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

winding during the off period of the nMOSFET. In prac-

tice, parasitic inductances and capacitors in the circuit,

such as leakage inductance of the flyback transformer,

cause voltage overshoot and ringing. Snubber circuits

are used to limit the voltage overshoots to safe levels

within the voltage rating of the external nMOSFET. The

snubber capacitor can be calculated using the following

equation:

C SNUB

=

2

×

L LK

× IPRIPEAK

VOUT 2

2

×

K

2

where LLK is the leakage inductance that can be

obtained from the transformer specifications (usually 1%

to 2% of the primary inductance).

The power to be dissipated in the snubber resistor is

calculated using the following formula:

PSNUB = 0.833 × LLK × IPRIPEAK 2 × fSW

The snubber resistor can be calculated based on the

following equation:

R SNUB

=

6.25 × VOUT 2

PSNUB × K 2

The voltage rating of the snubber diode is:

VDSNU=B

VINMAX

+

2.5

×

VOUT

K





Output-Capacitor Selection

X7R ceramic output capacitors are preferred in industrial

applications due to their stability over temperature. The

output capacitor is usually sized to support a step load

of 50% of the maximum output current in the application

so that the output-voltage deviation is contained to 3% of

the output-voltage change. The output capacitance can

be calculated as:

COUT

=

ISTEP

× tRESPONSE

∆VOUT

t RESPONSE

≅

(0.33

fC

+

1)

fSW

where ISTEP is the load step, tRESPONSE is the response

time of the controller, DVOUT is the allowable output-

voltage deviation, and fC is the target closed-loop cross-

over frequency. fC is chosen to be 1/10 the switching

frequency (fSW). For the flyback converter, the output

capacitor supplies the load current when the main switch

is on, and therefore, output-voltage ripple is a function of

load current and duty cycle. Use the following equation

to calculate the output-capacitor ripple:

∆VCOUT

= DNEW × IPRIPEAK − K × IOUT 2

2 × IPRIPEAK × fSW × COUT

where IOUT is load current and DNEW is the duty cycle at

minimum input voltage.

Input-Capacitor Selection

The MAX17498A is optimized to implement offline

AC-DC converters. In such applications, the input capac-

itor must be selected based on either the ripple due

to the rectified line voltage, or based on holdup-time

requirements. Holdup time can be defined as the time

period over which the power supply should regulate its

output voltage from the instant the AC power fails. The

MAX17498B /MAX17498C are useful in implementing

low-voltage DC-DC applications where the switching-

frequency ripple must be used to calculate the input

capacitor. In both cases, the capacitor must be sized to

meet RMS current requirements for reliable operation.

Capacitor Selection Based on Switching Ripple

(MAX17498B/MAX17498C): For DC-DC applications,

X7R ceramic capacitors are recommended due to their

stability over the operating temperature range. The ESR

and ESL of a ceramic capacitor are relatively low, so

the ripple voltage is dominated by the capacitive com-

ponent. For the flyback converter, the input capacitor

supplies the current when the main switch is on. Use the

following equation to calculate the input capacitor for a

specified peak-to-peak input switching ripple (VIN_RIP):

CIN

=

DNEW

×

IPRIPEAK 1− (0.5 ×

2 × fSW × VIN_RIP

D NEW )

2

Capacitor Selection Based on Rectified Line-Voltage

Ripple (MAX17498A): For the flyback converter, the

input capacitor supplies the input current when the diode

rectifier is off. The voltage discharge (VIN_RIP), due to the

input average current, should be within the limits speci-

fied:

CIN

=

0.5 × IPRIPEAK × DNEW

fRIPPLE × VIN_RIP

where fRIPPLE, the input AC ripple frequency equal to the

supply frequency for half-wave rectification, is two times

the AC supply frequency for full-wave rectification.

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