SC1933C datasheet(11/28 Pages) POWERINT

11 / 28 page

Rev. C 11/18

SC1933C/SC1936C

www.power.com

The turns ratio for the bias winding should be selected such that 7 V

is developed across the bias winding at the lowest rated output

voltage of the power supply at the lowest load condition. If the

voltage is lower than this, no-load input power will increase.

In USB PD or rapid charge applications, the output voltage range is

very wide. For example, a 45 W adapter would need to support 5 V,

9 V and 15 V and a 100 W adapter would have output voltages

selectable from 5 V to 20 V. Such a wide output voltage variation

results in a large change in bias winding output voltage as well.

A linear regulator circuit is generally required to limit the current

injected into the PRIMARY BYPASS pin of the SC1933C/SC1936C.

The bias current from the external circuit should be set to

approximately 300 mA to achieve lowest no-load power consumption

when operating the power supply at 230 VAC input (V

BPP > 5 V). A

glass passivated standard recovery rectifier diode with low junction

capacitance is recommended to avoid the snappy recovery typically

seen with fast or ultrafast diodes that can lead to higher radiated

EMI.

An aluminum capacitor of at least 22 mF with a voltage rating 1.2

times greater than the highest voltage developed across the capacitor

is recommended. Highest voltage is typically developed across this

capacitor when the supply is operated at the highest rated output

voltage and load with the lowest input AC supply voltage.

Line UV and OV Protection

Resistors connected from the UNDER/OVER INPUT VOLTAGE pin to

the DC bus enable sensing of input voltage to provide line under

voltage and overvoltage protection. For a typical universal input

application, a resistor value of 4 MΩ is recommended. Figure 12

shows circuit configurations that enable either the line UV or the line

OV feature only to be enabled.

SC1933C/SC1936C features a primary sensed OV protection feature

that can be used to latch-off the power supply. Once the power

supply is latched off, it can be reset if the UNDER/OVER INPUT

VOLTAGE pin current is reduced to zero. Once the power supply is

latched off, even after the input supply is turned off, it can take

considerable amount of time to reset the SC1933C/SC1936C controller

as the energy stored in the DC bus will continue to provide current to

the controller. A fast AC reset can be achieved using the modified

circuit configuration shown in Figure 13. The voltage across capacitor

CS reduces rapidly after input supply is disconnected reducing current

into the INPUT VOLTAGE MONITOR pin of the SC1933C/SC1936C IC

and resetting the SC1933C/SC1936C controller.

Primary Sensed OVP (Overvoltage Protection)

The voltage developed across the output of the bias winding tracks

the power supply output voltage. Though not precise, a reasonably

accurate detection of the amplitude of the output voltage can be

achieved by the primary-side controller using the bias winding

voltage. A Zener diode connected from the bias winding output to

the PRIMARY BYPASS pin can reliably detect a secondary overvoltage

fault and cause the primary-side controller to latch-off. It is

recommended that the highest voltage at the output of the bias winding

should be measured for normal steady-state conditions (at full load

and lowest input voltage) and also under transient load conditions. A

Zener diode rated for 1.25 times this measured voltage will typically

ensure that OVP protection will only operate in case of a fault.

Primary-Side Snubber Clamp

A snubber circuit should be used on the primary-side as shown in

Figure 6. This prevents excess voltage spikes at the drain of the

switch at the instant of turn-off of the switch during each switching

cycle though conventional RCD clamps can be used. RCDZ clamps

offer the highest efficiency. The circuit example shown in Figure 6

uses an RCD clamp with a resistor in series with the clamp capacitor.

This resistor dampens the ringing at the drain and also limits the

reverse current through the clamp diode during reverse recovery.

Standard recovery glass passivated diodes with low junction

capacitance are recommended as these enable partial energy

recovery from the clamp thereby improving efficiency.

Components for SC1933C/SC1936C

Secondary-Side Circuit

SECONDARY BYPASS Pin – Decoupling Capacitor

A 2.2 mF, 25 V multilayer ceramic capacitor should be used for

decoupling the SECONDARY BYPASS pin of the SC1933C/SC1936C IC.

Since the SECONDARY BYPASS pin voltage needs to be 4.4 V before

the output voltage reaches the regulation voltage level, a significantly

higher BPS capacitor value could lead to output voltage overshoot

during start-up. Values lower than 1.5 mF may not offer enough

capacitance, and cause unpredictable operation. The capacitor must

be located adjacent to the IC pins. A 25 V rating is necessary to

guarantee a minimum aperture value in operation since the

capacitance of ceramic capacitors drops with applied voltage 10 V

rated capacitors are not recommended for this reason. Capacitors

with X7R dielectrics should be used for best results.

When the output voltage of the power supply is 5 V or higher, the

supply current for the secondary-side controller is provided by the

OUTPUT VOLTAGE (VOUT) pin of the IC as the voltage at this pin is

higher than the SECONDARY BYPASS pin voltage. During start-up

and operating conditions where the output voltage of the power

supply is below 5 V, the secondary-side controller is supplied by

current from an internal current source connected to the FORWARD

pin. If the output voltage of the power supply is below 5 V and the

load at the output of the power supply is very light, the operating

frequency can drop significantly and the current supplied to the

secondary-side controller from the FORWARD pin may not be

sufficient to maintain the SECONDARY BYPASS pin voltage at 4.4 V.

For such applications, it is recommended that an additional active

preload be used as shown in Figure 7. This load is turned on by the

interface IC (or USB PD controller) when the output voltage of the

power supply is below 5 V.

Figure 7.

Active Pre-Load Circuit.

VOUT

To USB PD Controller GP10

PI-8414-082317

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