FA5502 데이터 시트보기 (PDF) - Fuji Electric

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FA5502

Power Supply Control IC

Fuji Electric 

FA5502P/M

9. Design advice

(1) Vcc circuit

Vcc voltage can be supplied from an auxiliary

winding of the inductor. An example circuit is shown in

Fig.15.

L

D1

Vac

(sub)

Co

R8

D3

Q1

Rs

C5

10

VCC

7

FA5502 GND

Fig.15 Vcc circuit (1)

In this circuit, R8 is a start up resistor. The start up

resistor R8 should be satisfied the following formula in

order to supply with at least 30µA of IC start up

current.

R8 <

2 × Vac(min) − 17.5

30 ×10−6

• • • • • (11)

Note that this formula is a minimum condition for

starting the IC. Practically, determine the value upon

taking into account the start up time required for

converter. The start up time must be determined upon

measurement at actual circuit operation.

In steady state, Vcc is supplied from the auxiliary

winding (sub) of inductor. When the IC is just starting

up, however, it takes time for the voltage from auxiliary

winding to rise enough. The value of capacitor C5

connected to Vcc pin should be determined to prevent

Vcc from falling below the OFF threshold voltage of

UVLO during this period. The capacity of C5 should be

determined by evaluation in the actual circuit because

the time lag is different in each circuit.

Vcc

UVLO

ON

UVLO

OFF

Vcc must not drop

below UVLO OFF.

Quality is our message

Even after PFC starts up, Vcc may fall due to step

changes of the load or inputs. To prevent the IC from

stopping in those cases, the circuit shown in Fig.17 is

effective to prolong the hold time of the Vcc voltage.

After the PFC converter starts up, Vcc is supplied

through C6. Therefore, you can prolong the hold time

of Vcc by using a large capacity for C6.

VCC

R8

D3 D4

sub

C5

C6

Fig.17 Vcc circuit (2)

In some case, the Vcc voltage cannot be supplied

enough in light load condition. In this case, the circuit

shown in Fig.18 may be effective to improve the Vcc.

The appropriate value of C7 and R9 should be

determined by evaluation in actual circuit because

they depend on each circuit.

VCC

R8

D3 C7 R9

sub

C5 D5

Fig.18 Vcc circuit (3)

(2) Supplying Vcc from external power supply

If Vcc is not supplied from the auxiliary winding of

inductor but from an external power supply, pay

attention to the followings.

• In order to start up the IC, Vcc must be above the ON

threshold voltage VTHUON (17.5V (max.)) of

undervoltage lockout circuit (UVLO). When starting

up, apply at least this VTHUON. After starting up,

the operation is available within the recommended

range of 10 to 28V.

• If a noise is applied to Vcc pin, it may cause

malfunction. To avoid a noise, connect a capacitor

near VCC pin even when Vcc is supplied from an

external power supply. To prevent a malfunction,

suppress the noise below about ±0.6V. And, make

sure there is no malfunction attributable to noise.

Auxiliary winding voltage

t

Fig.16 Vcc voltage at start up

18

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