Electronics > Projects, Designs, and Technical Stuff
Design of SMPS - Not sure what these spikes are being caused by.
ocset:
LTspice quick start guide attached if you need it....you easily have the knowledge to be able to run a LTspice sim and view the circuit voltages and currents.
If you want to run it, just open the .asc file and hit the running man icon....when youve finished, delete the large .wav file or else your hard disk will eventually fill up if you keep running different sims.
Zog:
HOLY HAND GRENADE BATMAN ! ;D ;D ;D
You my friend, are a true legend and not just in your own underpants !
You have been busy haven't you.
Time to have another crack at LTSpice.
I don't quite know how to thank you for this. A forum thanks does not seem adequate.
But THANKS ! :-+ :-+
I will of course keep you posted on how I go with this.
P.S. early versions of eagle will not open later versions as far as I know.
edit:
Mate I am loving this LTSpice !
Your quick start guide and the circuits you have supplied have made it so easy now to adjust component values.
I have been thinking of making the rsense divider network adjustable using a trim pot for example.
Now I just have to change the values to see what range I can adjust it to and look at the wave forms.
TI Workbench is rubbish compared to this.
Thank you so much for suggesting the UCC28C43 as a replacement too. It's datasheet layout guide is so much easier to understand that the chip I was using for the other design.
I have a few more questions if I may impose on you again.
Why would I use the constant frequency over the over the constant off time circuits. Does one have any advantages over the other ?
I don't see much difference in the output ?
Constant time off (no slope compensation)
Constant frequency (slope compensation)
The tapped boost is confusing me too. How would I get the pulse into the mosfet ? (custom pulse circuit ?)
ocset:
--- Quote ---Why would I use the constant frequency over the over the constant off time circuits. Does one have any advantages over the other ? I don't see much difference in the output ?
--- End quote ---
Yes in this case it doesn’t matter too much…COT is more helpful when there is a wide range of Vin (or wide range of vout)…COT means that in cases of wide vin range, the increase of duty cycle wont be as unmanageable as with a constant frequency converter….typically, as duty cycle increases, the off-time gets less with a const freq converter, and sometimes there is a limit to how low the off time can get…so if you have a COT converter……well, your off time is fixed to an acceptable level in the first place, so its never going to get ridiculously small…….however, it does mean that your switching frequency changes as the vin changes……with COT you have to examine all conditions of load and line to tell that your switching frequency never gets too ridiculously high.
Another point is that COT converters don’t ever need slope compensation…….which means you don’t have to bother with that.
Another point is that there's not many COT pwm controllers on the market off the shelf....the LT1248 that i show in the sim is a const freq converter that i hacked in the sim to work in COT.
The HV9910B is another COT chip
Onsemi do one also.
But there arent many others.
The Tapped Boost is attached here in LTspice sim.......it takes ages to run unfortunately.....and if you put in the leakage inductance by going "K L1 L1 0.99", it then takes even longer.
There arent FETs in LTspice with high enough voltage and low enough rdson........so i have to make a behavioural source and just drive loads of high rdson fets in parallel...(like i did in the previous abbreviated tapped boost sim)
With the tapped boost attached here, if you add in the leakage inductance , the fet then gets overvoltaged.....by the leakage spikes....so then you have to change to a 600v fet.....but then its rdson is too high....so you then have to go adding more in parallel ...and then of course theres too much gate capacitance for the pwm chip to drive...so you then have to add a behavioural source to mimic the pwm controller output.
T3sl4co1l:
Note the downside to constant-time and hysteretic converters: the ripple is all over the place, especially at the operating extremes. At high frequency, switching loss is more significant; at low frequency, reactive losses are more significant (inductor core/winding loss, filter cap ESR), and input and output voltage ripple/noise.
Controllers with reasonably well bounded frequency are easier to design filters for. They're also easier to observe on the oscilloscope, which is probably helpful in development.
Tim
ocset:
Oops sorry, i used the wrong diode in the tapped boost simulation, please find the corrected version here.
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