Yes. Qod=Qg(tot)-(Qgs+Qgd)Is it best to select nC from the graph including Qgs, Qgd, Qos? as stated in DRIVING THE MOSFET QG and IG Microchip 00786a, Does that roughly coincide/correlate with GQtot in the (FDD8447L) NMOS datasheet excerpt.
Yes. I have added a word in your sentence above.Transition time: I understand that is user-definable, it's basically waveshaping for speed to cut switching power losses, so long as the SMPS frequency is not faster than the MOSFET can turn on or off. Is that definition about right for a beginner?
Current is being taken from the driver during the delay time also and is accounted within the first part of Qgs. If you do not want to account for it, subtract that charge from Qg.When using rise time and/or fall time in formulas (such as the above about QG/t), do I need to add turn-on delay to rise time, and turn-off delay to fall time?
You should be spending most of your time mastering the control - sepic is a delayed action converter ( RHP zeroes ), when you suddenly turn the fet on for longer the Vout actually dips initially, before recovering - this action means the control must be generally slower than for other direct action converters ( e.g. buck derived)
You will need "phase lead" or speed up circuits to get the best transient performance, i.e. RC across the input sensing (volts) resistor, and extra damping in the control loop.
The control response should be in the few hundred Hz region - if you have large chokes and output caps.
p.s. your device FDD8447L seems to need at least 4V to be getting close to fully on - it will run at 3v3 / 0 but a little lossily
p.p.s. at Vin = 2.6V you might get the mosfet to switch (if there are no volt losses in your control / driving circuitry) but it will be a close thing ( and batch dependent ) if it did manage to work you need to take some of the output volts and route this to your control / driver to give it a stable 5V say to run and drive the fet... - without feeding power to your input line ...!!!
Current is being taken from the driver during the delay time also and is accounted within the first part of Qgs. If you do not want to account for it, subtract that charge from Qg.
Microchip's formula (transition time=Qg/Ig) is valid only if you are driving the gate with constant current (Ig=constant).
This is second time around for trying to understand SMPS via a SEPIC and yet again I sometimes wonder if a buck converter isn't perhaps a simpler entry point for a beginner who's just curious but would also like to make the thing and see it work correctly.
You asked: When using "Qg", do I need to add turn on/off delay time ? Yes, you need because you are using Qg.What do you mean by "if you don't want to account for it," wouldn't it be better to include that time/charge in all calculations?
Microchip and post #1 imply that Ig is constant throughout the whole switching time, which is incorrect. Post #4 attachment, by contrast, implies Ig is constant during the Qgd charge period, which is correct.The gate current is constant, just for a very short amount of time, periodically
Yes.Do you mean like in the Lazlo gate drive app note timing graph with t1 to t4 for on and off that appears in the first few pages somewhere?
No. Formulas are for the specified SEPIC converter and the compensation network formulas are widely known as they are Type II compensation with OTA (see here).the snva168e app note for SEPIC design has CMC compensation, but I think the formulas are for use with a specific driver IC in the design example.
Crossover freq does not help in selecting OP amp.Could someone be able to tell from a few graphs which op amp is most suitable re gbw and V/uS for a 90 kHz smps (sepic), with a crossover frequency of 227Hz? I still don't fully understand what I'm reading and seeing in op amp datasheets, so adult advice would be appreciated. My hunch is the OPA376 is too slow but the OPA322 is fast enough.
I can't see any problem. Document is not breaking any "rule"...What may I be doing wrong in my approach to resolving the compensation equations correctly and complying with divider resistor and Ri2 rule, please, if you can think of anything?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?