Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Estimating ripple current of 8 x paralel MLCC 500V 2.2nF for a resonant converter

Status
Not open for further replies.

rxpu

Member level 3
Joined
Mar 9, 2019
Messages
56
Helped
0
Reputation
0
Reaction score
0
Trophy points
6
Activity points
795
I am trying to estimate the ripple current of a capacitor bank of 8x MLCC cap 500V 2.2nF connected in paralel

Ctotal = 17.6 nF Irms = 8A

The datasheet have only ESR value: 0.009 Ohm up to 1MHZ, butno data for ripple current


Resonant freq of circuit = 200khz

If I calculate the resistive power dissipation for caps each carrying 1A

The resulting power per cap is 0.009Ohmx1x1 = 0.009W

Main questions:

The caps are CG0 type (+-%5), will the current distrubute evenly between the paralel connected caps?

Assuming an equal distrubution, is calculating the power dissipation through ESR enough?

Are there other effects to consider when estimating the maximum ripple current per cap.

Case is 1206.

Thank you for your input.
 

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
17,284
Helped
3,914
Reputation
7,826
Reaction score
3,778
Trophy points
113
Activity points
114,736
Hi,

The caps are CG0 type (+-%5), will the current distrubute evenly between the paralel connected caps?
There will always be tolerances when you have 8 capacitors, even if you use them all from the same production batch.
Thus the answer clearly is: No. There never will be a perfect match...
But the question is: what current distribution tolerance do you call as "equal" --> you need to consider this on your own.

Besides the part tolerance there will be a difference caused by the PCB layout.
A bad layout may cause additional impedance ... causing widely different currents...

Klaus
 

Easy peasy

Advanced Member level 5
Joined
Aug 15, 2015
Messages
2,410
Helped
889
Reputation
1,778
Reaction score
876
Trophy points
113
Location
Melbourne
Activity points
13,699
you need to know the dissipation factor of the caps at 200kHz ( D = 1/Q ) the VA each cap is doing, i.e. Vrms ac x Irms ac x D = heat in the caps in watts...

- - - Updated - - -

C0G / NPO is a good choice - make sure the V is 2x peak in ckt ...
 
  • Like
Reactions: rxpu

    rxpu

    points: 2
    Helpful Answer Positive Rating

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
46,775
Helped
13,884
Reputation
28,016
Reaction score
12,522
Trophy points
1,393
Location
Bochum, Germany
Activity points
272,973
Obviously capacitive impedance dominates the current sharing, respectively it will be almost equal. 1A rms should be no problem, but 360 Vrms @ 200 kHz is marginal for a 500V rated cap. 1000V rating suggested.
 
  • Like
Reactions: rxpu

    rxpu

    points: 2
    Helpful Answer Positive Rating

Easy peasy

Advanced Member level 5
Joined
Aug 15, 2015
Messages
2,410
Helped
889
Reputation
1,778
Reaction score
876
Trophy points
113
Location
Melbourne
Activity points
13,699
FvM is correct, 500V rating is too light ... the peak V will be in excess of 500V for 1A at 200kHz in 2.2nF ( V = Irms x Xc )
 

rxpu

Member level 3
Joined
Mar 9, 2019
Messages
56
Helped
0
Reputation
0
Reaction score
0
Trophy points
6
Activity points
795
Obviously capacitive impedance dominates the current sharing, respectively it will be almost equal. 1A rms should be no problem, but 360 Vrms @ 200 kHz is marginal for a 500V rated cap. 1000V rating suggested.
FvM is correct, 500V rating is too light ... the peak V will be in excess of 500V for 1A at 200kHz in 2.2nF ( V = Irms x Xc )
The topology is the capacitor-clamped half bridge LLC. Each cap is paralel connected with fast diodes.

These diodes act as voltage limiter (the diodes clamps the maximum cap. voltage to the bus voltage, any voltage voltage bigger than the bus voltage will be cut and crippled to the bus voltage )

At the same time this acts as a current limiter for the half bridge. Because limiting the cap voltage results in limiting the current flowing through the cap.

Do you think even under this clamping arrangement , a 1000V cap is needed? (in the simulation I see that for overload condition the maximum voltage clamps to the bus voltage ex: 350VDC)

If I you agree with 500V, which kinds of diodes should be used to be sure that the clamping sequence is fast enough, should these diodes be fast-soft recovery or fast-hard recovery?. I also see that regular 1N4007s may also be used. But I am not sure if 1N4007 has enough forward recovery to clamp the voltage rapidly.

Thnak you for your input.
 

