# Controlling current of SMPS

Discussion in 'The Projects Forum' started by rlm0360, Mar 14, 2016.

1. ### rlm0360 Thread Starter New Member

Mar 13, 2016
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0
I designed my first SMPS this morning but I have 3 questions. The first 2 deal with if there is an equation to find the right value of capacitor and inductor to give a smooth DC signal into the load.

The 3rd question is I want to limit the current to 1 amp. My thought was if the gain of the transistors was 50 (arbitrary number) and I have 20 milliAmps flowing into the base I would assume that would set a cap of 1 amp. But since the transistor is switching on and off, would that mean my actual current would be just shy of that? Say 700 milliAmps? If so, is there an equation to predict how much gain a transistor would need to set a true cap of 1 amp.

Thank you!!

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2. ### Papabravo Expert

Feb 24, 2006
10,152
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You can set practical conditions on your circuit, but trying for a precise result is probably impossible due to component value limitations and tolerances. Think in terms of inequalities.

In order to limit the current you have to measure the current. A normal SMPS will regulate voltage output and adjust the switch duty cycle to achieve that result. When the current limit is reached you need to decide what you want to do. Do you want to shut the supply off? Do you want to change the output voltage? You need to give something up in order to limit the current. No reasonable design would ever be based on a particular value of gain in a transistor. Gain cannot be controlled very well and it changes over time and with the load current.

3. ### Dodgydave Distinguished Member

Jun 22, 2012
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745
you need to be using the output of the op-amp to control the pwm on pin 5 of the 555 timer for voltage control, and for current limit you monitor the voltage drop across a series resistor, ideally you need to be using a deadicated pwm chip and not a timer and lm317.

4. ### rlm0360 Thread Starter New Member

Mar 13, 2016
13
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I suppose that would make more
Sense to bypass the AND gate and just use the op amp to power the 555. I will incorporate that idea into my next design.

so would it be better to leave out the LM317 all together and use a potentiometer to adjust the reference pin of the op amp? I realize now that the LM317 serves no purpose but to waste energy.

5. ### Dodgydave Distinguished Member

Jun 22, 2012
4,998
745
Correct, if you want to make a smps you need to use a deadicated chips, like TL494, Uc3842/5, Lm2596 with a pulse transformer, have a look at their datasheets, the 555 isn't the correct way to go, there are loads of ready-made pcb buck regulators on the Internet, cheaper and more reliable than making your own.

6. ### rlm0360 Thread Starter New Member

Mar 13, 2016
13
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I'm an electrical engineering student and I'm trying to do this project with what I have laying around. Could you explain why I would benefit more from a PWM chip. The switching speed should be fast enough on a 555.

7. ### Dodgydave Distinguished Member

Jun 22, 2012
4,998
745
It can be done with a 555, you need to control the sweep frequency with pin 5, using the opamp output to control pin 5 voltage, start by making an Astable centred around 10khz.

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8. ### ronv AAC Fanatic!

Nov 12, 2008
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First let me say your schematic make my neck sore.
There are design equations for smps, but I usually just cheat and use a calculator like this one.
http://www.daycounter.com/LabBook/BuckConverter/Buck-Converter-Equations.phtml
Buck converters like yours are easier than boost converters because they are more stable.
If your just starting look at hysteretic buck converters, they can be simple. I have a circuit somewhere if you would like to see it.
Often current limit can be achieved with a small sense resistor in the ground return to reduce the voltage just like the voltage control loop.

9. ### rlm0360 Thread Starter New Member

Mar 13, 2016
13
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I apologize for my schematic, it wouldn't flip. I'm very new at this as you can tell, but eager to learn!

I checked out the hysteretic buck converter and so basically it's deleting the need for PWM and I'm generating a triangle wave from a capactor discharging in parallel with the comparator? Wouldn't the triangle wave increase the heat in the transistor since it will spend more time being partially on?

Last thing is I did the math on a current regulator design and the resistors I need are nearly negligible. I suppose I could find a transistor with a highe Base-Emitter saturation voltage, but I don't think that is the right way to go.

Thanks!

10. ### Papabravo Expert

Feb 24, 2006
10,152
1,793
So basically your approach to current limiting is to reduce the voltage across the load. In you calculations you seem have come up with impractical resistor values, or what is worse values the must be realized with impossibly tight tolerances. I take the load is perfectly happy with any voltage from 0 up to and including Vcc across it. Is this correct?
Second question, what is the voltage across a shorted load?

11. ### rlm0360 Thread Starter New Member

Mar 13, 2016
13
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This is being used as a variable voltage power supply, so yes the arbitrary load I'm trying to power is happy with any voltage.

If you short a load then the voltage drop across that load become 0 volts