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AND8076 Datasheet

A 70W Low Standby Power Supply

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AND8076/D
A 70 W Low Standby
Power Supply with
the NCP120x Series
Prepared by: Christophe Basso
ON Semiconductor
http://onsemi.com
APPLICATION NOTE
INTRODUCTION
The NCP1200 represents one of the cheapest solutions to
build efficient and cost-effective Switch-Mode Power
Supplies (SMPS). As this design example will show, the part
definition does not confine the component in low-power
applications only, but it can actually be used in Flyback and
Forward supplies for virtually any output power. The below
example depicts a universal mains 90-260 VAC power
supply delivering 16.5 V @ 4.5 A.
Beside its ease of implementation, the NCP1200 excels in
true low standby power designs. This application note
details how an amazing standby power of less than 100 mW
can be reached at high line with a nominal 70 W board.
DSS or Not DSS?
The Dynamic Self Supply (DSS) lets you directly drive
MOSFETs from the high-voltage rail. This option brings
you several advantages, as stated below:
True overload detection: with UC384X-based systems,
the switching oscillations are stopped in case the Vcc
line drops below a given Undervoltage Lockout level
(UVLO). This principle considers a good coupling
between the primary auxiliary winding and the power
secondary winding. Unfortunately, leakage elements
often degrade this coupling and you only can detect true
short-circuit (when Vout is close to zero) and not
overload conditions. Thanks to the DSS, the NCP1200
does not need an auxiliary information to sense an
overload condition. By detecting a current setpoint
pushed to the maximum, the internal logic takes the
decision to enter into a safe burst operation,
auto-recovering when the default leaves. Precise
overload levels can thus be implemented.
Guaranteed operation at low output levels: the Vcc
delivered by an auxiliary winding moves with the
power output level because a coupling exists between
both windings. When the supply is used in battery
charging applications, Vout can move depending on the
charging state. That is to say, when the battery is nearly
empty, its voltage can be close to zero, forcing Vout at
this level. Thanks to the natural secondary / auxiliary
reflection, the primary auxiliary winding cannot
maintain a sufficient voltage on the control IC: Vcc
collapses and puts the controller in trouble, probably
entering an hiccup mode, similar to that of a startup
sequence. DSS being decoupled from Vout, you never
see that phenomenon.
As you can see, the DSS offers interesting features but, on
the other hand, it can sometimes compromise key design
parameters. Standby power and power dissipation are one of
these:
Standby power: the DSS standby power contribution
can easily be evaluated: VHV × Iavg with Iavg, the current
consumption taken by the controller and VHV, the
high-voltage supply rail. If Iavg equals 1 mA, then we
have a standby power of 350 mW at a 350 VDC voltage
rail. Tricks exist to slightly reduce it, like the half-wave
diode, but you will only gain between 20–30%.
Power dissipation: as stated above, all the current
consumed by the IC is seen through pin8. This is due to
the self-adaptive feature of the DSS. Should the IC
current move up or down, the DSS duty-cycle will
automatically adjust to deliver it. The controller current
depends on the internal IC consumption, but also on the
type of MOSFET connected to the output. It therefore
important to assess the total current drawn from the HV
rail and checks the right compatibility with the package
type. All details are given in the NCP1200 dedicated
data sheet and the application note AND8023/D.
As a result, the answer lies behind your design constraints.
If you would like to have a precise Over Current Protection
(OCP) trip point while driving a moderate size MOSFET,
DSS can be a good choice, provided low standby power is
not an absolute necessity. In our case, we want to drive a
large MOSFET for a better efficiency but we need to reach
the lowest possible standby power. We will thus adopt an
auxiliary winding configuration to permanently disable the
DSS. Solutions to various combinations of these constraints
are described in the application note “Tips and Tricks for the
NCP1200,” document number AND8069/D.
© Semiconductor Components Industries, LLC, 2003
April, 2003 - Rev. 2
1
Publication Order Number:
AND8076/D


  ON Semiconductor Electronic Components Datasheet  

AND8076 Datasheet

A 70W Low Standby Power Supply

No Preview Available !

AND8076/D
Self-Powering the Controller in Standby
An auxiliary winding does not usually cause any
self-supply problem with a continuous pulses flow. In
standby, whatever implemented frequency reduction
techniques (e.g. skip or frequency foldback), the recurrence
between pulses can become very low. By definition, the
feedback loop manages to keep the energy content in each
burst high enough to maintain the nominal output voltage.
However, on the auxiliary side, it can be difficult to keep the
Vcc above the controller’s UVLO. Remember, to
permanently disable the DSS, you need to guarantee a level
above VccON max. which is 11 V for the NCP1200. Failure
to do this will re-activate the DSS in no-load conditions and
standby power will be degraded. Figure 1 offers a view of a
typical bunch of pulses captured in standby at a 127 VDC
input voltage.
33 ms
auxiliary output gets clamped by a 15V Zener diode in
nominal operation. Figure 2 shows the option.
We measured a Vcc of 11.5 V @ 230 VAC and 12.2 V @
90 VAC. Rlimit on Figure 2 can easily be adjusted to move
these values up or down, depending on the final winding
ratios. Care must be taken to avoid over-dissipation of the
15 V Zener diode in nominal conditions.
Power Supply, Element-by-Element Design
Let’s first detail the specs of our power supply:
Vin: 90–265 VAC
Vout: 16.8 V @ 4.2 A (Pout = 70 W)
Short-circuit protection
Over-voltage protection
Efficiency > 80%
Pin = 70 / 0.8 = 87.5
The below sequence details step-by-step the calculation
procedure for every component of the power supply.
DC High-Voltage Rail
From these above numbers, we can deduce the level of the
high-voltage rail, neglecting the dual Vf drop:
VHV max + 265 @ Ǹ2 + 374 VDC
VHV min + 90 @ Ǹ2 + 127 VDC
200 µs
Vripple
Vinpeak
Figure 1. A Bunch of Auxiliary Pulses Captured
While the Supply Operates at No-Load
(Vin = 127 VDC)
18
2 NCP1200 7
36
45
CVcc
Rlimit
3
15 V
1N4148
21
Caux
Laux
VHVavg
10 ms
Figure 3. A Typical Ripple Voltage Over the Bulk
Capacitor
Figure 2. The Auxiliary Is Clamped to Avoid
Exceeding the 16 V Maximum Rating
As we previously stated, we want to deliver 70 W with a
16.5 V output level. The maximum rating for the NCP1200
states a level less than 16 V. As a result, the auxiliary Vcc
shall be less than 16V but also above VccON in any
conditions to ensure full DSS de-activation. A solution
consists in artificially raising the ratio between the power
winding and the auxiliary one to ensure adequate supply at
no-load. We successfully tested a 0.9 ratio, where the
Bulk Capacitor
Figure 3 portrays the typical waveform captured across a
bulk capacitor delivering power to a given charge. To
simplify the calculation, we will neglect the charging period
and thus consider a total discharge time equal to 1/(2 Fline).
From the design characteristics, we can evaluate the
equivalent current (Iload) drawn by the charge at the lowest
input line condition. Let’s us adopt a 40% ripple level, or a
50 V drop from the corresponding Vinpeak. To evaluate the
equivalent load current (which discharges Cbulk between
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Part Number AND8076
Description A 70W Low Standby Power Supply
Maker ON Semiconductor
Total Page 14 Pages
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