MPX2003

– UP TO 140kHz ALL-IN-ONE FLYBACK CONTROLLER

MPX2003 Rev. 1.0

MonolithicPower.com

26

8/5/2022

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2022 MPS. All Rights Reserved.

APPLICATION INFORMATION

Selecting the VCC Capacitor

Figure 13 shows the start-up circuit.

VCC

GND

HV

Figure 13: Start-Up Circuit

VCC is initially charged by HV. When VCC reaches

VCC-IOFF, HV stops charging VCC and the IC starts

to operate. To guarantee a successful start-up,

VOUT should be set up before VCC drops to VCC_MIN.

For most applications, choose a VCC capacitor

value that is about 22µF. To ensure safe

operation across the entire input and output

range (especially under no load conditions), VCC

should have some margin. Usually, the minimum

VCC is recommended to be about 1V greater than

the minimum operating voltage (VCC-MIN). The

recommended VCC capacitor (CVCC) value can

be calculated with Equation (1):

OP

START UP

VCC

CC IOFF

CC MIN

It

C

VV

−

−−



−



(1)

Designing the Transformer’s Magnetizing

Inductance

The MPX2003

’s maximum switching frequency

(fSW) is recommended to be 130kHz at maximum.

With internal slope compensation, the MPX2003

supports CCM when the duty cycle exceeds 50%.

Set a ratio (KP) for the primary inductor’s ripple

current amplitude vs. the peak current value to

0 < KP ≤ 1, where KP = 1 for DCM.

Figure 14 shows the relevant waveforms. A

larger inductor leads to a smaller KP, which can

reduce

the

RMS

current

but

increase

transformer size. An optimal KP value is between

0.6 and 0.8 for the universal input range, or 0.8

to 1 for the 230VAC input range.

IRIPPLE

KP = IRIPPLE / IPEAK

IPEAK

IAV

Figure 14: Typical Primary Current Waveform

The input power (PIN) at the minimum input can

be estimated with Equation (2):

OUT

OUT

IN

VI

P



=



(2)

Where VOUT is the output voltage, IOUT is the rated

output current, and ƞ is the estimated efficiency

(typically between 0.85 and 0.9 depending on

the input range and output application).

During

CCM

at

the

minimum

input,

the

converter’s maximum duty cycle can be

calculated with Equation (3):

OUT

FWD

MAX

OUT

FWD

IN(MIN)

PS

PS

(V

V

) N

D

(V

V

) N

V

+

=

+



+

(3)

Where VFWD is the secondary SR MOSFET

regulated

forward

voltage,

NPS

is

the

transformer’s primary-secondary turn ratio, and

VIN(MIN) is the minimum voltage on the bulk

capacitor.

The MOSFET turn-on time can be estimated with

Equation (4):

ON

S

t

D t

=

(4)

Where tS is the frequency jitter’s dominant

switching period. The relationship between tSMIN

and fSW-MAX can be calculated with Equation (5):

−

==

SW MAX

SMIN

1

f

130kHz

t

(5)

The average value for the primary current can be

estimated with Equation (6):

IN

AV

IN(MIN)

P

I

V

=

(6)