Is some cheaper/better way to drive high side up to 100% duty than transformer? - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: March 22, 2021, 01:14:26 pm
content: Hi folks.I have a question.Does exist some cheap switched capacitor pump or something other to deliver power for a high side driver when I want to have true 100% duty on it?I know a common way is to use a transformer and integrated driver for it. But both are relatively expensive parts.Or is the only way to limit the duty cycle and use simple bootstrap? And add that switching loss.It Will be used with a four switch buck boost, so no insulation needed.

author: xavier60
author_rank: Super Contributor
timestamp: March 22, 2021, 02:08:56 pm
content: There are high side drivers with a built in charge pump,http://ww1.microchip.com/downloads/en/DeviceDoc/20005677A.pdfThere is a thread about another one, I haven't' found it yet.Another one,https://www.analog.com/en/about-adi/news-room/press-releases/2017/fast-150v-high-side-n-channel-mosfet-driver-provides-100-duty-cycle-capability.html#

author: Miyuki
author_rank: Frequent Contributor
timestamp: March 22, 2021, 04:32:33 pm
content: Sorry, I forget to mention I need up to 400V input voltage.//edit: I can use discrete charge pump like on the picture, just positiveSomething that can be done with 555 and few passives.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 22, 2021, 05:03:37 pm
content: Can you use a P-channel, high-voltage mosfet? Digikey has some rated at 500V and 600V.

author: Miyuki
author_rank: Frequent Contributor
timestamp: March 22, 2021, 05:21:40 pm
content: Quote from: jpanhalt on March 22, 2021, 05:03:37 pmI need to high current for P MOSI'll use a separate charge pump for a 100% duty cycle as it won't need much currentAnd bootstrapping for the rest of the time

author: mvs
author_rank: Frequent Contributor
timestamp: March 22, 2021, 08:17:22 pm
content: 400V floating charge pump?You may look for optocouplers with photovoltaic output, like VOM1271, APV1121 or other. They are usually not very fast, but can be used as auxillary supply for 100% duty cycle.https://www.vishay.com/docs/83469/vom1271.pdf

author: Miyuki
author_rank: Frequent Contributor
timestamp: March 23, 2021, 06:26:46 am
content: Quote from: mvs on March 22, 2021, 08:17:22 pmDo you have any idea how to cooperate photovoltaic driver with fast bootstrapped driver? I won't see any simple solution now as fast one will pull gate down without power.And for providing power to gate driver is it too weak.IRF have even a reference design for charge pump, just connected this way. It looks like what I want. Just replace bottom resistor with constant current and will be happy to go.

author: Miyuki
author_rank: Frequent Contributor
timestamp: March 23, 2021, 06:57:03 am
content: blueskull: I need high speed variable PWM that can stay on indefinitely when needed

author: xavier60
author_rank: Super Contributor
timestamp: March 23, 2021, 08:37:41 am
content: Quote from: blueskull on March 23, 2021, 07:17:05 amThe IRF reference design shouldn't have that problem. The ICM755's supply is referenced to VS.

author: xavier60
author_rank: Super Contributor
timestamp: March 23, 2021, 09:10:46 am
content: One potential weakness is the 100K dropper. Manufacturer specs cant be trusted for high voltage resistors.I have found these to be reliable so far,https://www.ttelectronics.com/TTElectronics/media/ProductFiles/Resistors/Datasheets/VRW.pdfA constant current sink can be done with a high voltage Depletion Mode MOSFET and one resistor.https://www.ixys.com/Documents/AppNotes/IXAN0063.pdfMicrochip also.Edit; fixed IXYS link.and this,https://www.infineon.com/dgdl/Infineon-Application_Note_Applications_for_Depletion_MOSFETs-AN-v01_00-EN.pdf?fileId=5546d4624cb7f111014cd63d1a197d94

author: mvs
author_rank: Frequent Contributor
timestamp: March 23, 2021, 11:54:31 am
content: Quote from: Miyuki on March 23, 2021, 06:26:46 amPhotovoltaic driver needs to compensate only leakage at Vb (50µA at most for IR2125), bootstrap diode, cap and MOSFET gate. You will probably need multiple VOM1271 in series and parallel to do this.The IRF reference design looks quite ok.

author: filssavi
author_rank: Frequent Contributor
timestamp: March 23, 2021, 12:04:29 pm
content: In my opinion your application is a textbook example of where an isolated gate drive would make sense.You can’t use bootstrap alone since you need 100% duty cycle. If you add a separste dc/dc for that scenario, you might as well use it always and get rid of the bootstrap all together.Now I would also use a traditional transformer based converter, as it will be much better at rejecting common mode (provided the transformer is decent)If you go for something integrated (like traco TBA 2-0513) it will probably as expensive if not cheaper than the 2 supplies solution. If you want to learn you can implement it yourself

author: Miyuki
author_rank: Frequent Contributor
timestamp: March 24, 2021, 08:42:25 am
content: I was thinking about it a little more and I'll use a different solutionA high duty cycle is needed just on the low input voltage. I can feed bootstrap with something like 100V and then just add a linear regulator for 15V to the driver.It will be a solution with the lowest component count and the drive current is enough low to use just a linear dropper.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: March 24, 2021, 08:57:52 am
content: This is one of possible solutions /see attachment/.However isolated DCDC + isolated gate driver seems to more practical approach.

author: jbb
author_rank: Super Contributor
timestamp: March 24, 2021, 09:04:44 am
content: You know, whatever you do will cost something, whether it be parts and area or a bit more loss.You mentioned just limiting duty cycle to <100%, but this would indeed add switching losses (up to you to determine how much might be acceptable). Have you had a look at refreshing the bootstrap by giving that half bridge a brief off time every N switching cycles? That might get your gate driver to work without paying a switching loss penalty every single cycle.

author: Relaxe
author_rank: Regular Contributor
timestamp: August 18, 2025, 01:46:56 pm
content: Sorry to Necro this thread, but here is something I have done in the past for exactly this reason:This circuit has been in mass production, over 100k units produced without failure. Trust me, it works.It has also been used to power a back-to-back N-FET in a 54V battery application, with added fast turn-off:Note D502 is an Oring diode, there was three inputs to the fast turn-off.The components are the only limit to make this floating charge pump float higher.Changing the BAS70 (70V) to a 100V one would allow you to float 12V over 80V.If you have any MCU in your board, a 25kHz 50% duty pulse is an easy thing to procure. From memory, frequency was not that important either...I have lost my LTSpice file for that, but it is easy to reproduce. I highly recommend you do reproduce in simulation to probe and understand what is going on here.In all cases, this was the cheapest solution we could build at scale for a 100% on high-side N-FET at the time.Enjoy!