MP6005

– WIDE-INPUT, HIGH-EFFICIENCY FLYBACK AND FORWARD CONTROLLER

MP6005 Rev. 1.0

www.MonolithicPower.com

22

2/5/2021

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© 2021 MPS. All Rights Reserved.

leads to high switching loss, while a low QG

may

cause

fast

turn-on/off

speed,

which

determines the spike and kick.

The turn-on threshold voltage (VTH) is also

important. The GATE pin is powered by VCC,

so VTH must be lower than VCC.

Selecting

a

Transformer

for

Flyback

Application

A transformer is an important component of a

flyback converter because it determines the

duty cycle, peak current, efficiency, MOSFET

value, output diode rating, and more. A good

transformer should account for the winding ratio,

primary-side

inductance,

saturation

current,

leakage inductance, current rating, and core

selection.

The transformer winding ratio determines the

duty cycle, which can be calculated with

Equation (10):

OUT

OUT

IN

N V

D=

N V

V





(10)

Where N is the primary winding transformer to

output winding ratio, and D is the duty cycle.

For most applications, it is recommended to

have a duty cycle of about 45%.

The primary-side inductance affects the input

current ripple ratio factor. A high inductance

value results in a large transformer size and

high cost. A low inductance value results in a

high switching peak current and RMS current,

which can decrease efficiency. Choose a

primary-side inductor to set the current ripple

ratio factor between 30% and 50%. The

primary-side inductance can be estimated with

Equation (11):

SW

IN

2

IN

P

f

x

I

x

n

x

2

D

x

V

=

L

(11)

Where n is the current ripple ratio, IIN is the

input current, and LP is the primary inductance.

Calculate LP based on the minimum input

voltage condition.

The transformer should have a high saturation

current to support the switching peak current;

otherwise,

the

transformer

inductance

decreases sharply. The SENSE resistor can be

used to limit the switching peak current.

The energy stored in the leakage inductance

cannot couple to the secondary side, which

causes a high spike when the MOSFET turns

off. This decreases efficiency and increases

MOSFET stress. Normally, the transformer

leakage inductance is less than 3% of the

transformer inductance.

The current rating counts the maximum RMS

current, which allows current to flow through

each winding. Uncontrolled current density can

cause high resistive power loss.

Setting the Diode Conduct Time (Only for

PSR Flyback Mode)

In PSR mode, the MP6005 starts sampling the

auxiliary-winding

voltage

after

the

primary

power MOSFET turns off. A 300ns blanking

time helps avoid spike ringing from the leakage

inductance. To guarantee a sufficient sample

FB

period,

the

output

diode

’s

current-

conduction time (tCON) should not exceed 600ns

under

light-load

conditions.

Design

the

transformer to ensure tCON exceeds 700ns when

VSENSE_PK = 33mV, which can be estimated with

Equation (12):

700ns

)

V

+

(V

x

N

x

R

N

x

L

33mV x

DOF

OUT

P

SENSE

S

P

≥

(12)

Where VDOF is the output diode

’s forward-drop

voltage.

Resistance Capacitor Diode (RCD) Snubber

for Flyback Application

The transformer leakage inductance causes

spikes and excessive ringing on the drain

voltage waveform. The RCD snubber circuit

limits the voltage spike (see Figure 10).

CSN

DSN

NP

T1

RSN

AGND

NS

GATE

MP6005

MOSFET

PGND

Figure 10: RCD Snubber

The power dissipation of the snubber circuit can

be estimated with Equation (13):

2

SN

K

PEAK

SW

1

P

=

L

I

f

2







(13)