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NCP1200D60R2
PWM Current Mode Controller With Low Standby Power, 40/60/100kHz
AND8023/D
Implementing the NCP1200
in Low-Cost AC/DC
Converters
Prepared by: Christophe Basso
ON Semiconductor
INTRODUCTIONThe NCP1200 implements a standard current mode
architecture where the switch–off time is dictated by the
peak current setpoint. This component represents the
ideal candidate where low part–count is the key
parameter, particularly in low–cost AC/DC adapters,
auxiliary supplies etc. Thanks to its proprietary Very
High–Voltage Integrated Circuit (VHVIC) technology,
ON Semiconductor NCP1200 will please experts as well
as non–experts in the Switch–Mode Power Supplies
(SMPS) arena as the following features demonstrate:
No need of auxiliary winding: the VHVIC technologylets you supply the IC directly from the high–voltage DC
rail. We call it Dynamic Self–Supply (DSS). In battery
charger applications, you no longer need to design a
special primary circuitry to cope with the transient lack of
auxiliary voltage (e.g. when V out is low).
Short–circuit protection: by permanently monitoringthe feedback line activity, the IC is able to detect the
presence of a 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. For given applications (e.g. constant
output power supplies), you can easily disconnect this
protective feature.
Low standby–power: if SMPS naturally exhibit a goodefficiency at nominal load, they begin to be less efficient
when the output power demand diminishes. By skipping
un–needed switching cycles, the NCP1200 drastically
reduces the power wasted during light load conditions. In
no–load conditions, the NPC1200 allows the total
standby power to easily reach next International Energy
Agency (IEA) recommendations.
No acoustic noise while operating: instead of skippingcycles at high peak currents, the NCP1200 waits until the
peak current demand falls below a user–adjustable 1/3rd
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.
External MOSFET connection: by leaving the externalMOSFET external to the IC, you can select avalanche
proof devices which, in certain cases (e.g. low output
powers), let you work without an active clamping
network. Also, by controlling the MOSFET gate signal
flow, you have an option to slow down the device
commutation, therefore reducing the amount of
ElectroMagnetic Interference (EMI).
SPICE model: 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, INTUSOFT’s IsSpice and Spectrum–Software’s
μCap from the ON
www., NCP1200 related section.
Low external part–count: by integrating the principalelectronic blocks in one die, the final NCP1200
implementation reveals an obvious gain in component
reduction compared to other offers: Built–in clock generator without external R–C
elements. Operating frequencies at 40 kHz, 60 kHz or
100 kHz. The optocoupler is directly wired to the feedback pin,
the internal IC control taking care of the signal flow. The 250 ns Leading Edge Blanking (LEB) circuitry
also saves an external R–C network.
Dynamic Self–SupplyTo avoid the use of a dissipative resistor,
implements a controlled current source whose technology
allows a direct connection to the high–voltage rail (up to
450 VDC). Figure 1a depicts the component arrangement.
The current source always operates in the ON or OFF
states, either delivering 4 mA or zero. As a result, the
internal VCC pin ramps up and down with a 2 V ripple
centered around 10.6 V (Figure 1b).
