MIC3832BN 查看數據表（PDF） - Micrel

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MIC3832BN

Current-Fed PWM Controllers

Micrel 

MIC3832/3833

Micrel

The push-pull cycles are synchronized to run at half of the

PWM frequency, so a soft or zero voltage switch transition

may be obtained, reducing spikes and EMI. Feeding a

sample of the PWM oscillator ramp to the current-mode

comparator along with the input current sample allows slope

compensation to be obtained for PWM duty cycles above

50%, preventing subharmonic oscillation at low input volt-

ages.

Boost-Derived Current Fed

If a regulator is designed to run at higher push-pull voltage

than the input line voltage, a step-up (boost) PWM regulator,

with an inductor switched to ground, minus the normal output

capacitor, will provide a current limited input to the push-pull

stage. In this configuration all three power switches may be

operated low-side, simplifying their drive circuitry. An input

fuse is needed to guard against short-circuits since there is no

high-side series switch.

an inductor current instead of a voltage. This constant current

reduces cross conduction and catastrophic transformer core

saturation. Push-pull topologies are often used in 100W and

larger power supplies as they allow more efficient use of the

transformer. The entire range of the B-H curve is used in a

push-pull supply, so a transformer that is one-half the size of

a transformer used in a single-ended, forward-mode topology

can be used. This topology can be extended to a full bridge

where the two 50% duty cycle stages would be used to drive

two MOSFETs each, one for each half of the bridge.

S2

S1

Duty Cycle

Control

L

D1

50%

S3

50%

Construction Hints

Figure 3: Current-Fed Push-Pull Topology

Careful prototyping techniques are required to prevent oscil-

lations. Traditional solderless breadboards are a source of Current-Fed Multiple-Output SMPS

noise, and should be avoided. Use double-sided, copper-

clad boards with a large area used as a single-point ground

plane.

Figure 4 illustrates this topology. The absence of output

inductors improves cross-regulation and simplifies the con-

struction of isolated or high-voltage output supplies.

All timing and loop compensation capacitors and resistors

should be star connected to GND (ground). Wire lengths

S2

4

along the high-current path should be kept as short as

50%

possible, with appropriate wire gauges being used. Do not

S1

socket the switching transistors as this can add to the voltage

drop and power losses.

Current-Fed Push-Pull SMPS

Figure 3 illustrates this basic topology, a standard push-pull

L

Duty Cycle D1

Control

S3

50%

Additional

Outputs

configuration where the center tap of the primary is fed with

Figure 4: Current-Fed Multiple-Output Topology

April 1998

4-143

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