ECL: Why'd they use negative voltages?
in [Design]
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From: John Larkin on 8 Jun 2010 16:10 On Tue, 08 Jun 2010 14:42:13 -0500, Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: > > >John Larkin wrote: > > >> One of the old Motorola books has a class-D amp that uses bipolar >> supplies, half-bridge mosfets, LC filter, DC coupled to a grounded >> loudspeaker. The problem is that, if you're swinging the load, say, >> positive, you take power out of the + supply and pump power *into* the >> - supply, and blow up its filter caps. > >The problem is known as "rail pumping". > >> Their fix was cute: an idler >> circuit off to the side, a pair of mosfets switching at 50% duty >> cycle, pumping V+ and V- into a dummy grounded inductor. That >> automagically equalized the supply voltages. > >There is one small problem with this solution: it doesn't work. If >anything is slightly off balance, that creates virtually unlimited >current. Nothing is off balance, because the idler forces things to balance. The "virtually unlimited current" is what pulls down the higher supply and boosts the lower one. In fact, it transfers energy from the unused, higher voltage side of the supply to the actively-loaded, lower-voltage side. Sorta cute. They wouldn't have published it if it didn't work. John
From: Joel Koltner on 8 Jun 2010 16:14 "Vladimir Vassilevsky" <nospam(a)nowhere.com> wrote in message news:4K-dnR1EoP5zAZPRnZ2dnUVZ_rmdnZ2d(a)giganews.com... > There is one small problem with this solution: it doesn't work. If anything > is slightly off balance, that creates virtually unlimited current. Current-mode control would likely cost too much as well?
From: Vladimir Vassilevsky on 8 Jun 2010 16:22 John Larkin wrote: > On Tue, 08 Jun 2010 14:42:13 -0500, Vladimir Vassilevsky > <nospam(a)nowhere.com> wrote: > > >> >>John Larkin wrote: >> >> >> >>>One of the old Motorola books has a class-D amp that uses bipolar >>>supplies, half-bridge mosfets, LC filter, DC coupled to a grounded >>>loudspeaker. The problem is that, if you're swinging the load, say, >>>positive, you take power out of the + supply and pump power *into* the >>>- supply, and blow up its filter caps. >> >>The problem is known as "rail pumping". >> >> >>>Their fix was cute: an idler >>>circuit off to the side, a pair of mosfets switching at 50% duty >>>cycle, pumping V+ and V- into a dummy grounded inductor. That >>>automagically equalized the supply voltages. >> >>There is one small problem with this solution: it doesn't work. If >>anything is slightly off balance, that creates virtually unlimited >>current. > > > Nothing is off balance, because the idler forces things to balance. > The "virtually unlimited current" is what pulls down the higher supply > and boosts the lower one. In fact, it transfers energy from the > unused, higher voltage side of the supply to the actively-loaded, > lower-voltage side. Sorta cute. > Now think what happens if the duty cycle is not exactly 50/50. Or if the /+/ and /-/ supplies are slightly different. > They wouldn't have published it if it didn't work. Heh, haven't you seen published circuits which don't work? That things could work for low power stuff where the stray resistances allow for some slack. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: John Larkin on 8 Jun 2010 16:51 On Tue, 08 Jun 2010 15:22:15 -0500, Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: > > >John Larkin wrote: > >> On Tue, 08 Jun 2010 14:42:13 -0500, Vladimir Vassilevsky >> <nospam(a)nowhere.com> wrote: >> >> >>> >>>John Larkin wrote: >>> >>> >>> >>>>One of the old Motorola books has a class-D amp that uses bipolar >>>>supplies, half-bridge mosfets, LC filter, DC coupled to a grounded >>>>loudspeaker. The problem is that, if you're swinging the load, say, >>>>positive, you take power out of the + supply and pump power *into* the >>>>- supply, and blow up its filter caps. >>> >>>The problem is known as "rail pumping". >>> >>> >>>>Their fix was cute: an idler >>>>circuit off to the side, a pair of mosfets switching at 50% duty >>>>cycle, pumping V+ and V- into a dummy grounded inductor. That >>>>automagically equalized the supply voltages. >>> >>>There is one small problem with this solution: it doesn't work. If >>>anything is slightly off balance, that creates virtually unlimited >>>current. >> >> >> Nothing is off balance, because the idler forces things to balance. >> The "virtually unlimited current" is what pulls down the higher supply >> and boosts the lower one. In fact, it transfers energy from the >> unused, higher voltage side of the supply to the actively-loaded, >> lower-voltage side. Sorta cute. >> > > >Now think what happens if the duty cycle is not exactly 50/50. Or if the > /+/ and /-/ supplies are slightly different. If the power supplies are slightly different, the inductor sees a net average DC. So the inductor current rises. The inductor then extracts energy from one supply (the one with the higher magnitude) and pumps energy into the other one. That tends to equalize the supply voltage magnitudes. These are soft supplies, especially in the absorbing-energy direction. They aren't stiff, absolute voltages. Which is why it works. Of course, a true h-bridge solves the same problem more efficiently. John
From: Joerg on 8 Jun 2010 17:29
John Larkin wrote: > On Tue, 08 Jun 2010 09:05:42 -0700, Joerg <invalid(a)invalid.invalid> > wrote: > >> John Larkin wrote: >>> On Tue, 08 Jun 2010 09:12:24 +0200, Jeroen Belleman >>> <jeroen(a)nospam.please> wrote: >>> >>>> John Larkin wrote: >>>>> 10KH type ECL does work fine at Vcc=5, Vee=0. That's called "PECL" >>>>> mode, originally "pseudo ECL" and lately "positive ECL". One generally >>>>> references all the signals to a nice 5-volt copper pour. >>>> I once designed a VME module with a lot of PECL with signals terminated >>>> into +3V, and which also had some circuitry running between +3V and >>>> GND. The +3V net was shared. Since the combined current of all the PECL >>>> terminators largely exceeded the consumption of the stuff between +3V >>>> and GND, I used a negative regulator with its input connected to GND to >>>> make the +3V. >>>> >>>> That raised some eyebrows, but it worked fine. >>>> >>>> Jeroen Belleman >>> >>> It's fun to use regulators "upside down." >>> >>> We need a good bipolar-drive regulator. I use LM8261s for small stuff, >>> and occasionaly LT1010s for bigger stuff. >>> >> Check out the big fat audio amp hybrids. Of course, one has to be >> careful and pick one that isn't going obsolete next year because a >> particular car stereo was discontinued. >> >> Heck, if you want to go green on this you might even consider class-D. >> Then claim your carbon offset :-) > > A non-H-bridge class D amp can have circulating-current problems... it > takes current out of one supply rail and dumps it into the other. > Conservation of energy. > Thou shalt use an inductor towards the load :-) Essentially this is how synchronous buck converters work, and also class-C push-pull RF power stages. There is always an inductor first. I remember a design of such a power amp where the designer had followed it with a Pi-filter meaning cap first ... *PHOOMP* ... *POP* ... both FET literally pulverized upon power-up, looked like the ones from your FET multilation photo the other day. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |