device can
thus evacuate is:
18
27
3
45
+
CV
CC
P
+
max
T
J(max)
Transformer
* T
AMB(max)
R
qJA
which gives around 1 W for an ambient of 50°C. The losses
inherent to the MOSFET R
can be evaluated using the
DS(on)
following formula:
1
P
+
mos
2
@ I
@ D @ R
p
3
DS(on)
where Ip is the worse case peak current (at the lowest line
input), D is the converter operating duty−cycle and R
the MOSFET resistance for TJ = 100°C. This formula is only
valid for Discontinuous Conduction Mode (DCM)
.
operation where the turn−on losses are null (the primary
current is zero when you re−start the MOSFET). Figure 24
gives a possible layout to help dropping the thermal
resistance. When measured on a 35 mm (1 oz.) copper
thickness PCB, we obtained a thermal resistance of 75°C/W:
(eq. 12)
(eq. 13)
DS(on)
DC
Figure 24. A Possible PCB Arrangement to Reduce the Thermal Resistance Junction−to−Ambient
Design Procedure
The design of a SMPS around a monolithic device does
not differ from that of a standard circuit using a controller
and a MOSFET. However, one needs to be aware of certain
characteristics specific of monolithic devices:
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To Secondary Diode
1. In any case, the lateral MOSFET body−diode shall
never be forward biased, either during start−up
(because of a large leakage inductance) or in
normal operation as shown by Figure 25.
14
Page 15
NCP1015
ǒ
Ǔ
350
250
150
50.0
> 0 !!
−50.0
1.004M1.011M1.018M1.025M1.032M
Figure 25. The Drain−Source Wave Shall Always be Positive . . .
As a result, the Flyback voltage which is reflected on
the drain at the switch opening cannot be larger than
the input voltage. When selecting components, you
thus must adopt a turn ratio which adheres to the
following equation:
N @ (V
) Vf) t V
out
IN(min)
For instance, if you operate from a 120 V dc rail and
you deliver 12 V, we can select a reflected voltage of
100 VDC maximum: 120 − 100 > 0. Therefore, the
turn ratio Np : Ns must be smaller than 100 / (12 +
1) = 7.7 or Np : Ns < 7.7. We will see later on how
it affects the calculation.
2. Current−mode architecture is, by definition,
sensitive to subharmonic oscillations.
Subharmonic oscillations only occur when the
SMPS is operating in Continuous Conduction
Mode (CCM) together with a duty−cycle greater
than 50%. As a result, we recommend operating
the device in DCM only, whatever duty−cycle it
implies (max. = 65%).
3. Lateral Mosfets have a poorly doped body−diode
which naturally limits their ability to sustain the
avalanche. A traditional RCD clamping network
shall thus be installed to protect the MOSFET. In
some low power applications, a simple capacitor
can also be used since:
V
DRAIN(max)
+ Vin) N @ (V
) Vf) ) Ip@
out
Ǹ
where Lf is the leakage inductance, C
capacitance at the drain node (which is increased by
the capacitor you will wire between drain and
source), N the Np : Ns turn ratio, V
voltage, V
finally, I
the secondary diode forward drop and
f
the maximum peak current. Worse case
p
out
occurs when the SMPS is very close to regulation,
(eq. 14)
L
f
(eq. 15)
C
tot
the total
tot
the output
e.g. the V
target is almost reached and Ip is still
out
pushed to the maximum.
Taking into account all previous remarks, it becomes
possible to calculate the maximum power that can be
transferred at low line:
When the switch closes, V
inductance L
until the current reaches the level imposed by
p
is applied across the primary
in
the feedback loop. The duration of this event is called the ON
time and can be defined by:
Lp@ I
ton+
p
V
in
(eq. 16)
At the switch opening, the primary energy is transferred
to the secondary and the flyback voltage appears across L
reseting the transformer core with a slope of:
the t
time is thus:
off
N @ (V
t
+
off
N @ (V
) Vf)
out
L
p
Lp@ I
out
p
) Vf)
@ t
off
(eq. 17)
If one wants to keep DCM only, but still need to pass the
maximum power, we will not allow a dead−time after the
core is reset, but rather immediately re−start. The switching
time t
tsw+ t
can be expressed by:
sw
) ton+ Lp@ Ip@
off
1
)
V
in
N @ (V
1
out
) Vf)
(eq. 18)
The Flyback transfer formula dictates that:
P
out
1
+
h
@ Lp@ I
2
2
@ f
p
sw
(eq. 19)
which, by extracting Ip and plugging into Equation 19 leads to:
tsw+ L
p
Ǹ
out
h @ fsw@ L
1
ǒ
@
)
V
p
in
N @ (V
1
out
) Vf)
Ǔ
(eq. 20)
2 @ P
Extracting Lp from Equation 20 gives:
,
p
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15
Page 16
NCP1015
(Vin@ Vr)2@ h
@ (V
out
with Vr = N . (V
If L
critical gives the inductance value above which
p
L
+ Vf) and h the efficiency.
