10-09-2013, 04:18 PM
250 WATT POWER INVERTER
[attachment=57848]
Power FET's to the rescue
Power FET (field effect transistor) devices have gotten more versatile over the last few years and, at the same time.
The prices for them have plummeted. Nothing can match a FET in its ease of interfacing with logic signals. and for the
ease in which it can work in parallel with similar devices without the need for any extra components. `lb parallel the
FET's, all you have to do is tie the source leads together. When the they get warm, FET's exhibit a positive temperature
characteristic, which means as the temperature goes up, so does the resistance; as the resistance goes up, the current
through the device Is lowered. That makes FET's self-limiting when working in parallel.
FET's are now being produced with power ratings that can often make parallel operation unnecessary The ratings for
the IRFZ30s that are used in this project are amazing: they can handle a 30-amp load with 50 volts across the source-
drain leads and 75-watt power dissipation, all in a TO-220AB plastic package-for less than two bucks each when
purchased in small quantities.
Operation
Figure 1 shows the schematic of the inverter. A 555 timer, IC1, along with R3, R2, and C2, generates a 120-Hz (+/- 2
Hz) signal, as set by the value of potentiometer R3.
The output of IC1 at pin 3 is fed to the CLOCK input of a CD4013BE dual D-type flip-flop, IC2-a, which is wired to
divide the input frequency by two; that generates the 60-Hz clocking for the FET array (Q1-Q6). The output from flip-flop
IC2-a at pin I has a 50% duty cycle, which is necessary for the output transformer. The flip-flop also provides an inverted
output (/Q pin 2), which saves us from having to add additional components to invert the Q output. The second half of
IC2 (IC2-b) is not used, so all of its input pins are grounded.
The Q and /Q outputs from IC2a are each fed, via R5 and R4, to three inputs of IC3 a CMOS CD4050BE hex buffer.
Each group of three buffer outputs drives one bank of FET's in the power stage.
Construction
Some of the components mount on a small PC board, for which we've provided the foil pattern. The parts-placement
diagram is shown in Fig. 2. We recommend that you use sockets for the IC's. After soldering all components on the
board, apply 12 volts and measure the frequency on the pads marked J4 and J2. Adjust R3 for a reading of 60 Hz, and
make sure the voltage is very close to 1/2 of the supply voltage on each pad. That tells you that your duty cycle is 50%.
Now connect the rest of the components. The small offboard components can be mounted on a terminal strip.
However, be sure to mount the FET's on a heatsink. If the beatsink is at ground potential, also be sure to insulate the
FET's from it.
Operation
To operate the unit, plug the input power into
your cigarette lighter socket, turn on the power
switch, and turn on the appliance that's plugged
into the inverter. When you are not using the
inverter, be sure to turn it off, since the
transformer will draw about 2 amps even with no
load. That will drain your car battery fairly
quickly!
[attachment=57848]
Power FET's to the rescue
Power FET (field effect transistor) devices have gotten more versatile over the last few years and, at the same time.
The prices for them have plummeted. Nothing can match a FET in its ease of interfacing with logic signals. and for the
ease in which it can work in parallel with similar devices without the need for any extra components. `lb parallel the
FET's, all you have to do is tie the source leads together. When the they get warm, FET's exhibit a positive temperature
characteristic, which means as the temperature goes up, so does the resistance; as the resistance goes up, the current
through the device Is lowered. That makes FET's self-limiting when working in parallel.
FET's are now being produced with power ratings that can often make parallel operation unnecessary The ratings for
the IRFZ30s that are used in this project are amazing: they can handle a 30-amp load with 50 volts across the source-
drain leads and 75-watt power dissipation, all in a TO-220AB plastic package-for less than two bucks each when
purchased in small quantities.
Operation
Figure 1 shows the schematic of the inverter. A 555 timer, IC1, along with R3, R2, and C2, generates a 120-Hz (+/- 2
Hz) signal, as set by the value of potentiometer R3.
The output of IC1 at pin 3 is fed to the CLOCK input of a CD4013BE dual D-type flip-flop, IC2-a, which is wired to
divide the input frequency by two; that generates the 60-Hz clocking for the FET array (Q1-Q6). The output from flip-flop
IC2-a at pin I has a 50% duty cycle, which is necessary for the output transformer. The flip-flop also provides an inverted
output (/Q pin 2), which saves us from having to add additional components to invert the Q output. The second half of
IC2 (IC2-b) is not used, so all of its input pins are grounded.
The Q and /Q outputs from IC2a are each fed, via R5 and R4, to three inputs of IC3 a CMOS CD4050BE hex buffer.
Each group of three buffer outputs drives one bank of FET's in the power stage.
Construction
Some of the components mount on a small PC board, for which we've provided the foil pattern. The parts-placement
diagram is shown in Fig. 2. We recommend that you use sockets for the IC's. After soldering all components on the
board, apply 12 volts and measure the frequency on the pads marked J4 and J2. Adjust R3 for a reading of 60 Hz, and
make sure the voltage is very close to 1/2 of the supply voltage on each pad. That tells you that your duty cycle is 50%.
Now connect the rest of the components. The small offboard components can be mounted on a terminal strip.
However, be sure to mount the FET's on a heatsink. If the beatsink is at ground potential, also be sure to insulate the
FET's from it.
Operation
To operate the unit, plug the input power into
your cigarette lighter socket, turn on the power
switch, and turn on the appliance that's plugged
into the inverter. When you are not using the
inverter, be sure to turn it off, since the
transformer will draw about 2 amps even with no
load. That will drain your car battery fairly
quickly!