Electronics > Beginners
Review my Schematic
jhpadjustable:
--- Quote from: redgear on November 09, 2019, 06:37:44 am ---Sorry, but it was the best picture I got. That's why I included the actual datasheet in the attachments.
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Fair enough, but I meant your schematic screenshot. I generally use an A4 worksheet with all defaults so that, on a 1080p monitor after clicking "Zoom to fit", I can read labels and reference designators with slight difficulty. You might try the multiple/hierarchical sheet support and see how you like it. It's very handy when you're building an array of LEDs/ports/drivers/what have you.
--- Quote ---So, I can either use the oversized capacitor or pick a low ESR and a higher ripple current. Since the latter would cost more $$ they have used a oversized capacitor in the design, Correct?
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Electrolytic capacitors are rated to hold their specified capacitance after a specified lifetime under the specified conditions of maximum working voltage, ripple current, and temperature. For every 10°C reduction in operating temperature, the lifetime of the capacitor approximately doubles. For the first halving of the working voltage, the capacitor's lifetime approximately doubles (but the effect is much less with further halvings). There are similar effects in relation to ripple current and frequency, which vary by capacitor series. Capacitor manufacturers typically offer application notes that go into detail on how to forecast the lifetime of a capacitor under your chosen conditions. Some, like Illinois Capacitor and Nichicon, offer online lifetime calculators.
So you are correct, where the design can tolerate quite a bit of performance degradation while still meeting its mission, such as with C15. Their BOM seems to have been conservatively written with plenty of margin and lowest-common-denominator commodity-grade parts in mind, so that a junior engineer has a very good chance of a successful design with very good yield if they adhere to the BOM specs, no matter how crap caps the manufacturer down the street has in stock on any given day. Manufacturers love it when you buy their stuff and don't call them. :) You, as the design engineer, are free to rate your components more aggressively, balancing MTTF and performance against cost, size, etc. For example, you wouldn't take pains to derate the capacitor to the point where it outlasts the batteries by a factor of 25 or more. Now, I haven't done anything like translating the datasheet or performing an in-depth analysis of this design to calculate currents on Vsys, so take everything I say as "less than rigorous". ;)
redgear:
--- Quote from: jhpadjustable on November 09, 2019, 09:00:10 am ---Fair enough, but I meant your schematic screenshot. I generally use an A4 worksheet with all defaults so that, on a 1080p monitor after clicking "Zoom to fit", I can read labels and reference designators with slight difficulty. You might try the multiple/hierarchical sheet support and see how you like it. It's very handy when you're building an array of LEDs/ports/drivers/what have you.
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That's what I also use. I uploaded the schematic as PDF as well. The hierarchical sheet feature was what I was looking after but I did not know how to enable it.
--- Quote ---Electrolytic capacitors are rated to hold their specified capacitance after a specified lifetime under the specified conditions of maximum working voltage, ripple current, and temperature. For every 10°C reduction in operating temperature, the lifetime of the capacitor approximately doubles. For the first halving of the working voltage, the capacitor's lifetime approximately doubles (but the effect is much less with further halvings). There are similar effects in relation to ripple current and frequency, which vary by capacitor series. Capacitor manufacturers typically offer application notes that go into detail on how to forecast the lifetime of a capacitor under your chosen conditions. Some, like Illinois Capacitor and Nichicon, offer online lifetime calculators.
--- End quote ---
That's some cool info... Thank you for the links.
--- Quote ---You, as the design engineer, are free to rate your components more aggressively, balancing MTTF and performance against cost, size, etc. For example, you wouldn't take pains to derate the capacitor to the point where it outlasts the batteries by a factor of 25 or more.
--- End quote ---
That's what I intend to do, balancing MTTF and performance against cost but idk how. Where can I read more about it? What are things I can change from the above to achieve a good cost performance ratio?
Can the max ripple current be assumed to be three times that of the max DC load current? The max current in the above design will be 5A.
jhpadjustable:
--- Quote from: redgear on November 09, 2019, 10:10:42 am ---That's what I also use. I uploaded the schematic as PDF as well. The hierarchical sheet feature was what I was looking after but I did not know how to enable it.
--- End quote ---
Ah, that's a bit better.
--- Quote ---That's what I intend to do, balancing MTTF and performance against cost but idk how. Where can I read more about it? What are things I can change from the above to achieve a good cost performance ratio?
--- End quote ---
How many are you making, and how much do you consider your time to be worth? You're pushing pennies here.
--- Quote ---Can the max ripple current be assumed to be three times that of the max DC load current? The max current in the above design will be 5A.
--- End quote ---
I don't see where you're going to get 5A out of it. Third and fourth paragraphs of the overview, thanks to Google Translate:
--- Quote ---The IP5328P's synchronous switching boost system provides up to 18W of output capability, maintaining an efficiency of over 90% even when the battery voltage is low. Automatically enters hibernation when unloaded.
The IP5328P's synchronous switch charging system provides up to 5.0A of charging current. Built-in IC temperature, battery temperature and input voltage control loop to intelligently adjust the charging current.
--- End quote ---
Since there's only one of those boost systems in the chip, 18W is probably a whole-system specification, not a per-port spec. VSYS doesn't participate in charging, only output.
With that cleared up, according to p39 of a presentation in Avnet's 2012 Power Forum virtual conference, the output capacitor's rms current can be estimated by:
At the maximum boost of 12V at 1.5A with a worst-case 3V battery voltage, that works out to ~2.4A output cap ripple current for the entire bank. So, what's the ESR look like? :-//
Murata has built SimSurfing, a comprehensive selection and data tool for their capacitors. I chose GRM21BR61E226ME44, the general purpose line's 0805 22µF/25V for the caps on VSYS. At 500kHz, the given ESR is about 2mΩ each.