out
Pcritical
+
2 @ f
sw
@ [P
DCM operation is lost, there is another expression we can
write to connect L
, the primary peak current bounded by the
p
NCP1015 and the maximum duty−cycle that needs to stay
below 50%:
L
P(max)
P
max
+ t
+
sw
max
2
@ V
@ V
I
P(max)
IN(min)
IN(min)
2
@ V
@ t
r
sw
2
@ h @
(eq. 22)
(2L
P(max)
D
From Equation 22 we obtain the operating duty−cycle D:
Ip@ L
Vin@ t
sw
p
(eq. 24)
D +
This lets us calculate the RMS current circulating in the
MOSFET:
+ Ip@
Ǹ
3
(eq. 25)
I
D(rms)
D
From this equation, we obtain the average dissipation in
the MOSFET:
1
P
+
avg
2
@ I
@ D @ R
p
3
DS(on)
(eq. 26)
to which switching losses shall be added.
If we stick to Equation 23, compute Lp and follow the
above calculations, we will discover that a power supply
built with the NCP1015 and operating from a 100 Vac line
minimum will not be able to deliver more than 7 W
continuous, regardless of the selected switching frequency
(however the transformer core size will go down as f
sw
is
increased). This number grows up significantly when
operated from single European mains (18 W).
For more different flyback converters then are the below
examples we recommend use following support:
1) Application note AND8125/D “Evaluating the power
Example 1.: A 12 V 7.0 W SMPS Operating on a Large
Mains with NCP1015:
Vin = 100 Vac ÷ 250 Vac or 140 Vdc ÷ 350 Vdc once
rectified, assuming a low bulk ripple
Efficiency = 80%
V
= 12 V, I
out
= 65 kHz
f
sw
I
= 450 mA − 10% = 405 mA
P(max)
= 580 mA
out
2
) 2 @ Vr@ Vin) V
r
where V
IN(min)
(eq. 21)
2
)]
in
corresponds to the lowest bulk voltage,
hence the longest ton duration or largest duty−cycle. I
is the available peak current from the considered part, e.g.
450 mA typical for the NCP1015 (however, the minimum
value of this parameter shall be considered for reliable
evaluation). Combining Equations 21 and 22 gives the
maximum theoretical power you can pass respecting the
peak current capability of the NCP1015, the maximum
duty−cycle and the discontinuous mode operation:
f
P(max)
sw
VrV
IN(min)
) V
IN(min)
2
V
) 4L
r
2
)
(eq. 23)
Applying the above equations leads to :
Selected maximum reflected voltage = 120 V
with V
L
I
p
D
I
DRAIN(rms)
P
P
= 12 V, secondary drop = 0.5 V ³ Np : Ns = 1 : 0.1
out
critical = 3.9 mH
p
= 250 mA
= 0.39
max
= 90 mA
MOSFET
DSS
= 202 mW at R
= 1.2 mA x 350 V = 420 mW, if DSS is used
= 25 W (TJ > 100°C)
DS(on)
Secondary diode voltage stress = (350 x 0.1) + 12 = 47 V
(e.g. a MBRS360T3, 3 A / 60 V would fit)
Example 2.: A 12 V 16 W SMPS Operating on Narrow
European Mains with NCP1015:
Vin = 230 Vac ± 15%, or 276 Vdc ÷ 370 Vdc
Efficiency = 80%
= 12 V, I
V
out
= 65 kHz
f
sw
I
= 450 mA − 10% = 405 mA
P(max)
= 1.25 A
out
Applying the equations leads to :
Selected maximum reflected voltage = 250 V
with V
L
I
p
D
I
DRAIN(rms)
P
P
= 12 V, secondary drop = 0.5 V ³ Np : Ns = 1:0.05
out
= 7,2 mH
p
= 0.27 mA
= 0.41
max
= 100 mA
MOSFET
DSS
= 250 mW at R
= 1.2 mA x 370 V = 444 mW, if DSS is used below an
= 25 W (TJ > 100°C)
DS(on)
ambient of 50°C.
