Switching 50V DC/20A - Page 1
Forum section: electronics / projects

author: joe1057
author_rank: Newbie
timestamp: August 24, 2014, 01:25:41 pm
content: Hi everyone,as a student project we want to design a system for a battery driven bike. I will be in charge to design the safety electronics of the battery. In other projects we are using relays for switching on or off the Battery (see my drawing in attachment). For this project we have maximal ratings of 50V DC and 20A. Does anyone have any experience for switching voltages/currents like this. Is it useful to use relays or maybe TRIACS or IGBTs? I think this question is very fundamental before starting the actual system design.Cheers, Joe

author: Sebastian
author_rank: Regular Contributor
timestamp: August 24, 2014, 01:40:21 pm
content: I would use either MOSFETs or IGBTs with adequate drivers to switch them fast enough. TRIACs are used to switch AC, so that's not what you need.

author: WarSim
author_rank: Frequent Contributor
timestamp: August 24, 2014, 02:00:10 pm
content: The first thing you need to know is the motor you will be using. The type of motor will determine what types of control you will using. The type of control you are going to use will determine the switching element.If it says the motor type on the supplies schematic, I didn't translate it to find out.Sent from my iPad using Tapatalk

author: SeanB
author_rank: Super Contributor
timestamp: August 24, 2014, 02:28:04 pm
content: Safety wise you really want to use a fuse on the battery, rated to break the current the battery can deliver if the output is shorted. That is the final safety and is an absolute need. Then you can have optional safety like thermal switching to disable the unit on overheating of the battery unit, and then you can go to control devices.Relays are both cheap and robust, but have the problem of coil current being drawn continuously during operation. You can use a bistable relay to reduce this but this then has issues with not being a reliable disconnect method. You can use a mechanical switch that has a disconnect coil and use this as a power disconnect though.Then you use electronic switches like mosfets, which are the lowest voltage drop devices at high current, though for N channel devices operating on the positive rail you will need a small DC-DC converter to give 15V above the positive rail to switch them on. IGBT will have a similar problem, along with a voltage drop of around 2V or so irrespective of current. Mosfet will be almost a short wire if selected correctly.

author: German_EE
author_rank: Super Contributor
timestamp: August 24, 2014, 04:06:02 pm
content: A MOSFET to do this is surprisingly small, have a look for the SPP80P06PH device by Infineon which can handle 60V at 80A yet appears in a TO220 package.

author: nctnico
author_rank: Super Contributor
timestamp: August 24, 2014, 05:12:07 pm
content: I wouldn't like to run 80A through a TO220 package though.Another thing to consider is dealing with spikes when the MOSFETs switches off. Even switching big resistors can causes spikes strong enough to destroy semiconductors.

author: krish2487
author_rank: Frequent Contributor
timestamp: August 24, 2014, 05:52:43 pm
content: QuoteIt depends if he is using the FET as a electrical switch and not 'switching' the FET at a frequency.From information he has provided I d say he is using it as a dumb switch.Looking at the datasheet of the FET mentioned - 80 times 0.023(Rds on) equals a measly 1.84 watts.The fet does not even need a heatsink!!!

author: joe1057
author_rank: Newbie
timestamp: August 24, 2014, 06:05:42 pm
content: First of all: Thank you all for your good advices.Having a fuse at the battery: absolutely yes. I just forgot to mention it in the scratch i attached.About the actual motor we dont have much information yet. But i think the voltage-current ratings are enough for designing the switches.First thing i would do is to set up a demo board with the SPP80P06PH and run it with 50V/20A to find out about heating and so on. I think with this MOSFET a do not need anything like precharging, right? So it can switch the full load if needed?Also does anyone know if i would need a filter capacity parallel to the motor?Cheers, Joe

author: German_EE
author_rank: Super Contributor
timestamp: August 24, 2014, 07:56:53 pm
content: As the motor is an inductive load a back EMF diode would be a good idea.

author: joe1057
author_rank: Newbie
timestamp: August 24, 2014, 08:37:34 pm
content: I think the motor controller is taking care of that...

author: Jeroen3
author_rank: Super Contributor
timestamp: August 24, 2014, 09:05:21 pm
content: I've seen people run an mini electric bike on a pwm-ed half bridge using (if I recall correctly) four to-220 quality mosfets. (Heatsinked)When doing that, all that counts is the mosfet driver (and board design). You can buy the best mosfet or igbt on the market, but they will all burn when improperly driven.Yet if it is for safety switch-off. Get a circuit breaker, a relay guaranteed to open when commanded to.

author: Rick Law
author_rank: Super Contributor
timestamp: August 24, 2014, 09:14:05 pm
content: Quote from: nctnico on August 24, 2014, 05:12:07 pmThe OP needs only 20Amp@50V.German_EEwas just saying this can handle up to 80A so it should be a cake walk for 20A.

author: plesa
author_rank: Frequent Contributor
timestamp: August 24, 2014, 09:28:20 pm
content: You mentioned "safety electronics of the battery" so there can be some requirements for certification of the switch/relay or not?You can check some SSR internal shematic I suppose for inspiration.I will put two independent identical circuits in series for safety reasons.

author: Niklas
author_rank: Frequent Contributor
timestamp: August 24, 2014, 09:43:30 pm
content: Quote from: krish2487 on August 24, 2014, 05:52:43 pmP = R * I^2 => 0.023 * 80 * 80 = 147 W

author: plesa
author_rank: Frequent Contributor
timestamp: August 24, 2014, 10:03:22 pm
content: Do not forget to add into calculation the Rdson change when the temperature increases working condition, it can be 5 imes higher.

author: krish2487
author_rank: Frequent Contributor
timestamp: August 25, 2014, 11:21:07 am
content: QuoteSorry My bad!!!I was thinking of voltage drop across the FET and wrote down power dissipated instead.You are right.. 147 is the correct answer.However, given the 20A load required by the OP. It is likely to be around 10 watts.A heatsink might be in order after all!!!!

author: Artraze
author_rank: Contributor
timestamp: August 25, 2014, 03:46:03 pm
content: Quote from: Jeroen3 on August 24, 2014, 09:05:21 pmI will strongly second this remark.The main failure mode of a semiconductor switch is short-circuit. So if something goes wrong and you need to shut off the power, it's not all that unlikely that the failure will also kill your FET leaving you unable to switch off the power. Indeed this is a common warning in solid state relay datasheets: don't use them as fail-safes.Depending on the nature of your application, either a good relay or a circuit breaker would be the right tool for the job.Unfortunately relays for 50V @ 20ADCare tough to find (switching DC is much harder than AC) as are remotely operated circuit breakers. So if it were me (and keep in mind I don't really know what circumstances you are thinking of using this switch), I'd probably get one of those breaker+switch units as the main on-off and remotely trip it by shorting the power though a sacrificial(?) MOSFET if needed. That's some real hobby grade 'safety' to be sure, but it would still probably be more robust/reliable than a simple FET power switch.