MP6002 – MONOLITHIC FLYBACK/FORWARD DC-DC CONVERTER

MP6002 Rev. 1.0

www.MonolithicPower.com

11

12/4/2013

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2. Ripple Factor of the Magnetizing Current

The conduction loss in S, D2, the transformer,

the snubber, and in the ESR of the input/output

capacitors will increase as the ripple of the

magnetizing current increases. The ripple factor

ripple current vs. the average current as shown

in Figure 3.

M

M

r

I

I

K





output current;

)

D

1

(

N

I

D

I

I

0

IN

M









0

Figure 3—Magnetic Current of Flyback

Transformer (Reflected to Primary Side)

The input/output ripple voltage will also

increase with a high ripple factor, which makes

the filter bigger and more expensive. On the

other hand, it can help to minimize the turn-on

loss of S and reverse-recovery loss due to D2.

at 60%~120% for most DC-DC converters.

The primary side (or magnetizing) inductance

can be determined by:

M

r

S

IN

F

I

K

T

D

V

L









3. Core Selection

Pick a core based on experience or through a

catalog (Refer to http://www.ferroxcube.com).

Select an ER, EQ, PQ, or RM core to minimize

the transformer’s leakage inductance.

4. Winding Selection

Solid wire, Litz wire, PCB winding, Flex PCB

winding or any combination thereof can be used

as transformer winding. For low current

applications, solid wire is the most cost effective

choice. Consider using several wires in parallel

and interleaving the winding structure for better

performance of the transformer.

The number of primary turns can be determined

by:

E

MAX

P

F

P

A

B

I

L

N







effective area of the core.

The air gap can be estimated by:

F

E

2

o

L

A

N

Gap









5. Right Half Plane Zero

A Flyback converter operating in continuous

mode has a right half plane (RHP) zero. In the

frequency domain, this RHP zero adds not only

a phase lag to the control characteristics but

also increases the gain of the circuit. Typical

rule of thumb states that the highest usable

loop crossover frequency is limited to one third

the value of the RHP zero. The expression for

the location of the RHP zero in a continuous

mode flyback is given by:

2

F

2

LOAD

RHPZ

N

D

L

2

)

D

1

(

R

f















magnetizing inductance on transformer primary

side, and N is the transformer’s turn ratio.

Reducing the primary inductance increases the

RHP zero frequency which results in higher

crossover frequencies.