Author Topic: Ripple current / voltage, what is too much?  (Read 5197 times)

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Offline FriedMuleTopic starter

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Ripple current / voltage, what is too much?
« on: October 24, 2018, 10:05:09 am »
I am trying to learn how I determine max allowable power supply ripple for flawless functioning of a component, out from a datasheet, because I can't seem to find out.

Please help:-)

Here are two of the component I can't find out:

Opto PC619 component: https://www.datasheets360.com/pdf/-3876812092323290973
Opto TLP3543A https://www.google.dk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwi_kfXq6J7eAhXBlIsKHWFPCiIQFjAAegQICRAC&url=https%3A%2F%2Ftoshiba.semicon-storage.com%2Finfo%2Fdocget.jsp%3Fdid%3D60317%26prodName%3DTLP3543A&usg=AOvVaw3BW5ay3ild_na7FThE9tP6
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Offline r0d3z1

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Re: Ripple current / voltage, what is too much?
« Reply #1 on: October 24, 2018, 10:21:53 am »
it's a good question, the LED don't like ripple but it will be hard to find such a parameters in the DS
 

Offline FriedMuleTopic starter

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Re: Ripple current / voltage, what is too much?
« Reply #2 on: October 24, 2018, 10:24:57 am »
Ok so I am not blind:-)

But what would you say I shall go after, except NUL ripple? :-)
Are we talking of m, u, n or p ?
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Offline Mario87

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Re: Ripple current / voltage, what is too much?
« Reply #3 on: October 24, 2018, 10:32:30 am »
I would try to aim for approx 5%, so on a 12v PSU that would be between 11.4v - 12v, and on a 5v PSU it would be between 4.75v - 5v, but it depends on your load, as you may require some fairly hefty capacitors to be able to filter to within 5%, ultimately though, if you want to eliminate ripple you'll need to use a voltage regulator.
 

Offline FriedMuleTopic starter

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Re: Ripple current / voltage, what is too much?
« Reply #4 on: October 24, 2018, 10:40:17 am »
I do not know the exact voltage or current but 2-10V at 500mA is what we are talking about
I have thought about something like this to filter the voltage and current:
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Offline FriedMuleTopic starter

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Re: Ripple current / voltage, what is too much?
« Reply #5 on: October 24, 2018, 12:26:00 pm »
Sorry forgot to say that, I have still not got my first Oscilloscope or any other gear, so I am relaying on your expertise:-)

Therefore my questions that a scope easily could answer.
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Offline T3sl4co1l

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Re: Ripple current / voltage, what is too much?
« Reply #6 on: October 24, 2018, 02:58:15 pm »
Why would an opto be concerned about ripple?  Their purpose is to communicate signals, presumably if anything they would be rated not by a maximum, but by a ripple transmission spec?

it's a good question, the LED don't like ripple but it will be hard to find such a parameters in the DS

The LED can be subjected to any level of AC or DC that is within the bounds of the absolute maximum ratings.  If that counts as maximum ripple, then sure, but that would be a very special-case way to rate a very general component. :)

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Offline FriedMuleTopic starter

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Re: Ripple current / voltage, what is too much?
« Reply #7 on: October 24, 2018, 08:28:28 pm »
As I understand it, is an Opto made of a LED and a LDR or LDFET. Therefor do I think that a slight dimming or flashing LED can mean a lot for the function and the signal in the receiving part of the Opto.
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Offline not1xor1

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Re: Ripple current / voltage, what is too much?
« Reply #8 on: October 25, 2018, 10:38:12 am »
it's a good question, the LED don't like ripple but it will be hard to find such a parameters in the DS

then may be they like tea with both lemon and milk like the young Mr Feynman  :-DD
 

Offline not1xor1

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Re: Ripple current / voltage, what is too much?
« Reply #9 on: October 25, 2018, 10:47:12 am »
I do not know the exact voltage or current but 2-10V at 500mA is what we are talking about
I have thought about something like this to filter the voltage and current:

like others have stated: LEDs have no problem with ripple or even short pulses of current and unless you plan for at least 10V of dropout and a lot of power waste a mosfet as a capacitance multiplier makes little sense (unless you supply the gate voltage from elsewhere)


 

Offline cowana

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Re: Ripple current / voltage, what is too much?
« Reply #10 on: October 25, 2018, 12:24:31 pm »
it's a good question, the LED don't like ripple but it will be hard to find such a parameters in the DS

LEDs are perfectly happy being driven with PWM, which could be considered as 100% ripple.

The datasheet will state the maximum allowed current (usually in continuous, average and pulsed cases) - from there it is just a matter of considering the IV characteristics to convert that into a ripple specification.
 

Offline FriedMuleTopic starter

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Re: Ripple current / voltage, what is too much?
« Reply #11 on: October 25, 2018, 01:57:54 pm »
My knowledge is wary poor so I'd trust the smart people, like you on this forum and Mr. EEV:-)
My circuit is inspired from this EEV video: https://youtu.be/wopmEyZKnYo?t=633

Since I am new and don't have any lab gear yet, could anybody please tell me if my circuit will work at all and if so how much ripple it delivers?
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Offline T3sl4co1l

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Re: Ripple current / voltage, what is too much?
« Reply #12 on: October 25, 2018, 02:07:00 pm »
You've drawn it with a MOSFET, which doesn't multiply capacitance at all.  Or, actually, following the same design motivation, it still is, but the ratio is nearly infinite; but that's absurd, and in practice, that means the description breaks down and it's better called just a filtered source follower.

