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

PWM Comparator and Slope Compensation

An internal 275kHz oscillator determines the switching

frequency of the controller. At the beginning of each

cycle, NDRV switches the N-channel MOSFET on.

NDRV switches the external MOSFET off after the maxi-

mum duty cycle has been reached, regardless of the

feedback.

The MAX5942B uses an internal ramp generator for

slope compensation. The internal ramp signal is reset

at the beginning of each cycle and slews at 26mV/µs.

The PWM comparator uses the instantaneous current,

the error voltage, the internal reference, and the slope

compensation (MAX5942A only) to determine when to

switch the N-channel MOSFET off. In normal operation,

the N-channel MOSFET turns off when:

IPRIMARY × RSENSE > VEA - VREF - VSCOMP

where IPRIMARY is the current through the N-channel

MOSFET, VREF is the 2.4V internal reference, VEA is the

output voltage of the internal amplifier, and VSCOMP is a

ramp function starting at zero and slewing at 26mV/µs

(MAX5942A only). When using the MAX5942A in a for-

ward-converter configuration, the following condition

must be met to avoid control-loop subharmonic oscilla-

tions:

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 using the MAX5942B:

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:

VOUT

=

VREF

⎡⎣⎢1 +

R1 ⎤

R2 ⎦⎥

R1//R2 << 50kΩ

VREF = VSS_ SHDN = 2.4V

where VREF is the reference voltage of the shunt reg-

ulator, 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 MAX5942B (44%).

To enable the use of MOSFETs with drain-source

breakdown voltages of less than 200V, use the

MAX5942B 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).

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