Re: [neonixie-l] MC34063 SMPS questions
There are so many things wrong with this circuit that I scarcely know where to start. The basic operation of an up convertor is that a certain amount of power from the low voltage supply is stored as magnetic energy in the coil, and then released as high voltage when the field collapses. Hhopefully most of it goes into the output capacitor. You cannot of course get more power out than goes in, and every milliWatt of it has to be stored and then released by the magnetic core on the way. Let's start with C8, the cap directly on the coil. At the moment of turn on, this capacitor supplies essentially all of the coil current until it discharges - thus completely defeating the current limiting function of the 34063, which senses only the current flowing through R3-R6. The coil could saturate and the chip still think it has no current at all. The coil is 220uH and there is some series resistance, 0.5R in the FET, 0.25R in R3-R6 and some unknown series resistance in the PSU and elsewhere. Let's guess it's 1 ohm total. The time constant of the coil = L/R, or in this case 220us. Note that the bigger the series resistance, the smaller is the time constant - counter-intuitive, but true. The time constant is the time it takes the inductor current to rise to 63% of its final value, which in this case would be about 12V/1R or 12 amps. Unless you run it from a car battery the PSU probably can't source that much current, but that doesn't matter in the short term. 63% of 12A is 7.56A. If we assume the current rises in a linear fashion, which it does approximately, the straight line from 0 to 7.56 takes 220 microseconds. If you were to plot that on a graph you would see that the current passes the coil saturation point of 1.25A after 36 microseconds - actually it will be a bit less, because the current rises faster at the beginning. (If the current limit was working properly it would limit the current to about 1.2A.) The switching frequency is set by C3 and with 1nF, the frequency is about 33kHz. The maximum on period is therefore 30 microseconds, which is kind of close to the saturation point. You must not, therefore, make the timing capacitor C3 any larger than 1nF unless you also increase the size of the coil. Now, power-in versus power-out. If you want 180V at 35mA, that's 6.3W at the output. Allowing for 85% efficiency (the 34063 spec) that's about 7.5W at the input. From a 12V supply, that requires a mean forward current of 625mA in the coil. The current waveform is roughly a triangle and the maximum permissible peak current is less than twice the required mean - therefore, you can only just barely put enough energy into the inductor to get close but still short of the output you want, running flat out with no losses. To get more, you need to use a physically bigger inductor and reduce the current limit resistors, to input and store more energy. Let's look next at the losses. Only one thing really matters in a flyback convertor and that's how fast you can turn off the switching transistor, because every nanosecond the drain voltage takes to rise, it's dissipating magnetic energy that isn't going into the output. The dual emitter follower configuration Q1-Q3 in the schematic doesn't do a good job here. For a start, the NPN Q1 is not only unnecessary, it's harmful. We don't care how slowly the FET turns ON, only how fast it turns OFF. We want to wham that FET gate down to ground so hard and so fast it doesn't know what it it, and all Q1 does is fight it. Q1 could be replaced with a 4.7k resistor (and R12 removed). Then there's the configuration of Q3. Emitter followers are fast, but they have a very high input impedance (rie * hfe) - for the 2N3906 in this configuration, it would be in the region of 20-50k. Since the 34063 in this configuration can only source current and not sink it, any stray capacitance on the base side has to discharge through Q3 and R11, with R11 being dominant. If the stray capacitance is 25pF, the time constant is 125 nanoseconds, which is the time for the voltage to fall by 63%. At (100-63)% of 12V the base of Q3 is still at 4.5V. and its emitter is Vbe above that, ie at 5V - the FET is therefore still turned hard on. Then it gets worse, because the discharge curve flattens after 1 time constant - it takes about 5 time constants to lose 95% of the charge, so over the next microsecond the FET gate voltage falls slowly from 5V to 1V, and the FET channel resistance rises equally slowly. During that whole time, the magnetic energy in the coil has an easier path to ground than through the rectifier to the output. It's all wasted by making the FET hot. So without totally ripping up the circuit, here's my suggestion. - Remove C8, the cap across the coil, and replace C2. - Remove Q1 and replace it with a 4.7k resistor. Remove R12. - Replace R11 with a 680 ohm 1/4 watt to shorten the rise time. If you really want to rip up the circuit,
Re: [neonixie-l] MC34063 SMPS questions
