Bootstrap capacitor for mosfet

[chris414]

I've heard everyone talking about using "bootstrap capacitors" to provide the higher voltage required by a mosfet (in my case i need 34V relative to ground to give me the 10V from gate to source), but for the life of me I cannot find a circuit or explanation (without using inductors) on the internet that charges this capacitor to a higher voltage than my supply voltage (24V or 12V already available, could use a voltage regulator if another voltage is needed).

The closest I came to was Charge pump: Information from Answers.com, except it doesn't use transistors - I tried to recreate the circuit with transistors but cannot seem to get it right. Can anyone help me out here?

 

[Nigel Goodwin]

Bootstrap capacitors are commonly used in audio amplifiers, and link between the output and either the upper or lower drives, boosting that drive voltage by a maximum 50%.

 

[RCinFLA]

See attached diagram.

When the bottom N-ch output device is ON it also pulls Vs, the bottom side Vss of top high side driver to ground. The diode charges the 0.1 uF cap connected at Vb, the top high side Vdd to about 11 vdc.

When bottom N-ch releases, the 0.1uF connected to Vb and Vs becomes the power supply for top driver section. This top section driver actives the upper N-ch output device.

The 0.1 uF bootstrap cap must be refreshed periodically. If using a PWM drive this will always happen.

 

[chris414]

Thanks for the link. Id prefer to make one from discreet components now though rather so i can see how it works...?

 

[Speakerguy]

When the MOSFET output is low, the bootstrap capacitor is charged up to VCC (say 10V) through a diode. When the output stage transitions "high", the capacitor retains its voltage (10V) but its negative terminal is now connected to your output high voltage (say 24V for the power MOSFET). So you get VCC for the gate driver chip (10V) added to the output voltage (24V) for 34V. The diode the capacitor was charged through now prevents current from flowing back into VCC powering the gate driver chip.

Look at the data sheet for a part like the IR2104 and it will show how it works.

https://www.electro-tech-online.com/custompdfs/2009/12/ir2104.pdf

 

[ronsimpson]

Here is a switching power supply. The two MOSFETS make an ‘amplifier’ that drives the transformer. (also pin 6 Vs). The output is switched to 0 volts (ground) and VDC (400 volts)

The bottom one if easy to drive. There is an amplifier from pin 1 IN to pin 5 LO. It drives the bottom MOSFET, 0 volts = off, Vdd = on. Vdd=15 volts.

The top MOSFET needs a signal on it’s gate that is complicated. The voltage from Gate to Source will be 0 and +14 volts. (14 not 15) While the source voltage is 0 and 400 volts. So the gate voltage will be 0 and 414 volts. There is an inverting amplifier from pin 1 IN to pin 7 HO. This amplifier gets it’s supply from pin 8 Vb and pin 6 Vs. While the bottom MOSFET is on VS=0 volts. Vdd through Dboct charges Cboct to 14 volts. Bottom FET goes off. Top amplifier goes on, Pin 7 HO pulls to 14 volts and turns on the top FET which pulls VS up to 400 volts VDC. This pulls up the top amplifier up. VS=400, VB=414, VH=414, Dboct blocks the 400 volts.

Simply, the bottom MOSFET stands on ground and pulls down. The top MOSFET ‘grabs it’s own boot string and pulls it’s self in to the air’. In real life I found this works best after much vodka. I got to 30 feet once before my shoestring broke.

 

[RCinFLA]

Attached is an old discrete parts 300 watt modified sinewave inverter. Most all new inverters use an I.C. like shown in previous post.

The caps C12 and C13 are the bootstrap power supply caps to drive the upper stage N-ch MOSFETs.

 

[chris414]

Thanks for all the help. I came across this page **broken link removed** and was wondering about the first circuit they show - is that just a floating supply? Would I be able to run a floating supply from a 12V battery using that circuit for the high side driver and use the same battery (without the floating supply) to drive the low side mosfets?

 

[RCinFLA]

Separte floating supply works but general object is to avoid the extra supply.

If you need static d.c. control then viable option.

