AN8026 データシートの表示（PDF） - Panasonic Corporation

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AN8026

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

Panasonic Corporation 

AN8026

Voltage Regulators

s Application Notes (continued)

[2] Operation descriptions

1. Start/stop circuit block

• Start mechanism

When AC voltage is applied and the sup-

ply voltage reaches the start voltage through

the current from start resistor, the IC starts

After AC rectificatio

Start resistance

R1

VCC

C8

VOUT

GND

operation. Then the power MOSFET driving

starts. Thereby, bias is generated in the trans-

former and the supply voltage is given from

Before start

Start

the bias coil to the IC. (This is point a in figure 1.) Start

Voltage supplied

a

from bias coil

During the period from the time when the voltage

start voltage is reached and the voltage is gen-

Start condition

erated in the bias coil to the time when the IC

Stop

voltage

is provided with a sufficient supply voltage,

b

c Start failure

the supply voltage of the IC is supplied by the

capacitor (C8) connected to VCC .

Figure 1

Since the supply voltage continuously decreases during the above period (area b in figure 1), the power supply

is not able to start (state c in figure 1), if the stop voltage of the IC is reached before the sufficient supply voltage

is supplied from the bias coil.

• Function

The start/stop circuit block is provided with the function to monitor the VCC voltage, and to start the operation

of IC when VCC voltage reaches the start voltage (14.9 V typical), and to stop when it decreases under the stop

voltage (8.6 V typical). A large voltage difference is set between start and stop (6.3 V typical), so that it is easier

to select the start resistor and the capacitor to be connected to VCC .

Note) To start up the IC operation, the startup current which is a pre-start current plus a circuit drive current is necessary.

Set the resistance value so as to supply a startup current of 350 µA.

2. Oscillation circuit

The oscillation circuit generates the pulse signal for turning on/off the power MOSFET by using charge/

discharge of the C2, R2 and C3 connected to TOFF (pin 2), TON (pin 3) respectively.

The concept of constant voltage control at the time of making up the switching power supply is fixing the off-

period of the power MOSFET and achieving the control by changing the on-period. This on-period control is

performed by directly changing the output pulse width of the oscillation circuit.

During the on-period of the power MOSFET, the C2 is charged to the constant voltage (approximately 0.9 V).

On the other hand, the C3 is charged from almost 0 V by the charge current from the TON terminal. When the

voltage across the both ends of the C3 reaches approximately 0.7 V (Ta = 75°C), the oscillation circuit output is

reversed and the power MOSFET is turned off. At the same time, the C3 is rapidly discharged by the discharge

circuit inside the IC and its voltage across the both ends becomes almost 0 V. The charge current from the TON

terminal is changed by the feedback signal to the FB terminal (pin 9). (Described later.)

On-period Off-period

200 Ω

0.9 V

Voltage across

both ends of C2 0.1 V

0.7 V

Voltage across

both ends of C3 0 V

C3 C2

R2

IC output

0V

Figure 2. Oscillation circuit operation

6

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