On 15/06/14 17:23, Chris Stalin wrote: 1. I also added a 220uF cap near the coil. I noticed the voltage jump about 3 volts. I have kept the 220uF cap near the IC. Does any one recommend I keep the IC cap or remove it o adjust its value? When removed the voltage drops about 1 volt. So clearly adding or removing caps adjust the voltage - just not sure why. None of my caps are low ESR yet. From the circuit, it's probably providing an alternate source of current for L1 which bypasses R3..R6. What you're seeing is the capacitor charging while Q2 is off then discharging into L2 when Q2 is on, supplementing the current coming from R3..R6. The 34063 notices that there's less voltage going through the current sense resistors (via Ipk Sense) and reacts by increasing the power through Q2 by increasing the duty cycle. (if I remember correctly the 34063 is an almost-PWM controller). 2. When my system is not loaded and just idling I see about 182.5V on the 180V line and 535V on the 540V line. When I load down my 180V line with 5mA of resistance my voltage jumps 555V on the 540 line. 180V line stays the same. Can any one explain why this is happening? I would assume a load on the system would make voltage sag not drop. You have a Cockroft-Walton divider comprised of C6, D2, D3, and the other unnamed parts. What you're probably seeing is the divider responding differently to a change in input frequency and/or voltage. In short, The 34063 is regulating the 180V rail, but not the 540V rail. If your feedback was coming from the 540V rail you'd expect to see the 180V rail jumping around while the 540V remains stable. Are you loading down the 540V rail at all? 3. Are the cap values critical? I need to pick up a couple more caps as I ran out of HV caps. I currently have 2.2uF caps x 2 and the rest are 4.7uF and 10uF 450V caps - Just for testing purposes only. ( cap values from 1-2.2uF throughout the HV section is acceptable ? ) I'll let someone else field this one as I don't have an answer. Sorry! :( Though ideally for a C-W multiplier all the capacitors and all the diodes should be identical. If they're not, this might be why you're seeing the voltage rise slightly. Thanks, -- Phil. phil...@philpem.me.uk http://www.philpem.me.uk/ -- You received this message because you are subscribed to the Google Groups neonixie-l group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/539DE01E.7030901%40gmail.com. For more options, visit https://groups.google.com/d/optout.
Re: [neonixie-l] MC34063 SMPS questions
Hi Philip, A friend suggested that extra cap just before the coil. Surprise surprise :) The explanation for the voltage ripple on the 540V line makes sense, With no real feedback there can't be any regulation. I have much bigger issue on of heavy sag on the 180V line. I removed the tripler all together for testing. I plan to have identical caps @ 600V when I figure out this sagging issue. Thanks for the tips and your time! On Tuesday, June 17, 2014 5:47:42 AM UTC-6, Philip Pemberton wrote: On 15/06/14 17:23, Chris Stalin wrote: 1. I also added a 220uF cap near the coil. I noticed the voltage jump about 3 volts. I have kept the 220uF cap near the IC. Does any one recommend I keep the IC cap or remove it o adjust its value? When removed the voltage drops about 1 volt. So clearly adding or removing caps adjust the voltage - just not sure why. None of my caps are low ESR yet. From the circuit, it's probably providing an alternate source of current for L1 which bypasses R3..R6. What you're seeing is the capacitor charging while Q2 is off then discharging into L2 when Q2 is on, supplementing the current coming from R3..R6. The 34063 notices that there's less voltage going through the current sense resistors (via Ipk Sense) and reacts by increasing the power through Q2 by increasing the duty cycle. (if I remember correctly the 34063 is an almost-PWM controller). 2. When my system is not loaded and just idling I see about 182.5V on the 180V line and 535V on the 540V line. When I load down my 180V line with 5mA of resistance my voltage jumps 555V on the 540 line. 180V line stays the same. Can any one explain why this is happening? I would assume a load on the system would make voltage sag not drop. You have a Cockroft-Walton divider comprised of C6, D2, D3, and the other unnamed parts. What you're probably seeing is the divider responding differently to a change in input frequency and/or voltage. In short, The 34063 is regulating the 180V rail, but not the 540V rail. If your feedback was coming from the 540V rail you'd expect to see the 180V rail jumping around while the 540V remains stable. Are you loading down the 540V rail at all? 3. Are the cap values critical? I need to pick up a couple more caps as I ran out of HV caps. I currently have 2.2uF caps x 2 and the rest are 4.7uF and 10uF 450V caps - Just for testing purposes only. ( cap values from 1-2.2uF throughout the HV section is acceptable ? ) I'll let someone else field this one as I don't have an answer. Sorry! :( Though ideally for a C-W multiplier all the capacitors and all the diodes should be identical. If they're not, this might be why you're seeing the voltage rise slightly. Thanks, -- Phil. phi...@philpem.me.uk javascript: http://www.philpem.me.uk/ -- You received this message because you are subscribed to the Google Groups neonixie-l group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/059bb0ea-1845-480b-9212-4845015596c3%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
