MIC3833 Просмотр технического описания (PDF) - Micrel

Номер в каталоге

Компоненты Описание

производитель

MIC3833

Current-Fed PWM Controllers

Micrel 

MIC3832/3833

If a voltage-divided portion of the timing capacitor ramp (from

CT) is fed to CMR (to slope compensate for current-mode

subharmonic oscillation, for example), the corresponding

maximum duty cycle control voltage must be proportionally

reduced to achieve the same duty cycle control.

Soft Start

This feature prevents damage due to large inrush currents

generated upon initial application of system power or when

the device attempts to restart after an overcurrent shutdown

by the current limit function. When soft start is activated, the

PWM comparator output duty cycle will increase slowly, with

a time constant determined by the size of the external

capacitor connected to MDC/SS. (Timing is RTHC, where

RTH is the Thevenin equivalent resistance seen by this pin.)

Overcurrent Sensing and Shutdown

Overcurrent sensing and shutdown is accomplished via a

current sense transformer or an external sense resistor

connected from the switching element (power transistor) to

ground. The current ramp is fed into the noninverting input of

two sensing comparators (SHDN). If the sensed voltage

equals or exceeds 1.0V, the corresponding input to the logic

gates is pulled low, and the PWM comparator output is

overridden. This provides a current limited output. If 1.25V

is exceeded, the other comparator is also tripped activating

the soft start feature.

Application Information

Voltage Mode

Voltage mode control has a single feedback path, comparing

the oscillator voltage ramp with the output of an error amplifier

which is comparing a sample of the dc output voltage to a

reference. The MIC3832/3 may be operated in voltage mode

by connecting CT directly to CMR. Excessive current may be

controlled indirectly by driving SHDN. Input voltage changes

are sensed as output voltage changes, with delayed re-

sponse. The ESR (effective series resistance) of each output

capacitor adds a pole, requiring a compensating zero or low-

frequency roll-off in the error amplifier. Loop gain varies with

input voltage.

Current Mode

Current-mode control samples the inductor current wave-

form. It provides feedback from the output stage, limits peak

switching transistor current, removes one pole (the LC filter

pole) from the output, provides input voltage feedforward with

good rejection of input line transients, and reduces the

problem of core saturation. The CMR pin monitors the

inductor current.

Current-mode control uses a current sense resistor or trans-

former to provide a voltage ramp which is compared the

output of an error amplifier/comparator which is comparing a

sample of the dc output voltage to a reference.

The MIC3832/3 may be operated in current mode by connect-

ing the current sample to the CMR pin. Input voltage

variations affect the inductor current slope, providing fast

Micrel

response. Two feedback loops complicate circuit analysis.

The error amplifier controls the output current, with a single

pole from the output filter.

Multiple supplies may be connected in parallel without con-

cern for current-hogging.

Slope Compensation

At duty cycles above 50% subharmonic oscillations may

occur due to the negative resistance effect of the input, for

example, current decreasing as input voltage increases.

Slope compensation, adding a portion of the oscillator ramp

to the CMR pin, is used to remove this error. Power circuit

resonances may introduce instability due to output current

variations. Internal front edge blanking reduces the effect of

leading edge inductive current spikes.

Push-Pull Cross Conduction

Push-Pull power stages have a problem when both power

switches are on simultaneously, creating a short-circuit path

from rail to rail. In order to eliminate this cross-conduction, or

shoot-through, a dead-time is usually added at each transi-

tion, allowing the energized switch to fully turn off before the

opposite switch is energized. This dead time decreases

efficiency as the available duty cycle time is reduced, making

the input current larger than optimum for a given input

voltage, with higher conduction losses.

In a push-pull (forward) converter, an output inductor oper-

ates like a buck converter to store and provide energy to the

load and output capacitor during the deadtime. Both output

rectifier diodes are pulled into conduction by the output

inductor during the deadtime, draining the magnetic field from

the output transformer. At the start of each power cycle the

energized input switch sees a virtual short-circuit in the

transformer, until the opposite diode is pulled out of conduc-

tion.

Current Fed

If a constant current source is added to the feedpoint of a

push-pull power stage, no deadtime is needed between

power cycles, since the switches may be designed to handle

the limited cross-conduction current. No output inductor is

needed to store energy during the deadtime, and the related

problem of simultaneously conducting output rectifiers is

eliminated.

Buck-Derived Current Fed

Refer to Figure 5.

If a supply is designed to operate from a widely varying input

voltage, such as the power line, a PWM step-down (buck)

regulator may be used as the constant current input to the

push-pull stage by omitting the customary output capacitor

from the buck circuit. A bifilar wound pulse transformer is

used to provide high-side drive to the PWM switch. A slightly

overlapping drive is provided to the push-pull switches, so the

output side of the buck inductor will swing toward ground

during cross-conduction, limiting dissipated power.

4-142

April 1998

Share Link: