Joerg wrote:> No smoothing inductors at the rectifiers? The current peaks when the > caps get replenished "whambam style" are often not too friendly towards > analog stuff in the neighborhood. Like a Harley at night :-)That is usually not a big problem if convertor runs at fixed duty cycle close to 100%. Stray inductances/resistances would take care of that. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
isolated DC/DC converter
Started by ●November 3, 2011
Reply by ●November 7, 20112011-11-07
Reply by ●November 7, 20112011-11-07
On Mon, 07 Nov 2011 10:50:22 -0600, Vladimir Vassilevsky <nospam@nowhere.com> wrote:> > >Joerg wrote: > > >> No smoothing inductors at the rectifiers? The current peaks when the >> caps get replenished "whambam style" are often not too friendly towards >> analog stuff in the neighborhood. Like a Harley at night :-) > >That is usually not a big problem if convertor runs at fixed duty cycle >close to 100%. Stray inductances/resistances would take care of that. > > >Vladimir Vassilevsky >DSP and Mixed Signal Design Consultant >http://www.abvolt.comThe Coiltronics DRQ-series dual-winding inductors have M's of just around 0.99, so I can expect a couple of uH of leakage inductance to soften things up, just about the right amount maybe. And I can always slow down the fets a bit, too. John
Reply by ●November 7, 20112011-11-07
Vladimir Vassilevsky wrote:> > > Joerg wrote: > > >> No smoothing inductors at the rectifiers? The current peaks when the >> caps get replenished "whambam style" are often not too friendly towards >> analog stuff in the neighborhood. Like a Harley at night :-) > > That is usually not a big problem if convertor runs at fixed duty cycle > close to 100%. Stray inductances/resistances would take care of that. >This one runs at 50%. For the DRQ127 inductors John is using he'll need a version with lots of inductance so it won't saturate much at 24V and 100-200kHz. Thiose have very little leakage inductance. I'd at least put the inductor in, and bridge with zero-ohms it if not desired. Often the noise siuation makes it rather desirable though :-) -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●November 7, 20112011-11-07
Joerg wrote:> Vladimir Vassilevsky wrote: > >> >>Joerg wrote: >> >> >> >>>No smoothing inductors at the rectifiers? The current peaks when the >>>caps get replenished "whambam style" are often not too friendly towards >>>analog stuff in the neighborhood. Like a Harley at night :-) >> >>That is usually not a big problem if convertor runs at fixed duty cycle >>close to 100%. Stray inductances/resistances would take care of that. >> > > This one runs at 50%.Push-pull, which means 100%.> For the DRQ127 inductors John is using he'll need > a version with lots of inductance so it won't saturate much at 24V and > 100-200kHz. Thiose have very little leakage inductance. > > I'd at least put the inductor in, and bridge with zero-ohms it if not > desired. Often the noise siuation makes it rather desirable though :-)Just slow down the FETs so they will operate kinda linear at the edges. This will also absorb the inductive turn-off spikes on the drains. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●November 7, 20112011-11-07
Vladimir Vassilevsky wrote:> > > Joerg wrote: > >> Vladimir Vassilevsky wrote: >> >>> >>> Joerg wrote: >>> >>> >>> >>>> No smoothing inductors at the rectifiers? The current peaks when the >>>> caps get replenished "whambam style" are often not too friendly towards >>>> analog stuff in the neighborhood. Like a Harley at night :-) >>> >>> That is usually not a big problem if convertor runs at fixed duty cycle >>> close to 100%. Stray inductances/resistances would take care of that. >>> >> >> This one runs at 50%. > > Push-pull, which means 100%.Can't get to zero transition time. At 100kHz even a mere 500nsec per transition adds up to 10%.> >> For the DRQ127 inductors John is using he'll need >> a version with lots of inductance so it won't saturate much at 24V and >> 100-200kHz. Thiose have very little leakage inductance. >> >> I'd at least put the inductor in, and bridge with zero-ohms it if not >> desired. Often the noise siuation makes it rather desirable though :-) > > Just slow down the FETs so they will operate kinda linear at the edges. > This will also absorb the inductive turn-off spikes on the drains. >Those aren't necessarily the problem. What I like to keep muffled are the current spikes into the secondary caps after each polarity reversal on the primary side. This stuff tend to magnetically couple into things where one really doesn't want to see it. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●November 7, 20112011-11-07
