SC1933C datasheet(10/28 Pages) POWERINT

10 / 28 page

Rev. C 11/18

SC1933C/SC1936C

www.power.com

Below the CC threshold, the device operates in constant voltage

mode. During constant voltage mode operation, output voltage

regulation is achieved through sensing the output voltage via divider

resistors R15 and R16. The voltage across R16 is fed into the FB pin

with an internal reference voltage threshold of 1.265 V. Output

voltage is regulated so as to achieve a voltage of 1.265 V on the FB

pin. Capacitor C13 provides noise filtering of the signal at the FB pin.

During CC operation power supply allows output current regulation to

be maintained V

OUT(AR). Output current is sensed by monitoring the

voltage drop across resistor R21 between the IS and SECONDARY

GROUND pins. A threshold of approximately 35 mV reduces losses.

C12 provides filtering on the IS pin from external noise. Once the

internal current sense threshold is exceeded the device regulates the

number of switch pulses to maintain a fixed output current.

Key Application Considerations

Output Power Table

The data sheet output power table (Table 1) represents the maximum

practical continuous output power level that can be obtained under

the following conditions:

1. The minimum DC input voltage is 90 V or higher for 85 VAC input,

220 V or higher for 230 VAC input or 115 VAC with a voltage-

doubler. Input capacitor voltage should be sized to meet these

criteria for AC input designs.

2. Assumed >90% efficiency.

3. Transformer primary inductance tolerance of ±7%.

4. Reflected output voltage (VOR) is set to maintain K

P = 0.8 at

minimum input voltage for universal line and K

P ≥ 1 for high input

line designs.

5. Maximum conduction losses for adapters is limited to 0.6 W, 0.8 W

for open frame designs.

6. Increased current limit is selected for peak and open frame power

columns and standard current limit for adapter columns.

7. The part is board mounted with SOURCE pins soldered to a

sufficient area of copper and/or a heat sink to keep the SOURCE

pin temperature at or below 110 °C.

8. Ambient temperature of 50 °C for open frame designs and 40 °C

for sealed adapters.

Below a value of 1, K

P is the ratio of ripple to peak primary

current. To prevent reduced power delivery, due to premature

termination of switching cycles, a transient K

P limit of ≥0.25 is

recommended. This prevents the initial current limit (I

INT) from

being exceeded at switch turn-on.

Primary-Side Overvoltage Protection (Latch-Off Mode)

Primary-side output overvoltage protection provided by the SC1933C/

SC1936C IC uses an internal latch that is triggered by a threshold

current of ISD into the PRIMARY BYPASS pin. In addition to an

internal filter, the PRIMARY BYPASS pin capacitor forms an external

filter helping noise immunity. For the bypass capacitor to be effective

as a high frequency filter, the capacitor should be located as close as

possible to the SOURCE and PRIMARY BYPASS pins of the device.

The primary sensed OVP function can be realized by connecting a

series combination of a Zener diode, a resistor and a blocking diode

from the rectified and filtered bias winding voltage supply to the

PRIMARY BYPASS pin. The rectified and filtered bias winding output

voltage may be higher than expected (up to 1.5X or 2X the desired

value) due to poor coupling of the bias winding with the output

winding and the resulting ringing on the bias winding voltage

waveform. It is therefore recommended that the rectified bias

winding voltage be measured. This measurement should be ideally

done at the lowest input voltage and with highest load on the output.

This measured voltage should be used to select the components

required to achieve primary sensed OVP. It is recommended that a

Zener diode with a clamping voltage approximately 6 V lower than the

bias winding rectified voltage at which OVP is expected to be

triggered be selected. A forward voltage drop of 1 V can be assumed

for the blocking diode. A small signal standard recovery diode is

recommended. The blocking diode prevents any reverse current

discharging the bias capacitor during start-up. Finally, the value of

the series resistor required can be calculated such that a current

higher than I

SD will flow into the PRIMARY BYPASS pin during an

output overvoltage.

Reducing No-load Consumption

The SC1933C/SC1936C IC can start in self-powered mode, drawing

energy from the BYPASS pin capacitor charged through an internal

current source. Use of a bias winding is however required to provide

supply current to the PRIMARY BYPASS pin once the SC1933C/

SC1936C IC has started switching. An auxiliary (bias) winding

provided on the transformer serves this purpose. A bias winding

driver supply to the PRIMARY BYPASS pin enables design of power

supplies with no-load power consumption less than 30 mW. Resistor

R8 shown in Figure 6 should be adjusted to achieve the lowest

no-load input power.

Secondary-Side Overvoltage Protection (Auto-Restart Mode)

The secondary-side output overvoltage protection provided by the

SC1933C/SC1936C IC uses an internal auto restart circuit that is

triggered by an input current exceeding a threshold of I

BPS(SD) into the

SECONDARY BYPASS pin. The direct output sensed OVP function can

be realized by connecting a Zener diode from the output to the

SECONDARY BYPASS pin. The Zener diode voltage needs to be the

difference between 1.25 × V

OUT and 4.4 V − the SECONDARY BYPASS

pin voltage. It is necessary to add a low value resistor in series with

the OVP Zener diode to limit the maximum current into the

SECONDARY BYPASS pin.

Selection of Components

Components for SC1933C/SC1936C

Primary-Side Circuit

BPP Capacitor

A capacitor connected from the PRIMARY BYPASS pin of the

SC1933C/SC1936C IC to GND provides decoupling for the primary-

side controller and also selects current limit. A 0.47 mF or 4.7 mF

capacitor may be used. Though electrolytic capacitors can be used,

often surface mount multi-layer ceramic capacitors are preferred for

use on double sided boards as they enable placement of capacitors

close to the IC. Their small size also makes it ideal for compact

power supplies. 16 V or 25 V rated X7R dielectric capacitors are

recommended to ensure that minimum capacitance requirements are

met.

Bias Winding and External Bias Circuit

The internal regulator connected from the DRAIN pin of the switch to

the PRIMARY BYPASS pin of the SC1933C/SC1936C primary-side

controller charges the capacitor connected to the PRIMARY BYPASS

pin to achieve start-up. A bias winding should be provided on the

transformer with a suitable rectifier and filter capacitor to create a

bias supply that can be used to supply at least 2 mA of current to the

PRIMARY BYPASS pin.

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