MP8007 ― IEEE 802.3AF PD WITH 13W FLYBACK / BUCK CONVERTER

MP8007 Rev.1.0

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17

6/27/2016

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buck mode, and do not place a resistor

between MODE and GND in flyback mode.

Converter Switching Work Principle

After startup, DCDC converter works in

discontinuous conduction mode (DCM). The

second switching cycle will not start until the

inductor current drops to 0A. In each cycle, the

internal MOSFET is turned on, and the current-

sense circuit senses the current IP(t) internally.

Use Equation (2) to calculate the rate at which

the current rises linearly in flyback mode:

p(t)

IN

M

dI

V

dt

L



(2)

When IP(t) rises up to IPK, the internal MOSFET

turns off (see Figure 5). The energy stored in

the primary-side inductance transfers to the

secondary-side through the transformer.

Figure 5—Primary-side current waveform

The primary-side inductance (LM) stores energy

in each cycle as a function of Equation (3):

2

MPK

1

EL I

2



(3)

Calculate the power transferred from the input

to the output with Equation (4):

2

MPK S

1

PL I F

2



(4)

Where FS is the switching frequency. When IPK

is constant, the output power depends on FS

and LM.

Use Equation (5) to calculate the rate at which

the current rises linearly in buck mode:

p(t)

IN

OUT

M

dI

VV

dt

L





(5)

The internal MOSFET turns off when IP(t) rises

to IPK (see Figure 6). The output current is

calculated with Equation (6):

OUT

PK

1

IDI

2



(6)

Where, D is the inductor current conducting

duty cycle.

Figure 6—Inductor current waveform

Converter Light-Load Control

In flyback mode (if the load decreases), DCDC

converter

stretches

down

the

frequency

automatically to reduce the power transferring

while keeping the same IPK in each cycle. An

approximate 10 kHz minimum frequency is

applied to detect the output voltage even at a

very light load. During this condition, the

switching IPK jumps between 20 percent of the

normal IPK and 100 percent of the normal IPK to

reduce the power transferring. The DCDC

converter still transfers some energy to the

output even if there is no load on the output due

to the 10 kHz minimum frequency. This means

that some load is required to keep the output

voltage in regulation, or else VOUT will rise and

trigger OVP.

In buck mode, the DCDC converter has no

minimum frequency limit, so it stretches down

to a very low frequency and regulates the

output automatically even there is no load on

the output.

Frequency Control

By monitoring the auxiliary winding voltage in

flyback mode or monitoring the SW voltage in

buck mode, the DCDC converter detects and

regulates the inductor current in DCM. The

frequency is controlled by the peak current, the

current ramp slew rate, and the load current.

The maximum frequency occurs when the

DCDC converter runs in critical conduction

mode, providing the maximum load power. The

DCDC converter switching frequency should be

lower than 200 kHz in the design.