Joerg <invalid@invalid.invalid> wrote:> Those aren't necessarily the problem. What I like to keep muffled are > the current spikes into the secondary caps after each polarity > reversal on the primary side. This stuff tend to magnetically couple > into things where one really doesn't want to see it.This is of general interest for low level work. Part of the problem is the huge difference between the supply voltage, 24V, and the LSB with a 20 dB op amp input gain. The lsb with a 2v p-p adc is 2/2^12 = 488uV. With 20dB input buffer gain, the lsb is 48uV. The supply is 24V, so the total attenuation required is greater than 20*log(24/48e-6) = 113.979 dB That is going to be extremely tough on a pcb when the source and victim are only 2 inches apart. The issue is to minimize fast transients, either voltage (capacitive charge/discharge) or current (diode snap off, turnon). I wonder if we are examining all the alternatives. For example, synchronous rectifiers, buck converters, and other methods of dropping the supply voltage to the desired level. For example, here are some descriptions of synchronous rectification with some examples using buck converters: Synchronous Rectification Tutorial "http://scholar.lib.vt.edu/theses/available/etd- 173510281975580/unrestricted/chapter2.pdf" Maxim: AN652-1 Synchronous Rectification Aids Low-Voltage Power Supplies http://pdfserv.maxim-ic.com/en/an/AN652.pdf Synchronous Rectification in High�Performance Power Converter Design: http://www.national.com/assets/en/appnotes/national_power_designer112.pdf Control Driven Synchronous Rectifiers In Phase Shifted Full Bridge Converters: http://www.ti.com/lit/an/slua287/slua287.pdf The Implication of Synchronous Rectifiers to the Design of Isolated, Single-Ended Forward Converters http://www.ti.com/lit/ml/slup175/slup175.pdf Another approach might work if the adc readings are not needed continuously. Then it might be possible to charge a fairly large capacitor during the off time, then turn the power conversion off while the adc is active. A simple linear regulator could be used to keep the supply to the ADC constant during the measurement. The best approach is to control the noise at the source. If you can eliminate it, the SNR is infinite. No amount of filtering can achieve that. Mike
Reply by ●November 7, 20112011-11-07
On Tue, 08 Nov 2011 01:13:16 GMT, Mike <spam@me.not> wrote:>Joerg <invalid@invalid.invalid> wrote: > >> Those aren't necessarily the problem. What I like to keep muffled are >> the current spikes into the secondary caps after each polarity >> reversal on the primary side. This stuff tend to magnetically couple >> into things where one really doesn't want to see it. > >This is of general interest for low level work. > >Part of the problem is the huge difference between the supply voltage, >24V, and the LSB with a 20 dB op amp input gain. The lsb with a 2v p-p >adc is 2/2^12 = 488uV. With 20dB input buffer gain, the lsb is 48uV. The >supply is 24V, so the total attenuation required is greater than > >20*log(24/48e-6) = 113.979 dB > >That is going to be extremely tough on a pcb when the source and victim >are only 2 inches apart.The ADC is an LTC2242-12. Its input range is 2 volts p-p, and I catually have 3V p-p coming into the board. I gain it up a little and throw away half in a 70 MHz lowpass filter, so I never have a bunch of gain. Yup, an LSB is 488 uV at tha ADC. Heck, cell phones have switching regulators and work with microvolt inputs. See my other post. John
Reply by ●November 7, 20112011-11-07
On Wed, 02 Nov 2011 20:51:39 -0700, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:>This > >http://www.panoramio.com/photo/61564837 > >might work for powering a thing I'm doing. I can use a standard cheap >Coiltronix dual-coil inductor as the transformer. The complementary >emitter followers will have no shoot-through and can have controllably >slow switching edges, since they will just follow the base drive. > >Anybody got ideas for the base driver device? Ideally it would be >self-oscillating, set with some R-C; have a moderate slew rate; swing >to the rails. I'm thinking roughly 150 KHz maybe, a few watts output. > >Maybe an LM8261 opamp? I'd have to see if it winds up when it rails. >It might not. > >Some sort of fet gate driver would be OK, but few go to 24 volts. > >Something discrete maybe, like a 2N7002 to 24-, and maybe a >bootstrapped pullup? > > >John > > >This looks pretty good: http://www.panoramio.com/photo/61831924 