VOLTAGE CONVERTER 12VDC / 220VAC WITH 250W POWER.
The designed converter is a system enabling the supply of devices requiring 220V
AC power supply in the event of a power grid failure or in situations where there
is no access to it. The advantages of the inverter will be appreciated by people
spending their holidays in a tent or camping.
The converter was tested practically and it worked properly with the following
receivers:
• TV and radio receivers,
• bulbs, fluorescent lamps,
• circulation pumps of the central heating furnace,
• power tools (drills, grinders),
• computers,
• power supplies.
PRINCIPLE OF OPERATION
The schematic diagram of the converter is shown in Fig. 1. The 220VAC voltage
was obtained thanks to the alternating keying of the TS1 transformer windings.
Two power MOSFET transistors with n channel - T2 and T3 work as keys. These
transistors
are controlled directly by the US3 integrated circuit (SG 3524). This circuit is an
integrated PWM generator designed for use in voltage converters. The frequency
at which it works properly is quite wide and ranges from 10 Hz to 300 kHz.
In the case of the converter described here, this frequency is 50 Hz, i.e. it
corresponds to the operating frequency of the power grid. This frequency is set by
items C6 and R15.
The possibility of regulating the width of the pulses generated by the US3 system
was used to stabilize the voltage output 220VAC.
One of the voltage stabilization circuits are the elements D6, D7 and the resistive
divider R12 and R13, from which the voltage signal goes to the input of the error
amplifier (IN- terminal) of the US3 system. This voltage is compared with a
suitably divided reference voltage available at the terminal Vref. This allows for
"Tuning" the generator to the voltage at the battery terminals.
The second stabilization circuit consists of elements D8, R6, PR1, US2, R7, R8, and
C4, which are responsible for providing the signal feedback, proportional to the
voltage at the output of the converter.
A converter without stabilization would provide an output voltage depending on
the load power and the battery discharge level. This voltage would range from
170V ~ to 270V ~. Thanks to the stabilization circuits mentioned, the voltage the
inverter output should be 220 V ~ in practically all conditions. Assuming the most
unfavorable situation, which may be a partial discharge of the battery and loading
the converter with full power of 250 W, the output voltage should not drop below
220 V ~.
The keying transistors T2 and T3 are blocked by capacitances C8 and C9, whose
task is to reduce the overvoltage pulses generated when the T2 and T3 keys are
turned off. The reverse diodes present in the structures of MOSFET transistors are
also an additional protection.
When the converter is loaded with a power of 200W or 250W, significant currents
flow through the terminal transistors. They can cause the temperature of the
connectors to rise to dangerously high temperatures. Therefore, the converter is
equipped with an active cooling system. If the heat sink temperature reaches 40 °
C (approximating that the structure temperature is 70 ° C) the resistance of the
temperature sensor - which is a PTC thermistor - will increase. This will cause the
US5 comparator to change the output state to the opposite, thus driving the T4
transistor in the circuit of the collector there is a cooling fan mounted on a heat
sink.
The converter system is protected against improper connection of the battery
poles. This protection was implemented by the D1 diode in the control system of
the PK1 relay. This circuit also serves as a protection from over-discharging the
battery. If the value of the input supply voltage drops below 10.5 V, the supply
voltage will appear at the pin 6 of US1, thus blocking the operation of the T1
transistor and, consequently, the PK1 relay contacts disconnecting.
The D5 diode signals the converter operation, while the D4 diode informs about
the fact that the battery is excessively discharged.
The converter's overload protection system has been consciously abandoned
here, because in most cases the system works with only one load, the power of
which is appropriately selected. However, short-circuit protection in the form of
B1 and B2 fuses should be sufficient.
ASSEMBLY AND START-UP OF THE SYSTEM
ATTENTION!! The inverter generates 220V alternating voltage, which is as
dangerous to life and health as the voltage of the power grid. Therefore, the
commissioning of the system should be performed with particular care.
Preferably in the presence of a second person who is able to turn off the device
and provide first aid.
The converter circuit is assembled on one printed circuit board, which is shown in
Fig. 2. The arrangement of elements is shown in Fig. 3.
The keying transistors T2 and T3 are not mounted on the board, but mounted on
a suitable heat sink and connected to a transformer.
The PTC thermistor is also mounted directly on the heat sink. The situation is
similar with B1 and B2 fuses that are accessible from the outside.
A converter built of working elements should work immediately after connecting
the battery. The entire regulation of the system comes down to the appropriate
setting of the PR1 and PR2 potentiometers. Using PR1, set the voltage of 220V ~
on the converter's output. It is worth paying attention to the problem of correct
measurement of the rms voltage of 220V ~ from the converter.
Since the output waveform is not sinusoidal but close to square wave, some
instruments may misread the RMS value of the output voltage. Therefore, to
measure this voltage, it is best to use a meter equipped with a "True RMS"
converter, or to use an oscilloscope.
Set the second potentiometer PR2 so that the fan works at the heat sink
temperature of approx. 40 ° C.
The transformer used is a dual voltage toroidal transformer symmetrical 10 V
(secondary winding) and one 220 V (primary winding). The converter without
load should consume a current of approx. 300mA. If the consumed current is
much higher, it may indicate an imbalance of the TS1 transformer windings or a
different opening time of the transistors-keys T2, T3.
Finally, a few words about the cooperation between the inverter and the battery.
When selecting the battery for the converter, pay special attention to two
parameters. The first is the maximum current drawn from the battery, which
depends on the power of the device powered by the converter.
Every 10 W load inverter means 1A current consumption from the battery. The
second element is the battery capacity. The running time of the inverter must
take this important battery parameter into account.
Rezystory: Kondensatory: Półprzewodniki:
R1 – 470 Ω
R2 – 1 kΩ
R3 – 10 kΩ
R4 – 820 Ω
R5 – 1 kΩ
R6 – 130 kΩ
R7 – 5,1 kΩ
R8 – 2 kΩ
R9 – 2 kΩ
R10 – 2 kΩ
R11 – 4,7 kΩ
R12 – 10 kΩ
R13 – 5,1 kΩ
R14 – 6,8 kΩ
R15 – 130 kΩ
R16 – 6,8 kΩ
R17 – 2 kΩ
R18 – 10kΩ
R19 – 10kΩ
R20 – 10 kΩ
R21 – 10 kΩ
PR1 – 220 kΩ
PR2 – 10 kΩ
C1 – 100 μF /16V
C2 – 220 μF/16V
C3 – 100 nF
C4 – 47 μF/16V
C5 – 4,7 μF
C6 – 100 nF/63V
C7 – 100 nF /400V
C8 – 100 nF /100V
C9 – 100 nF /100V
D2 – Dioda Zenera C10V
D3 – 1N4007
D4 – LED (red)
D5 – LED (green)
D6 – 1N4007
D7 – 1N4007
D8 – 1N4007
D9 – 1N4007
D10 – 1N4007
D11 – 1N4007
D1 – 1N4007
Other elements:
PK1 – RM 83Z
B1 – 30A (ammobile)
B2 – T2A/250V
TS1 – Usec=2x10V, Upri=220V, P=250VA
PTC – Termistor 10kΩ
Wł – Switch
M – Fan 12VDC
US1 - LM741
US2 – CNY17
US3 – SG3524
US5 – LM741
T1 – BC308
T2 – IRFP 240
T3 – IRFP 240
T4 – BC 337