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From: Bitrex on 12 Jul 2010 05:49 > I made a blocking oscillator for a high voltage supply, > http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/Tubescope_Supply.png > which, due to the large capacitance of the HV winding, runs in a resonant mode, with slightly clipped peaks due to the diodes. > I would be interested to know what your thought process is when you design a blocking oscillator such as the one in the above supply. Do you use trial-and-error, computer simulation, or a particular set of arcane equations? I have never had much luck finding a "design guide" for blocking oscillators. The one thorough analysis of such a circuit I've seen was in a journal about a modification to the circuit to generate a "broad band chaos" by adding a resistor and capacitor in the emitter leg. Their preliminary analysis consisted of setting up the system of nonlinear differential equations that described the circuit and doing a simultaneous numerical solution. I don't know how much practical use doing that would be for designing a power supply, though...:)
From: Bitrex on 12 Jul 2010 07:26 Bitrex wrote: > I have never had much luck finding a "design guide" for blocking > oscillators. The one thorough analysis of such a circuit I've seen was > in a journal about a modification to the circuit to generate a "broad > band chaos" by adding a resistor and capacitor in the emitter leg. Correction: I believe the capacitor may have been from collector to ground, rather than the way I described it above. I'll have to find the paper and take another look.
From: Jan Panteltje on 12 Jul 2010 08:15 On a sunny day (Mon, 12 Jul 2010 02:39:23 -0500) it happened "Tim Williams" <tmoranwms(a)charter.net> wrote in <PEz_n.24314$Zi.17971(a)newsfe14.iad>: >> ------------------- +12.5 >> | | Scottky >> 6k8 |------ ------- a k ----> +3.3 >> |___ ||( | ( | >> | | ||( 10t === ( === >> | 1t ) ||( | 150 nF ( --- >> | | | ----- --------------| >> | | c | >> --- - b /// >> === e BC547 >> | 1u | >> /// /// > >That's a blocking oscillator, operated in a non-blocking mode. Assuming = >"blocking" refers to "it's not always oscillating". > >With big C on the load, when the "flyback" is done discharging through = >the load, voltage goes down around zero (or +V at the collector), but = >because of the capacitor, it undershoots, which forward biases the = >transistor again. Resonant or quasi-resonant circuits are hard to make = >blocking; if you underbias them, then you'll get squegging instead. The = >trick then is to make C small enough so the RC time constant roughly = >matches the LC time constant, so the squegging rate is close to the = >cycle rate and it runs as a throttled sinewave oscillator. > >I made a blocking oscillator for a high voltage supply, >http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/Tubescope_Supp= >ly.png >which, due to the large capacitance of the HV winding, runs in a = >resonant mode, with slightly clipped peaks due to the diodes. But I cannot follow you there, sure it works, but you lose power in that snubber? I did the same for a 300 MHz transistor scope, had some more turns, but used the above tuned circuit. But I went up from 12V DC to a couple of kV IIRC. No power losses in snubbers, no hot components! Also have a look at the old TV CRT H output stages, boost diode configurations. Anytime I see a 'snubber' my alarm goes off :-) Here a nice very old HV supply with build in rectifier: ftp://panteltje.com/pub/HeNe_laser_hv_supply_img_2070.jpg side view: ftp://panteltje.com/pub/HeNe_laser_HV_supply_sideview_img_2069.jpg This thing produces great sparks, it is a 555 timer switching a TIP140 power transistor. The grey thing holds the HV transformer and HV diode. Runs on 6 or 12 V (do not remember), bought it in the eighties for a helium neon laser I had. Lost the diagram... But you will note the absence of any 'snubbers', in fact nothing gets hot. Snubbers indicates low efficiency :-) >> In the long ago past I cheated with this system by adding a series >> regulator on >> the input, it dropped only a little bit of voltage so got hardly warm = >:-) > >LDOs can be very handy for cleaning up ridiculously noisy SMPS. I did that once in a design for the army, they did acceptance testing and could not measure the ripple... LOL
From: Jan Panteltje on 12 Jul 2010 10:43 I wrote: >Here a nice very old HV supply with build in rectifier: > ftp://panteltje.com/pub/HeNe_laser_hv_supply_img_2070.jpg >side view: > ftp://panteltje.com/pub/HeNe_laser_HV_supply_sideview_img_2069.jpg These waveforms may be of interest to you, I connected it to 6V and 12 V, note the parabolic waveform. ftp://panteltje.com/pub/HeNe_laser_supply_collector_voltage_img_2079.jpg Base drive to the TIP140 from the 555 timer: ftp://panteltje.com/pub/HeNe_laser_supply_base_drive_img_2080.jpg Output good for a few kV, and short ciruit proof: ftp://panteltje.com/pub/HeNe_laser_supply_arc_img_2078.jpg The parabolic waveform is what you want, I soldered out the grey object, it turns out to be just a potted voltage multiplier, no HV transformer inside (measures infinity), and when it is out then it shows that same collector waveform. The small transformer is a the step up, it has a many turns secondary that measures about 500 Ohms DC.
