How do I size these components? - Page 1
Forum section: electronics / beginners

author: redgear
author_rank: Frequent Contributor
timestamp: September 25, 2019, 05:24:06 am
content: I am building a Battery Management System myself from scratch by using TI's reference schematics. I have a few noob questions.Here is the schematic I am followingI have a question on the following components are sized.Diodes : D1, D5, D6, D2, D3, D4Capacitors : C6, C7, C3, C4, C2, C1, C5Resistors : R7, R6, R16, R14, R17, R18, R15I posted a question on TI's forum and got the following reply:QuoteAnd going by the reply,I am planning to a use a TVS rated 33.3v Breakdown and a flyback diode rated at 100v.All my capacitors, resistors and diodes will have a 50% derating from nominal voltage.Do you have any suggestions? Does this look fine? Is there a better way to do it?

author: redgear
author_rank: Frequent Contributor
timestamp: September 26, 2019, 04:49:58 am
content: BUMP

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 26, 2019, 07:35:17 pm
content: The gate resistors should be split so that there is one resistor per MOSFET. Then the ferrite beads don't matter.The BQ- looks to have zeners inside so D2 and D3 aren't necessary. D4 is superfluous anyway. D6 is superfluous as, as I mentioned in the other thread, a unidirectional TVS (D5) is already a diode.If the voltage range is up to 60V, why would you choose a 33V TVS?Tim

author: redgear
author_rank: Frequent Contributor
timestamp: September 27, 2019, 05:39:04 am
content: Hi Tim,Thank you for taking the time to answer. I am a beginner and I am sorry if the questions below are stupid.Quote from: T3sl4co1l on September 26, 2019, 07:35:17 pmSo, I should just add two more gate resistors with the same value of R6 and R16, such that each MOSFET has a resistor connected to its gates and remove the ferrite beads. Correct?Quote from: T3sl4co1l on September 26, 2019, 07:35:17 pmOne of their app note says under headings `Protecting the FET Gate and DSG Pin` and `CHG Circuit` it says:Code:A zener diode may be desired between the gate and source to protect the FET VGS from excessive voltageMay be it's designed for redundancy. Thanks for pointing it out, I will get it clarified with them.I was also thinking if there would be a better protection than a TVS diode?Diode D6 is a flyback diode, its purpose would be to clamp PACK+ near PACK-. It would be a pulse current when the discharge FET opens under load, have a fast response, and stand off the normal operation voltage of the battery plus any transient. (they have mentioned it in their reply they have posted above)Quote from: T3sl4co1l on September 26, 2019, 07:35:17 pmThe above schematic I attached is a reference schematic from Ti. My system is a different from theirs. I am a beginner and felt it is best to modify the above schematic to my requirements than to create one from scratch. I will be using a battery that will measure 28.7v when fully charged. I would like to limit the voltage build up to a 30.1v. But I could not find a TVS diode rated at 30.1 so I went with the closest value that is 33v.I haven't accounted for transients while choosing the diode, Should I?Edit : Can I apply 50% derating on the capacitors from the nominal voltage - 40v ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 27, 2019, 11:48:13 am
content: Quote from: redgear on September 27, 2019, 05:39:04 amCorrect.QuoteWeird. Note that, if you do use a zener or TVS to protect gate voltage: put it "outside" of the gate resistors. The resonant loop created between a zener's capacitance and an active MOSFET typically causes oscillation during switching. Making sure that the loop between MOSFET and zener includes a resistor, prevents oscillation.I don't know what they would be protecting against, so I might suggest one of:1. Nothing2. A simple zener diode like 1N5244 (or an SMT equivalent)3. A TVS like P6KE15A or SMAJ15AQuoteYeah, I got that. That doesn't say anything about the diodes being in parallel, or the one diode doing the same thing already.The one diode will do both already, D6 isn't needed.QuoteAh. 33V will be fine then.What is this powering, or being charged by? What kind of transients are you expecting?If a charger is connected which is higher voltage or current, and attempts to overcharge the battery, what happens? Should the diode absorb all that excess energy? Should the pack voltage be allowed to float up so that no charging current is drawn? What is that hypothetical charger likely to run at?What kinds of loads will the pack see? Inductive, capacitive, motors, controllers, converters? Regenerative or dumping? (Most controllers and converters will have capacitance at their inputs, due to local filter capacitors needed for their switching circuitry.)Capacitors can be rated for:- Electrolytic or polymer: 35 or 50V- Ceramic or film: 50-100V- Don't use tantalumMost capacitors will be ceramic, because they're small.C2 I'm guessing can be rated for less voltage. RTFDS to see what that pin charges up to. At 1.5uF it will be a larger ceramic (1206 size?) and may be rated 16-50V. Electrolytic would probably be okay, too.Ceramic: prefer X7R for general purpose, and C0G under 1nF. For X7R, check the characteristics. If you can't find a char. sheet, don't use it. (Note, some brands keep these data on their website, not linked from the supplier. So you have to poke through their listings to be sure...) You're looking for a plot of capacitance versus DC bias voltage. Make sure it has adequate capacitance at the voltage used. -30% is a typical threshold.Tim

author: redgear
author_rank: Frequent Contributor
timestamp: September 28, 2019, 09:35:19 am
content: Quote from: T3sl4co1l on September 27, 2019, 11:48:13 amCool. The schematic above has resistors connected between zener and MOSFET -- R13, R14, R15. Are they enough?Quote from: T3sl4co1l on September 27, 2019, 11:48:13 amD4 is to reduce DSG switching loss, D3 is to protect the DSG FET gate, and D2 is for CHG FET gate protection.QuoteOkQuoteThe entire battery pack will be powering a ebike including motor, motor controller, micro controller and a TFT display. It was advised in one Ti's app notes to use 10V transient per cell in series. I will using a 7s design.QuoteThis is a part of BMS circuit. The above schematic is that of the High Side Gate Driver which will be controlled by a Gauge IC. The gauge IC has programmed limits for charging and discharging currents and volatges based on which it opens or closes the FETs using the BQ76200 High side driver(in schematic above)Sorry, I should have linked to the entire schematic in my thread. Here is a link to theentire schematicQuoteThe loads will be low powered BLDC motors(250W) and regen capable controllers. My motors would operate at 10A constant with max of 15A. I guess the current pulled by the controller and display won't matter much as they will be very less.QuoteThank You!C2 is a charge pump capacitor for pre-charging purposes. It will typically have the charge pump voltage, less than 15V. I'm still trying to figure out its capacitance as the information about it in the datasheet is vague.