Simple class A amplifier for a different reason

[Salvador]

Hi, so i have built some amps and power supplies before , even some real simple smps from a working circuit schematic but I have two high power npn transistors laying around and some old half baked circuit boards.
I am interested can I make a real simple smps out of a 555 oscillator chip, and a few transistors , I took a basic A class design (I know it's an amplifier but most smps circuits are basically certain frequency amplifiers and their load is a transformer)

I made a multisim simulation about the schematic , the ne555 chip is simulated with the bipolar voltage source in the simulation.
I want to know what you think could such a circuit work?What elements would you add to make it better?
the main push pull npn transistors I would use would be BUX98A , I used these ones in the simulation because the database didn't have any more powerful NPN's.

 

[Svein]

I do not like the concept of class A in a SMPS. Class A means that there will be a current through the transformer all the time. This has two consequences:

1. A lot of unnecessary heat will be generated
2. A DC component through a transformer will change the magnetic operating point

For a SMPS I would strongly recommend class C (which is the normal mode for SMPS).

 

[Salvador]

Ok right, I forgot about the fact that in this schematic I can't regulate the bias of the transistors so I don't have the chance to let them be turned off when not conducting, thanks for pointing that out. I will check class C.

actually I just did a search and the internet is quite silent about the class c circuits as compared to the amount one can find about class D or AB for example.
could you please be so kind as to show me some basic class C circuits that I could then edit and use as the basis for my simple SMPS?

thank you.

 

[Svein]

could you please be so kind as to show me some basic class C circuits that I could then edit and use as the basis for my simple SMPS?

The simplest example is to remove the biasing resistors on the output transistors (the resistor going from the base to "+") and increasing the the resistor from base to ground to 22k or thereabouts.

By the way, you can read a lot about switching regulators here http://cds.linear.com/docs/en/application-note/an44fa.pdf and here http://www.onsemi.com/PowerSolutions/content.do?id=16599 .

 

[Salvador]

So basically this is what you suggested + I added a capacitor in the output stage so that if somehow one of the transistors fails or stays open the full current wouldn't go through it from +ve to ground. even though the ones I plan to use are the bux98a which have a CE curret of about 30 amps.

what do you think?

 

[Svein]

what do you think?

Sorry, this circuit won't work. Connect both BUX's the same way (like you did in your original circuit), just with the differences I mentioned (22k to ground, no pullup).

 

[Salvador]

let me guess , in order to have the output with a capacitor like this which is then charged discharged recharged constantly one needs to have something like an AB push pull circuit correct?
I wonder why in this case it wouldn't work? I assume its because the capacitor is blocking charge flow and this circuit can't live without it or something along those lines.

also what do you think are the resistor values ok?

 

[Salvador]

Ok what about this particular case , routing back the output to the mains filter caps at the input ?
would this work? weirdly multisim shows it works but then again I don't always think multisim is 100% accurate in all cases.

 

[Svein]

Now I think it is OK. The trouble with your previous circuit was that when you turned the lower transistor ON, the upper was turned OFF, so no current could flow through the transformer at any time.

Have you looked at the links I gave you?

 

[Salvador]

Hmm since your reply was so close to mine I will ask once more to be sure about which picture you were referring to.
are you saying ok to the one were i routed the primary middle back to the input filter caps? in other words the picture I have attached to this reply?

Yes I did look at those links but they seemed mainly talking about a particular device as they were made by the very company that produces them , or maybe I missed something.

 

[Svein]

Ok what about this particular case , routing back the output to the mains filter caps at the input ?
would this work? weirdly multisim shows it works but then again I don't always think multisim is 100% accurate in all cases.

Well, it will work after a fashion. The upper BUX (Q2) is "upside down", so it will work as a zener diode with a voltage drop of about 6V.

 

[Svein]

Hmm since your reply was so close to mine I will ask once more to be sure about which picture you were referring to.

I was referring to the picture in post #7. Do not try the circuit in post #8.

 

[Salvador]

so the post #8 drawing doesn't work ?
maybe I am confusing something but i have seen smps topologies that do exactly this, take for example a simple two transistor driving a transformer primary , the upper transistor comes on and charges a capacitor , then the upper transistor shuts off the lower one comes on and discharges the capacitor, or does this in series with the high frequency transformer primary inductance can create peak voltages well above the +325 volt supply and destroy the transistors,?

