# Power Supply Ripple Measurement

Started by August 9, 2015
```Hi,

1. I am trying to measure the output voltage ripple of a 5 Volt , 1A power supply. The input to the power supply is 120V, 60Hz.

Can someone please suggest an appropiate Differential probe to do this measurement?

Plus I am expecting the ripple to be 2mV peak to peak. If I do not get 2mV peak to peak then what should I do to reduce it. I can not use Voltage regulator at the output.

2. I am also looking at the following part

http://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/FAN301.pdf

It's a PWM ocntroller for power charger. I am trying to understand the Figure2 on page 2 and trying to determine the output ripple on paper.

I do not have the chip yet. Can someone advise how can I calculate the ripple by the data given in the data sheet?

I am sorry if I have asked a stupid question.

melissa
```
```walrave...@gmail.com wrote:
>
>
> 1. I am trying to measure the output voltage ripple of a 5 Volt,
> 1A power supply. The input to the power supply is 120V, 60Hz.
>
> Can someone please suggest an appropiate Differential probe to
> do this measurement?

** You do not need one.

> Plus I am expecting the ripple to be 2mV peak to peak.

** So it's a switching supply.

Hook you scope ( with AC coupling) across the output with a load attached an see what you get.

> If I do not get 2mV peak to peak then what should I do to reduce it.

** If you have such a problem, come back then and describe it.

> 2. I am also looking at the following part
>
> http://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/FAN301.pdf
>
> It's a PWM ocntroller for power charger. I am trying to understand the Figure2 on page 2 and trying to determine the output ripple on paper.
>
> I do not have the chip yet. Can someone advise how can I calculate the ripple by the data given in the data sheet?
>
> I am sorry if I have asked a stupid question.

** Not stupid, but outside the realm of "basic" electronics.

The actual p-p ripple voltage seen will depend on load current, circuit layout, stray inductances in PCB tracks plus the actual ESR the electros in the filter.

....   Phil
```
```Hi,

What is the purpose of the transformer TX1 in Figure 1 of the schematic?

http://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/FAN301.pdf

melissa
```
``` walrave...@gmail.com wrote:

>
> What is the purpose of the transformer TX1 in Figure 1 of the schematic?
>
> http://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/FAN301.pdf
>
>

** O - M - G  . . . . .  :-0

I suppose this new Q is at least  "on topic " for SEB.

...  Phil

```
```It is to isolate the load from the AC line.  Also I would assume that it is
to cut the voltage from the AC line down to a voltage more compatiable with
the battery it is charging.

Notice the ground sysmbles on the left side is not the same as the chassies
ground onthe right.

<walravenmelissa@gmail.com> wrote in message
> Hi,
>
> What is the purpose of the transformer TX1 in Figure 1 of the schematic?
>
> http://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/FAN301.pdf
>
>
> melissa

```
```On Mon, 10 Aug 2015 04:09:55 -0700, walravenmelissa wrote:

> Hi,
>
> What is the purpose of the transformer TX1 in Figure 1 of the schematic?
>
> http://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/FAN301.pdf

That question, coupled with the lethal voltages on the left of the
schematic, is rather alarming.

What Ralph said: it isolates the output from the line (because you can't
trust either pin of the line to be "safe"), and it probably has a many:1
turns ratio to help cut the voltage while keeping the power supply
efficient.

Have fun.  Learn.  Don't electrocute yourself or burn your house down.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
```
```On Sun, 09 Aug 2015 19:33:37 -0700, walravenmelissa wrote:

> Hi,
>
> 1. I am trying to measure the output voltage ripple of a 5 Volt , 1A
> power supply. The input to the power supply is 120V, 60Hz.
>
>
> Can someone please suggest an appropiate Differential probe to do this
> measurement?
>
> Plus I am expecting the ripple to be 2mV peak to peak. If I do not get
> 2mV peak to peak then what should I do to reduce it. I can not use
> Voltage regulator at the output.
>
>
> 2. I am also looking at the following part
>
> http://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/FAN301.pdf
>
> It's a PWM ocntroller for power charger. I am trying to understand the
> Figure2 on page 2 and trying to determine the output ripple on paper.
>
> I do not have the chip yet. Can someone advise how can I calculate the
> ripple by the data given in the data sheet?
>
> I am sorry if I have asked a stupid question.

The output ripple is the amount of voltage that the output capacitors (Co1
and Co2) sag when D6 is not conducting.  To a first approximation, this
is close to the output current divided by the switching frequency times
the total output capacitance.

But there's a hell of a lot going on in that circuit, and first
approximations may not work.  If you can find an example circuit that's
already laid out and has all the part numbers called out for it you'll
get a huge boost in having a successful outcome.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
```
```<walravenmelissa@gmail.com> wrote:

> Hi,
>
> What is the purpose of the transformer TX1 in Figure 1 of the schematic?
>
> http://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/FAN301.pdf
>
>
> melissa

careful that the drawing of the chip in figure 2 is mirrored respect of
reality (figure 5).

Bye Jack
--
Yoda of Borg am I! Assimilated shall you be! Futile resistance is, hmm?
```
```On 2015-08-10, Ralph Mowery <rmowery28146@earthlink.net> wrote:
> It is to isolate the load from the AC line.  Also I would assume that it is
> to cut the voltage from the AC line down to a voltage more compatiable with
> the battery it is charging.
>
> Notice the ground sysmbles on the left side is not the same as the chassies
> ground onthe right.

infact the ground symbols on the left don't mean ground at all.
they'll be over 100V different from protective earth

--
\_(&atilde;&fnof;&bdquo;)_
```
```Tim Wescott wrote:

>
> The output ripple is the amount of voltage that the output capacitors (Co1
> and Co2) sag when D6 is not conducting.  To a first approximation, this
> is close to the output current divided by the switching frequency times
> the total output capacitance.
>
>

** The FAN301 switches at 45kHz, a frequency where the ESR of typical electros dominates the impedance ( ESR meters test electros at this kind of frequency ) plus the overall impedance is rising with frequency due to stray inductance in the leads and body of the cap.

To a first approximation, the ripple voltage is the *ESR* times the load current.

> But there's a hell of a lot going on in that circuit, and first
> approximations may not work.
>

** For sure.

> If you can find an example circuit that's
> already laid out and has all the part numbers called out for it you'll
> get a huge boost in having a successful outcome.

** See p32, further down the same pdf.

...  Phil

```