Buck converter question

[dru]

Hello,
I'm working on a buck converter for a QCW coil. I have a schematic I plan on using, but I'm wondering if I may be doing something wrong. For the MOSFET or IGBT that is being driven by my microcontroller, is it correct that this should be an N channel device? I have gotten satisfactory ramps when running sims, but it was always necessary to drive the MOSFET at vcc to get it into linear region. This makes sense because it is in a high side configuration. What are some practical methods of amplifying the low voltage signal from my mcu to vcc + ~5v or so? Does it make sense to use a high side driver for this?

[davekni]

For the MOSFET or IGBT that is being driven by my microcontroller, is it correct that this should be an N channel device?

NFETs have higher current ratings for a given cost and drive power.  A PFET is possible to use.  IGBTs aren't available in P variety AFAIK.

Does it make sense to use a high side driver for this?

Yes, that is what high side drivers are designed to do.

[dru]

NFETs have higher current ratings for a given cost and drive power.  A PFET is possible to use.  IGBTs aren't available in P variety AFAIK.


Yes, that is what high side drivers are designed to do.

Great, thank you Dave!

[dru]

Hello I have another question about the buck converter. Is there a reason a buck converter is used for the voltage ramps instead of a boost converter? It's much easier to drive the switch in the boost converter than the buck. Also, if I want to generate a 400v ramp it seems counterintuitive to use a buck converter. I get 110vac mains/155vdc when rectified, and I have a variac that can output 230vac. I would have to put the 230vac through at least one voltage doubler for it to make sense to use a buck. So im wondering if there is some advantage in using the buck, it seems a lot easier to use a boost if I can get adequate current from it. This is for a qcw ramp, not drsstc bus. Thank you!

[davekni]

Good question.
I think the big issue is too-high starting voltage for QCW ramp.  Pure boost converters (not boost/buck combinations) have minimum output voltage equal to input voltage.
A boost converter will also require a higher current switch (IGBT or FET).  If filter inductor is reasonably large, buck converter switch peak current is only a little higher than output current.  Boost converter switch current is higher by ratio of output to input voltage.  In both topologies, peak switch current will be higher with smaller inductance (higher ripple current).
One other reason:  QCW coils require large bulk capacitance to store energy for the ramp.  Cost per joule for electrolytic capacitors is higher at low voltage.  I think ~350V electrolytic caps are optimum for cost/energy and other parameters.

[dru]

Good question.
I think the big issue is too-high starting voltage for QCW ramp.  Pure boost converters (not boost/buck combinations) have minimum output voltage equal to input voltage.
A boost converter will also require a higher current switch (IGBT or FET).  If filter inductor is reasonably large, buck converter switch peak current is only a little higher than output current.  Boost converter switch current is higher by ratio of output to input voltage.  In both topologies, peak switch current will be higher with smaller inductance (higher ripple current).
One other reason:  QCW coils require large bulk capacitance to store energy for the ramp.  Cost per joule for electrolytic capacitors is higher at low voltage.  I think ~350V electrolytic caps are optimum for cost/energy and other parameters.

That makes sense thank you! It would likely be unreasonable then to input a very low voltage and step it up to ~350v, the current draw would be enormous and I'd imagine there would be high losses for such a high conversion. I've taken inventory and logged everything in a Jupyter notebook, I have 17 400-420v electrolytic caps of capacities ranging from 200uf to 880uf. I'm aware of the challenges of building a bank with non-identical caps so I've been browsing mouser, digikey to see if I can get a good price for a set to avoid dealing with the mismatches. I've seen recommendations for a bus capacity of 5-15mf, I will probably start on the lower end maybe 8mf. I'm not super familiar with buck converters so this question may seem silly, but does the capacitor bank act as the bucks supply (positioned directly after full wave rectification), or is the bank placed where the capacitor is typically positioned in a buck converter- parallel to diode and load? Also, is using a voltage doubler viable or am I limited by the 230rms from my variac? If the latter, should I still get higher voltage caps to save money, or should I get caps rated 300v? Thank you for your help!

[davekni]

I'm aware of the challenges of building a bank with non-identical caps

Not difficult, especially if building a single parallel set.  Your caps are all 400V or slightly higher.  Just parallel them for a 400V larger capacitor.
If you want a higher voltage with two banks in series, just make the two banks roughly equal in total capacitance.  As long as the banks are fed from a normal full-wave voltage doubler (each bank supplied by diode from line), there's no need for balancing with bleed resistors.  A bleed resistor across each bank is still recommended for safety after power is removed.

does the capacitor bank act as the bucks supply (positioned directly after full wave rectification)

Yes.  A smaller capacitor is necessary on buck-converter output (on H-bridge supply) to handle both H-bridge switching frequency and buck converter ripple current.  That output capacitor needs to be large enough to handle ripple currents, but much smaller than above input capacitors so as to avoid wasting energy charging the output capacitor during ramp.

Also, is using a voltage doubler viable or am I limited by the 230rms from my variac?

Either is fine.  Depends mostly on the voltage of your switches (FETs or IGBTs), for both buck converter and H-bridge.

If you decide to purchase new caps, 350V seems to be about the optimum for electrolytic caps, so anywhere in the 300V to 400V range is great.  Easiest to make your bulk caps the doubler caps too.  Two banks of 300V or 350V caps is series if your buck converter can handle ~600V input.

Thank you for your help!

You're welcome.  Have fun learning!