Secondary diode voltage stress = (370 x 0.05) + 12 = 30.5 V
(e.g. a MBRS340T3, 3 A / 40 V)
P(max)
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16
Page 17
NCP1015
Please note that these calculations assume a flat DC rail
whereas a 10 ms ripple naturally affects the final voltage
available on the transformer end. Once the Bulk capacitor
has been selected, one should check that the resulting ripple
(min V
?) is still compatible with the above calculations.
bulk
As an example, to benefit from the largest operating range,
a 7 W board was built with a 47 mF bulk capacitor which
ensured discontinuous operation even in the ripple
minimum waves.
CVcc
HV
1
8
7
2
3
6
45
NCP1015
A
C
Figure 26. Different Options to Clamp the Leakage Spike
HV
Rclamp
CVcc
MOSFET Protection
As in any Flyback design, it is important to limit the drain
excursion to a safe value, e.g. below the MOSFET BVdss
which is 700 V. Figures 26A, B, and C present possible
implementations:
HV
1
2
3
45
NCP1015
B
Cclamp
D
8
7
6
CVc
c
Dz
1
2
3
45
NCP1015
C
D
8
7
6
Figure 26A: The simple capacitor limits the voltage
according to Equation 15. This option is only valid for low
power applications, e.g. below 5 W, otherwise chances exist
to destroy the MOSFET. After evaluating the leakage
inductance, you can compute C with Equation 15. Typical
values are between 100 pF and up to 470 pF. Large
capacitors increase capacitive losses.
Figure 26B: The most standard circuitry called the RCD
network. You calculate R
clamp
and C
clamp
using the
following formulas:
R
+
clamp
V
is usually selected 50−80 V above the reflected
clamp
value N x (V
2 @ V
@ (V
clamp
C
+
clamp
+ Vf). The diode needs to be a fast one and
out
clamp
L
leak
V
ripple
* (V
2
@ I
p
V
clamp
@ fsw@ R
) Vfsec) @ N)
out
@ f
sw
clamp
(eq. 27)
(eq. 28)
a MUR160 represents a good choice. One major drawback
of the RCD network lies in its dependency upon the peak
current. Worse case occurs when I
and V
is close to reach the steady−state value.
out
and Vin are maximum
p
Figure 26C: This option is probably the most expensive of
all three but it offers the best protection degree. If you need
a very precise clamping level, you must implement a zener
diode or a TVS. There are little technology differences
behind a standard zener diode and a TVS. However, the die
area is far bigger for a transient suppressor than that of zener.
A 5 W zener diode like the 1N5388B will accept 180 W peak
power if it lasts less than 8.3 ms. If the peak current in the
worse case (e.g. when the PWM circuit maximum current
limit works) multiplied by the nominal zener voltage
exceeds these 180 W, then the diode will be destroyed when
the supply experiences overloads. A transient suppressor
like the P6KE200 still dissipates 5 W of continuous power
but is able to accept surges up to 600 W @ 1 ms. Select the
zener or TVS clamping level between 40 to 80 volts above
the reflected output voltage when the supply is heavily
loaded.
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17
Page 18
NCP1015
Typical Application Examples
A 6.5 W NCP1015−based Flyback converter. (For
evaluation a universal NCP1012 demo−board can be used)
Figure 27 shows a converter originally built with a
NCP1012 which can be easily used for evaluation of
NCP1015 device delivering 6.5 W from a universal volts
D1D2
1N40071N4007
E1
10 m/400 V
E2
10 m/63 V
47 R
J1
CEE7.5/2
R1
1
2
2
D3D4
1N40071N4007
Figure 27. A NCP1012−based Flyback Converter Delivering 6.5 W
input range. The board uses the Dynamic Self−Supply and
a simplified zener−type feedback. This configuration was
selected for cost reasons and a more precise circuitry can be
used, e.g. based on a TL431:
D6
8
7
6
5
B150
E3
470 m/25 V
ZD1
11 V
R3
100 R
2
1
CZM5/2
R4
180 R
R2
150 k
1
2
3
7
IC1
NCP1012
V
CC
GND
GND
GND
C1
2n2/Y
DRAIN
GND
FB
D5
U160
5
4
4
8
TR1
1
4
PC817
2n2/Y
IC2
C2
J2
The converter built according to Figure 28 layouts, gave
the following results:
Figure 28. The NCP1012−based PCB Layout and its Associated Component Placement
• Efficiency at V
• Efficiency at V
= 100 Vac and P
in
= 230 Vac and P
in
= 6.5 W = 75.7%
out
= 6.5 W = 76.5%
out
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18
Page 19
NCP1015
A 7.0 W NCP1015−based Flyback Converter Featuring Low Standby Power
Figure 29 shows another typical application showing a
NCP1015−65 kHz operating in a 7 W converter up to 70°C
of ambient temperature. We can grow−up the output power
since an auxiliary winding is used, the DSS is disabled, and
Vbulk
1N4148
D4
C10
+
33 mF/25 V
R4 22
T1
Aux
C8
10 nF
400 V
thus offering more room for the MOSFET. In this
application, the feedback is made via a TLV431 whose low
bias current (100 mA min) helps to lower the no−load
standby power.
