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Old 28th March 2011, 03:10 PM   #11
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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).
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Old 3rd April 2011, 08:45 PM   #12
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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.
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Old 3rd April 2011, 08:47 PM   #13
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Quote:
Originally Posted by levelshift View Post
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.
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Old 3rd April 2011, 08:59 PM   #14
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Thanks, so there are no good formulas to help me choose from just a couple?
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Old 3rd April 2011, 09:10 PM   #15
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Sorry its a large bitmap but this shows my level translator using an MPSA92.
Click the image to open in full size.

Last edited by nigelwright7557; 3rd April 2011 at 09:13 PM.
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Old 4th April 2011, 08:55 AM   #16
Eva is offline Eva  Spain
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Quote:
Originally Posted by AP2 View Post
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.
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Old 4th April 2011, 10:22 AM   #17
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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.
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