Difference between boot strap and level shift

[levelshift]

Hi. Is boot strap a kind of level shift or what's the difference? I see some circuits using both these methods, why? Thanks.

 

[nigelwright7557]

Boot strap is usually where positive feedback is used to increase the input impedance.

 

[Pafi]

In ClassD the bootstrap usually refers to a power supplying method of high side gate driver. Level shifting is a different thing, but often they go together.

 

[nigelwright7557]

Level shifting in class d is altering the levels of the signals to match the mosfet driver.
The output of the comparator is usually based around zero volts and the input to the mosfet driver is based around B-.

 

[levelshift]

Thanks but not enough to make me understand though, could you try to explain a bit more, treat me like a noob, cause that's what I am.

 

[AP2]

Hi,
Pafi says right on the bootstrap method.
Level shift is used when there is a potential difference between the stage of the mosfet and the stadium before (this is often referred to GND).
usually resolves with two bjet the low side MOSFET (VN)

 

[nigelwright7557]

Hi,
Pafi says right on the bootstrap method.
Level shift is used when there is a potential difference between the stage of the mosfet and the stadium before (this is often referred to GND).
usually resolves with two bjet the low side MOSFET (VN)

This can usually be done with a single transistor stage.

 

[AP2]

It depends on what is the potential difference.
consider that this stage is critical for the transient response, and then must work with low load resistances. this (depending on the voltage difference that must absorb) the energy will dissipate.

 

[nigelwright7557]

With my discrete class d amp I had to shift grom +12v/0v to -35v/-20v and a simple transistor stage worked fine. I did put a diode to the -20v rail and a resistor in series with it to clip the voltage. Worked very well.
I have also seen another design with a very similar circuit.

 

[levelshift]

Ok, so a level shift is not necessary to drive the upper transistor, the boot strap is? And a level shift might also be needed in some cases? I'm sorry it's a bit hard to understand, all these English terms.. If anyone has time and wants, a picture might help. Sorry for being slow.

 

[Eva]

Electric signals (voltages) only make sense when referenced to a known potential, usually ground but not always.

To level shift a signal means to create a perfect copy of that signal but referenced to another potential and optionally with some gain or attenuation.

In most class D amplifiers, the first level shifting that usually takes place consists in creating a copy of the ground-referenced PWM signal, usually 0-5V, but referenced to the negative supply rail (-Vcc, so it becomes -Vcc to -Vcc+5V).

The second level shifting that usually takes place involves converting the -Vcc referenced 0-5V PWM signal into two 0-12V (typ.) complimentary signals to drive the N-channel output MOSFET, one of them again referenced to -Vcc and the other referenced to the switching node, which is no longer a simple task (because the potential at the switching node changes very fast from -Vcc to +Vcc and back on every switching event, resulting in self disturbance easily).

Bootstrapping in class D is used to keep the gate driver of the high-side N-channel MOSFET powered. A virtual 12V (typ.) power supply rail referenced to the switching node is created that way. While the low side MOSFET is turned on, a capacitor is charged to 12V (typ.) through a diode, then this capacitor keeps the high side driver powered when it's on. That cap should be sized right (accounting for voltage drop due to discharge between one charge period and the next).

 

[levelshift]

Thanks a lot, now I understand completely. We've chosen the boot strap parts, but I can't find any info on what transistor to use as a level shift. I'm guessing that it's pretty important to choose the right one. Any help? Thanks.

 

[nigelwright7557]

but I can't find any info on what transistor to use as a level shift. I'm guessing that it's pretty important to choose the right one. Any help? Thanks.

I found a bog standard MPSA42/92 worked as well as anything.

 

[levelshift]

Thanks, so there are no good formulas to help me choose from just a couple?

 

[nigelwright7557]

Sorry its a large bitmap but this shows my level translator using an MPSA92.

An externally hosted image should be here but it was not working when we last tested it.

 

[Eva]

It depends on what is the potential difference.
consider that this stage is critical for the transient response, and then must work with low load resistances. this (depending on the voltage difference that must absorb) the energy will dissipate.

Translation (with some gap filling):

The transient response of the level shift stage is important because it determines class D distortion to some extent. Delay must be made as independent from pulse width and the history of previous pulses as possible.

In transistor based level shift stages the power dissipated has to be considered, since a fast response demands a low collector load resistance and a few milliamperes of current. On the other hand, the voltage drop across the level shift transistor can be quite high, but a power transistor is not usually suitable because it's slower than a small-signal transistor.

 

[levelshift]

Thanks a lot! Now I got some info on what transistor we should use. I love you nigel and eva. I'll try to simulate this and I will return with results.