inverting dc-dc converter circuit
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From: miso on 30 Jan 2010 16:35 On Jan 28, 9:27 pm, John Larkin <jjSNIPlar...(a)highTHISlandtechnology.com> wrote: > This afternoon we were talking with Phil Hobbs about an > electro-optical thingie we're doing. The prime power is a +15 volt > wall wart, and we need -30 internally to power some photodiodes and > opamps and stuff, 30 mA maybe. Our design currently has a cute > homebrew single-inductor flyback converter, which circuit I've posted > here some time back. We are concerned about having such a potentially > noisy gadget on the same small board with nanoamp signals. > > I proposed a different circuit: imagine eight opto-SSRs and two > capacitors. A low frequency clock, 400 Hz maybe, switches 4 of them on > and 4 off, alternately. The arrangement connects the two caps in > parallel to the +15 supply, charging them up. Then it disconnects them > and then restacks them in series such as to make -30 to ground. The > low frequency and fairly soft switching edges should make this pretty > quiet. > > Phil named this the Groucho Marx Generator. > > John There is always the philosophical question as to just how big is an inductor. If it is not shielded, well, it's influence can be felt way beyond it's borders. If shielded, less so, but nothing is perfect. Slapping caps on a rail with a soft charge is probably the right approach, though you really don't have to get exotic here. You could go to a tripler scheme if it is "on demand" so to speak (i.e. regulated), If the flying cap to reservoir is sized appropriately, you can keep the step sizes small, then linearly regulate the output. Here is a good question. How do they generate the phantom voltage in battery powered microphone line amps. Surely they have the same issues.
From: Joerg on 30 Jan 2010 16:50 miso(a)sushi.com wrote: > On Jan 28, 9:27 pm, John Larkin > <jjSNIPlar...(a)highTHISlandtechnology.com> wrote: >> This afternoon we were talking with Phil Hobbs about an >> electro-optical thingie we're doing. The prime power is a +15 volt >> wall wart, and we need -30 internally to power some photodiodes and >> opamps and stuff, 30 mA maybe. Our design currently has a cute >> homebrew single-inductor flyback converter, which circuit I've posted >> here some time back. We are concerned about having such a potentially >> noisy gadget on the same small board with nanoamp signals. >> >> I proposed a different circuit: imagine eight opto-SSRs and two >> capacitors. A low frequency clock, 400 Hz maybe, switches 4 of them on >> and 4 off, alternately. The arrangement connects the two caps in >> parallel to the +15 supply, charging them up. Then it disconnects them >> and then restacks them in series such as to make -30 to ground. The >> low frequency and fairly soft switching edges should make this pretty >> quiet. >> >> Phil named this the Groucho Marx Generator. >> >> John > > There is always the philosophical question as to just how big is an > inductor. If it is not shielded, well, it's influence can be felt way > beyond it's borders. If shielded, less so, but nothing is perfect. > > Slapping caps on a rail with a soft charge is probably the right > approach, though you really don't have to get exotic here. You could > go to a tripler scheme if it is "on demand" so to speak (i.e. > regulated), If the flying cap to reservoir is sized appropriately, > you can keep the step sizes small, then linearly regulate the output. > > Here is a good question. How do they generate the phantom voltage in > battery powered microphone line amps. Surely they have the same > issues. > The ones I came across are just an oscillator and a Cockcroft-Walton cascade, or with a little ferrite core transformer. Then there are the really crude "solutions" like a stack of 9V batteries (seriously ...). -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: mook Johnson on 30 Jan 2010 18:33 On 1/30/2010 12:05 PM, John Larkin wrote: > On Sat, 30 Jan 2010 09:58:57 -0600, mook Johnson<mook(a)mook.net> > wrote: > >> On 1/28/2010 11:27 PM, John Larkin wrote: >>> >>> >>> This afternoon we were talking with Phil Hobbs about an >>> electro-optical thingie we're doing. The prime power is a +15 volt >>> wall wart, and we need -30 internally to power some photodiodes and >>> opamps and stuff, 30 mA maybe. Our design currently has a cute >>> homebrew single-inductor flyback converter, which circuit I've posted >>> here some time back. We are concerned about having such a potentially >>> noisy gadget on the same small board with nanoamp signals. >>> >>> I proposed a different circuit: imagine eight opto-SSRs and two >>> capacitors. A low frequency clock, 400 Hz maybe, switches 4 of them on >>> and 4 off, alternately. The arrangement connects the two caps in >>> parallel to the +15 supply, charging them up. Then it disconnects them >>> and then restacks them in series such as to make -30 to ground. The >>> low frequency and fairly soft switching edges should make this pretty >>> quiet. >>> >>> Phil named this the Groucho Marx Generator. >>> >>> >>> John >>> >> >> What kind of regulation do you need on the -30V? Is that 30mA constant >> or does it move around a lot between 0 and 30mA? >> > > Load is pretty constant and stability of maybe a half a volt would be > OK. The +15 is well regulated, so a non-regulated charge pump would > work if it was fairly stiff. > > In real life, we'll probably use the inverting flyback with a very > well-managed layout and an option for a deep-drawn aluminum cover in > case it's needed. > > I still want to do something with the Groucho circuit. It can also be > a floating supply, up to the SSR voltage limits. > > John > A brute force way I generated a low noise AUX voltage was to make a 50Khz sine wave generator run that through a buffer and drive the primary of a toroid. A high speed bridge rectifier on the back side. Since the signals was sinusoidal much less radiated swithcing edges flying around. Efficiency was about 40% but like you I only needed less than 1 watt.
