Description Of Operating Sequence; Variable Power Cooking Control; Inverter Power Supply Circuit; Inverter Defrost - Philips NN-SD681S Service Manual

2 DESCRIPTION OF OPERATING SEQUENCE

2.1. Variable power cooking
control
High Voltage Inverter Power Supply (U) controls output power
by the signal from Digital Programmer Circuit (DPC). Power
relay always stay on, but PWM (Pulse Width Modulation) signal
controls microwave output power.
NOTE:
The ON/OFF time ratio does not correspond with the
percentage of microwave power since approximately 2
seconds are required for heating of magnetron
filament.
Variable Power Cooking
POWER SETTING OUTPUT
POWER(%)
APPROX.
HIGH P10
P9
P8
MEDIUM-HIGH P7
MEDIUM P6
P5
P4
MEDIUM-LOW P3
P2
P1
2.2.

Inverter power supply circuit

The Inverter Power Supply circuit powered from the line
voltage, 120V 60Hz AC input supplies 4,000V DC to the
magnetron tube, and functions in place of the H.V. transformer,
the H.V. capacitor and H.V. diode.
1. The AC input voltage 120V 60Hz is rectified to DC voltage
immediately.
2. DC voltage will be supplied to the switching devices called
IGBT. These devices are switched ON-OFF by the 20 to 40
kHz PWM (pulse width modulation) signal from the
microcomputer in the DPC.
3. This drives the High voltage transformer to increase voltage
up to 2,000V AC.
4. Then the half-wave doubler voltage rectifier circuit,
consisting of the H.V. diodes and capacitors, generates the
necessary 4,000V DC needed for the magnetron.
5. Output power of the magnetron tube is always monitored by
the signal output from the current transformer built into the
inverter circuit.
6. This signal is fed back to the microcomputer in the DPC to
determine operating conditions and output necessary to
control PWM signal to the Inverter Power Supply for control
of the output power.
NN-SD681S / NN-SN671S / NN-SN661S / NN-SA661S / NN-SA651S / NN-SN651B / NN-SN651W / NN-SA631B / NN-SA631W
MANUAL MICROWAVE
DUTY
ON(SEC) OFF(SEC)
100% 22 0
90% 22 0
80% 22 0
70% 22 0
60% 22 0
50% 22 0
40% 22 0
30% 22 0
20% 15 7
10% 8 14
2.3.

Inverter defrost

When the Auto Control feature is selected and the Start pad is
tapped:
1. 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.
Inverter Turbo Defrost
SELECTED WEIGHT
1.0 LB
2. When cooking time in the display window has elapsed, the
oven turns off automatically by a control signal from the
digital programmer circuit.
2.4. Sensor cooking (Only for
sensor models)
Auto sensor cooking without setting a power level or selecting
a time. All that is necessary is to select an Auto Sensor
Program before starting to cook.
Understanding Auto Sensor Cooking
As the food cooks, a certain amount of steam is produced.
If the food is covered, this steam builds up and eventually
escapes from the container. In Auto Sensor Cooking, a
carefully designed instrument, called the steam sensor
element, senses this escape of steam. Then, based upon
the Auto Sensor
automatically determine the correct power level and the
proper length of time it will take to cook the food.
NOTE:
Auto Sensor Cooking is successful with the foods and
recipes found in the Auto Sensor Cooking Guide.
Because of the vast differences in food composition,
items not mentioned in the Cooking Guide should be
prepared in the microwave oven using power select
and time features. Please consult Variable Power
Microwave Cookbook for procedures.
7
COOKING TIME
4 min.00 sec.
Program selected, the unit will