Cheaper method to turn on highside mosfets than isolated DCDCmodules (DC no pwm) - Page 1
Forum section: electronics / projects

author: Psi
author_rank: Super Contributor
timestamp: December 10, 2021, 11:11:15 am
content: I'm looking for a way to cost down a product that uses 4x DCDC isolated modules to turn on the mosfets in 3 half-bridges. There is no PWM here, just turning on and off the mosfets at human timescales (>500ms) for DC control.1W isolated DCDC modules seems like overkill here, but obviously any solution does need to be isolated from the input for the high side fetsAre there any other cheaper solutions to this problem? So far I have thought of the following- Custom transformer with 4 output windings. But that is a custom part so i'd rather not do that.- Some sort of isolation that uses capacitor switched been input and output?- Some sort of photocoupler that has enough photocells in series on the output side to act like a solar panel and turn on a fet without any external power input? Do these exist, my guess is they are not cheap.Can anyone think of other solutions that might be cheaper than 4x 1W DCDC modules?

author: Jeroen3
author_rank: Super Contributor
timestamp: December 10, 2021, 11:19:52 am
content: A DC-DC isolated module is $2.50 today, how much cheaper do you need to go?What is the high side voltage? Can you use a booster/sepic instead of a DC-DC?If they are the same high side voltage, then you only need one DC-DC.Can you use a bootstrap circuit or is there no low side switching?

author: PartialDischarge
author_rank: Super Contributor
timestamp: December 10, 2021, 11:27:49 am
content: Quote from: Jeroen3 on December 10, 2021, 11:19:52 amKeep in mind that most widely available 1kV dc-dc converters are not meant to support 1kV nor 200V long term.

author: PartialDischarge
author_rank: Super Contributor
timestamp: December 10, 2021, 11:28:24 am
content: How much are you paying for those dc-dc converters and what is your rail voltage?

author: Marco
author_rank: Super Contributor
timestamp: December 10, 2021, 11:30:39 am
content: P-channel MOSFETs aren't an option?

author: Marco
author_rank: Super Contributor
timestamp: December 10, 2021, 11:32:38 am
content: Quote from: Psi on December 10, 2021, 11:11:15 amVOM1271, it's going to be a bit slow to turn on.

author: Psi
author_rank: Super Contributor
timestamp: December 10, 2021, 11:36:59 am
content: Quote from: Jeroen3 on December 10, 2021, 11:19:52 amAt the moment its just a question of "is there a cheaper option" the answer might be no.Quote from: Jeroen3 on December 10, 2021, 11:19:52 amInput is 12V, mosfets are switching 12V, so need 24V for turning the high side on fully.Quote from: Jeroen3 on December 10, 2021, 11:19:52 amHow do you use only 1 to control 3 independent high side fets?Quote from: Jeroen3 on December 10, 2021, 11:19:52 amcorrect, no low side switching to keep boost strap cap chargedQuote from: MasterTech on December 10, 2021, 11:27:49 amNot an issue here, only 24v isolated needed

author: Psi
author_rank: Super Contributor
timestamp: December 10, 2021, 11:39:53 am
content: Quote from: Marco on December 10, 2021, 11:30:39 amNa the RDS is too high. Needs to be <1.5mR

author: Psi
author_rank: Super Contributor
timestamp: December 10, 2021, 11:43:57 am
content: Quote from: MasterTech on December 10, 2021, 11:28:24 am~$1.8 and 12V rail voltage

author: Marco
author_rank: Super Contributor
timestamp: December 10, 2021, 11:46:31 am
content: Quote from: Psi on December 10, 2021, 11:36:59 amWhy not just use a spare I/O to get a squarewave for a voltage doubler, then use the doubled voltage with any old level shifting halfbridge driver?At these voltages you don't need galvanic isolation, you just need an alternative for the standard bootstrap circuit.

author: Psi
author_rank: Super Contributor
timestamp: December 10, 2021, 11:51:49 am
content: Quote from: Marco on December 10, 2021, 11:46:31 amProbably worth revising that option, an early prototype did have a half-bridge driver but I had issues with it. So went to opto-isolating mosfet gate drives running from DCDC modules.From what i remember, the bridge driver IC had shoot through issues. would be worth testing some other fet bridge drivers with separate high/low inputsThanks.

author: Alti
author_rank: Frequent Contributor
timestamp: December 10, 2021, 12:52:40 pm
content: Quote from: Psi on December 10, 2021, 11:36:59 amwith low side commoned.You just needone24V bus that pulls high side gates high. Use some bipolar pnps, if you pull their bases, these are going to clamp gates to nearly 24V. You can use IL7660 to double the voltage but I am not sure if there are 24V versions available. Or, as suggested above, build discrete voltage doubler.

author: Jeroen3
author_rank: Super Contributor
timestamp: December 10, 2021, 03:34:01 pm
content: You can use a LED booster driver. Eg: KTC218 or similar.But overall it may not be cheaper. At worst you lose flexibility to drop in any other SIP dc-dc.Are Infineon high side switches not an option?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 10, 2021, 07:45:57 pm
content: This your high current bridge thingy?Charge pump. Basically hook up a 555 to a bootstrap gate driver, using a "half wave voltage doubler", and you're good.Take care to mind dV/dt (peak current), provide enough series resistance, clamping diodes, etc. to keep everything happy.Or get an integrated one like HIP4080, which is suitable for your voltage range I think.Tim

author: langwadt
author_rank: Super Contributor
timestamp: December 10, 2021, 08:05:00 pm
content: https://www.infineon.com/dgdl/Infineon-Simple_High_Side_Drive_Provides_Fast_Sw-Article-v01_00-EN.pdf?fileId=5546d46254e133b401554de58b225d15