Lifetime of electrolytic capacitor that is over-ripple currented?

[Flyback]

Hello,
My boss has asked us to design a 60W offline flyback (Vout=24V; Vin=90-265VAC).
He insists that we use the Rubycon, 120uF, 400V (18mm x 30mm) electrolytic capacitor as our primary side smoothing capacitor(datasheet and lifetime equation below).
However, this capacitor is only rated for 670mA of ripple current, and in the application, at 100VAC input, the capacitor will see 1.4A of ripple current.
We have tried explaining this but he will not listen.
It is not possible for us to calculate this capacitor’s lifetime with this much ripple current in it because the surface temperature rise of the capacitor goes above 20degC, and the given lifetime equation does not apply if the surface case temperature goes above 20degC.

*** Rubycon 120uF, 400V electrolytic capacitor:
https://www.rubycon.co.jp/en/catalog/e_pdfs/aluminum/e_kxw.pdf

***Capacitor lifetime equation for rubycon 120uF, 400V capacitor:
https://www.rubycon.co.jp/en/products/alumi/pdf/Life.pdf

Please find attached a representative schematic, and a LTspice simulation of the 60W flyback.
We know we should not use this capacitor, but how do we prove to our boss that we should not use it?
(The SMPS is for a shower pump, and there is some belief that we can get round the over-ripple current problem by simply controlling the SMPS to only operate say 5 minutes out of every 15 minutes)

 

[alec_t]

What justification does your boss give for using that particular cap?

 

[Tony Stewart]

Run two pumps at the same time and predict when the caps blow up by running at maximum ambient +20'C. When he says what was that... say it was his cap on an accelerated life test.

 

[tvtech]

Hello,
My boss has asked us to design a 60W offline flyback (Vout=24V; Vin=90-265VAC).
He insists that we use the Rubycon, 120uF, 400V (18mm x 30mm) electrolytic capacitor as our primary side smoothing capacitor(datasheet and lifetime equation below).
However, this capacitor is only rated for 670mA of ripple current, and in the application, at 100VAC input, the capacitor will see 1.4A of ripple current.
We have tried explaining this but he will not listen.
It is not possible for us to calculate this capacitor’s lifetime with this much ripple current in it because the surface temperature rise of the capacitor goes above 20degC, and the given lifetime equation does not apply if the surface case temperature goes above 20degC.

*** Rubycon 120uF, 400V electrolytic capacitor:
https://www.rubycon.co.jp/en/catalog/e_pdfs/aluminum/e_kxw.pdf

***Capacitor lifetime equation for rubycon 120uF, 400V capacitor:
https://www.rubycon.co.jp/en/products/alumi/pdf/Life.pdf

Please find attached a representative schematic, and a LTspice simulation of the 60W flyback.
We know we should not use this capacitor, but how do we prove to our boss that we should not use it?
(The SMPS is for a shower pump, and there is some belief that we can get round the over-ripple current problem by simply controlling the SMPS to only operate say 5 minutes out of every 15 minutes)

Popular misunderstanding with SMPS. The Main Smoothing cap never runs hot. Very little heating smoothing 100 or 120Hz.

Secondary caps are another story altogether. That is where heating occurs. Smoothing high frequency DC from the Chopper.....

 

[Flyback]

the boss wants to use it, rather than eg an epcos one which can handle that ripple current, because it is smaller and cheaper than the epcos one.
This KXW series cap is definitely over ripple currented.

 

[Tony Stewart]

If pump is a 2 Hz planetary water pump, current swings will be huge and cap will burn out unless SMPS can handle all the peak surge currents with water under high pressure.

 

[crutschow]

Popular misunderstanding with SMPS. The Main Smoothing cap never runs hot. Very little heating smoothing 100 or 120Hz.
...............................

Not the whole story. The primary cap not only sees ripple from the input frequency but will also have high frequency ripple current. When the SMPS chopper switch turns on and off it will draw pulses of current from the primary cap.

 

[crutschow]

the boss wants to use it, rather than eg an epcos one which can handle that ripple current, because it is smaller and cheaper than the epcos one.
.......................

Your boss is the reason engineers lose their hair.
Is he related to Dilbert's boss?

 

[Flyback]

the strange thing is that the rubycon lifetime equation sheet (top post), doesn't give any means of calculating esr at any other frequency than 120hz...so it wouldn't actually be possible to use it to calculate a lifetime anyway. However, doing an FFT, and using the value of 100Hz ripple alone gives a capacitor temperature rise so high that the lifetime equation doesn't even apply, as the temperature rise is too high.

 

[Nigel Goodwin]

Instead of reading datasheets and playing with maths, why not simply build it, and measure the REAL results. As you're forecasting an extremely high temperature rise it should be pretty conclusive.

You could also try Tony's suggestion, and run two pumps off it and see what happens?.

 

[tvtech]

Not the whole story. The primary cap not only sees ripple from the input frequency but will also have high frequency ripple current. When the SMPS chopper switch turns on and off it will draw pulses of current from the primary cap.

If the Main Smoothing Cap is rated properly...it will not stress.
Normally the first to go (sustained over voltage)....or the very last under normal working conditions. Trust me. I did not dream this up

Regards,
tv

 

[Tony Stewart]

Flyback I have told you several times here and there.