Let's look at a "low impedance" cap example, Panasonic's FC series, claiming 1/2 the impedance of the regular high-temperature HA series. The impedance of our 220µF 25V cap in a 10x10mm case is given as 0.15Ω, and permissible ripple current at 100kHz at 105°C is 670mA. For the standard high-temperature HA series, let's take their word for it and figure 0.3Ω. Not accounting for copper losses which are on the order of the ceramic caps' ESR, we can figure the ceramic caps are each going to see 0.3/0.002 = 150x the ripple current of the elcap, leaving 2.4/451 = 5mA for the elcap. No problem here.
Having done all that I can say that the use of the 220µF/16V cap is perfectly fine from the ripple current standpoint, but has a slight impact on service life.
So how much ripple voltage will we see? Our boost converter is rated for 18W. I'm gonna handwave and use the common rule-of-thumb inductor ripple current estimate of 40%p-p of the maximum inductor current, which is about (18W/3V)=6A*0.4=2.4A. Now let's shove that up the 66µF ceramics for about (1/300kHz) = 3.3µS * (3/12) = ~0.8µs * 2.4A = ~1.9µC of charge applied. A capacitance of one farad will increase its voltage by one volt when one coulomb of charge is applied to it. Our 2µC/66µF = 1/33V = 30mVp-p. Most devices won't have a problem with that!
Now let's multiply by 1.5A max current = 45mWp-p = ~15mWrms. That's a fair bit of power to have to deal with from an EMC standpoint. Poorly dressed, cheap cables might cause interference and undesired operation. The extra capacitance will reduce the ripple to about 30mVp-p * 66µF/(66µF+220µF) = 6mVp-p = ~3mWrms. So that 220µF is probably there to control ripple on the output, and therefore EMI. The elcap is 6 cents in 5-packs at LCSC. The ceramics are 8 cents in 10-packs. How hard do you really want to push on that, and do you have a scope to experiment and validate? :)
Any greyerbeards want to tell me how far off my tree I am?
redgear:
--- Quote from: jhpadjustable on November 09, 2019, 01:19:30 pm ---How many are you making, and how much do you consider your time to be worth? You're pushing pennies here.
--- End quote ---
1k units... Maybe $2 or $3 on the end product. I wanted to learn about it as it will help me in my future projects..
--- Quote ---Since there's only one of those boost systems in the chip, 18W is probably a whole-system specification, not a per-port spec. VSYS doesn't participate in charging, only output.
With that cleared up, according to p39 of a presentation in Avnet's 2012 Power Forum virtual conference, the output capacitor's rms current can be estimated by:
(Attachment Link)
At the maximum boost of 12V at 1.5A with a worst-case 3V battery voltage, that works out to ~2.4A output cap ripple current for the entire bank. So, what's the ESR look like? :-//
Murata has built SimSurfing, a comprehensive selection and data tool for their capacitors. I chose GRM21BR61E226ME44, the general purpose line's 0805 22µF/25V for the caps on VSYS. At 500kHz, the given ESR is about 2mΩ each.
Let's look at a "low impedance" cap example, Panasonic's FC series, claiming 1/2 the impedance of the regular high-temperature HA series. The impedance of our 220µF 25V cap in a 10x10mm case is given as 0.15Ω, and permissible ripple current at 100kHz at 105°C is 670mA. For the standard high-temperature HA series, let's take their word for it and figure 0.3Ω. Not accounting for copper losses which are on the order of the ceramic caps' ESR, we can figure the ceramic caps are each going to see 0.3/0.002 = 150x the ripple current of the elcap, leaving 2.4/451 = 5mA for the elcap. No problem here.
Having done all that I can say that the use of the 220µF/16V cap is perfectly fine from the ripple current standpoint, but has a slight impact on service life.
--- End quote ---
Thanks, that's pretty detailed... What is the 451 and how did you arrive at that figure?
--- Quote ---So how much ripple voltage will we see? Our boost converter is rated for 18W. I'm gonna handwave and use the common rule-of-thumb inductor ripple current estimate of 40%p-p of the maximum inductor current, which is about (18W/3V)=6A*0.4=2.4A. Now let's shove that up the 66µF ceramics for about (1/300kHz) = 3.3µS * (3/12) = ~0.8µs * 2.4A = ~1.9µC of charge applied. A capacitance of one farad will increase its voltage by one volt when one coulomb of charge is applied to it. Our 2µC/66µF = 1/33V = 30mVp-p. Most devices won't have a problem with that!
Now let's multiply by 1.5A max current = 45mWp-p = ~15mWrms. That's a fair bit of power to have to deal with from an EMC standpoint. Poorly dressed, cheap cables might cause interference and undesired operation. The extra capacitance will reduce the ripple to about 30mVp-p * 66µF/(66µF+220µF) = 6mVp-p = ~3mWrms. So that 220µF is probably there to control ripple on the output, and therefore EMI. The elcap is 6 cents in 5-packs at LCSC. The ceramics are 8 cents in 10-packs. How hard do you really want to push on that, and do you have a scope to experiment and validate? :)
Any greyerbeards want to tell me how far off my tree I am?
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Thank you! Looks like I would be better using the specs on the BOM than wasting time to save a few cents.
While selecting a capacitor do I also need to check their temperature coefficents? They are just too many to choose from X5R,X7r, etc.
I am planning to buy a scope around $500 but I can extend my budget till $1k. Do you have any suggestions?
aix:
How much is known about IP5328P's I2C registers?
The datasheet describes the chip's slightly convoluted I2C electrical interface, but doesn't seem to even list the registers.
Anyone here knows more?
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