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HIGH-VOLTAGE INVERTER POWER SUPPLY (U) controls output

power by the signal from Digital Programmer Circuit (DPC). Power relay

1 stays on but the inverter drive signal to control it’s output power.

This Inverter Power Supply Circuit supplies 4,000V DC to the magnetron

tube from the line voltage, 220-240V 50Hz / 120V 60Hz AC input. functions

as the H.V. transformer, the H.V.capacitor and H.V.Diode.

(A) The AC input voLtage 220-240V 50Hz / 120V 60Hz is rectified to DC

voltage immediately.

(B) DC voltage will be supplied to the switching devices called IGBT.

These devices will be switched ON-OFF by the 20 to 40 kHz PWM.

(pulse width modulation) signal from the microcomputer in the DPC.

(C) This drives the High voltage transformer to increase up to 2,000V

AC and approximately 3V AC by means of transformer.

(D) Then the half-wave doubler voltage rectifier circuit, consisting of the

HV diodes and Capacitors, generates the necessary 4,000V DC

needed for the magnetron.

(E) Output power of the magnetron tube is always monitored by the

signal output from the current transformer built into the inverter

ciruit.

(F) Then this signal will be fed back to the microcomputer in the DPC to

determine operating conditions and output necessary to control PWM

signal to the inverter Power Supply to control output power.

When this Auto Control feature is selected and the Start Pad is tapped:

(A) The digital programer circuit determines the power level and

cooking time to complete cooking and indicates the operating state

in the display window. Table shows the corresponding cooking

times for respective serving by categories.

(B) When cooking time the display window has elapsed, the oven

tums off automatically by a control signal from the digital

programmer circuit.

The digital programmer circuit controls the ON-OFF time of the heater

in order to control oven cavity temperature.

(A) After the start pad is tapped with the desired Bake/Broil program

set, an 12V DC singal comes out of the digital programmer circuit

and is applied to coil of power relay (RY4).

(B) When the contacts of power relay 4 close, power source voltage

is applied to the heater and the heater turns on.

(C) When the oven temperature reaches the set temperature, the

digital programmer circuit senses the temperature through oven

temp sensor and stops supplying and 12V DC signal to the coil of

power relay 4 and the heater turns off.

(D) After the heater turns off, the oven temperature will continue

increasing a while and then decrease as shown in Figure. When

the oven temperature drops below the set temperature, the digital

programmer circuit senses the signal and starts supplying an 12C

DC  signal to the coil of power relay again.

- 9 -

POWER SETTING

OUTPUT

POWER(%)

APPROX.

HIGH

MEDIUM-HIGH

MEDIUM

MEDIUM-LOW

LOW

DEFROST

100%

70%
55%

30%

10%
30%

stay ON

stay ON
stay ON

15s ON / 7s OFF

8s ON / 14s OFF
16s ON / 6s OFF

stay ON

stay ON
stay ON

ON / OFF

ON / OFF
ON / OFF

INVERTER

CONTROL

SIGNAL

RY-1

OVEN TEMP

HEATER

(RY4)

CONVECTION

FAN MOTOR

(RY5)

OVEN TEMP

SELECTED OVEN TEMP

START

ON               OFF        ON       OFF      ON

ON

WEIGHT

SELECTED

1.0KG

COOKING TIME

 14min. 36 sec.

Combination cooking is accomplished  by microwave and convection

cooking being done alternately during one combination cooking cycle.

One combination cooking cycle is 22 seconds.

(A) During combination cooking, the digital programmer circuit

controls ON-OFF time of both power relay 1 and 4 as shown in

Figure.

(B) When the power relay 1 (RY1) is turned on, heater turns off and

after the power relay 1 turns off, power relay 4 turns on.

- 10 -

Combination

No.

1

2

3

Microwave (RY 1)

ON (SEC)

OFF (SEC)

8

5

12

14

17

10

OVEN TEMP

HEATER

(RY4)

CONVECTION

FAN MOTOR

(RY5)

FAN MOTOR

(RY3)

MICROWAVE

(RY1)

SELECTED OVEN TEMP

START

8S

OFF

8S

ON

STOP

14S

ON

ON

14S

OFF

ON/OFF

Unlike many other appliances, the microwave oven is high-voltage, high

current equipment.  Though it is free from danger in ordinary use, extreme

care should be taken during repair.