You aren't apparently filtering anything the LM317 won't handle, so I don't get the point in the first place.

How does an opto fit into this?

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Offline FriedMuleTopic starter

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Re: Ripple current / voltage, what is too much?
« Reply #13 on: October 25, 2018, 02:19:34 pm »
Dave it talking about the advantages of using a N-FET in that circuit in his video, don't know if I have misunderstood.

The Opto arrives for two reasons, first I can't find out how to read max ripple for a component, don't know what to look for and the Opto is something I want to use in a smps.
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Offline mvs

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Re: Ripple current / voltage, what is too much?
« Reply #14 on: October 25, 2018, 05:54:29 pm »
Dave it talking about the advantages of using a N-FET in that circuit in his video, don't know if I have misunderstood.
Large gain of MOSFET is advantage for this circuit, but it comes together with a large voltage drop.
MOSFETs are usually enhancement mode devices with VGS(th) >2V. Single silicon BJT or Sziklai pair will drop only 0.6V.

Quote
Since I am new and don't have any lab gear yet, could anybody please tell me if my circuit will work at all and if so how much ripple it delivers?
You can get some SPICE simulator (LT-SPICE, PSPICE, TINA-TI, etc.) and simulate your circuit or at least parts of it.
LT-SPICE calculates around 1.2mV peak-tp-peak ripple after your capacitor multiplier for 10V 60Hz AC input and 10 Ohm load (please see attachment).
 

Offline FriedMuleTopic starter

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Re: Ripple current / voltage, what is too much?
« Reply #15 on: October 25, 2018, 06:58:02 pm »
Wow thanks a LOT!!

I have tried to use LTSpice but it do seem hard to use I think.

Sorry for my questions but a drop of 2V or more, that will go t heat, right?

Dave tells that the FET solution mean that you can get wary high caps-"multiplication" since it don't use any current on the gate and therefor the resistor can be wary high.

So the resulting ripple is about 1.2 mV?
If I drop the FET and use two BJT in one of the configurations, that should give less heat, but also the same ripples?
Can I get even better data in both heat and ripple with other solutions?
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Offline T3sl4co1l

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Re: Ripple current / voltage, what is too much?
« Reply #16 on: October 25, 2018, 06:59:40 pm »
Note that the output resistance is 1/Gm, or around an ohm for typical MOSFETs at typical load currents.

Output resistance is defined as a change in voltage for a change in (load) current.  So, you will get ripple voltage produced from a ripple-current-drawing load.

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Offline mvs

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Re: Ripple current / voltage, what is too much?
« Reply #17 on: October 25, 2018, 08:53:49 pm »
Sorry for my questions but a drop of 2V or more, that will go t heat, right?
Yes, its linear circuit, P=U*I.

Quote
So the resulting ripple is about 1.2 mV?
No, it is only after cap. multiplier.
LM317 does some ripple rejection too... Voltage regulators have PSRR typicaly at least 60dB (1:1000) at 100/120Hz, meaning you do not even need this cap. multiplier stage to get ripple down to few mV.
 

Offline FriedMuleTopic starter

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Re: Ripple current / voltage, what is too much?
« Reply #18 on: October 25, 2018, 11:41:45 pm »
Okay I se but how low do ripple have to be if I maybe want to power a sensitive chip?

Sorry I am a noob:-)
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Offline T3sl4co1l

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Re: Ripple current / voltage, what is too much?
« Reply #19 on: October 26, 2018, 12:35:18 am »
What chip is sensitive?

Tim
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Offline FriedMuleTopic starter

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Re: Ripple current / voltage, what is too much?
« Reply #20 on: October 26, 2018, 01:00:53 am »
It could be chips like ADCs and DACs, but also other kinds.
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Offline FriedMuleTopic starter

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Re: Ripple current / voltage, what is too much?
« Reply #21 on: October 26, 2018, 08:19:52 am »
Is the common mode choke placed correct or where would you place it?
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Offline FriedMuleTopic starter

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Re: Ripple current / voltage, what is too much?
« Reply #22 on: October 26, 2018, 12:12:16 pm »
I have tried to simulate my PSU in LTSpice, what have I done wrong to get that strange result?
Have never simulated before!
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Offline mvs

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Re: Ripple current / voltage, what is too much?
« Reply #23 on: October 26, 2018, 03:54:38 pm »
I have tried to simulate my PSU in LTSpice, what have I done wrong to get that strange result?
Ground connection is missing at lower R1 terminal.

To simulate common mode choke you need to flip one inductor and add following SPICE directive (Edit->Spice Directive):
K1 L1 L2 1

Start with normal resisistor as load. Pulsed current source is a bit complex.


Is the common mode choke placed correct or where would you place it?
Common mode choke is typicaly used in mains input filter of switched mode power supplies to limit EMI.
If you are designing power supply with 50Hz transformer, you do not need it.
« Last Edit: October 26, 2018, 04:44:43 pm by mvs »
 

Offline FriedMuleTopic starter

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Re: Ripple current / voltage, what is too much?
« Reply #24 on: October 26, 2018, 04:52:41 pm »
Thank you a lot:-)

It changed it but how can the ripple be so low, where is my error now?

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