Easy peasy

Advanced Member level 5
Joined
Aug 15, 2015
Messages
2,410
Helped
889
Reputation
1,778
Reaction score
876
Trophy points
113
Location
Melbourne
Activity points
13,699
4007 would be a bad choice, 600V with Trr < 35nS a far better choice - TO-220 package, on a heatsink ...
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
46,775
Helped
13,884
Reputation
28,016
Reaction score
12,522
Trophy points
1,393
Location
Bochum, Germany
Activity points
272,973
Instead of the long winded explanation I would appreciate a schematic. For the time being, if you have 1A@200kHz through 2.2 nF, you get 360 Vrms, if you clamp the voltage somehow, it's not 1A.
 

rxpu

Member level 3
Joined
Mar 9, 2019
Messages
56
Helped
0
Reputation
0
Reaction score
0
Trophy points
6
Activity points
795
Instead of the long winded explanation I would appreciate a schematic. For the time being, if you have 1A@200kHz through 2.2 nF, you get 360 Vrms, if you clamp the voltage somehow, it's not 1A.
I send the schematic. It was a rough estimation. The simulation result is Vcap=287Vpeak over 38nF capacitor @100khz @I=5A. (0.28A per 2.2nF)

1A estimation was a rough exaggeration to calculate power dissipation of the cap with a very large safety margin.


capclamp.png

- - - Updated - - -

4007 would be a bad choice, 600V with Trr < 35nS a far better choice - TO-220 package, on a heatsink ...
I think the diode kicks in only when there is an overload, In the normal working condition there should be little or negligible current flow through the diodes.

I used for each clamping diode , 2 smd diodes combination (D2PAK) in paralel with large copper area for cooling : 6cm2 x2 double sided with vias)

I think forward recovery time of the diode may also be as important as the reverse recovery time. I can not find the forward recovery time of the diodes in datasheet. I read that it may be not negligible for non-shottky diodes such as fast or ultrafast reverse recovery pn-diodes.
Any idea how to guess the forward recovery performance of a diode in the datasheet?
 
Last edited:

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
46,775
Helped
13,884
Reputation
28,016
Reaction score
12,522
Trophy points
1,393
Location
Bochum, Germany
Activity points
272,973
O.K. this looks much more comfortable. The specifications of post #1 are apparently wrong, including the total capacitance. 500V capacitor is just right for 360V DC bus.

You should know if the clamping diodes are ever forward biased, if this happens periodically, they should surely expose low trr.
 
  • Like
Reactions: rxpu

    rxpu

    points: 2
    Helpful Answer Positive Rating

Easy peasy

Advanced Member level 5
Joined
Aug 15, 2015
Messages
2,410
Helped
889
Reputation
1,778
Reaction score
876
Trophy points
113
Location
Melbourne
Activity points
13,699
because volts cannot change instantaneously - the forward recovery is not such an issue ...
 

rxpu

Member level 3
Joined
Mar 9, 2019
Messages
56
Helped
0
Reputation
0
Reaction score
0
Trophy points
6
Activity points
795
because volts cannot change instantaneously - the forward recovery is not such an issue ...
I got it, But what happens when the reverse recovery is not fast?

Because as I understand it is important that the diode kicks in if the volatge rises above a threshold.

But why is it critical that the diode shuts down fast?

My uncertain answer: If diodes shut down slowly , there may be a shoot through of the DC bus when one diode kicks in and the other can not shut down rapidly. Is it true?

- - - Updated - - -

O.K. this looks much more comfortable. The specifications of post #1 are apparently wrong, including the total capacitance. 500V capacitor is just right for 360V DC bus.

You should know if the clamping diodes are ever forward biased, if this happens periodically, they should surely expose low trr.
In fact it will not happen peridocally. It will happen only when the primary or the seconadry of the transformer is shorted. The half bridge will limit its current through limiting the Vcap.

But anyhow, I think low trr may also be mandatory even if the diodes are rarely forward biased.
 

mtwieg

Advanced Member level 5
Joined
Jan 20, 2011
Messages
3,434
Helped
1,224
Reputation
2,454
Reaction score
1,197
Trophy points
1,393
Activity points
25,706
I typically use ceramic caps from TDK, as they provide very detailed data on each part number (ESR vs frequency, temperature vs Irms and frequency, capacitance vs DC bias, etc), and they make this info fairly easy to find.

However I've noticed that the Irms ratings (from all manufacturers, not just TDK) are lower than expected. For example, here's their data on a 1uF, 100V, X7S in a 0805 package. They suggest that at 1MHz, the ESR is about 8.2mohm, and a ripple of 2.27A rms will give a temperature rise of 20C. But 2.27A rms into 8.2mohm is just 42mW of dissipation. Compare that to an 0805 resistor which is typically rated for 125mW. And I would expect a MLCC to have much better thermal performance than a resistor due to its interleaved metal electrodes.

Of course 20C isn't very high of a temperature rise. But I'm not sure this data is valid for establishing a maximum permissible ripple current. I generally don't see such a maximum specified for ceramic caps, except maybe for ATC and other specialty vendors.
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
46,775
Helped
13,884
Reputation
28,016
Reaction score
12,522
Trophy points
1,393
Location
Bochum, Germany
Activity points
272,973
If a manufacturer gives maximum AC current ratings for capacitors (either film or ceramic type), the values are possibly rather arbitrary. I did a research about the AC current rating of PP capacitors that could be used for LLC converters and found large variation between manufacturers without significant differences in ESR and measured temperature rise. At this point we decided to make our own specification of useful current rating based on empirical determined temperature rise.
 

Status
Not open for further replies.
Toggle Sidebar

Part and Inventory Search


Welcome to EDABoard.com

Sponsor

Sponsor

Design Fast


×
Top