Problem with MOSFET dc/ac converter

Thread Starter

jeka616

Joined Jan 14, 2009
131
Thanks my friend.

Can you suggest better mosfet drive circuits that I can adopt? I've searched around forum, didn't find anything suitable.

thanks for reply
 

Thread Starter

jeka616

Joined Jan 14, 2009
131
Do you think I can get CD4047 to drive mosfets directly , by using it in monostable mode?
 

Thread Starter

jeka616

Joined Jan 14, 2009
131
I think by changing LM358 to a comparator, and getting fets driving voltage and current high enough, i can get this circuit running the way i want it.

just need get hold of OSC. just don't have em now..
 

Audioguru

Joined Dec 20, 2007
11,251
Why are you making an old square-wave inverter that was sold 30 years ago (except with power transistors instead of Mosfets)? It is still used in 3rd-world countries.

Many electronic products don't work properly from a square-wave. They expect the mains to be a sine-wave.

A powerful inexpensive modern inverter has a voltage-regulated sine-wave output. It uses a high frequency stepup converter that has a lightweight ferrite-core transformer and uses a PWM IC to drive high voltage Mosfets in many high frequency steps. The steps are smoothed into a sine-wave with a small lightweight high frequency LC filter.
 

Thread Starter

jeka616

Joined Jan 14, 2009
131
:) This will be my second project. For now, I just experiment with this square wave. For me this will work also.

IF you have some suggestions about the sine-wave inverter schematics, please post em here, I'll have a look, as 2nd step was to build true s.w. inverter.
 

Thread Starter

jeka616

Joined Jan 14, 2009
131
You know, theoretically the circuit should work, and it does. Except the regime is not proper, so i get components hot..mosfets don't even warm up..

One comment that L.Chung pointed is, Vcc common -1.5v. Need clarity liittle bit more what this means, as electronics is just my hobby, not background, so I was just building circuits and getting them to work by experiments, not going deep into theory...by registering here, in this site, I've learned a lot of theoretical background. So thanks to every one , who contributed
 

eblc1388

Joined Nov 28, 2008
1,543
One comment that L.Chung pointed is, Vcc common -1.5v. Need clarity liittle bit more what this means,
That means the inputs to an opamp is not necessarily can allow a voltage as high as Vcc. Some opamps allow that but the LM358 don't.

It is the same for 0V as well. The LM358 allows 0V while other opamps don't.

Exceeding that limit the opamp either don't work, work incorrectly or even suffer damage.

It all in the datasheet.
 

mik3

Joined Feb 4, 2008
4,846
First, you drive the MOS transistors in a non very good way and they spend much time in the linear region and get overheated. Look at the attachment for a better driver circuit. Also, replace the 78L12 with a 7812 which can supply up to 1.5Amps.
Also, consider that when the MOS are turned on the current through the transformer increases almost linearly. The more time you leave the MOS on the higher the current becomes. Thus you have to take care of it. What is more, consider the fact that the transformer core saturates and its inductance decreases, as Audioguru mentioned.
 

Attachments

Audioguru

Joined Dec 20, 2007
11,251
The inverter circuit from Electronics-Lab was messed up (not corrected) by MP:
1) The current in each 2N3055 output transistor is 25A when the load is 500W. But the absolute max allowed current in a 2N3055 transistor is 15A and they perform poorly at more than 10A. My version (that MP deleted) used twice as many output transistors.
2) The output transistors must have their gain and base-emitter voltages matched. My version used 0.1 ohm series emitter resistors to help make the transistors the same.
3) The output and driver transistors do not have a resistor to turn them off and spike protection diodes are missing. My version had the resistors and protection diodes.
4) The CD4047 oscillator has an electrolytic capacitor with a value way too high and a timing resistor with a value way too low. My version used an accurate film capacitor and a much higher value resistor as recommended in the datasheet of the CD4047.
5) Two LM324 quad opamps are used. My version used a single LM358 dual opamp.
6) A very important fuse is missing in the wire from the battery. My version had a 60A fuse and on-off switch.
 

