hi all I have a question which I am struggling with please help - Power supply question

Mario777

Joined Nov 17, 2023
12
If I hook up a DC to DC converter 12V to 1.4V with a load of 2A to the circuit shown, how long converter will operate if it turns off when the supply drops below 4V? please help me out

WBahn

Joined Mar 31, 2012
29,885
If I hook up a DC to DC converter 12V to 1.4V with a load of 2A to the circuit shown, how long converter will operate if it turns off when the supply drops below 4V? please help me out View attachment 307834
Hook it up how?

Where is this 2 A load?

When what supply drops below 4 V?

Is the circuit you show powering the DC to DC converter? Or the other way around? Or what?

Mario777

Joined Nov 17, 2023
12
If I hook up a DC to DC converter 12V to 1.4V with a load of 2A to the circuit shown, how long converter will operate if it turns off when the supply drops below 4V? please help me out
If I hook up a DC to DC converter 12V to 1.4V with a load of 2A to the circuit shown, how long converter will operate if it turns off when the supply drops below 4V? please help me out View attachment 307834
Hook it up how?

Where is this 2 A load?

When what supply drops below 4 V?

Is the circuit you show powering the DC to DC converter? Or the other way around? Or what?
Yes this circuit powers up the converter via U1(+) terminal I am not given any information what converter that is only that it is a step down with a load of 2A

Mario777

Joined Nov 17, 2023
12
Hook it up how?

Where is this 2 A load?

When what supply drops below 4 V?

Is the circuit you show powering the DC to DC converter? Or the other way around? Or what?
My fault sorry supply drops below 4V the converter switches of how long will it stay on when I disconnect the main power supply

Audioguru again

Joined Oct 21, 2019
6,620
Your circuit does not convert 12V to 1.4V. Instead its output voltage is determined by the load current.
The polarity of the zener diode is backwards so its forward voltage is about 1V when 2A is flowing through it.

Mario777

Joined Nov 17, 2023
12
Your circuit does not convert 12V to 1.4V. Instead its output voltage is determined by the load current.
The polarity of the zener diode is backwards so its forward voltage is about 1V when 2A is flowing through it.
i am not saying the circuit is the converter what i am saying is if you were to connect a converter between U1 and gnd. I am aware of the V drop across the diode and simple RC network what I can not grasp is this converter (not shown on the pic).and behaviour of the circuit when power is diconnected. Should I just look at the diode just from the perspective of a v drop across it and the discharge of the cap. That's what I was thinking but I am not sure hence the question here.

Mario777

Joined Nov 17, 2023
12
i am not saying the circuit is the converter what i am saying is if you were to connect a converter between U1 and gnd. I am aware of the V drop across the diode and simple RC network what I can not grasp is this converter (not shown on the pic).and behaviour of the circuit when power is diconnected. Should I just look at the diode just from the perspective of a v drop across it and the discharge of the cap. That's what I was thinking but I am not sure hence the question here.
this is how I think about it and I would appreciate if somebody could correct me if I am wrong. The cap is just a smooting capacitor hence if the converter is drawing 2A at 1v4 from simple ohms law get the power assuming the converter is 100% efficient(ideal) Pin=Pout therefore Iin=2.8/12 and that is the current from the cap at full load power goes off load still pulling with the same current.

WBahn

Joined Mar 31, 2012
29,885
You need to show us a schematic with everything, at least at a decent block level.

As it is, it's very hard to figure out what you are actually trying to do.

Mario777

Joined Nov 17, 2023
12
You need to show us a schematic with everything, at least at a decent block level.

As it is, it's very hard to figure out what you are actually trying to do.
That is my straggle I have only a theoretical explanation of the hook up. Further to my previous simplistic explanation I want to add that if you were to treat the converter as a simple buck converter one could think of the timing ( switching frequency) Vout/Vin -duty cycle. since there is a diode lets assume 1Vf of the diode 1v4/11=0.13 and 1v4/4=0.35 for the two extremes of the rails (A,B). If the converter does not sense the power loss the controller will try to adjust the duty cycle of PWM controlling mosfets to make up for the change. Therefore one could express the time it takes for the converter to stay operational (after the power goes off) as a difference in the duty cycle of PWM. Am I on the right track or I missing something important?

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BobTPH

Joined Jun 5, 2013
8,686
What is the purpose of the resistor, diode and capacitor? All it does is waste power. If you want to disable the the converter when its supply voltage is less than 4V, then use a reference and a comparator to do so.

Mario777

Joined Nov 17, 2023
12
What is the purpose of the resistor, diode and capacitor? All it does is waste power. If you want to disable the the converter when its supply voltage is less than 4V, then use a reference and a comparator to do so.
It is not about controlling the disconnection threshold of the converter. My question is when the power goes off (remove the source battery) how long the converter will operate for?

Audioguru again

Joined Oct 21, 2019
6,620
The buck converter circuit has a large capacitor at its output to reduce ripple. It will add time as it discharges by your load.

WBahn

Joined Mar 31, 2012
29,885
It is not about controlling the disconnection threshold of the converter. My question is when the power goes off (remove the source battery) how long the converter will operate for?
There's no way to know, because it will be determined by the design of the DC-DC converter. It may have circuitry that disconnects the output if the input voltage drops below a certain level. It has internal capacitors at the input and output whose sizes are unknown.

BobTPH

Joined Jun 5, 2013
8,686
In addition to what @WBahn said, we don’t know what the buck converter does when it can no longer regulate the output voltage.