AN4138 datasheet(8/20 Pages) FAIRCHILD

Part #	AN4138

Description	Design Considerations

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Manufacturer	FAIRCHILD [Fairchild Semiconductor]

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AN4138

APPLICATION NOTE

8

In the snubber design in this section, neither the lossy

discharge of the inductor nor stray capacitance is considered.

In the actual converter, the loss in the snubber network is less

than the designed value due to this effects.

Figure 12. MOSFET drain voltage and snubber capacitor

voltage

(12) STEP-12 : Design the Control circuit.

In general, a battery charger employs constant current (CC) /

constant voltage (CV) control circuit for an optimal charge

of a battery. This design note presents two basic CC/CV

control circuits for FPS flyback converters. A simple, low

cost circuit using a transistor and shunt regulator (KA431) is

presented first. The second circuit features highly accurate

current control using an op amp together with a shunt

regulator (KA431) and secondary bias winding. In the circuit

analysis, it is assumed that the CTR of the opto-coupler is

100%.

(a) Transistor and regulator (KA431) scheme

Figure 13 shows the CC/CV control circuit using a transistor

and KA431 for 5.2V/0.65A output application. This circuit is

widely used when low cost and simplicity are major

concerns. Since the transistor base-emitter voltage drop

depends on the temperature, a temperature compensation

circuit is required for temperature stability. To turn on the

transistor (Q), about 0.7V voltage drop across the sensing

resistor (Rsense) is required and this current control circuit

should be used for output currents below 1A due to the

power dissipated in current sense resistor. For output currents

greater than 1A, or if output current accuracy and

temperature stability are a key factor, the op amp current

control circuits shown in Figure 15 should be used.

Figure 13. Transistor and KA431 CC/CV control

Constant voltage (CV) control : The voltage divider

network of R1 and R2 should be designed to provide 2.5V to

the reference pin of the KA431. The relationship between R1

and R2 is given by

where Vo is the output voltage.

By choosing R1 to be 2.2k

Ω, R2 is obtained as

The feedback capacitor (CF) introduces an integrator for CV

control. To guarantee stable operation, CF of 470nF is

chosen.

The resistors Rbias and Rd should be designed to provide

proper operating current for the KA431 and to guarantee the

full swing of the feedback voltage for the FPS device chosen.

In general, the minimum cathode voltage and current for the

KA431 are 2.5V and 1mA, respectively. Therefore, Rbias and

Rd should be designed to satisfy the following conditions.

0 V

V

DC

max

V

RO

V

sn2

Effect of stray inductance (5-10V)

BVdss

Voltage Margin > 10% of BVdss

N

S

D

R

C

O

KA431

817A

R

d

R

bias

R

1

R

2

C

F

V

O

L

P

C

P

R

sense

R

base

Q

R

TH

C

B

v

FB

1:1

FPS

R

B

GND

I

FB

I

o

250uA

510Ω

Ω

56Ω

Ω

470nF

KSP2222

10kΩ

Ω

510Ω

Ω

1Ω

Ω

5.2V / 0.65A

2.2kΩ

Ω

2kΩ

Ω

R

2

2.5 R

1

⋅

V

o

2.5

–

---------------------

=

(28)

R

2

2.5 2.2k

Ω

⋅

5.2V

2.5V

–

-------------------------------

2k

Ω

==

V

o

V

OP

–

2.5

–

Rd

--------------------------------------

I

FB

>

(29)

V

OP

R

bias

--------------1mA

>

(30)

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