Thread Starter

jeka616

Joined Jan 14, 2009
131
Thanks gents.

Audiouru, I got your versio also, it says corrected by R.T, right?

Now, my version is exactly the same as your corrected one, up to LM358. I just used 7812 regulator, SD888 and then driving MOS, rather than trans.

In your post 17, you mention that CD4047 is on/off same time, is this intentionally made for transistor drive, so can't be done for MOSFETS? Either both of us are wrong, or there must be a reason why I can't drive mos with your circuit.Please explain. Just need to get my head around current flow during switching...
 

Thread Starter

jeka616

Joined Jan 14, 2009
131
And another thing....

my MOSFETs aren't even getting warm. Feels like they are not working at all. THe inverter uses 85% efficiency. Everything is ok. But snubbers get really warm after a while.

Need to identify why...
 

Thread Starter

jeka616

Joined Jan 14, 2009
131
Another circuit here, shows driving MOSFETS thorugh the small transistor..

http://www.hobby-elec.org/e_ckt30.htm

Look at Circuit Describtion chapter..it shows how things are amplified, and the current flow thorught the transformer. Pretty nice.

Now, in my case...I do have ~5v at outputs of IC;s, but I don't have 12v amplification at driver trans. collector. When I disconnect the base resistor, it shows 12v, but drops to ~6v when 8.2k base resistor is connected.

I've removed LM358 for experiments, and hooked up CD4047 directly to transistors...no luck.

I just can't get ~10v to FETS gate...this is the problem..
 

Thread Starter

jeka616

Joined Jan 14, 2009
131
That means the inputs to an opamp is not necessarily can allow a voltage as high as Vcc. Some opamps allow that but the LM358 don't.

It is the same for 0V as well. The LM358 allows 0V while other opamps don't.

Exceeding that limit the opamp either don't work, work incorrectly or even suffer damage.

It all in the datasheet.
The outputs of the CD4047 switch at exactly the same time. There is no dead time to allow one Mosfet to turn off before the other Mosfet turns on. So there are moments when both Mosfets are turned on and then they short the supply.

An extremely simple square-wave inverter will not power many electronic products properly and has very poor voltage regulation.
The circuit is not properly designed and will not work satisfactorily:

1. CD4047 is used as a astable, its Q and /Q changes simultaneously and as other users have pointed out, there is no dead time for two drive legs of the inverter transformer. There is no way to make the chip output a dead time.

2. LM358 is an opamp and not a comparator. You are using it as a voltage follower but has exceeded its allowable common mode input voltage range of Vcc-1.5 maximum. This is not good. Why it is needed in the first place is questionable.

3. The gate drive to each MOSFET is effectively through 2.4K+82Ω so the gate charge current is very limited. The MOSFET turns ON slowly.

4. I have no idea of how the existing snubber works as it just charges up the 100uF when the leg is not conducting but discharge all its charge through the 0.5Ω into the MOSFETs to 0V, wasting energy.
The inverter circuit from Electronics-Lab was messed up (not corrected) by MP:
1) The current in each 2N3055 output transistor is 25A when the load is 500W. But the absolute max allowed current in a 2N3055 transistor is 15A and they perform poorly at more than 10A. My version (that MP deleted) used twice as many output transistors.
2) The output transistors must have their gain and base-emitter voltages matched. My version used 0.1 ohm series emitter resistors to help make the transistors the same.
3) The output and driver transistors do not have a resistor to turn them off and spike protection diodes are missing. My version had the resistors and protection diodes.
4) The CD4047 oscillator has an electrolytic capacitor with a value way too high and a timing resistor with a value way too low. My version used an accurate film capacitor and a much higher value resistor as recommended in the datasheet of the CD4047.
5) Two LM324 quad opamps are used. My version used a single LM358 dual opamp.
6) A very important fuse is missing in the wire from the battery. My version had a 60A fuse and on-off switch.

So gents,

1) is LM358 placed correctly?
2) What is the difference for driving MOS or trans. using this PWM?
 
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