NDRV did not switch enough times to build up sufficient

voltage across the tertiary output to power the device.

The device will go back into UVLO and will not start. Use

a low-leakage ceramic or film capacitor for C2 and C3.

As a rule of thumb, off-line power supplies keep typical

startup times to less than 500ms even in low-line condi-

ply the maximum startup bias of the IC (85µA) plus the

additional current required for charging the capacitors

C2 and C3 in less than 500ms. This resistor dissipates

continuous power in normal operation, despite the fact

that it is only used during the startup sequence.

Therefore it must be chosen to provide enough current

for the low-line condition as well as have an appropriate

power rating for the high-line condition (265VAC). In

most cases, split the value into two resistors connected

in series for the required voltage of approximately

400VDC.

The typical value for C2 and C3 is 220nF. The startup

rent of 85µA and the charging current for C2 and C3.

Bypass capacitor C3 charges to 9.5V and C2 charges

to 24V all within the desired time period of 500ms, for

an overall average charging current of 15µA. Hence,

the startup resistor must provide a total of at least

100µA. Developing 100µA from an input voltage of

Current-Mode PWM Controllers for Isolated

Power Supplies

6

_______________________________________________________________________________________

D1

AC

L

N

G

CENTRAL SEMICONDUCTOR

CBR1-D100S

AC

L2

470

µH

MAX5022

OPTO

FAIRCHILD

CNY17-3

OPTO

NOTE: ALL RESISTORS ARE 5% UNLESS OTHERWISE SPECIFIED.

CTX03-15256

T1

D2

3A, 40V

ON SEMICONDUCTOR

MBRS340T3

R6

10

Ω

C4

150

µF

6.3V

C7

1000pF

C6

0.1

µF

C9

10

µF

400V

C1

10

µF

400V

R3

1k

Ω

1%

R4

24.9k

Ω

1%

R10

10

Ω

8T

3T

480

µH, 60T

R1

360k

Ω

R8

1.2k

Ω

R2

360k

Ω

R11

10

Ω

+5V OUT

C5

0.01

µF

C8

8200pF

C2

0.22

µF

N1

INTERNATIONAL RECTIFIER

IRFRC20

C3

0.22

µF

85VAC TO

265VAC IN

D1

250mA, 75V

CENTRAL SEMICONDUCTOR

CMPD914

R5

8.06k

Ω

1%

1.78

Ω

1%

R9

240k

Ω

TEXAS

INSTRUMENTS

TLV431AIDBV

OPTO

NDRV

U1

CS

GND

Figure 1. Universal 5W Off-Line Standby Power Supply

0

5

15

10

20

25

TIME (ms)

0

100

50

150

200

NDRV BEGINS SWITCHING

IC COMES OUT OF UVLO

(WAKEUP)

WINDING (NORMAL OPERATION)

FULLY CHARGED