MP020-5 datasheet(10/20 Pages) MPS

10 / 20 page

MP020-5 – OFFLINE, PRIMARY-SIDE REGULATOR WITH CC/CV CONTROL AND A 700V FET

MP020-5 Rev. 1.05

www.MonolithicPower.com

1/23/2014

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

OPERATION

NP_AU

VAUX

Figure 2: Simplified Flyback Converter

Startup

Initially, the IC is self-supplying through the

internal high-voltage current source, which is

drawn from the Drain pin. The internal high-

voltage current source will turn off for better

efficiency when VCC reaches the VCC ON

threshold. Then the transformer’s auxiliary

winding takes over as the power source. When

VCC falls below the VCC OFF threshold, the IC

stops switching and the internal high-voltage

current source turns on again. See Figure 3 for

the start-up waveform.

VCCH

VCCL

Vcc

Drain

Switching Pulses

High-voltage

current source

OFF

Figure 3: VCC UVLO

Working Principle

After startup, the internal MOSFET turns on and

the current sense resistor (RCS) senses the

primary current iP(t) internally. The current rises

linearly at a rate of:

di (t)

IPK

Figure 4: Primary Current Waveform

As illustrated in Figure 4, when iP(t) rises up to

IPK, the internal MOSFET turns off. Then, the

energy stored in the inductor transfers to

secondary-side through the transformer.

The inductor, LM, stores energy with each cycle

as a function of:

MPK

So the power transferred from the input to the

output is:

MPK

××

Where fS is the switching frequency. When IPK is

constant, the output power depends on fS.

Constant-Voltage Operation

The MP020-5 detects the auxiliary winding

voltage from the FB pin and operates in

constant voltage (CV) mode to regulate the

output voltage.

Assume the secondary winding is the master

and the auxiliary winding is the slave. When the

secondary-side diode turns on, the FB pin

voltage is:

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