MP6005

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

MP6005 Rev. 1.0

www.MonolithicPower.com

24

2/5/2021

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© 2021 MPS. All Rights Reserved.

D)

-

(1

x

N

x V

D

=

V

N

I

R

(19)

The QF maximum VDS voltage (VF) can be

calculated with Equation (20):

N

V

=

V

IN

F

(20)

Where N is the transformer primary winding to

output winding ratio, and D is the primary duty

cycle. Some margin is typically necessary.

The MOSFET current rating should exceed its

maximum RMS current and peak current. The

QR RMS current (IR) can be calculated with

Equation (21):

2

OUT

PP

OUT

R

)

I

I

(

x

3

1

+

1

x

D

x

I

=

I

(21)

The QF RMS current (IF) can be calculated with

Equation (22):

2

OUT

PP

OUT

F

)

I

I

(

x

3

1

+

1

x

D

-

1

x

I

=

I

(22)

Where IPP is the peak-to-peak current.

Since

one

MOSFET

’s drain is the other

MOSFET

’s gate, the QR gate driver voltage is

equal to VF, and the QF gate driver voltage is

equal to VR. If the driver voltage exceeds the

maximum GATE voltage, a clamp circuit is

required. The turn-on resistance determines the

conduction loss, and QG determines the circuit

driver loss. Both the turn-on resistance and QG

should remain low to achieve high efficiency

and low temperature rise.

Selecting an Output Inductor for Forward

Applications

The forward output inductor supplies constant

current to the output load while the main power

MOSFET turns on. A larger-value inductor

results in less ripple current and a lower output

voltage ripple. However, larger-value inductors

are larger in size, and have a higher series

resistance and lower current saturation. To

determine the inductance, allow the peak-to-

peak ripple current in the inductor to be

approximately 30% to 50% of the maximum

output current. The inductance value can be

calculated with Equation (23):

OUT

OUT

SW

L

IN

V

V

x N

L =

x (1-

)

f

x

ΔIV

(23)

Where VOUT is the output voltage, VIN is the

input voltage, fSW is the switching frequency,

and ΔIL is the peak-to-peak inductor ripple

current.

Choose an inductor that does not saturate

under the maximum inductor peak current.

Selecting an Input Capacitor

An input capacitor is required to supply the AC

ripple current to the inductor while limiting noise

at the input source. A low-ESR capacitor is

required to minimize noise. It is recommended

to use ceramic capacitors, but tantalum or low-

ESR electrolytic capacitors can also be used.

For

ceramic

capacitors,

the

capacitance

dominates the input ripple at the switching

frequency.

In flyback mode, the input ripple can be

estimated with Equation (24):

)

V

+

x V

(N

x

C

x

f

V

x

I

=

ΔV

IN

OUT

IN

SW

IN

IN

IN

(24)

Where

ΔVIN is the input voltage ripple, IIN is the

input current, and CIN is the input capacitor.

In forward mode, the input ripple can be

estimated with Equation (25):

OUT

IN

IN

SW

IN

IN

V

x N

I

ΔV =

x (1-

)

f

x C

V

(25)

Equation (25) omits the primary-side inductance

current that makes the current ripple smaller.

Selecting an Output Capacitor

The output capacitor maintains the DC output

voltage. For the best results, it is recommended

to

use

ceramic

capacitors

or

low-ESR

capacitors to minimize the output voltage ripple.

For

ceramic

capacitors,

the

capacitance

dominates the output ripple at the switching

frequency.

In flyback mode, the output ripple can be

estimated with Equation (26):

OUT

OUT

SW

OUT

IN

OUT

OUT

C

I

x

f

x

)

x V

N

+

(V

x V

N

=

ΔV

(26)

If the voltage ripple is too high, place two

capacitors on either side of the inductor to