R7
100 k/
1 W
D2
MBRS360T3
T1
++
C6 C8
470 mF/16 V
L2
22 mH
12 V @
0.5 A
+
100 mF/16 V
C7
GND
C2
47 mF/
450 V
R2
3.3 k
NCP1015
+
+
100 mF/10 V
C3
18
V
GND
CC
NC
2
3
4
C9
1 nF
NC
FB
NC
DRAIN
D3
MUR160
7
5
IC1
SFH6156−2
C5
2.2 nF
Y1 Type
R3
1 k
C4
100 nF
IC2
TLV431
R5
39 k
R6
4.3 k
Figure 29. A Typical Converter Delivering 5 W from a Universal Mains
Measurements have been taken from a demonstration
board implementing Figure 12 12’s sketch and the following
results were achieved, with either the auxiliary winding in
place or through the Dynamic Self−Supply:
V
= 230 Vac, auxiliary winding, P
in
V
= 100 Vac, auxiliary winding, P
in
= 230 Vac, Dynamic Self−Supply, P
V
in
= 0, Pin = 60 mW
out
= 0, Pin = 42 mW
out
= 0, Pin =
out
300 mW
V
= 100 Vac, Dynamic Self−Supply, P
in
= 0, Pin =
out
130 mW
P
= 7 W, h = 81% @ 230 Vac, with aux winding
out
= 7 W, h = 81.3% @ 100 Vac, with aux winding
P
out
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For a quick evaluation of Figure 29 application example,
the following transformers are available from Coilcraft:
A9619−C, L
= 3 mH, Np : Ns = 1: 0.1, 7 W application on
p
universal mains, including auxiliary winding, NCP1015−
65 kHz
A0032−A, L
= 6 mH, Np : Ns = 1: 0.055, 10 W application
p
on European mains, DSS operation only, NCP1015−65 kHz
Coilcraft
1102 Silver Lake Road
CARY, IL 60013
Email: info@coilcraft.com
Tel. : 847−639−6400
Fax.: 847−639−1469
19
Page 20
NCP1015
ORDERING INFORMATION
Frequency
Device Order Number
NCP1015AP065G65PDIP−7
NCP1015AP100G100PDIP−7
NCP1015ST65T3G65SOT−223
NCP1015ST100T3G100SOT−223
†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging
Specifications Brochure, BRD8011/D.
(kHz)
Package TypeShipping
(Pb−Free)
(Pb−Free)
(Pb−Free)
(Pb−Free)
50 Units / Rail
4000 / Tape & Reel
†
R
DSon
(W)
11450
11450
11450
11450
Ipk (mA)
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20
Page 21
NOTE 3
−T−
SEATING
PLANE
H
58
B
14
F
A
C
N
D
G
0.13 (0.005)B
NCP1015
PACKAGE DIMENSIONS
PDIP−7
AP SUFFIX
CASE 626A−01
ISSUE O
L
K
M
M
A
T
M
NOTES:
1. DIMENSIONING AND TOLERANCING PER ANSI
Y14.5M, 1982.
2. CONTROLLING DIMENSION: MILLIMETER.
3. PACKAGE CONTOUR OPTIONAL (ROUND OR
SQUARE CORNERS).
4. DIMENSION L TO CENTER OF LEAD WHEN
FORMED PARALLEL.
*For additional information on our Pb−Free strategy and soldering
details, please download the ON Semiconductor Soldering and
Mounting Techniques Reference Manual, SOLDERRM/D.
SENSEFET is a trademark of Semiconductor Components Industries, LLC (SCILLC).
ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice
to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.
“Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All
operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights
nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications
intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should
Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates,
and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death
associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal
Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.
PUBLICATION ORDERING INFORMATION
LITERATURE FULFILLMENT:
Literature Distribution Center for ON Semiconductor
P.O. Box 5163, Denver, Colorado 80217 USA
Phone: 303−675−2175 or 800−344−3860 Toll Free USA/Canada
Fax: 303−675−2176 or 800−344−3867Toll Free USA/Canada
Email: orderlit@onsemi.com
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USA/Canada
Europe, Middle East and Africa Technical Support:
Phone: 421 33 790 2910
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Phone: 81−3−5773−3850
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Order Literature: http://www.onsemi.com/orderlit
For additional information, please contact your local
Sales Representative
NCP1015/D
22
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