AN8026 Ver la hoja de datos (PDF) - Panasonic Corporation

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componentes Descripción

Fabricante

AN8026

Self-excited RCC pseudo-resonance type AC-DC switching power supply control IC

Panasonic Corporation 

Voltage Regulators

AN8026

s Application Notes (continued)

[2] Operation descriptions (continued)

2. Oscillation circuit (continued)

An equation for approximate calculation of the maximum on-period TON(max) of the power MOSFET is as follows:

TON(max) = 6 500 × C3

When the power MOSFET is off, the TOFF terminal becomes a high impedance state and the C2 starts

discharging by the R2. When the voltage across the both ends of C2 decreases to approximately 0.1 V, the oscillation

circuit output is reversed again to turn on the power MOSFET. At the same time, the C2 is rapidly charged to

approximately 0.9 V. An equation for approximate calculation of the minimum off-period TOFF(min) is as follows:

TOFF(min) = 2.2 × C2 × R2

However, when the voltage-period fed back to the TDL terminal (pin 1) is longer than the TOFF period determined by

the C2 and R2, the off-period in the pseudo-resonance circuit operation described below is determined by the former.

By repeating the above operation, the power MOSFET is turned on and off continuously. Figure 2 shows the

oscillation waveform at the time when the TDL terminal is pulled down to the GND.

3. Power supply output control system (FB: feedback)

The constant voltage control of the power supply output is achieved by fixing the off-period of the power

MOSFET and changing the on-period. The control of on-period is performed by changing the charge current

from the TON terminal to the C3 through the following process: the photocoupler connected to the FB terminal

(pin 9) absorbs, from the FB terminal, the feedback current corresponding to the output signal of the output

voltage detection circuit provided in the secondary side output. A current approximately 8 times of the current

flowing out of the FB terminal flows out of the TON terminal as the charge current for the C3. (Refer to figure 3.)

The higher the AC input voltage of the current becomes, or the smaller the load current becomes, the larger the

current flowing out of the FB terminal becomes. When the current flowing out of the FB terminal becomes larger,

the charging to C3 becomes faster and the on-period becomes shorter.

In addition, the system has cancellation capability of about 150 µA for the dark current of the photocoupler.

(Refer to figure 4.)

1:8

200 Ω

PC

Secondary side power

supply output

AN1431T/M

PC C3 C2

R2 Primary Secondary

side

side

Figure 3. Power supply output control system

ITON (mA)

−10

−8

−6

−4

−2

Dark

current

0

− 0.2

− 0.4

− 0.6

− 0.8

−1.0

IFB (mA)

Figure 4. Feedback current versus charge current characteristics

7

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