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Re: LF: DC restoration

from [Andy Talbot] [Permanent Link][Original]
To: "[email protected]" <[email protected]>
Subject: Re: LF: DC restoration
From: Andy Talbot <[email protected]>
Date: Wed, 17 Jul 2019 19:50:39 +0100
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I assume we're talking about the series C from driver to gate, then a high value leakage R from gate to ground, and a reverse biassed diode in parallel with this.

AC coupling without any DC restoration means the MEAN voltage on the gate is zero.  For LF transmitter uses, where the drive has a 50% duty cycle (or very close to) this is not too much of an issue as there will still be a significant positive swing.  So provided Vdd  / 2 of the driver stage is more than sufficient to fully turn-on the FET than AC coupling with just a C and leakage R is OK.    Transformer coupling - allowing galvanic isolation - works very well and that givs true AC drive.

The problem comes in SMPSUs where the duty cycle is not zero.   Assume a 20% duty cycle and a drive waveform of 0/15V.   The mean voltage on the gate still has to  0V, so the get this from a 0/15V wavefomr means a positive excursion of 3/4 * 15V = 12V and a negative excursion of 3V.  That will still work OK
But an 80% duty cycle is +3V and -12V that certainly won't.

By adding a DC restoration diode, the negative excursion is clamped to around -0.6V (ish)) and the positive lifted to 14.4V (ish) WHATEVER the duty cycle.   

SO even at 50% the drive amplitude is doubled.

I think earlier IGBJT devices needed a negative voltage to turn off properly - I doubt that is the case with later ones.   And IGBJTs don't seem to have been adopted for LF transmitters anyway.

I remember learning about DC restoration when I was quite young and being shown how TVs (monochrome at that time) worked.   It was a simple way of getting a constant black level on AC coupled video amps, by clamping the recovered sync pulses to 0V, then biassing to get black level, whnere teh video signal have a varying DC component.   Later designs clamped the back porch at true black level.  Exactly the same circuit arrangement



On Wed, 17 Jul 2019 at 19:26, DK7FC <[email protected]> wrote:
Hi Chris,

Maybe it helps to show a concrete schematic together with your question.
I find reverse diodes quite useless, at least when MOSFETs are used. It
may be a different thing with bipolar transistors...

73, Stefan

Am 17.07.2019 15:38, schrieb Chris Wilson:
>
> Hello  rsgb,
>
>    Some Class D LF / MF amps use a diode to DC restore the waveform
>    after the FET driver chip to the gates. Why do they do this, I am
>    thinking the negative part of the waveform would turn OFF the gate
>    harder, assuming the positive part is adequate to turn the gates ON
>    hard enough? Is there any point in using the modern FET's designed
>    to have a negative rail to turn the gates off hard? Cree do the
>    Wolfspeed ones with this feature. I have also noticed that with amps
>    where the input to the driver is capacitively coupled adding a DC
>    restoration diode makes the waveform pretty much exactly 50 / 50
>    mark  space,  whereas without one this is not the case. Good idea to
>    use these restoration diodes there? Thanks.
>
>   

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