Hello Andy and Stefan,
Thanks for the replies, it's a lot clearer now, I appreciate your
time!
Wednesday, July 17, 2019, 7:50:39 PM, you wrote:
> 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
> Andy
> www.g4jnt.com
> 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.
>>
>>
>
>
--
Best regards,
Chris mailto:[email protected]
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