# Thread: Anyone try a Forward Converter?

1. Originally posted by Charles Enwesi:
<STRONG>damnit if u guys dont post a schematic or something i will close this post. I have being waiting and it seems like Jeff and PreeLab has this thing under control now. PressLab, If i am not mistaken, reading ur post. You simply took an ATX psu, ripped out the AC transformer part and reduced/increase another transfer to drop the voltage to a 12v......ok i am lost. Someone explain this electronic stuff in simpler form for me. Sounds like i can do this stuff...i hope </STRONG>
Have a little patience man.. Yes, I just took out the transformer part, as well as the electronics on the AC side, and replaced it the my own creation.

Okay a transformer works on turns ratio. Let's say you have 120VAC and you want 12VAC. Your turns ratio would be primary:secondary, or in this case, 120:12 = 10:1. So as you see, the number of turns, as well as the input voltage determine the output voltage. Example web page, not mine

Let's say you wanted both 12VAC and 24VAC on the output. You would then have two secondary windings, with the 24VAC one having twice the number of turns as the 12VAC one. The voltage output is not related to current (actually it is a little bit because of resistance in the wire), so with the two windings, you will have 12VAC on one, and 24VAC on the other, regardless on how much current each one is using.

The turns ratio is a little more complicated with a switching supply, but you get the idea.

So, why do they use this "switching" power supply? It basically ups the frequency of the wall outlet, 60Hz, to somethen from 20kHz to 500kHz. This allows the use of much smaller transformers because it reduces the chance of core saturations at high loads. This happens with the core can't hold any more magnetic energy, and your windings basically turn them self into little heaters instead of energy converters (inefficiency).

So what happens if our input voltage changes, but we want a constant output voltage? With the turns ratio, our output will be ratiometric with the input, right? That's where the PWM comes in. This changes the duty cycle, or the on vs off time of the primary windings. So with a 8V input your duty cycle would be 40%, and with 16V input your duty cycle would be 20%.

This maintains the constant output voltage by monitoring the output voltage, and comparing it to a fixed reference voltage. These two voltages are fed into an error amplifier. The output of the error amplifier then controls the PWM modulator, which will increase the duty cycle if the output voltage is too low, and decrease the duty cycle if the output is too high. This maintains the correct output voltage, if the duty cycle is not too large, &gt; 50%, or too small, &lt; 5% or so. If your duty cycle is too small, your transistors don't fully turn on and turn off, so they get hot, or it does't work at all.

Anyone correct me if I'm wrong.

Presslab

2. if anyone need some background info on power suppplies here is a nice site explaining almost everything. http://www.pcguide.com/ref/power/sup/index.htm

3. Just to get this topic on the main page...

Uhm I still don't get much of what is said here which is really sad, as I majored in electronics about 6 years ago (but then again I haven't done anything with it since)

Ok I get the transformer part (windings & stuff) what I don't get a stable output. lets say, just as example, the windings ratio is 1:1. When the engine is running input is 14,4 and therefore the output too right? 14,4V is ok using a maxim part you can regulate the voltage down. but when my engine is off the current is 10V orso. so than it must be transformed up?

As may be apparent I don't get any of all of this. Maybe if my technical-english was a bit better...

hm forget what I said about getting the transformer stuff:

The transformer has three primary windings, each 3 turns, 18GA doubled up. The secondary windings are all 22GA, they are 8 turns doubled up, 4 turns tripled up, and 2 turns tripled up. It is VERY important to keep the turns ratio, and have exactly 2 turns for example. (Bend the wire at 90deg when done with the winding back to the bobbin pin.) Also the layer of each turn is important too. I will post all this info when I make it pretty.
what is 18GA and 22GA? doubled up and tripled up?

sorry I don't get it at all

[ 08-06-2001: Message edited by: marsjell ]

4. This looks like a great idea!
But, even small transformers I've played with always end up shocking the **** out of me, so I think I will be sticking with a Inverter, or I may sell it on ebay and get a DC-Dc premade.
But, I may want to keep my inverter, cause heck, its 600watts! That can come in pretty handy.

5. http://www.hills2.u-net.com/electron/smps.htm#forward

Are we all sure what the difference is between forward and flyback converter???

6. Hi all,

Sorry no schematics yet, my schematic capture software crashed on me, and my CD is here at work. So tonight I'll work on it!

Presslab

7. Originally posted by marsjell:
<STRONG>Just to get this topic on the main page...

Uhm I still don't get much of what is said here which is really sad, as I majored in electronics about 6 years ago (but then again I haven't done anything with it since)

Ok I get the transformer part (windings & stuff) what I don't get a stable output. lets say, just as example, the windings ratio is 1:1. When the engine is running input is 14,4 and therefore the output too right? 14,4V is ok using a maxim part you can regulate the voltage down. but when my engine is off the current is 10V orso. so than it must be transformed up?

As may be apparent I don't get any of all of this. Maybe if my technical-english was a bit better...

hm forget what I said about getting the transformer stuff:

what is 18GA and 22GA? doubled up and tripled up?

sorry I don't get it at all

[ 08-06-2001: Message edited by: marsjell ]</STRONG>

The PWM circuit changes the duty cycle of the primary windings to compensate for a changing input voltage. The maximum duty cycle is 50%. So let's say your minimum operating input voltage is 8V. You would be at ~50% duty cycle. At an input voltage of 16V, the duty cycle would be ~25% and so on, as to maintain a stable output. It has feedback that monitors the output voltage, and changes the duty cycle accordingly.

On the windings stuff. 18GA means 18 gauge wire. 22GA means 22 gauge wire. The doubled and tripled up means that the wire is wound together with one, or two more wires in parallel. For instance, with doubled up windings, instead of just winding one wire on the bobbin, you would put two wires next to each other and then wind it the same as you would a single wire.

Presslab

8. Originally posted by lstrunk:
<STRONG>This looks like a great idea!
But, even small transformers I've played with always end up shocking the **** out of me, so I think I will be sticking with a Inverter, or I may sell it on ebay and get a DC-Dc premade.
But, I may want to keep my inverter, cause heck, its 600watts! That can come in pretty handy.</STRONG>
There is no possible shock hazard from this, the windings could not create that much voltage from a 12V input. (24V is the maximum voltage in the system.)

A lot of small transformers are designed for LCD cold cathode lamps, and they WILL shock the hell out of you. They have a much higher voltage (some around 600V), and they have enough current to easily perforate your skin.

Presslab

9. Originally posted by Jeff Mucha:
<STRONG>http://www.hills2.u-net.com/electron/smps.htm#forward

Are we all sure what the difference is between forward and flyback converter???</STRONG>
The forward converter has a extra winding to 'drain' the magnetic flux in the core on every cycle. This allows you to run much higher power without core saturation after only a few cycles. Meaning, smaller transformer and more power.

The inductor and the diode on the secondary work as a sort of buck converter, smoothing out the current and voltage transients.

Presslab

[ 08-06-2001: Message edited by: PressLab ]

10. (24V is the maximum voltage in the system.)
do you mean the difference between the +12V and the -12V... I really hope so, I just tought I was starting to understand everything and than I read this....

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