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

Error-Amplifier Compensation Design

In the CCM flyback converter, the primary inductance

and the equivalent load resistance introduces a right

half-plane zero at the following frequency:

fZRHP

=

2×

π

(1− DMAX)2 × VOUT

× DMAX × LPRI × IOUT

×K2

The loop-compensation values are calculated as:

= R Z

200 ×IOUT ×

(1− DMAX )

1

+

5fR×HfPP







2

where fP, the pole due to output capacitor and load, is

given by:

fP

=

(1+ DMAX) × IOUT

2 × π × COUT × VOUT

The above selection sets the loop-gain crossover

frequency (fC, where the loop gain equals 1) equal to 1/5

the right half-plane zero frequency:

fC

≤

fZRHP

5

With the control-loop zero placed at the load pole frequency:


CZ

=

1

2π × RZ

×

fP

With the high-frequency pole placed at 1/2 the switching

frequency:

CP

=

π

1

×RZ ×

fSW

DCM Boost

In a DCM boost converter, the inductor current returns to

zero in every switching cycle. Energy stored during the

on time of the main switch is delivered entirely to the load

in each switching cycle.

Inductance Selection

The design procedure starts with calculating

the boost converter’s input inductor so that it oper-

ates in DCM at all operating line and load conditions.

The critical inductance required to maintain DCM

operation is calculated as:

LIN

≤

(VOUT

−

VINMIN) ×

VINMIN 2





IOUT × VOUT 2 × fSW

×

0.4

where VINMIN is the minimum input voltage.

Peak /RMS-Current Calculation

To set the current limit, the peak current in the inductor

can be calculated as:

where IPK is given by:

IL= IM IPK × 1.2

IPK =

2





×

(VOUT − VINMIN) × IOUT

LINMIN × fSWMIN







LINMIN is the minimum value of the input inductor, taking

into account tolerance and saturation effects. fSWMIN is

the minimum switching frequency for the MAX17498B

from the Electrical Characteristics section.

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 boost converter, the output

capacitor supplies the load current when the main switch

is on, and therefore, the output-voltage ripple is a func-

tion of duty cycle and load current. Use the following

equation to calculate the output-capacitor ripple:

∆VCOUT

= IOUT × LIN × IPK

VINMIN × COUT

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