|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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|References:||<[email protected]> <[email protected]>|
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:
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