 

[mone]

hey hi,am sorry for the interrupt.i went thro above posts.i would like to have a detail if any one could help me in solving a doubt.
In basic power supply circuit,on using a power regulator Ic say fr example: LM2596s-ADJ/NOPB IC,we choose the input capacitor based on the RMS current and twice the maximum of input volatage .Similarly,i've used a motor ic a3986 ,38PIN tssoP PACKAGE LEAD FREE.i JUST WANTED TO KNOW HOE TO CALCULATE THE CAPACITOR VALUE FOR vbb =30v.??..plz help???

 

[joyee]

hi all, I want to know the concept of bootstrapping. please explain.

 

[large_ghostman]

Not the best on you tube but have a look at this

 

[crutschow]

hey hi,am sorry for the interrupt.i went thro above posts.i would like to have a detail if any one could help me in solving a doubt.
In basic power supply circuit,on using a power regulator Ic say fr example: LM2596s-ADJ/NOPB IC,we choose the input capacitor based on the RMS current and twice the maximum of input volatage .Similarly,i've used a motor ic a3986 ,38PIN tssoP PACKAGE LEAD FREE.i JUST WANTED TO KNOW HOE TO CALCULATE THE CAPACITOR VALUE FOR vbb =30v.??..plz help???

This a hijack of a thread and is considered poor form. Please start a new thread for your question.

 

[crutschow]

Here is a simulation using LTspice of a discrete bootstrap driver.
Zener D3 voltage is selected to keep the Vgs of the MOSFET between 10V and its maximum rating (typically 20-30V).
Q2 speeds up the risetime of the Vgs signal.


Bootstrap Mosfet Driver HV2.asc

 

[large_ghostman]

This a hijack of a thread and is considered poor form. Please start a new thread for your question.

Your telling him off 4 years too late!!

 

[RCinFLA]

Tall boots may have a tab, loop or handle at the top known as a bootstrap, allowing one to use fingers or a boot hook tool to help pulling the boots on. The saying "to pull oneself up by one's bootstraps"[ was already in use during the 19th century as an example of an impossible task. The idiom dates at least to 1834, when it appeared in the Workingman's Advocate: "It is conjectured that Mr. Murphee will now be enabled to hand himself over the Cumberland river or a barn yard fence by the straps of his boots."

In this case it refers to the process of creating the power supply for the high side MOSFET driver. The positive side of the upper driver power supply filter capacitor, which will power the floating high side MOSFET driver circuit, is charged through a blocking diode from a 12 to 15vdc supply when the bottom (lowside) MOSFET is ON. When the high side driver, now powered by the storage filter capacitor, drives the high side MOSFET ON, the high side driver power supply is jacked up along with the conduction of high side MOSFET. "jacked up" equals "bootstrapping" in this reference. You are pulling the high side driver supply (filter cap) "up by its bootstraps".

This method only worked when there is regular cycling of the output, like for a D.C. to A.C. power inverter. For long duration static, push-pull bridge motor direction control, this method will not work because the 'bootstrapped' filter cap which supplies power to the upper driver will eventually discharge beyond point of powering upper driver stage. The filter supply capacitor for the upper (high side) driver must be regularly refreshed, by low side MOSFET conduction, to provide enough supply voltage to power the upper, floating driver.

The floating upper driver gets its input logic control by a high voltage, output current sourcing input buffer. The source to drain breakdown voltage on this logic input transistor must be able to take the full voltage range. 400 to 600 volt breakdown voltage is common for a D.C. to A.C. power inverter.

 

[joyee]

Thanks all.....

 

[crutschow]

Your telling him off 4 years too late!!

Darn, I got bitten by those practicing necromancy again. I keep forgetting to look at the dates of the post.

 

[large_ghostman]

Gets me every time lol, I didnt notice until after I posted lol. Trouble is people google a question then land on the thread, few read the date and just post in it. In some ways though its good to get some the old threads turned over to the top.

 

[lillian]

Hello can someone help me to calculate the value of the bootcap thank u