From: Tim Williams on 12 Jul 2010 13:05
"Bitrex" <bitrex(a)de.lete.earthlink.net> wrote in message news:ifCdnaRETYcue6fRnZ2dnUVZ_g-dnZ2d(a)earthlink.com... > I would be interested to know what your thought process is when you > design a blocking oscillator such as the one in the above supply. Do > you use trial-and-error, computer simulation, or a particular set of > arcane equations? The first two. As for the third, only crude approximations, the ones you'd use in any switching circuit, nothing specific to this. I learned by trial-and-error, so I have an intuitive grasp of things; I can pick a few values, calculate a few other, and get close on the first try. Usually bias and timing capacitance are the problems (since it's easy to get everything else right). The critical part of finding the bypass capacitor Cbb http://myweb.msoe.edu/williamstm/Images/Blocking%20Oscillator.gif is in considering its purpose. In this circuit, there are two ways it turns off: one, collector current just rises so damned high that the transistor desats, and feedback ends the cycle; two, the voltage on Cbb decays due to base current, to a point where the transistor kind of falls off (so it desats again, but maybe at a lower current). This makes it hFE dependent. Realistically, a combination of the two will occur, and you can help the process by adding a current-limiting resistor, like this has: http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/Fast_DCDC.png Such a transistor is required for MOSFET operation. You can control the power output in Discontinuous Current Mode (DCM) (this circuit) by varying bias; the transistor stays on for constant time (i.e., until inductor current reaches the peak value, then the transistor turns off), and in the process, Cbb (C3) gets discharged, so you just charge it back up at a rate determined by power consumption. It's an electronic hit-n'-miss engine. The other way is to modulate the turn-off current, which is how you do BCM (Boundary CM, the next cycle starts when inductor current goes to zero): http://myweb.msoe.edu/williamstm/Images/Discrete_Boost.png In all of these circuits, the next cycle starts when the switching transistor is forward biased. In Cbb-timed circuits (the DCM example), it's normally biased off, then charges up, then switches on (assisted by positive feedback). In continuous circuits (BCM, resonant and quasi-resonant), the undershoot caused by the LC resonance starts the next cycle. Of note, core type is important. Ferrite has low loss, so it tends to resonate very easily. This is good for resonant/BCM circuits. Powered irons are very resistive and well damped. Fast_DCDC uses a powdered iron core, and behaves as an ideal DCM circuit. If you put a ferrite core and lots of capacitance in a DCM circuit, you'll end up with squegging, because it keeps cycling until Cbb has a voltage lower than the undershoot peak. This is usually a few volts negative. Note there isn't much seperating DCM from BCM, and over a wide range of line, bias or load conditions, you may get squegging from an otherwise well-behaved circuit. > I have never had much luck finding a "design guide" for blocking > oscillators. The one thorough analysis of such a circuit I've seen was > in a journal about a modification to the circuit to generate a "broad > band chaos" by adding a resistor and capacitor in the emitter leg. > Their preliminary analysis consisted of setting up the system of > nonlinear differential equations that described the circuit and doing a > simultaneous numerical solution. I don't know how much practical use > doing that would be for designing a power supply, though...:) Hah. It's definitely a complex system and quite capable of generating chaos. I don't remember what conditions I had, but I've definitely had the transformer making what sounds like white noise. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms |