SC1933C datasheet(13/28 Pages) POWERINT

13 / 28 page

Rev. C 11/18

SC1933C/SC1936C

www.power.com

SR FET Operation and Selection

Although a simple diode rectifier and filter works for the output, use

of an SR FET enables the significant improvement in operating

efficiency often necessary to meet the European CoC and the U.S.

DoE energy efficiency requirements. The secondary-side controller

turns on the SR FET once the flyback cycle begins. The SR FET gate

should be tied directly to the SYNCHRONOUS RECTIFIER DRIVE pin

of the SC1933C/SC1936C IC (no additional resistors should be

connected in the gate circuit of the SR FET). The SR FET is turned

off once the V

DS of the SR FET reaches VSR(TH).

A FET with 6 mΩ R

DS(ON) is appropriate for a 20 V, 3.25 A output, and

two SR FET’s in parallel with 6 mΩ R

DS(ON) is suitable for designs rated

with a 20 V, 5 A output. The SR FET driver uses the SECONDARY

BYPASS pin for its supply rail, and this voltage is typically 4.4 V. A

FET with a high threshold voltage is therefore not suitable; SR FETs

with a threshold voltage of 1.5 V to 2.5 V are ideal although FETs with

a threshold voltage (absolute maximum) as high as 4 V may be used

provided their data sheets specify R

DS(ON) across temperature for a

gate voltage of 4.5 V.

There is a slight delay between the commencement of the flyback

cycle and the turn-on of the SR FET. During this time, the body diode

of the SR FET conducts. If an external parallel Schottky diode is

used, this current mostly flows through the Schottky diode. Once the

SC1933C/SC1936C IC detects V

SR(TH) across SR FET it turns off the SR

gate and any remaining portion of the flyback cycle is completed with

the current commutating to the body diode of the SR FET or the

external parallel Schottky diode. Use of the Schottky diode parallel

to the SR FET may provide another ~0.1% - 0.2% higher efficiency.

The voltage rating of the Schottky diode and the SR FET should be at

least 1.4 times the expected peak inverse voltage (PIV) based on the

turns ratio used for the transformer. 60 V rated FETs and diodes are

suitable for most 5 V designs that use a V

OR < 60 V, and 100 V rated

FETs and diodes are suitable for up to 20 V designs.

The interaction between the leakage reactance of the output

windings and the SR FET capacitance (COSS) leads to ringing on the

voltage waveform at the instance of voltage reversal at the winding

due to primary switch turn-on. This ringing can be suppressed using

an RC snubber connected across the SR FET. A snubber resistor in

the range of 4.7 Ω to 47 Ω may be used (higher resistance values

lead to noticeable drop in efficiency). A capacitance value of

1 nF to 2.2 nF is adequate for most designs.

Output Capacitor

Low ESR aluminum electrolytic capacitors are suitable for use with

most high frequency flyback switching power supplies though the use

of aluminum-polymer solid capacitors have gained considerable

popularity due to their compact size, stable temperature

characteristics, extremely low ESR and high RMS ripple current rating.

These capacitors enable the design of ultra-compact chargers and

adapters.

Typically, 200 mF to 300 mF of aluminum-polymer capacitance per

ampere of output current is adequate. The other factor that

influences choice of the capacitance is the output ripple. Ensure that

capacitors with a voltage rating higher than the highest output

voltage plus sufficient margin be used.

Output Voltage Feedback Circuit

The output voltage FEEDBACK pin voltage is 1.265 V [V

FB]. A voltage

divider network should be connected at the output of the power

supply to divide the output voltage such that the voltage at the

FEEDBACK pin will be 1.265 V when the output is at its desired

voltage. The lower feedback divider resistor should be tied to the

SECONDARY GROUND pin. A 330 pF (or smaller) decoupling capacitor

should be connected at the FEEDBACK pin to the SECONDARY

GROUND pin of the SC1933C/SC1936C IC. This capacitor should be

placed close to the SC1933C/SC1936C IC.

Interfacing with USB PD and Rapid Charge Controllers

A microcontroller can be used to alter the feedback voltage divider in

order to increase or decrease the output voltage. The interface IC

can also use the signal from the SC1933C/SC1936C ISENSE pin to

sense output current and provide current, power limiting or

protection features.

Figure 12. (a) Line OV Only; (b) Line UV Only.

PI-8405a-102218

1N4148

VOUT

BPP

SC1933C/36C

(a)

(b)

PI-8406a-102218

6.2 V

VOUT

BPP

SC1933C/36C

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