# Dc/dc transformer

Discussion in 'The Projects Forum' started by Madstudio, Nov 2, 2014.

1. ### Madstudio Thread Starter New Member

Nov 1, 2014
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Hey guys. I try to make simple dc/dc converter . I tried with buck converter chips but all of them are regulators. I would like to have fixed ratio converter, or when the Vin changes Vout follows the same by choosen ratio. I presume that I need fixed duty PWM to achieve this, but it looks pretty complex and "current hungry". My voltages are 7-40 Vin , Vout@36v=7v/125ma. Input current should be lower than 40ma. Can you advice what type of circuit can achieve this?
Cheers. Milan

2. ### ronv AAC Fanatic!

Nov 12, 2008
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So you want 7 volts out with 35 volts in and about 1.5 volts out with 7 volts in?
How well regulated?

3. ### #12 Expert

Nov 30, 2010
17,842
9,179
Basic concept: Divide the input voltage by 5. Compare that to the output voltage. Give that result to the "voltage satisfied" pin of the buck regulator.

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4. ### Madstudio Thread Starter New Member

Nov 1, 2014
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Yes, something like that. Ratio 5.14:1. . Sorry, how you mean "regulated"? If you ask for ratio tollerance, then it's not critical, most important is that input current never goes more than 40ma and with voltage @vin 36v is Vout 7v +\- 1%. With lower Vin values tollerance can be less acurate. I hope that I described well enaugh :/

5. ### Madstudio Thread Starter New Member

Nov 1, 2014
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Very interesting i have to try this with LM2597 which is right now on my bench. Thanks

6. ### #12 Expert

Nov 30, 2010
17,842
9,179
Looks like 61% efficiency required. Entirely possible.

No guarantees that you can finish this circuit, but I am trying to present a concept. If I seem to be suffering from cranio-rectal inversion, the other denizens here at AAC will surely point it out for us. We have a very bright crowd here.

7. ### Madstudio Thread Starter New Member

Nov 1, 2014
7
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Thanks again. i will post my result A.S.A.P

8. ### Madstudio Thread Starter New Member

Nov 1, 2014
7
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hey guys, I have one more question before i burn something Do I miss something or simple opamp comparator will do the job on the #12 idea circuit? If yes, then do I have to pay attention on CM voltages. The worst scenario can be, 7v at -in and 7V at +in. since I can supply the opamp with single +15v do I play in "opamp safe area"?

9. ### #12 Expert

Nov 30, 2010
17,842
9,179
Yes, common mode voltage limits are a requirement of op-amps. Choose the right chip or your switching regulator dies a horrible death.

I have to confess, fast switchers are outside my area of expertise. For instance, this one needs a feed-forward capacitor that I have no idea how to calculate for a variable supply and the noise radiation depends heavily on the current loop area of the regulator board. A comparator is faster than most op-amps, but you need to arrange the circuit to add positive voltage to pin 4 to stop the voltage rise. An op-amp can be as simple as a differential pair and a voltage reference ( drawing attached) or you might find Op-amp IC's that outperform anything you can build with one transistor at a time.

I'm experienced enough to see the concept but I'm not educated enough to design the circuit. I'd hope for MikeML or Crutschow to try to get this to run on a simulator program (LTSpice, maybe) but it's really out of my league.

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10. ### Madstudio Thread Starter New Member

Nov 1, 2014
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Here's my sim attempt with Tina

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11. ### Madstudio Thread Starter New Member

Nov 1, 2014
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Oops, sorry, I uploaded the wrong jpg. here is the right one comparator is LM2903

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