 

[Salvador]

I would still love to hear about my previous question but I just wanted to say that I built the tlc555 board, a small package that sitting on the main board on which I just made a zener voltage regulator for the 555 IC.I know this sounds a bit crazy but I took the very rectified mains 325v DC which is waiting to be driven through the transformer pirmary and just used that as the power source for the 555 with the help of a few 5w wire wound 3.6Kohm resistors and 4 zeners in parallel , the zeners hold the voltage to 14.7v.

I wonder will the 555 output be powerful enough to drive the schematic I added in previous posts ? I mean ther are only two stages , so the output would have to drive the mje15032 transistor , what do you think would it suffice or would i need to add one more stage before the mje15032? like some single mje340 or something along the line of that?

thanks.

 

[Salvador]

Svein ?

 

[Svein]

I think the circuit in post #7 might work after a fashion. It depends a great deal on the transformer. And - if you do not keep the leads between the transistors and the transformer very short, you are going to spew out a lot of illegal RF noise.

Now to the problem of driving the output transistors. The output from the emitter of the driver transistor is probably good enough, but the output from the collector is probably too weak. If so, insert an emitter follower between the collector and the output transistor. By all means - try it (and remember to be careful around 325V, it bites!).

 

[Salvador]

well I added one more stage of an mje340 before the mje15032, just in case the 555 chip isn;t feeling strong enough to drive the driver transistor directly , so you would still even with this modification I just mention say that the driver transistor could do with a emitter follower, and since the emiter of the driver is good enough but the collector could fall short of it's glory , then I should connect the follower transistors base to the driver transistors collector correct?
beacuse to keep the 180 degrees out of phase signal the correct way i couldn't just drive one output transistor from the emitter and then the follower alos from the emitter correct?

 

[Svein]

then I should connect the follower transistors base to the driver transistors collector correct?

Correct.

 

[Salvador]

Now I started to solder in some of the parts for this but before i continue i want to ask whether my parts values are fine.
also i tried to simulate the two transistor schematic were the driver transistor has a follower transistor from collector and then each of the emitters of each of the transistors drive one NPN output transistor , but the simulation didnt show any sucess, when i just have one driver transistor and its collector attached to one output NPN while it's amitter attached to the other everything shows to be working fine.
What could be wrong here ? also on google i don't find much examples of a collector follower, basically everything there is is about emitter followers.

Ok I'm interested what you think.
I have attached both pictures, one is with the oridnary circuit , the other is with the follower, now in that second with the follower i have no base resistor to the follower transistor but ion the simulation i tried many ways none seemed to work.

P.S. One thing some posts back I asked why it wouldn't work if the transformer primary was driven with a capacitor or just attached to the middle point of two smoothing capacitors used at the mains rectifier , and I got no answer to that but I would still like to know , as in many other smps this sort of thing works, I even built one very simple some time ago were the very exactthing was used.

 

[Svein]

I am not sure I like your versions. I have created a slightly different version (http://Schematic.pdf ) with the following assumptions:

• You need a low voltage supply for your square wave generator (I have assumed +5V)
• The output of the square wave generator goes from 0V to V_CC (+5V)

 

[Svein]

I have created a slightly different version

Oops - forgot a capacitor between the square wave generator and PNP base. 100nF should do the trick.

 

[Salvador]

hmm , I see your schematic , but i don't have a 5v source at present on the board, my idea was to power everything from the rectified mains.I already achieved that I used some robust wound resistors and a 15v zener to power the 555 timer chip giving me the square wave.it already works only the output is kinda messed up more resembles a saw waveform than a square i guess will have to adjust the regulation resistors on the chip pins.
well then the waveform is fed via a capacitor into the transistors , you think that wouldn't work? but multisim shows me the voltages and currents are on acceptable levels both for the 300V ce mje340 and 250 volt CE mje15032, the output BUX98A are definitely up to the job with 400 volts and some 30amps current.

or do you think the 555 chip doesn't have enough power to drive the next satge in my schematic ?

 

[Svein]

i don't have a 5v source at present on the board

For "+5V" substitute "the supply voltage to the 555".

 

[Salvador]

sorry I didnt't understand , but what would that +5v line change ? i have the chip running from +15v now which is it's maximum but it's also the zener voltage of the diode i had at hand so i just went with it.
I guess i will just try the chematic i made , not sure whether it's the best way.

after all no one seems to be interested in anything here recently , have to experiment myself , folks only answer to those ordinary questions one gets from going through a high school course or university.