Measuring capacitor ripple current with scope

[cupoftea]

Hi,

Please help to measure ripple current in an electrolytic capacitor in our SMPS?
Its PFC’d, so we want to measure the ripple current in this capacitor over an n x 10ms interval. (n = 1 or 2 or 3 or ……)

So anyway, we are checking out a novel 150W offline , isolated single stage PFC converter.
I have no schem or BOM as we haven’t payed for it yet. “We are just sampling it” from the supplier.
Its kind of like a Half Bridge, but each switching cycle “stroke” switches from a different capacitor.

One capacitor that gets drawn from is a 33uF, 500V electrolytic in the primary side. Hence we wish to measure the ripple current (RMS) in this capacitor.
I am sensing the ripple current in this capacitor with a 100:1 current sense transformer. Secondary of which is simply a 10R burden resistor. So its sensing at 0.1V/A. (I used a 10:1 probe to measure the burden resistor voltage, and the scope was on 10:1)

I had the scope on 2ms/div, and there’s 10 divisions on the scope window.

At 100VAC input to the converter, and 150W output loading, the “cycle RMS” feature on the scope showed “0.317V”.
…since we are at 0.1V/A, may I assume that this means an AC RMS ripple current of 3.17A? This would seem odd since the capacitor only felt at room temperature (20degC) to touch after I’d switched off.

Anyway, at 240VAC, the “cycle RMS” feature on the scope showed “0.103V”….so again, may we assume this means 1.03A AC RMS ripple current?

Anyway, I wasn’t happy with these readings, so repeated them but this time with the MATH function…So I did the integral with respect to time (dt) of (C1*C1), where C1 was channel 1, the scope probe channel that I was reading the burden resistor voltage with.

Anyway…at 100VAC and 150W, the integral over the 10 divisions of the scope window, (at 2ms/div) gave “167uV^2.sec”.

So we divided 0.000167 by 20ms….giving 0.00835. The square root of this was then taken giving 0.091. But since our system is at 0.1V/A, then this comes out as 910mA AC RMS ripple current?

This is not the same as the 3.17A that was gotten using the “cycle RMS” feature.
How would you interpret these different scope readings?

I must admit, i'm surprised the scope just doesnt let you do......integral with respect to time of i^2 between two cursor points, and use the cursor-to-cursor time interval to do the division....then the answer could be square rooted , and job done. (cursor-to-cursor time being one 10ms period).

The SDS2104x scope has a memory depth of 28MPts.....so for the actual measurement done today, thats one sample every 20ms/28e6 seconds = every 714ps (since there was 10 divisions on the scope screen and it was set to 2ms/div)...that surely should have been enough samples?

The scope used was an Siglent SDS2104x…
SDS2104x User Manual:

https://int.siglent.com/upload_file/user/SDS2000X/SDS2000X_UserManual_UM0102X-E02B.pdf

 

[cupoftea]

Thanks, i see what you mean, but then maybe we just need to add a bigger input filter to it.
The only input filter it has in the AC section is a 3uF X2 cap at the front end..then small 47uH inductor...then small 1.5cm torroid (diam) common mode choke.

After the bridge, it has the 5u6 450v film cap...and then connected to the top fet of the totem pole, there is the 33uF 500v capacitor, in parallel with a 1uF film cap.....this 33uf cap is also fed from the post rectifier bus from a diode.

 

[Easy peasy]

There's an old saying about trying to make a silk purse out of a sow's ear ...

 

[Easy peasy]

here is our single switch 300W solution, single stage, Blue = 230Vac, green = Iin

We think it is quite beautiful - simple fixed pwm drive for fixed load

under normal control a feedback loop compensates for load and line and temp and current limit

just FYI

 

[cupoftea]

There's an old saying about trying to make a silk purse out of a sow's ear ...

Thanks, though it passed mains harmonics.

Please also could you give the approx size of your 300W single switch solution at all? (100-265vac)

 

[Easy peasy]

There is harmonics - then there is RF EMC cond & radiated

How small do you want it ? we are looking at GaN for this and moving to 250kHz ( also a dual GaN version for 600W )

The std min min cost version uses Si and 100kHz - the pcb is 90 x 60 x 35 high, totally convection cooled at full power ( emc filter start at 300kHz - but as you can see from the picture - it is inherently very quiet )

the GaN version will likely be about 70 x 35 x 31mm H - again convection only

 

[cupoftea]

Thanks, That sounds like what this place are looking for.
They want preferably on wall wart, and ~400w peak, poss ~200W average. (they dont exactly know just yet)
Preferably totally enclosed in plastic.
May i ask is it eg a PFC'd Flyback single stage, or one of these "very modified half bridge" SMPS's that i have with me now?
I searched for PPS on the web but couldnt find it any more?

 

[Easy peasy]

It is neither - just very clever use of existing methodologies, that have reduced the control to the simplest possible.

 

[Easy peasy]

@ cupoftea - FYI:

Single stage 300W, AC to 30V, 10A, hybrid design.

As well as the multi 100kW stuff - we also design the more common-place higher volume stuff, such as single stage solutions for mass manufacture, that confer a number of benefits to the user as well as the manufacturer. Our latest is a hybrid design, with input current seen above - for a fixed PWM o

www.linkedin.com

 

[cupoftea]

Thanks,
So, to Cap iRipple again...

...If you want to measure the ripple current in the output capacitor bank of a Flyback PFC, you can either buy a Pearson 410 Wideband current sense transformer (£600) and a Fluke 8920A True RMS meter (£800)…..
OR
….as in the attached, (LTspice and jpeg) you can settle for a little less accuracy and use the MATH function on a scope….and do an integral w.r.t time (10ms) of the i^2…….and to get the “i” you can use…
(1)….A $3 current sense transformer (eg CST2010-100L) with eg a 10R burden resistor…use this to get the switching frequency ripple current
(2)….A $1 sense resistor of a few milliOhms or so…..Filter its signal with say 1k and 47nF……use this to measure the 100Hz ripple.
When you have the above two readings, then find the total capacitor ripple current by doing the SQRT{(1)^2 + (2)^2}.

Which way would you choose? £1400 or $4 ?

(LTspice and jpeg as attached)