LC5546AD datasheet(21/30 Pages) SANKEN

21 / 30 page

LC5546AD/LD

LC5546AD/LD-DSE Rev.3.0

SANKEN ELECTRIC CO., LTD.

Dec. 12, 2022

https://www.sanken-ele.co.jp/en/

Surge voltage pulse width at

turning on

VROCP

OCP detection period

tON(LEB)

S/GND

Figure 9-29.

OCP Pin Voltage (Converted from Power

MOSFET Drain Current by ROCP)

9.6.2. Input Compensation Function

The IC has the input compensation function that

compensates

the

Overcurrent

Detection

Threshold

Voltage, VOCP, according to AC input voltages. In an

application using a quasi-resonant converter with

universal input (85 VAC to 265 VAC), the peak drain

current of the power MOSFET decreases due to a higher

operation frequency caused by an increased input voltage

under a condition of constant output power.

Figure 9-30 shows the relationship between AC input

voltage and the output current at OCP activation, IOUT(OCP),

with the OCP input compensation function being

activated or deactivated. When the IC without the input

compensation, IOUT(OCP) increases as the AC input voltage

increases (see “IOUT without input compensation” in

Figure 9-30). To regulate IOUT(OCP) at the maximum AC

input voltage, add the OCP input compensation circuit

(DX1, DZX1, RX1) as shown in Figure 9-31. The values of

these components should be adjusted so that IOUT(OCP) at

the maximum AC input voltage exceeds the target output

current,

IOUT

(see

“IOUT with appropriate input

compensation” in Figure 9-30).

Figure 9-31 shows the OCP input compensation circuit,

and Figure 9-32 shows the Vfw1 and Vfw2 voltages

according to the AC input voltage. The compensation

amount of VOCP depends on values of the input

compensation current, I, RX1, R3, and ROCP. The input

compensation current, I, is calculated by the following

equation:

I =

Vfw1 − VZX1 − VFX1

RX1 + R3 + ROCP

(6)

Where:

I is the input compensation current,

Vfw1 is the forward voltage of the auxiliary winding, D

proportional to input voltage,

VFX1 is the forward voltage of the rectifier diode DX1,

and

VZX1 is the Zener voltage of the Zener diode DZX1.

The Overcurrent Detection Threshold Voltage, VROCP'

applied with OCP input compensation is calculated by the

following equation:

VROCP

′

= −(|VOCP| + |R3 × IOCP| − R3 × I) .

(7)

Where:

I is the input compensation current,

R3 is the value of R3,

VOCP is the overcurrent detection threshold voltage

(−0.6 V), and

IOCP is the OCP pin source current (−40 μA).

As the input voltage, VIN, increases, the voltage drop

across R3 by the input compensation current, I (i.e., R3 ×

I) increases. Then, the input compensation amount also

increases, and the absolute value of VROCP' decreases.

The compensation start voltage of the OCP input

voltage is determined by the Zener voltage (VZX1) of the

Zener diode (DZX1). Set VZX1 to the same voltage as Vfw1

at the OCP input compensation starting point. The OCP

pin voltage, including surge voltage, must be regulated

within the rated voltages (−2.

V to 5. V) under a

condition of the maximum AC input voltage.

IOUT without input compensation

IOUT with appropriate input compensation

IOUT with excessive input

compensation

IOUT cannot be acquired

IOUT

85 V

265 V

AC Input Voltage (V)

IOUT (target output current)

Figure 9-30.

AC Input Voltage vs. Output Current at

OCP (with or without OCP Input Compensation

Function)

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