http://www.panoramio.com/photo/61831918 http://www.panoramio.com/photo/61831931 http://www.panoramio.com/photo/61715323 That little IR driver chip is really very nice. It has a 1 us anti-shoot-through delay, and with 270 ohm gate resistors everything is nice and trapezoidal, sort of what I had in mind with the original circuit idea. Thanks for the discussion and ideas. ======================= As soon as we started programming, we found to our surprise that it wasn't as easy to get programs right as we had thought. Debugging had to be discovered. I can remember the exact instant when I realized that a large part of my life from then on was going to be spent in finding mistakes in my own programs. � Maurice Wilkes discovers debugging, 1949 ====================== I'm thinking that I may spend a large part of my life from now on designing power supplies. John
Reply by ●November 8, 20112011-11-08
On Sun, 06 Nov 2011 08:45:03 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:>On Sun, 06 Nov 2011 09:45:41 -0500, legg <legg@nospam.magma.ca> wrote: > >>On Sat, 05 Nov 2011 10:59:48 -0500, Vladimir Vassilevsky >><nospam@nowhere.com> wrote: >> >>> >>> >>>John Larkin wrote: >>> >>>>>Vladimir Vassilevsky wrote: >>>>>>John Larkin wrote: >>>>>> >>>>>>>This >>>>>>> >>>>>>>http://www.panoramio.com/photo/61564837 >>>>>>> >>>>>>>might work for powering a thing I'm doing. >>>>>>>Anybody got ideas for the base driver device? >>> >>>>>>You might want to check IR2085S >>>> Actually, I ordered some IRS2153D's, a very simlar part. >>> >>>So, getting a self oscillating thing to work seems too complicated ? >>>(http://www.abvolt.com/misc/self_osc.jpg) >>> >> >>No, it's the built-in simplicity that's hard to find, as an integrated >>feature. >> >>Most IC designers just don't get it. >> >>RL > >"Most" IC Designers are laughing up their sleeve ;-) > > ...Jim ThompsonThe three terminal pass regulator showed up as soon as it was physically possible to integrate the pass element. They could have stuck 5 or more pins on it and some bright sparks did just that, and continue to do so with varying success. Then there's the 431 shunt regultor........ The 3842 came out in a field flooded with more elaborate alternatives. It sparked a flood of pin-compatibles, with moderate improvements or variations. Designers seem to be afraid to do the same basic thing with the integrated buck converter, or any other simple power conversion function - even simple power drive circuits. Either the market share isn't attractive enough, or there are too few linear start-ups with both the IP, and the need to grab market share. You may laugh at the 555, but........ I'm not arguing for internal simplicity......just external functionality first, with small signal or 'intelligent' features to follow as optional; let their value be determined by subsequent sales volume. RL
Reply by ●November 8, 20112011-11-08
On Sun, 06 Nov 2011 10:09:56 -0500, Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:>On Sun, 06 Nov 2011 10:25:04 -0500, the renowned legg ><legg@nospam.magma.ca> wrote: > >>On Sat, 05 Nov 2011 09:40:53 -0700, John Larkin >><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >> >>>On Sat, 05 Nov 2011 01:53:47 -0500, legg <legg@nospam.magma.ca> wrote: >>> >><snip> >>>I did find a weird asymmetry that turned out to be a feature of LT >>>Spice. If you specify a pulse generator of >>> >>>Ton 5 us >>>Tr 1 us >>>Tf 1 us >>>Tper 10 us >>> >>>you get an asymmetric waveform. You need Ton of 4u to get symmetry. >>> >>> >>>John >>> >> >>It means a lot of decimal places, if you're trying to build a longer >>pattern within a set period, but I prefer this to some unknown >>homogenization going on, at the SW writer's half-baked whim. >> >>Realistic rise-times are best developed by identifiable elements in >>the simulation. >> >>I guess you're wondering, right about now, why there isn't an >>integrated, self-oscillating transformer driver in SO8 or even >>SOT23-5. >> >>Rail pins, output, timing and inhibit pins. > >There are a few, but they're a bit oddball and expensive. > >MAX253 >MAX845 >MAX13256 > >This one is a bit more complex, but claims to be ultra-low noise: >LT3439 > > > >> >>No doubt there will be one, someday, but it will require 1-Wire serial >>programming and run on 2.65 - 3.6V supply rails. It'll only be >>available in 9-pin (3mm x 2mm) DFN. It will still require external >>schottkys, but this will not be discovered till 6 months after product >>release. >> >>RL > > >Best regards, >Spehro PefhanyI generally just use a cmos inverter and buffer it. RL
