MAX5942BCSE データシートの表示（PDF） - Maxim Integrated

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MAX5942BCSE

IEEE 802.3af Power-Over-Ethernet Interface/PWM Controller for Power Devices

Maxim Integrated 

IEEE 802.3af Power-Over-Ethernet

Interface/PWM Controller for Power Devices

VOUT = Output voltage (5V).

VD1 = Voltage drop across D1 (typically 0.5V for

power Schottky diodes).

DMAX = Minimum value of maximum operating duty

cycle (44%).

VIN_MIN = Minimum input voltage (30V).

In this example:

NS ≥ 5V +(0.5V × 0.44) = 0.395

NP

0.44 × 30V

Choose NP based on core losses and DC resis-

tance. Use the turns ratio to calculate NS, rounding

up to the nearest integer. In this example, NP = 14

and NS = 6.

For a forward converter, choose a transformer with a

magnetizing inductance in the neighborhood of

200µH. Energy stored in the magnetizing inductance

of a forward converter is not delivered to the load

and must be returned back to the input; this is

accomplished with the reset winding.

The transformer primary to secondary leakage

inductance should be less than 1µH. Note that all

leakage energy is dissipated across the MOSFET.

Snubber circuits may be used to direct some or all

of the leakage energy to be dissipated across a

resistor.

To calculate the minimum duty cycle (DMIN), use the

following equation:

=

DMIN

=

VOUT

⎡⎢VIN_MAX

⎣

×

NS

NP

⎤

⎥

⎦

- VD1

where VIN_MAX is the maximum input voltage (67V).

4) The reset winding turns ratio (NR/NP) needs to be

low enough to guarantee that the entire energy in

the transformer is returned to V+ within the off cycle

at the maximum duty cycle. Use the following equa-

tion to determine the reset winding turns ratio:

NR

≤

NP

×

1- DMAX

DMAX ′

′

where:

NR/NP = Reset winding turns ratio.

DMAX’ = Maximum value of maximum duty cycle:

NR

≤

14

×

1- 0.5

0.5

=

14

Round NR to the nearest smallest integer.

The turns ratio of the reset winding (NR/NP) determines

the peak voltage across the N-channel MOSFET.

Use the following equation to determine the maxi-

mum drain-source voltage across the N-channel

MOSFET:

⎛

VDSMAX ≥ VIN_MAX × ⎝⎜1 +

NP

NR

⎞

⎠⎟

VDSMAX = Maximum MOSFET drain-source voltage.

VIN_MAX = Maximum input voltage:

VDSMAX ≥ 67V × ⎛⎝⎜1

+

14 ⎞

14 ⎠⎟

= 134V

Choose MOSFETs with appropriate avalanche

power ratings to absorb any leakage energy.

5) Choose the tertiary winding turns ratio (NT/NP) so that

the minimum input voltage provides the minimum

operating voltage at VDD (13V). Use the following

equation to calculate the tertiary winding turns ratio:

VDDMIN + 0.7

VIN_MIN

× NP

≤ NT

≤

VDDMAX + 0.7

VIN_MAX

× NP

where:

VDDMIN is the minimum VDD supply voltage (13V).

VDDMAX is the maximum VDD supply voltage (30V).

VIN_MIN is the minimum input supply voltage (30V).

VIN_MAX is the maximum input supply voltage (67V

in this design example).

NP is the number of turns of the primary winding.

NT is the number of turns of the tertiary winding:

13.7

30

× 14

≤

NT

≤

36.7

67

× 14

6.39 ≤ NT ≤ 7.67

Choose NT = 7.

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