NCP1205(2002) Просмотр технического описания (PDF) - ON Semiconductor

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производитель

NCP1205
(Rev.:2002)

Single Ended PWM Controller Featuring QR Operation and Soft Frequency Foldback

ON Semiconductor 

NCP1205

If we now enter this formula into a spreadsheet, we can easily plot the switching frequency versus the output power demand:

250000

200000

150000

Fmax

100000

Transition

BCM to VFM

Fmax

50000

VCO Action

0

0

5

10

15

20

OUTPUT POWER (W)

Figure 12. A Typical Behavior of Free Running Systems

with a Smooth Frequency Foldback with the NCP1205

The typical above diagram shows how the frequency

moves with the output power demand. The components used

for the simulation were: Vin = 300 V, Lp = 6.5 mH,

Vout = 10 V, Np/Ns = 12.

The red line indicates where the maximum frequency is

clamped. At this time, the VCO takes over and decreases the

switching frequency to the minimum value.

VCO Operation

The VCO is controlled from the Verr voltage. For Verr

levels above 1.0 V, the VCO frequency remains unchanged

at 125 kHz. As soon as Verr starts to decrease below 1.0 V,

the VCO frequency decreases with a typical small–signal

slope of –175 kHz/mV @ Verr = 500 mV down to

zero (typically at FB ≈ 3.3 V). The demagnetization

synchronization is however kept when the Toff expands.

The maximum switching frequency can be altered by

adjusting the Ct capacitor on pin 5. The 125 kHz maximum

operation ensures that the fundamental component stays

external from the international EMI CISPR–22

specification beginning.

The following drawing explains the philosophy behind

the idea:

Internal Verr

VCO Frequency

is Fixed at 130 kHz

VCO Frequency

can Decrease

BCM Mode

Peak current

can change

Peak Current is Fixed

3V

1V

0.75 V

Figure 13. When the Power Demand goes Low, the Peak Current is Frozen and the Frequency Decreases

Zero Crossing Detector

To detect the zero primary current, we make use of an

auxiliary winding. By coupling this winding to the primary,

we have a voltage image of the flux activity in the core.

Figure 13 details the shape of the signal in BCM.

The auxiliary winding for demagnetization needs to

be wired in Forward mode. However, the application

note describes an alternative solution showing how to wire

the winding in Flyback as well. As Figure 13 depicts, when

the MOSFET closes, the auxiliary winding delivers

(Naux/Np . Vin). At the switch opening, we couple the

auxiliary winding to the main output power winding and

thus deliver: (–Naux/Ns . Vout). When DCM occurs, the

ringing also takes place on the auxiliary winding. As soon

as the level crosses–up the internal reference level

(65 mV), a signal is internally sent to re–start the MOSFET.

Three different conditions can occur:

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