MAX5941BCSE(2003) データシートの表示（PDF） - Maxim Integrated

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MAX5941BCSE
(Rev.:2003)

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

Maxim Integrated 

IEEE 802.3af-Compliant Power-Over-Ethernet

Interface/PWM Controller for Power Devices

NS × k × RSENSE × VOUT = 26mV / µs

NP

L

where k = 0.75 to 1, and NS and NP are the number of

turns on the secondary and primary side of the trans-

former, respectively. L is the output filter inductor. This

makes the output inductor current downslope as refer-

enced across RSENSE equal to the slope compensa-

tion. The controller responds to transients within one

cycle when this condition is met.

N-Channel MOSFET Gate Driver

NDRV drives an N-channel MOSFET. NDRV sources

and sinks large transient currents to charge and dis-

charge the MOSFET gate. To support such switching

transients, bypass VCC with a ceramic capacitor. The

average current as a result of switching the MOSFET is

the product of the total gate charge and the operating

frequency. It is this current plus the DC quiescent cur-

rent that determines the total operating current.

Applications Information

Design Example

The following is a general procedure for designing a

forward converter (Figure 5) using the MAX5941B:

1) Determine the requirements.

2) Set the output voltage.

3) Calculate the transformer primary to secondary

winding turns ratio.

4) Calculate the reset to primary winding turns ratio.

5) Calculate the tertiary to primary winding turns

ratio.

6) Calculate the current-sense resistor value.

7) Calculate the output inductor value.

8) Select the output capacitor.

The circuit in Figure 5 was designed as follows:

1) 30V ≤ VIN ≤ 67V, VOUT = 5V, IOUT = 10A, VRIPPLE ≤

50mV. Turn-on threshold is set at 38.6V.

2) To set the output voltage, calculate the values of

resistors R1 and R2 according to the following

equation:

VREF = R2

VOUT R1 + R2

where VREF is the reference voltage of the shunt

regulator, and R1 and R2 are the resistors shown in

Figures 5 and 6.

3) The turns ratio of the transformer is calculated based

on the minimum input voltage and the lower limit of

the maximum duty cycle for the MAX5941B (44%).

To enable the use of MOSFETs with drain-source

breakdown voltages of less than 200V, use the

MAX5941B with the 50% maximum duty cycle.

Calculate the turns ratio according to the following

equation:

( ) NS ≥ VOUT + VD1 × DMAX

NP

DMAX × VIN_MIN

where:

NS/NP = Turns ratio (NS is the number of secondary

turns and NP is the number of primary turns).

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 will be dissipated across the MOS-

FET. 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

= 17.7

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

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