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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)

8. Output block

In order to drive the power MOSFET which is

a capacitive load at high speed, this IC is adopt-

ing the totem pole (push-pull) type output circuit

which performs the sink and source of the current

with the NPN transistor as shown in figure 10.

The maximum sink/source current is ±0.1 A

(DC) and the current at peak is ±1.0 A (peak). The

Schottky barrier

diode

circuit is provided with the sink capability even if

the supply voltage VCC is under the stop voltage so

that it turns off the power MOSFET without fail.

Figure 10

The peak current capability is mainly required and a particularly too large current is not required constantly.

Because the power MOSFET which becomes a load for the output is capacitive, a large peak current is required

for driving it at a high speed. However, after the charge and discharge, a particularly large current is not required

to keep such condition. In the case of this IC, the peak current capability of ±1 A is ensured by taking a

capacitance value of the power MOSFET used into account.

The parasitic LC of the power MOSFET may produce ringing to decrease the output pin under the GND

potential. When the voltage decrease of the output pin becomes larger than the voltage drop of diode and its

voltage becomes negative, the parasitic diode consisting of the substrate and collector of the output NPN turns on.

This phenomenon can cause the malfunction of device. In such a case, the Schottky barrier diode should be

connected between the output and GND.

[3] Design reference data

• How to start the soft start function by external parts

The power supply rises under overload condition due to the After AC rectification

capacitor connected to the power supply output. In this condi-

tion, since the voltage of the power supply output is low, the

normal constant voltage control attempts to rise the power sup-

ply output at the maximum duty. The control uses the pulse-by-

Start resistor

R1

VCC

C8

pulse overcurrent protection (CLM), attempting to limit the cur-

rent. However, the pulse can not be brought down to zero due to

delay of filter, etc. As a result, a large current flows into the main

switch (the power MOSFET) or the diode in the secondary side,

and in the worst case these parts are damaged. For this reason,

the soft start function is used to suppress the rush current at start

of the power supply.

As the method of installing the soft start function, the R3 and

C4 are connected between the OVP terminal (pin 8) and the

CLM terminal (pin 4) as shown in figure 11. When the supply

OVP

VOUT

CLM

GND

C4

R3

C6 R6

R7

voltage of the IC reaches the start voltage and the start circuit

begins to operate, an open bias of approximately 6.5 V at the

Figure 11

OVP

terminal is outputted. By this voltage, the charge current flows into the C4 and the CLM terminal voltage rises. The

CLM terminal voltage decreases with the lapse of time since it changes in proportion to the charge current of the

C4. The CLM circuit operates by the sum of the voltage across the both ends of R7 created by the current flowing

into the power MOSFET when turning on power and the voltage across the both ends of R6 created by the charge

current of the C4.

Therefore, since the current which flows into the power MOSFET when turning on power gradually increases,

the rush current can be suppressed.

11

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