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Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
I'm going to be building a mains inverter in the not too distant future, to convert 12VDC to 230VAC 50Hz.
I'm looking at maybe 300 watts to start with, or whatever my ferrite core (from a 17" monitor smps) will handle.
It's going to just be a basic PWM controller switching the primary at 20Khz ish, rectified and smoothed output to give 325VDC, some kind of feedback to the PWM controller to keep it at 325V, and then a 555, inverter stage and a fullbridge to chop the 325V into 50Hz AC, and finally some filtering on the output. I'm also adding a low battery alarm and shutdown, maybe thermal/overload shutdown as well, but those will be finishing touches that will only happen if I get the time.
Now onto the questions...
1.Firstly does the TL494 sound like a good chip to use, or should I go for a more specialised SMPS controller? Any suggestions...?
2.Transformer turns ratio - this has me confused. I've dismantled a few smps transformers, and all of the windings appear to be roughly the same length, with only the thickness being the variant. How do I calculate the turns ratio, and number of turns required? All searches so far just tell me that the output is proportional to the input turns, ie ordinary transformer behaviour. I know the output is regulated with feedback, but how do you achieve any voltage increase if the turns ratio is equal?
3.For the switching topology on the primary, I'm not sure whether to go for a single MOSFET or IGBT swithching the 12volts into the primary, a push-pull with a center tapped primary, or a halfbridge. Which would be best? I'm thinking halfbridge.
4. Is there any advantage of running the transformer at resonance? I'm thinking not, so what would be a good way to determine the optimum operating frequency, or doesn't it matter as long as it's inaudible?
The biggest challenge will probably be getting the regulation/feedback working properly, but I just wanted to make sure I was doing the first bit properly before I start throwing things together.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
1. TL494 is OK. I only prefer SG3525 sometimes because of better output stage. I suggest you to go for some 50kHz frequency since it seems to be a compromise between losses and power.
For output stage, you should also use a PWM controller in order to make deadtime between output squarewaves (otherwise you will end with 325V RMS instead of 230!). Reactive filter in series with output could polish that to nice sinewave.
2.transformer ratio can be calculated like a normal transformer.
If I remember ATX transformer uses pretty varied number of turns, I recall 6 for 12V secondary, and since primary is 160V I remember some 30-40 turns for primary (at full duty cycle transformer can output 25V+)
You could probably use ATX transformer as-is, switching the primary at 12V, and then doubling the output with 2 diodes and caps (those from ATX supply will be good).
Diodes will need to be some fast recovery HV diodes.
3. I would use halfbridge, as flyback techniques (single transistor and push pull) produce high voltage spikes, troubling your output voltage and your mosfets wich would now need to withstand voltage much higher than supply voltage.
4. I don't get this one too clearly. Things like mazzili driver could work like a lowcost, high power and efficient solution, but with cost of no regulation. Resonant drive usually doesn't make a big difference to the transformer itself.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
Thanks for the comments firkragg, I will start the building soon!
wrote ... it's not really worth the effort!
It is to me though... I'm not really doing it because I want an inverter, but I want the satisfaction and knowledge I'll gain from building and debugging it myself. Plus I thought all these igbts I have deserved a chance to a decent life
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
One inverter that I dismantled a while back used two SG3525s: one to drive the DC-DC front end, and another to generate a 50Hz "square wave with bites out" for the fullbridge DC-AC back end.
Another used a similar front end (can't remember the exact controller chip) and for the back end it used a quartz crystal (for a precise 50Hz) and 4060 divider chip, followed by a ring counter and diode logic to generate the square wave with bites out. This drove a MOSFET fullbridge through discrete bootstrap drivers made with HV transistors. (You can't use an ordinary gate drive transformer for the output fullbridge, as they can't handle the 50Hz component of the gate drive waveform you need. You'd need either a solid-state high-side driver like the IR2181, or optoisolated drivers like the TLP250, or an iron cored GDT...)
Both DC-DC front ends used the push-pull converter with centre tapped primary. It seems to be preferred over the halfbridge for lower voltages (each MOSFET sees twice the voltage and half the current compared to the halfbridge) I guess you could use the transformer from an ATX PC power supply for this without any modification.