From: Glen Walpert on 31 Jan 2010 10:29 On Sat, 30 Jan 2010 10:05:13 -0800, John Larkin wrote: > On Sat, 30 Jan 2010 09:58:57 -0600, mook Johnson <mook(a)mook.net> wrote: > >>On 1/28/2010 11:27 PM, John Larkin wrote: >>> >>> >>> This afternoon we were talking with Phil Hobbs about an >>> electro-optical thingie we're doing. The prime power is a +15 volt >>> wall wart, and we need -30 internally to power some photodiodes and >>> opamps and stuff, 30 mA maybe. Our design currently has a cute >>> homebrew single-inductor flyback converter, which circuit I've posted >>> here some time back. We are concerned about having such a potentially >>> noisy gadget on the same small board with nanoamp signals. >>> >>> I proposed a different circuit: imagine eight opto-SSRs and two >>> capacitors. A low frequency clock, 400 Hz maybe, switches 4 of them on >>> and 4 off, alternately. The arrangement connects the two caps in >>> parallel to the +15 supply, charging them up. Then it disconnects them >>> and then restacks them in series such as to make -30 to ground. The >>> low frequency and fairly soft switching edges should make this pretty >>> quiet. >>> >>> Phil named this the Groucho Marx Generator. >>> >>> >>> John >>> >>> >>What kind of regulation do you need on the -30V? Is that 30mA constant >>or does it move around a lot between 0 and 30mA? >> >> > Load is pretty constant and stability of maybe a half a volt would be > OK. The +15 is well regulated, so a non-regulated charge pump would work > if it was fairly stiff. > > In real life, we'll probably use the inverting flyback with a very > well-managed layout and an option for a deep-drawn aluminum cover in > case it's needed. > > I still want to do something with the Groucho circuit. It can also be a > floating supply, up to the SSR voltage limits. > > John Have you read Linear AN's 70 and 85, where Jim Williams presents a bunch of example designs for low current bias supplies with less than 100 uV noise? The flyback was the worst of the topologies discussed from a noise perspective.
From: MooseFET on 31 Jan 2010 12:26
On Jan 31, 7:29 am, Glen Walpert <nos...(a)null.void> wrote: > On Sat, 30 Jan 2010 10:05:13 -0800, John Larkin wrote: > > On Sat, 30 Jan 2010 09:58:57 -0600, mook Johnson <m...(a)mook.net> wrote: > > >>On 1/28/2010 11:27 PM, John Larkin wrote: > > >>> This afternoon we were talking with Phil Hobbs about an > >>> electro-optical thingie we're doing. The prime power is a +15 volt > >>> wall wart, and we need -30 internally to power some photodiodes and > >>> opamps and stuff, 30 mA maybe. Our design currently has a cute > >>> homebrew single-inductor flyback converter, which circuit I've posted > >>> here some time back. We are concerned about having such a potentially > >>> noisy gadget on the same small board with nanoamp signals. > > >>> I proposed a different circuit: imagine eight opto-SSRs and two > >>> capacitors. A low frequency clock, 400 Hz maybe, switches 4 of them on > >>> and 4 off, alternately. The arrangement connects the two caps in > >>> parallel to the +15 supply, charging them up. Then it disconnects them > >>> and then restacks them in series such as to make -30 to ground. The > >>> low frequency and fairly soft switching edges should make this pretty > >>> quiet. > > >>> Phil named this the Groucho Marx Generator. > > >>> John > > >>What kind of regulation do you need on the -30V? Is that 30mA constant > >>or does it move around a lot between 0 and 30mA? > > > Load is pretty constant and stability of maybe a half a volt would be > > OK. The +15 is well regulated, so a non-regulated charge pump would work > > if it was fairly stiff. > > > In real life, we'll probably use the inverting flyback with a very > > well-managed layout and an option for a deep-drawn aluminum cover in > > case it's needed. > > > I still want to do something with the Groucho circuit. It can also be a > > floating supply, up to the SSR voltage limits. > > > John > > Have you read Linear AN's 70 and 85, where Jim Williams presents a bunch > of example designs for low current bias supplies with less than 100 uV > noise? The flyback was the worst of the topologies discussed from a > noise perspective. The Cuk converter is the best for inverted supplies. Still, the capacitance of the inductors lets some of the fast edges get through to the output capacitor. The layout of the windings can help by putting the moving end of the output side near the nonmoving end of the input side. |