Great regulation, as TL431 always provides. Efficiency probably around 50% at full output, not great but reasonable given the control method. PSRR isn't all that great, simply because the feedback network is VCC-referenced. Output ripple is in the mV.
Hmm, occurs to me Sloman will probably appreciate this. Though the distortion is obviously not very good, which isn't the point.
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
Do you need the inductor?
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Yeah, and lose the series 914s and use single UF400x diodes.
Waffor? 914s are low capacitance, low leakage and very short recovery time. Keeps switching noise down (as does the series inductor). The additional voltage drop is negligible.
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
Hey, let's optimize it. Cost, efficiency, parts count, reliability, ...
List the design requirements:
9 volts in, 100 volts out @ 3 ma., regulation, noise, ...Anyway, two parts vs. four? Board space? Holes? Both are fast, both are low C, Both about the same cost though the 914 might beat it out with quantity.
It looks rather like a Baxandall class-D oscillator
but the two 2N4401 switching transistors won't be getting enough base-drive to saturate, and will presumably run hot.
It might make more sense to PWM the voltage applied to the 270uH inductor - switching it between 0V and 9V - which is a configuration I've simulated, but the feedback would be a bit of a nightmare.
-- Bill Sloman, Nijmegen
At low power levels, you get away with murder. Actually, at 500mW, the
50% efficiency is pretty good, considering the large number of parts.Couldn't figure out if you were getting regulation by increasing the cross-conduction - a boost converter - or through base current starvation. I guess it has to be the latter because the frequency would normally be set by core saturation in this topology, and because you need it to start up (a handy feature in any supply).
Did you model it first? Will it model?
RL
They run linear for light loads, which isn't terrible as it's a light load. Efficiency vs. load and vs. supply voltage will vary more than otherwise.
I've tried it before on other occasions; you need a huge inductance if the switching frequency is not considerably higher than the oscillating frequency. I know of (relatively small) induction heaters which do this (replacing the transistors with IGBTs and a PLL); they use a 15kg choke to keep ripple low enough.
Also, the extra switching may introduce undesirable noise, say if it's used for a photodiode or avalanche supply.
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
Assuming low bias and small signal conditions, "cross conduction" will look more like a differential amplifier with lots of feedback. It never really runs out of voltage, so it'll tend to look more like an alternating beta-dependent current source.
Timing is provided by the 220pF on the secondary; surprisingly, frequency doesn't change much under load, despite the squashed waveform that would tend to suggest inductive or saturation induced commutation.
The frequency is on the low side for an FT50-43 at this number of turns, but the full-output waveform looks clean (a sine wave, somewhat skewed due to core nonlinearity and hysteresis losses I think).
Of course I did, on the best simulator available... Nature 1.0 ;-)
If it's switching due to saturation, it may still switch due to inductance alone, without using a nonlinear core model. I've seen that before with blocking oscillators. Saturation will commutate quicker, though.
One thing the simulation is almost guaranteed to miss is the squirrelies. At low power, the waveforms look funky due to myriad resonances (it doesn't maintain a clean/clamped sine appearance).
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
MMBD204 is a nice hv dual diode.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
You could drive the center tap of the transformer from an emitter follower off +9, essentially linear regulate the input, still using the bandgap thing for error feedback.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
=20
d.=20
9;ve=20e=20
=20
o=20
Not knowing the exact currents, I can't say anything definitive, but my inc= lination would have been to PWM at twice the resonant frequency of the tran= sformer-capacitor LC - use a phase-locked loop to lock that frequency to th= e VCO in a 4046 running at close to 10MHz, then divide down to get a wavefo= rm or two which could be used to switch the 2N401s, and separately to get a= PWM waveform synch'd to twice that frequency.
Feed-back would control the on/off ratio on the voltage applied to the indu= ctor.
That would basically fiddle with the second harmonic current you've got flo= wing through the inductor anyway, and if you got the phase right you could = probably minimise that. Switching back and forth between the two almost rig= ht PWM on/off ratio's to get the output voltage right would introduce some = ripple, but not a great deal.
