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| Número de pieza | AND8488 | |
| Descripción | Tips and Tricks | |
| Fabricantes | ON Semiconductor | |
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AND8488/D
Tips and Tricks for the
NCP1250
Prepared by: Christophe Basso
ON Semiconductor
Despite a limited number of pins, the NCP1250 can be
used in a variety of applications in the ac−dc or dc−dc power
conversion field. Besides the available literature on the
controller itself, this application note reviews a few tricks to
help you improve the performance of the part in particular
operating conditions.
Reducing the Standby Power
This is THE subject of discussion when tackling ac−dc
converters for the consumer market: ”what standby power
in no−load condition can your part reach?”. Well, if we
disconnect the start−up resistors and the 2 MW
X2−capacitors discharge resistors string in the NCP1250
demonstration board, we reach an amazing 35 mW when
powered at 230 Vrms and fully unloaded on the secondary
side (Vout = 19 V, Iout = 0). The main contributors to this
consumption are the following ones:
1. X2−capacitor choice and discharge elements: as a
substantial amount of reactive current circulates in
this capacitor, it can induce dielectric losses, in
particular with cheap components. Besides this
loss, the voltage on its terminals must decrease at a
sufficient pace when you unplug the power cord so
that the available level becomes benign for a user
touching the plug after 1 s. This is the reason why
discharge resistors are connected in parallel with
the filtering capacitor. These elements are selected
so that the time constant involving the
X2−capacitor and the resistors is 1 s, as specified
by the IEC−950 standard. For a 0.47 mF capacitor,
you must install a 2 MW resistors string,
dissipating 26 mW at 230 Vrms. If you chase the
tens of mW, you will be happy to discover an
active circuitry described in this application note.
2. Bulk capacitor leakage: it is an often forgotten
parameter because designers are not used to
chasing tens of mW when thinking about a 65 W
adapter. However, depending on the adopted
brand, you can experience a consumption of a few
mW, sometimes up to 10, just because the bulk
capacitor is leaky. Please pay attention to this
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APPLICATION NOTE
parameter if you plan to beat no−load consumption
records.
3. Controller consumption: there is nothing you can
do on this one. It depends on the design and
technical choices the semiconductor vendor made
when developing the device. The NCP1250 has
been the object of a particular care in this domain.
When supplied from a 12 V auxiliary source while
driving a 6 A/600 V MOSFET, the controller only
draws 550 mA typically. From the 12 V line, it is a
bare 6 mW. Difficult to do less...
4. Feedback currents: if you selected a TL431, you
must inject at least 1 mA in the device to get it
working properly. If you do not, you will
experience a poor output impedance, leading to an
unacceptable transient performance. With a 19 V
output, a 1 mA bias associated with the regular
feedback current generates a significant
primary−side loss. If your output voltage is below
18 V, you can use a TLV431 whose minimum
injected current is down to 100 mA. If your buyer
imposes a TL431, one of the proposed tricks will
help getting rid of this extra consumption.
5. Start−up resistors: with low−voltage controllers,
this is always the problem. How to combine a
start−up time less than 3 s at low−line while
consuming the least current on the mains at high
line? You can always add an external bipolar
transistor network to get rid of the start−up
network, but why not taking advantage of the X2
discharge resistors presence to crank the
controller? This is what is proposed in the
following lines.
6. Output LED: needless to say that the addition of a
LED in the adapter output can ruin all the efforts
you put in saving the tens of mW! The best is, of
course, to explain that the presence of the LED go
against power saving initiatives and it would be
better to abandon its implementation. In some
cases, however, a light is needed and a solution has
to be found to minimize its impact.
© Semiconductor Components Industries, LLC, 2011
March, 2011 − Rev. 0
1
Publication Order Number:
AND8486/D
1 page  
AND8488/D
stopped and the auxiliary Vcc starts to come down. When it
reaches VCCmin, the circuit enters sleep mode and reduces
its total consumption below 15 mA. The start−up current now
recharges the Vcc capacitor and lifts Vcc up towards the
start−up level of 18 V. With a weak start−up current (to
minimize losses), the operation can take several hundred of
ms at the lowest input line. The part then re−starts and pulses
for another 100 ms period in a recurrent manner. This is the
so−called hiccup operation that appears in Figure 5.
Vcc (t )
t1
T
t1 = 100ms
T = 1.7s
Iout (t )
Vin = 90Vrms
D [6%
Figure 5. In Hiccup Mode, the Controller Tries to Re−start but Stops After the Timer Has Elapsed Since the
Short−Circuit or the Overload is Still Present
As the circuit pulses for a 100 ms, a current circulates in
the output cable and the load. It is a square wave as shown
in Figure 6.
ISC
Icable (t )
t1
T
Figure 6. The Current Circulating in the Cable While the Load is Shorted Offers a Square Envelope
The Root Mean Square value of the signal envelope is
given by:
Iout,rms
+
ISC
Ǹt1
T
(eq. 3)
In the picture, the current peaks to 5 A. Applying
Equation 3, the rms current amounts to:
ǸIout,rms + 5
0.1
1.7
+
1.2
A
(eq. 4)
Unfortunately, in high−line conditions, the recharge time
of the VCC capacitor is significantly shortened, naturally
reducing the off−time duration and thus the recurrence T: the
rms current in the cable while undergoing a short−circuit is
increased.
One way to reduce the rms content is to act upon the
secondary−side peak current. Alternatively, the off−time T
can also be lengthened by an external means. The solution
that appears in Figure 7 adopts this solution. The principle
is simple: at start−up, in absence of auxiliary voltage, Q1 is
blocked and Vcc normally takes off thanks to the current
delivered by the start−up network. When the part starts to
pulse, the auxiliary voltage biases Q1 that interrupts the
charging current. As the part is already operating and owing
to the diode D3, it has no influence on the converter. When
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5
5 Page 
AND8488/D
This circuit has been added to the NCP1250 65 W
demonstration board and has proved to work ok. With the
component values put in the schematic, the converter
starts−up at Vin = 78 Vrms and Iout = 3.3 A. For a 3 A load, it
stops working for Vin = 65 Vrms, going down to 41 Vrms for
Iout = 2 A.
Conclusion
This application note describes how a 6−pin controller
housed in a TSOP6 package expands its capabilities when
adding a few components around it. The proposed ideas are
just a few examples our application engineering thought
about when dealing with day−to−day customers problems.
As more application ideas are tested and documented, we
will update this application note to make it an evolving
document.
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
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Phone: 303−675−2175 or 800−344−3860 Toll Free USA/Canada
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Phone: 81−3−5773−3850
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11
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Order Literature: http://www.onsemi.com/orderlit
For additional information, please contact your local
Sales Representative
AND8488/D
11 Page | ||
| Páginas | Total 11 Páginas | |
| PDF Descargar | [ Datasheet AND8488.PDF ] | |
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