If the Cap is rated ONLY in ripple current , do NOT use it for SMSP design except when system ESR guarantees not to exceed this.

Motor Pumps winding resistance+ low ESR MOSFETS + Choke ESR will never exceeded this value even if Cap shorts out. ( which is what will happen if you use this inferior cheaper cap material.

They dont spec ESR for a reason because it is not well controlled. You can infer that the ripple current I_squared ESR is a thermal limit of the thermal insulating electrolytic. Anyone who disagrees, shows some inexperience here unless they have proof.

RIpple current Caps are rated for AC Lines freq bridges because the ripple current is 1/x% or inverse with the x% pulse duration and x% ripple voltage. Hence the duty cycle is guaranteed to be lower when ripple current is high. THis is NOT the case with SMPS as it is high pulse rate already and then when you put a 2Hz cycle motor pump that Peak current results is double the average power of the motor load going at twice the RPM with intake + exhaust.

If you dont bother testing with a closed loop resticted flow hose to maintain water pressure as I suggested elsewhere (other forums) , since you dont have 2 pumps... I am going to give up on you.... now do the right thing.

 

[Flyback]

Thanks,
Interestingly, the following is one reason why the boss wants to use this capacitor....

**broken link removed**

...it is a 65W, 100-265vac offline flyback which uses a quasi resonant flyback , so much more ripple current in the capacitors than our ccm flyback design.
Have st.com messed up with their application note design?

 

[The Electrician]

Here's some more information on lifetime calculations:

https://www.cde.com/resources/catalogs/AEappGUIDE.pdf

This is also good:

https://www.cde.com/resources/technical-papers/multipliers.pdf

 

[Tony Stewart]

Thanks,
Interestingly, the following is one reason why the boss wants to use this capacitor....

**broken link removed**

...it is a 65W, 100-265vac offline flyback which uses a quasi resonant flyback , so much more ripple current in the capacitors than our ccm flyback design.
Have st.com messed up with their application note design?

Thanks,
Interestingly, the following is one reason why the boss wants to use this capacitor....

**broken link removed**

...it is a 65W, 100-265vac offline flyback which uses a quasi resonant flyback , so much more ripple current in the capacitors than our ccm flyback design.
Have st.com messed up with their application note design?

Maybe, but its ok as long as the step load does not exceed rated output current, as your motor does.

ST are using 1000uF from ESR= tan δ*Xc= tan δ*2πfC with DF=tan δ=0.12 for f= 120 Hz , ESR=90mΩ, yet Digi-key thinks it is 21mΩ which is not stated in the spec.

Caps rated at 25V for 19V out @ 3.42A probably getting 50% ripple current or 1.71A with cap rated at 2.36A.

There is a reason they use 3 significant figure for Amps.
No room for design slack as it can cause thermal runaway to exceed specs.

The following gives a good comparison for SMPS ESR and ripple current in an 85W 19V laptop charger.

If you want a more reliable design specify the ESR in every component at worst case internal ambient and reduce the stress on the each part and measure peak and RMS current at max load conditions, and full step load.

Note the ratio of output current 4.2Adc to the ripple current above and stronger consider an asymmetrical half bridge (AHB) design over the others. 0.13A rms secondary.vs 0.89 or 2.52A rms. AHB is only 2% ripple current for a resistive max load current.

But, your load is not resistive. What is the motor ESR?

 

[Flyback]

Thanks, Motor windings is 1 ohm.

 

[Tony Stewart]

Ok, so the stall current is 24A on your motor under worst case pressure from your 24V 2.5A? at 60W SMPS. In other words 10x your capacity.

Actual load current depends on pressure and pump/ motor velocity within each rotation with cycling peaks given to its rated mean current, determined by back EMF , velocity and ESR.

 

[Flyback]

Thanks, yes the stall current is in theory 24A, but as you know, with a flyback with a suitable sized sense resistor, it wont be able to deliver 24v at 24a.....and the l6566b controller would shutdown on overload after a second or so.

 

[Tony Stewart]

Thanks, yes the stall current is in theory 24A, but as you know, with a flyback with a suitable sized sense resistor, it wont be able to deliver 24v at 24a.....and the l6566b controller would shutdown on overload after a second or so.

Which is why SMPS is a poor power source for the starting and loaded power cycles of slow cycle pump motors.

Battery with charger is far better FARAD/ESR ratio than any SMPS. Perhaps 100x to 10kx better.
The only exception is a massive and expensive low voltage double layer polymer capacitor which comes closest and only has the advantage of many deep discharge life cycles. Now used in portable Panasonic/Epscos/TDK cap powered electric irons.

These are only avail. to large ODM's in Japan.

https://www.digikey.com/product-search/en?FV=fff40002,fff8000c&k=Double layer&mnonly=0&newproducts=0&ColumnSort=-13&page=1&stock=0&pbfree=0&rohs=0&quantity=&ptm=0&fid=0&pageSize=500

 

[AnalogKid]

Unless you are going to need thousands of supplies, I can't imagine that it is cheaper to design, proto, test, certify, and build one than to buy one from, what, a dozen manufacturers. Two dozen?

ak