ECL: Why'd they use negative voltages?
in [Design]
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From: Joerg on 8 Jun 2010 18:07 John Larkin wrote: > On Tue, 08 Jun 2010 14:29:38 -0700, Joerg <invalid(a)invalid.invalid> > wrote: > >> 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 :-) > > Of course. The inductor causes the problem. If a bipolar half-bridge > drove a resistive load, there would be no circulating current. > I've never had any issues there and driven quite big loads. Except in cases where motors began generating because forces kept acting on the shaft while I had the half-bridge in coast. That can make the rail voltage rise. So I didn't use coast. >> Essentially this is how synchronous buck converters work, > > They use unipolar bulk supplies, and can dump their circulating > current into ground. > They don't dump anything, the inductor current just keeps flowing. Unless you leave CCM, then it just stops. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 8 Jun 2010 18:10 John Larkin wrote: [...] > A synchronous rectifier in the supplies doesn't help. If you push > power back into a supply, as the half-bridge likes to do, then energy > has got to go somewhere. A synchronous rectifier just pushes it uphill > into the caps of the bulk supply. > Hah! The American solution: Use an inverter and sell that power back to the utility :-)) [...] -- SCNR, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 8 Jun 2010 18:17 On Tue, 08 Jun 2010 15:01:19 -0700, Joerg <invalid(a)invalid.invalid> wrote: >Vladimir Vassilevsky wrote: >> >> >> John Larkin wrote: >> >>> 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. >> >> So the lower rail is getting powered from the higher rail through the >> idler circuit. With the corresponding circulating high currents, losses >> and asymmetry. It looks worse if you consider the waveform of the idler >> current. >> >> BTW, once I made similar circuit. FETs were controlled by the CPU; that >> allowed to optimize the operation (For any considerable currents and >> voltages, don't think of running this in uncontrollable mode). It worked >> reasonably well, however I didn't like it. >> >>> >>> Of course, a true h-bridge solves the same problem more efficiently. >> >> H-bridge needs twice as many FETs, drivers, protections and feedbacks; >> that is pricey. Also, it is impossible to connect two channels in a >> bridge configuration to double the output power. For those reasons, half >> bridge is usually preferred in audio. >> Synchronous rectifiers in the power supplies are good solution for this >> problem and for the efficiency reasons, but they come with a price also. >> >> Bottom line is everybody just uses big capacitor tanks; this is dumb, >> cheap and good enough for the job. >> > >Pumping is normally only an issue with low frequency AC drive. There is >another trick: If you have two amps (usually the case with audio) >operate them 180 degree out of phase down there. Full H-bridge! John
From: Spehro Pefhany on 8 Jun 2010 18:23 On Tue, 08 Jun 2010 15:10:18 -0700, Joerg <invalid(a)invalid.invalid> wrote: >John Larkin wrote: > >[...] > >> A synchronous rectifier in the supplies doesn't help. If you push >> power back into a supply, as the half-bridge likes to do, then energy >> has got to go somewhere. A synchronous rectifier just pushes it uphill >> into the caps of the bulk supply. >> > >Hah! The American solution: Use an inverter and sell that power back to >the utility :-)) > >[...] Hmm .. if they force the utilities to buy "green" power at high rates, maybe a 'wind farm' is just the ticket. Just waste a bit of power spinning the blades to keep things looking legit..
From: Joerg on 8 Jun 2010 18:29
John Larkin wrote: > On Tue, 08 Jun 2010 15:01:19 -0700, Joerg <invalid(a)invalid.invalid> > wrote: > >> Vladimir Vassilevsky wrote: >>> >>> John Larkin wrote: >>> >>>> 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. >>> So the lower rail is getting powered from the higher rail through the >>> idler circuit. With the corresponding circulating high currents, losses >>> and asymmetry. It looks worse if you consider the waveform of the idler >>> current. >>> >>> BTW, once I made similar circuit. FETs were controlled by the CPU; that >>> allowed to optimize the operation (For any considerable currents and >>> voltages, don't think of running this in uncontrollable mode). It worked >>> reasonably well, however I didn't like it. >>> >>>> Of course, a true h-bridge solves the same problem more efficiently. >>> H-bridge needs twice as many FETs, drivers, protections and feedbacks; >>> that is pricey. Also, it is impossible to connect two channels in a >>> bridge configuration to double the output power. For those reasons, half >>> bridge is usually preferred in audio. >>> Synchronous rectifiers in the power supplies are good solution for this >>> problem and for the efficiency reasons, but they come with a price also. >>> >>> Bottom line is everybody just uses big capacitor tanks; this is dumb, >>> cheap and good enough for the job. >>> >> Pumping is normally only an issue with low frequency AC drive. There is >> another trick: If you have two amps (usually the case with audio) >> operate them 180 degree out of phase down there. > > Full H-bridge! > Demasiados Dolares :-) But seriously, what would be the problem driving the two subwoofers 180 degrees out of phase and reverse the connections for one of them? To the end customer it'll be the same tchk tchk *BOOM* sound. In the rare case of music that is highly asymmetrical down there a simple limiter could kick in. So far I've not encountered H-bridges in lower-end class-D amps but then again I don't deal with audio much, I typically re-purpose this stuff. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |