MAX17498A(2012) Ver la hoja de datos (PDF) - Maxim Integrated

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Fabricante

MAX17498A
(Rev.:2012)

AC-DC and DC-DC Peak Current-Mode Converters for Flyback/Boost Applications

Maxim Integrated 

MAX17498A / MAX17498B / MAX17498C

AC-DC and DC-DC Peak Current-Mode Converters

for Flyback/Boost Applications

Capacitor Selection Based on Hold-Up Time

Requirements (MAX17498A): For a given output power

(PHOLDUP) that needs to be delivered during hold-up

time (tHOLDUP), DC bus voltage at which the AC supply

fails (VINFAIL), and the minimum DC bus voltage at which

the converter can regulate the output voltages (VINMIN),

the input capacitor (CIN) is estimated as:

CIN

=

3

× PHOLDUP × tHOLDUP

(VINFAIL 2 − VINMIN2)

The input capacitor RMS current can be calculated as:

IINCRMS

=

0.6

×

VINMIN × (DMAX)2

fSW × LPRI

External MOSFET Selection

MOSFET selection criteria includes the maximum drain

voltage, peak /RMS current in the primary, and the

maximum allowable power dissipation of the package

without exceeding the junction temperature limits. The

voltage seen by the MOSFET drain is the sum of the

input voltage, the reflected secondary voltage on the

transformer primary, and the leakage inductance spike.

The MOSFET’s absolute maximum VDS rating must be

higher than the worst-case drain voltage:

VDSMAX

= VINMAX

+





VOUT +

K

VD





×

2.5



The drain current rating of the external MOSFET is

selected to be greater than the worst-case peak

current-limit setting.

Secondary-Diode Selection

Secondary-diode-selection criteria includes the maxi-

mum reverse voltage, average current in the secondary,

reverse recovery time, junction capacitance, and the

maximum allowable power dissipation of the package.

The voltage stress on the diode is the sum of the output

voltage and the reflected primary voltage.

The maximum operating reverse-voltage rating must be

higher than the worst-case reverse voltage:

VSECDIODE= 1.25 × (K × VINMAX + VOUT)

The current rating of the secondary diode should be

selected so that the power loss in the diode (given as

the product of forward-voltage drop and the average

diode current) should be low enough to ensure that the

junction temperature is within limits. This necessitates

that the diode current rating be in the order of 2 x IOUT

to 3 x IOUT. Select fast-recovery diodes with a recovery

time less than 50ns, or Schottky diodes with low junction

capacitance.

Error-Amplifier Compensation Design

The loop compensation values are calculated as:

R=Z

450 ×



1+





0.1×fPfSW







2







×

VOUT

×

IOUT

2 × LPRI × fSW

where:

fP

=

π×

IOUT

VOUT × COUT

CZ

=

π

1

×RZ

×

fP

CP

=

π

1

×RZ ×

fSW

fSW is the switching frequency of the devices and can

be obtained from the Electrical Characteristics section.

CCM Flyback

Transformer Turns-Ratio Calculation (K = Ns /Np)

The transformer turns ratio can be calculated using the

following formula:

K = (VOUT + VD) × (1− DMAX)

VINMIN × DMAX

where DMAX is the duty cycle assumed at minimum

input (0.35 for MAX17498A/MAX17498C and 0.7 for

MAX17498B).

Primary-Inductance Calculation

Calculate the primary inductance based on the ripple:

L PRI

=

(VOUT + VD) × (1− DNOM)

2 × IOUT × β × fSW

×

K

where DNOM, the nominal duty cycle at nominal operating

DC input voltage (VINNOM), is given as:

DNOM

=

(VOUT + VD) × K

VINNOM + (VOUT + VD)

× K

The output current, down to which the flyback converter

should operate in CCM, is determined by selection of

the fraction A in the above primary inductance formula.

For example, A should be selected as 0.15 so that the

converter operates in CCM down to 15% of the maximum

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