SC1933C datasheet(9/28 Pages) POWERINT

9 / 28 page

Rev. C 11/18

SC1933C/SC1936C

www.power.com

FL1

EQ25

FL2

InnoSwitch3

BPP

SC1933C-H114

2.2 nF

630 V

4.7 µF

16 V

6.8 µF

63 V

330 µF

25 V

C10

2.2 µF

25 V

C11

2.2 µF

25 V

C12

1000 pF

50 V

330 µF

25 V

C15

10 µF

35 V

C17

1 µF

100 V

C13

330 pF

100 V

100 µF

400 V

680 kΩ

20 Ω

DFLR1800-7

BAV3004WS-7

300 V

BAV19WS

100 V

R11

10 Ω

1 nF

200 V

680 pF

250 VAC

BAV19WS

100 V

R15

169 kΩ

1/16 W

R16

11.5 kΩ

1/10 W

R13

0.0 Ω

1/10 W

R14

0.009 Ω

1/2 W

3 kΩ

1/10 W

47 Ω

AON6220

R12

47 Ω

1/10 W

2.00 MΩ

1.80 MΩ

CONTROL

BR1

GBL06

600 V

04291-T231

18 mH

250 µH

330 nF

275 VAC

1 MΩ

3.15 A

90 - 265

VAC

CAPZero

CAP200DG

PI-8770-081718

Figure 6. 20 V, 3.25 A Notebook Adapter.

The circuit shown in Figure 6 is a 20 V, 3.25 A adapter using SC1933C.

This design is DOE Level 6 and EC CoC 5 compliant.

Fuse F1 isolates the circuit and provides protection from component

failure, and the common mode choke L1 and L2 with capacitor C1

attenuation for EMI. Bridge rectifier BR1 rectifies the AC line voltage

and provides a full wave rectified DC across the filter capacitor C2.

Capacitor C3 is used to mitigate the common mode EMI.

Resistors R1 and R2 along with U2 discharges capacitor C1 when the

power supply is disconnected from AC mains.

One end of the transformer (T1) primary is connected to the rectified

DC bus; the other is connected to the drain terminal of the switch

inside the SC1933C IC (U1). Resistors R3 and R4 provide Input

voltage sense protection for under voltage and over voltage

conditions.

A low cost RCD clamp formed by diode D1, resistors R5, R6, and R7,

and capacitor C4 limits the peak drain voltage of U1 at the instant of

turn off of the switch inside U1. The clamp helps to dissipate the

energy stored in the leakage reactance of transformer T1.

The IC is self-starting, using an internal high-voltage current source

to charge the BPP pin capacitor (C6) when AC is first applied. During

normal operation the primary-side block is powered from an auxiliary

winding on the transformer T1. Output of the auxiliary (or bias)

winding is rectified using diode D2 and filtered using capacitor C5.

Resistor R8 limits the current being supplied to the BPP pin of the

SC1933C IC (U1).

Zener diode VR1 along with R9 and D3 offers primary sensed output

over voltage protection. In a flyback converter, output of the

auxiliary winding tracks the output voltage of the converter. In case

of over voltage at output of the converter, the auxiliary winding

voltage increases and causes breakdown of VR1 which then causes a

Applications Example

current to flow into the BPP pin of SC1933C IC U1. If the current

flowing into the BPP pin increases above the ISD threshold, the U1

controller will latch off and prevent any further increase in output

voltage.

The secondary-side of the SC1933 IC provides output voltage, output

current sensing and drive to a SR FET providing synchronous

rectification. The secondary of the transformer is rectified by SR FET

Q1 and filtered by capacitors C8 and C9. Capacitors C15 and C17 are

used to reduce the high frequency output voltage ripple. High

frequency ringing during switching transients that would other- wise

create radiated EMI is reduced via a RCD snubber R11, C7 and D4.

Diode D4 was used to minimize the dissipation in resistor R11.

The gate of Q1 is turned on by secondary side controller inside IC U1,

based on the winding voltage sensed via resistor R12 and fed into the

FWD pin of the IC.

In continuous conduction mode of operation, the SR FET is turned off

just prior to the secondary-side commanding a new switching cycle

from the primary. In discontinuous mode of operation, the power

SR FET is turned off when the voltage drop across the SR FET falls

below a threshold of V

SR(TH). Secondary-side control of the primary-

side power switch avoids any possibility of cross conduction of the

two switches and provides extremely reliable synchronous

rectification.

The secondary side of the IC U1 is self-powered from either the

secondary winding forward voltage or the output voltage. Capacitor

C10 connected to the BPS pin of IC U1 provides decoupling for the

internal circuitry. When the output voltage is below 5 V during

start-up, the device will directly power itself from the secondary

winding. During the on-time of the primary-side power switch, the

forward voltage that appears across the secondary winding is used to

charge the decoupling capacitor C10 via resistor R12 and an internal

regulator. Capacitor C11 provides decoupling for the VO pin.

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