MP6005

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

MP6005 Rev. 1.0

www.MonolithicPower.com

23

2/5/2021

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Where LK is the leakage inductance, and IPEAK is

the peak switching current.

Since RSN consumes the leakage inductance

power

loss,

RSN

can

be

calculated

with

Equation (14):

SN

2

SN

SN

P

V

=

R

(14)

Where VSN is the expected snubber voltage on

CSN.

The snubber capacitor (CSN) achieves an

appropriate voltage ripple on the snubber,

which can be calculated with Equation (15):

SW

SN

SN

SN

SN

f

x

C

x

R

V

=

ΔV

(15)

A 15% ripple is acceptable for most applications.

Selecting an Output Diode for Flyback

Applications

The flyback output rectifier diode supplies

current to the output capacitor when the

primary-side MOSFET is turned off. Use a

Schottky diode to reduce power loss due to the

diode

’s forward voltage and recovery time. The

diode should have a reverse voltage 1.5 times

greater than VDIODE, which can be calculated

with Equation (16):

IN

DIODE

OUT

V

VV

N



(16)

The average current rating must exceed the

maximum expected load current, and the peak

current rating must exceed the output winding

peak current.

It is recommended to use an RC snubber circuit

for the output diode.

Selecting

a

Transformer

for

Forward

Applications

The forward transformer transfers energy to the

output.

The

forward

transformer

’s

key

parameters are the winding ratio, primary

winding turns, current rating, and core selection.

The transformer winding ratio determines the

duty cycle, which can be calculated with

Equation (17):

OUT

IN

V

N

D=

V



(17)

Where N is the transformer primary winding to

output winding ratio.

A 45% duty cycle is recommended for most

applications.

When the power MOSFET turns on, the

transformer transfers energy to the output,

while VIN generates a primary-side inductance

current in the transformer. The transformer

requires enough primary winding to avoid

saturation. The primary-side inductance peak

current can be calculated with Equation (18):

SW

P

OUT

EXC

f

x

L

x

2

N

x

V

=

I

(18)

Where IEXC is the primary-side inductance peak

current, and LP is the primary inductance.

Using IEXC to calculate the primary winding and

enough margins is needed for some extreme

conditions, such as load transient or OCP.

The current rating depends on the maximum

RMS

current,

which

flows

through

each

winding.

A

controlled

current

density

is

necessary to avoid high resistive power loss.

Selecting a SYNC MOSFET for Forward

Applications

The MP6005 supports active clamp forward

applications. The active P-channel MOSFET

clamp should be equal to the main power

MOSFET

’s maximum voltage, and its maximum

current

should

exceed

the

primary-side

inductance peak current and RMS current.

Selecting an Output MOSFET for Forward

Applications

The forward output requires two diodes to

conduct current. If higher efficiency is required,

the MOSFETs can replace the diodes (see

Figure 11).

VOUT

COUT

QF

NS

QR

L

Figure 11: Forward Output MOSFET

The MOSFET voltage rating should exceed the

VDS maximum. The QR maximum VDS voltage

(VR) can be calculated with Equation (19):