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Multi-Purpose Inverter

Multi-purpose Inverter

Ever notice how many projects use the same sort of oscillator -> gate driver -> half-bridge setup? If not, flybacks, induction heaters and SSTCs are what I'm thinking of. Since they can all use the same basic driver why not make one universal driver to use for all these projects? Fueled by this thought I made a TL494 based oscillator which has separate pulse width and frequency control. This then runs a simple gate driver stage and finally a half-bridge. To ensure that I could keep mosfet deaths to a minimum I put some safety features in place. First a primary current detection transformer is used to monitor the current through the load, and shut down the inverter if some threshold is reached. Second a thermistor is placed directly on one of the mosfets to monitor their current temperature, again once a threshold is reached the inverter is shut down. This eliminated two major causes of catastrophic failure, though that's not to say things don't still go wrong. ;-) This driver was used in my offline flyback driver, and is basis of the breadboarded driver used in the series resonant induction heater setup. I now use my multi-inverter for both projects whenever I need a quick demonstration.

Schematic

The TL494 part is pretty straight-forward, pretty much just taken from a datasheet. The internal comparators are in parallel and set up as voltage followers. The voltage at pin 1/16 (non-inverting inputs) is varied between 0,6 and 3,0V using a potentiometer and some resistors. This alters the duty cycle from maximum to a predetermined "safe" minimum based on the sharpness of the switching waveforms. The discrete gate driver is a full-bridge, allowing the use of 1:1:1 GDTs. It shorts the core during dead-time, which effectively shorts the gates of the mosfets preventing spurious drive signals. The CT part of the circuit is simple analog circuitry. If voltage through a transformer is stepped up, current is stepped down, so if 14A are flowing through the current transformer's primary, and it has a ratio of 1:35, the current is 400 mA. Knowing Ohm's law, this implies that the voltage over the burden resistor must be U= R*I, so 6,8Ω * 0,4A = 2,72 V. The rectifying diode steals about 0,13 volts (measured) from this, resulting in 2,59V at the comparator. With the voltage divider on the other input set to 3V this means the over current latch would still be roughly 0,4V (2A) from triggering. I've made an open-office spreadsheet to simplify customizing the design. Don't bring the reference value over 3,5V, or the shutdown state won't latch! Blame the LM324 for this... The reason 16A is selected despite the 14A rating of IRFP450s is due to the anticipated current waveform the inverter will be switching. The average value of the current should remain well under 14A in most conditions. An idea for future iterations would be an external current limit potentiometer. The thermistor section is just a couple of voltage dividers and comparator. The resistor values were found by heating the thermistor while on a heatsink, until it was deemed hot enough. The resulting thermistor resistance was roughly 3,3k.

inside of inverter  Completed Inverter
The sponge is for isolating the filter- and halfbridge capacitors.

PCB layout I've designed a PCB for this project, and the files can be downloaded here. The PCB in the image is of revision 1, which had some errors in the shutdown circuitry caused by the LM324 not swinging above 3,5V. The errors are fixed now, hence the different appearance of the PCB layout in the zip. Not specified in the schematic are heatsink requirements. When switching a 14A load the IRFP450s will dissipate a total of 85W, and on top of that the rectifier will contribute with another 5W or so. In comparison the average PC draws 120W, most of which is turned into heat. This power needs to be removed somehow, and we know how much effort goes into cooling a PC. Though my setup isn't ideal, it's not designed for continuous use so the puny fan/heatsink should suffice. Another unmentioned aspect is construction of the GDT (gate drive transformer) and CT (current transformer). Since it's used in so many projects I've made a small article about them, which you can read here. I usually scavenge CTs from ATX supplies and adjust the number of turns to my requirements. When purchasing one however the same points apply as when selecting a GDT core.


Multipurpose Inverter Mega

I've since built an upgraded inverter for driving even larger loads. Utilizing a full-bridge with advanced discrete gate drive, it's my most powerful inverter yet. Currently it is only used to power the Big Mofo HV transformer.

schematic of MKII

The lightbulb serves to limit inrush current to the filter capacitors, which could otherwise destroy the rectifier if it has an insufficient pulse current rating. Most importantly it prevents the breaker from tripping when turning this beast on, since 680µF or more of filter capacitance is quite the short when uncharged! Otherwise the circuit should be similar to the MKI shown above. For powering loads requiring more than 1kW, use even more filter capacitance. The over-current shutdown is set at 17A peak, rather than 12A. IRFP450s can withstand 18A peak, or 14A RMS, but require heavy cooling to do so continuously.

Fully constructed inverter.  Backside.
 


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Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.


Creative Commons License This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.


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