   

Do not operate on a 2-wire extension cord.  The microwave oven is

    designed to be used when grounded.  When it is inoperative, make

    sure it is grounded properly before beginning repair work.

CAUTION
Servicemen should remove their watches whenever
working close to or replacing the magnetron.

This High Voltage Inverter Power Supply circuit supplies

very high voltage and very high current for the magnetron

tube.  Though it is free from danger in ordinary use, extreme

care should be taken during repair.  As you can see, it looks

like a TV flyback transformer, however the current is

extremely large and so danger exists because of its high

current and high voltages.

The aluminum heat sink is also energized with high voltage

(HOT), so do not touch when AC input terminal is connected

to the power line because one of the IGBT  switching power

devices (Collector) is directly connected to the Aluminum

heat sink.

The Aluminum heat sink may be HOT from heat energy;

therefore, extreme care should be taken during servicing

and replacing.

Check the High Voltage Inverter Power Supply circuit grounding.

This High Voltage Inverter Power Supply circuit board must have

a proper chassis ground by the grounding bracket to the chassis

ground; otherwise, this H.V. Inverter circuit board will expose

very high voltage and cause extreme DANGER!  Be sure to have

proper grounding by the grounding plate and screws.

Warning about the electric charge in the high voltage capacitors.
For about 30 seconds after the oven is turned off, an electric charge
remains in the high voltage capacitors in the inverter power supply
circuit board.
When replacing or checking parts, remove the power plug from
the outlet and short the Inverter output terminal of the magnetron
filament terminals to the chassis ground with an insulated handle
screwdriver to discharge.  Please make sure to touch chassis
ground side first then short to the output terminals.

There is high-voltage present, with high-current capabilities in the

circuits of the primary, and secondary windings, choke coil and

heat sink of the Inverter.  It is extremely dangerous to work on or

near these circuits with oven energized.

DO NOT measure the voltage in the high voltage circuit including

filament voltage of magnetron.

Never touch any circuit wiring with your hand nor with an

insulated tool during operation.

(A) This is mandatory. Refer to “Measurements and Adjustments”

for these switches.

(B) When replacing the fuse, confirm that it has the appropriate

rating for these models.

(C) When replacing faulty switches, be sure mounting tabs are

not bent, broken or otherwise deficient in their ability to hold

the switches.

Never insert a wire, nail or any other metal object through the

lamp holes on the cavity or any other holes or gaps, because

such objects may work as an antenna and cause microwave

leakage.

INVERTER
GROUND WIRE

Touch chassis side first then short to the high voltage diode terminal.

- 11 -

When the 18 Amp. fuse if blown due to the operation of

short switch, you must replace Primary latch switch and

short switch. Also replace power relay B (RY1) when the

continuity check reads shorted contacts (1-2).

HIGH
VOLTAGE
CAPACITORS

SHORT FOR
DISCHARGE
HIGH VOLTAGE
CAPACITORS

DANGER OF HIGH VOLTAGE
AND HIGH TEMPERATURE
HOT/LIVE OF HEAT SINK

HIGH VOLTAGE REMAINS
IN HIGH VOLTAGE CAPACITORS

SHORT FOR
DISCHARGING
HIGH VOLTAGE
REMAINS IN
HIGH VOLTAGE
CAPACITORS

CHOKE COIL

CURRENT
TRANSFORMER

VR 701
* DO NOT

ADJUST

GROUNDING
HOLE

(A) After repair or replacement of parts, make sure that the screws of the

oven, etc. are neither loose nor missing.

Microwaves might leak if screws are not properly tightened.

(B) Make sure that all electrical connections are tight before inserting the

plug into the wall outlet.

(C) Check for microwave energy leakage. (Refer to procedure for measuring

microwave energy leakage.)

DO NOT BECOME EXPOSED TO RADIATION FROM THE

MICROWAVE GENERATOR OR OTHER PARTS CONDUCTING

MICROWAVE ENERGY.

1. The following components have potentials above 250V while the

appliance is operating..

h Magnetron

h High voltage transformer (Located on Inverter (U))

h High voltage diodes (Located on Inverter (U))

h High voltage capacitors (Located on Inverter (U))

Pay special attention on these portions.

2. When the appliance is operated with the door hinges or magnetron

fixed incorrectly, the microwave leakage can reach more than

5mW/cm

2

. After repair or exchange, it is very important to check if

magnetron and the door hinges are correctly fixed.

- 12 -

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