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MZ3500

9,

POWER

CIRCUIT

DESCRIPTION

1.

BLOCK

DIAGRAM

(Block

diagram)

A.

+5V and

+12V

supplies

1.

Functions

a.

Supply

voltage

is

first

rectified

in the

rectifier

circuit

and

sent

out to the

switching

regulator

via the

over-

current

detector

provided

in the

overcurrent

protect

circuit.

b.

Next,

the

voltage

is

converted

to the

+5/+12V

output

in the

switching regulator

and

sent

out to the

noise

•

Nfilter.

c.

Change

in the

switching

regulator

output

voltage

is

sensed

by the

control

circuit

and is fed

back

to the

switching regulator after being

amplified

in the

amplifier

located

in the

control

circuit,

for

maintaining

the

output

voltage

to a

constant level.

d. The

signal

from

the

oscillator

is

supplied

to the

switch-

ing

regulator

through

the

control

circuit

for

driving

the

switching regulator.

e.

For

prevention

of

overcurrent,

the

protect

circuit

is

used

for

stopping

the

oscillator when

an

overcurrent

is

met,

and

it

makes

the

switching regulator

to

halt

in

order

to

shut

off

+12V/+5V

supply.

2.

Description

of

each

block

a.

Overcutrent protect (control/protect)

circuit

When

an

overcurrent

is met in the

+5V/+12V

circuit,

it

causes

to

increase

the

voltage

at

both

ends

of the

over-

current

detector

resistor

R1,

which

in

turn

causes

to

increase

the Q3

collector current, for, there

arises

larger

voltage difference between

the

emitter

and

base

of the

transistor

Q3.

This

makes

the

gate voltage

of the

thyns-

tor

increased

owing

to

activation

of SR.

Witr,

jctivation

of SR it

makes

the

oscillator

voltage dropped

to the

GND

level

at the

point

"a" to

stop oscillation,

which

also

makes

the

switching

regulator stopped

by the de-

activation

of the

transistor

Q5

oscillation.

This

causes

the

transistor

Q5

inactive,

and it

shuts

off the

+5V/

+ 12V

supply,

b.

Oscillation

circuit

As

the Q1

emitter

voltage

is at

almost

GND

level

whe-

the

transistor

Q1 is

active,

the Q2

base

voltage tem-

porarily

drops

close

to the GND

level

by

means

of C6,

which

in

turn

makes

Q2

inactive

and the Q2

emittei

voltage increases.

Then,

the Q2

base

voltage comes

to

rise

as C6

begins

to

be

charged

through

R6, and the

transistor

Q2

starts

to

activate again.

With

activation

of the

transistor

Q2, the

Q2

emitter

voltage starts

to

drop

and the Q1

base

voltage

is

temporarily

dropped

by

means

of C5, to

shut

off the

transistor

Ql,

which

causes

to

increase

the

transistor

Q1

emitter

voltage.

Next,

as C5 is

charged

by R5, it

makes

the Q1

base

voltage

increased

which

puts

the

transistor

Ql

into

activation.

In

this manner,

transistors

Q1 and O2 are

alternately

turned

on and off to

keep oscillating.

C5 and C6 are

charged

through

R5 and R6 by

on/off

action

of the Q1 and Q2, and

discharged

through

Ql and

Q2.