Personally if I was building an inverter I'd go the extra mile to make it a true sine wave output. True sine wave units are a lot more expensive to buy, so you save more by building it. And the cheap "square wave with bites out" units hate capacitive loads because the fast edges of the waveform force huge currents through the capacitance. Getting true sine wave output just means driving the fullbridge back end with the PWM waveform that makes a sine wave, and bunging a L-C filter on the output.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
I totally forgot about how I was going to drive the fullbridge at 50Hz, thanks for the tips! Depending on how long it takes me to get the regulation working at 325VDC, I might be tempted to go for true sine also. I'm going to rewind the transformer now with a new center tapped multi filar primary, and drive the thing with 2 large IGBTs!
As for the controller chip, it looks like the SG3525 is quite popular for inverters, as I found one in my 150W as well. If I can find that chip in one of my old ATX PSUs I'll use that, but it looks like I might have to use TL494 at first.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Both DC-DC front ends used the push-pull converter with centre tapped primary. It seems to be preferred over the halfbridge for lower voltages (each MOSFET sees twice the voltage and half the current compared to the halfbridge) I guess you could use the transformer from an ATX PC power supply for this without any modification.
'push pull' drive are basically two flyback converters in antiparalell. WIthout some kind of resonant drive each switch is going to see large voltage spikes (very large, you can get 100's of volts frem 12V supply). Produced voltage depends simply on leakage inductance and dI/Dt of primary. Those need to be consumed by some kind of snubber network, and in order not to dissipate large amounts of power on it good regulation must be established beforerunning the thing at full power.
Ofcourse it can be made to work (and very well, + no GDT and just 2 switches) altough just a H bridge seems easier in some cases.
Getting true sine wave output just means driving the fullbridge back end with the PWM waveform that makes a sine wave, and bunging a L-C filter on the output.
In lack of bootstrapped drive you could try using P-channel mosfet's, altough it seems hard to find ones capable for 500V.
If you like to isolate low-frequency drive you can rectify a free winding on the HF transformer.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
[quote]
Both DC-DC front ends used the push-pull converter with centre tapped primary. It seems to be preferred over the halfbridge for lower voltages (each MOSFET sees twice the voltage and half the current compared to the halfbridge) I guess you could use the transformer from an ATX PC power supply for this without any modification.
'push pull' drive are basically two flyback converters in antiparalell. WIthout some kind of resonant drive each switch is going to see large voltage spikes (very large, you can get 100's of volts frem 12V supply). Produced voltage depends simply on leakage inductance and dI/Dt of primary. Those need to be consumed by some kind of snubber network, and in order not to dissipate large amounts of power on it good regulation must be established beforerunning the thing at full power.
Ofcourse it can be made to work (and very well, + no GDT and just 2 switches) altough just a H bridge seems easier in some cases.
Getting true sine wave output just means driving the fullbridge back end with the PWM waveform that makes a sine wave, and bunging a L-C filter on the output.
In lack of bootstrapped drive you could try using P-channel mosfet's, altough it seems hard to find ones capable for 500V.
If you like to isolate low-frequency drive you can rectify a free winding on the HF transformer.
As for the controller chip, it looks like the SG3525 is quite popular for inverters, as I found one in my 150W as well. If I can find that chip in one of my old ATX PSUs I'll use that, but it looks like I might have to use TL494 at first.
I somehow prefer SG3525. It has nicer output stage adn can drive most MOSFET's and GDT's directly at lower frequencies.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Don't worry, Push/pull transistors only 'see' just over twice the supply voltage, the positive voltage on one side of the windings is clamped by the diode on the other side via to 0v.
There is some leakage inductance 'spike' but if you wind bi-filar this will be small.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
As you say yourself, you don't have an ideal transformer there so spike will appear anyway; energy must go somewhere, and since it needs time to rectify trough another-winding diode or drain trough secondary you can see some pretty high voltage appearing there in reality (L*dI/dt as maximum)
Snubber network can help it but it usually ends wasting large amounts of power unless coupling is made best possible.
Running at low duty cycles when it isn't needed also helps saving power for somewhat.
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switch mode mains inverter questions