The digital logic would all fit into a smallish programmable logic device -= something bigger than a 22V10, but not all that much bigger. =20
ed=20
They survive. You've got to filter like hell after any kind of inverter. Yo= u can play trick to make the switching noise easier to filter out, but ther= e's always some ripple to get rid of.
--=20 Bill Sloman, Nijmegen
Or, even better, drive the inductor from the emitter follower, taking advantage of the rather quieter switching of the Class-D Baxandall inverter, which was reputedly invented to make tolerably quiet high-voltage supplies for photo-multiplier tubes.
-- Bill Sloman, Nijmegen
So your model was close. How did it model start up?
Oh. I get it. A little slow today. NatureSpice.....
RL
inclination would have been to PWM at twice the resonant frequency of the transformer-capacitor LC - use a phase-locked loop to lock that frequency to the VCO in a 4046 running at close to 10MHz, then divide down to get a waveform or two which could be used to switch the 2N401s, and separately to get a PWM waveform synch'd to twice that frequency.
through the inductor anyway, and if you got the phase right you could probably minimise that. Switching back and forth between the two almost right PWM on/off ratio's to get the output voltage right would introduce some ripple, but not a great deal.
something bigger than a 22V10, but not all that much bigger.
can play trick to make the switching noise easier to filter out, but there's always some ripple to get rid of.
This is a 500mW circuit.
RL
=20
What might be awkward is that each of the 2N4401 switching transistors is g= oing be be running as base-current-controlled current sink when it's on. Th= ere's no guarantee that they'll have the same current gain, so they'll pull= down one side of the transformer harder than the other. Presumably the cir= cuit sorts itself out, but it isn't exactly surprising that it gets "squirr= elly" under light loads.=20
se.=20
've =20
the=20
=20
is =20
e=20
inclination would have been to PWM at twice the resonant frequency of the t= ransformer-capacitor LC - use a phase-locked loop to lock that frequency to= the VCO in a 4046 running at close to 10MHz, then divide down to get a wav= eform or two which could be used to switch the 2N401s, and separately to ge= t a PWM waveform synch'd to twice that frequency.
nductor.
flowing through the inductor anyway, and if you got the phase right you cou= ld probably minimise that. Switching back and forth between the two almost = right PWM on/off ratio's to get the output voltage right would introduce so= me ripple, but not a great deal.
e - something bigger than a 22V10, but not all that much bigger.=20
=20
You can play trick to make the switching noise easier to filter out, but t= here's always some ripple to get rid of.
Ripple doesn't matter if the power levels is lower than 600mW?
--=20 Bill Sloman, Nijmegen
On 7/27/2012 1:51 PM, legg wrote:
You can model it in LTSpice
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I'm not sure that I've got the transformer inductances right - I found the relevant toroids on the web, and plugged in the numbers of turns, but the simulation oscillates at 8MHz, which seems a bit quick.
I've not gone to any trouble to get the resistances of the windings right -the extra inductor/ferrite bead at L8 is needed to damp ringing in L6, but a realistic winding resistance would probably do the same job.
The circuit - as modelled - isn't very efficient. The current through L6 settles down at about 106mA, which makes it a 1W circuit, rather than 500mW.
If Tim comes back to us with more realistic values for the component parameters, we should get closer to what he seems to have seen.
-- Bill Sloman, Nijmegen
I've now cleaned up my original model, by adding the ferrite beads L9 and L10 to stop the high voltage generator messing up the inverter so that the inverter now oscillates at a reasonable 135kHz, rather than
8MHz. The rest of the extra components stop all the other oscillations (more or less). 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0 100m 0 10n startup
-- Bill Sloman, Nijmegen
Apparently, you did not run the simulation out to 100ms. Pretty crappy waveform out there.
e e
I most certainly did run it out to 100msec, and I really didn't like the four cycle burst of 3.4MHz every time the Q1 and Q2 switched over.
It's very obvious on Ic(Q3).
If I'd put in even more time I might have been able to get rid of that too, but the circuit was already crawling with enough extra components that I really didn't see much point in struggling on.
Check out the new thread "A less squirrely cute little circuit" which follows up John Larkin's proposal.
I still rather like my pulse-width-modulated approach, but that's hard to model in LTSpice.
-- Bill Sloman, Nijmegen
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