Re: rectifier waveform
- From: Terry Given <my_name@xxxxxxxx>
- Date: Mon, 29 May 2006 08:27:55 +1200
John Larkin wrote:
Following a discussion that John Fields and I have going in a.b.s.e...
Consider a conventional power transformer running off the AC line.
Assume a 12 volt RMS untapped secondary that drives a bridge
rectifier, filter cap, and a modest grounded load, producing roughly
+15 out.
What is the waveform at either end of the transformer secondary?
John
it bounces up and down.
lumping all inductance into the xfmr (so none elsewhere), then at any point when current is flowing through the rectifiers, the transformer voltage is clamped. one winding will be a diode drop above +Vdc, the other a diode drop below -Vdc, and it swaps over every half cycle. its a source of common mode noise - although its primarily LF, the rectifier recovery generates HF CM noise.
when no current flows through the rectifiers, the transformer voltage will be reflected primary volts (which, by definition, is less than output voltage + diode drops) and it will be bouncing around as a function of whatever strays are going on.
Try synthesising a 3-phase waveform with three half-bridges (for simplicity assume split DC bus). no matter how hard you try, the sum of the three half-bridges will always = +/- Vdc/2, and it bounces around not at 50Hz, but at the switching frequency, which is a BIG problem with EMI. Even at a modest IGBT switching frequency (10kHz now for big ones), because the amplitude is about 300V peak, there is still rather a lot at, say, 150kHz (ooh, around 20V or so). fails EMI, ever so slightly.
Cheers
Terry
.
- References:
- rectifier waveform
- From: John Larkin
- rectifier waveform
- Prev by Date: Re: RoHS AKA Woah, Hoss!
- Next by Date: Re: RoHS AKA Woah, Hoss!
- Previous by thread: Re: rectifier waveform
- Next by thread: Re: rectifier waveform
- Index(es):
Relevant Pages
|
