Forums

boost-converter trouble

Started by Winfield Hill November 20, 2018
On Thu, 22 Nov 2018 16:20:10 -0600, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

>"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message >news:taqdvd983sbci31sulfkda0fagihjvm603@4ax.com... >> What is the cheapest uP that has an ADC input? One of those and a >> three cent mosfet would make a nice boost switcher; just needs a >> little code. 30 cents for the uP and fet and a diode maybe. Serial >> interface is probably free. > >Guessing, cheapest with >200kSps isn't going to be as competitive as you're >thinking. > >That's basically what the stupid phaux-analog controllers are anyways. >>:( > >Tim
I don't think you'd need to sample that fast. The cheapest uP on Digikey is 25 cents in quantity. It has a 10-bit ADC. -- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On 11/22/2018 11:27 PM, John Larkin wrote:
> On Thu, 22 Nov 2018 16:20:10 -0600, "Tim Williams" > <tiwill@seventransistorlabs.com> wrote: > >> "John Larkin" <jjlarkin@highlandtechnology.com> wrote in message >> news:taqdvd983sbci31sulfkda0fagihjvm603@4ax.com... >>> What is the cheapest uP that has an ADC input? One of those and a >>> three cent mosfet would make a nice boost switcher; just needs a >>> little code. 30 cents for the uP and fet and a diode maybe. Serial >>> interface is probably free. >> >> Guessing, cheapest with >200kSps isn't going to be as competitive as you're >> thinking. >> >> That's basically what the stupid phaux-analog controllers are anyways. >>> :( >> >> Tim > > I don't think you'd need to sample that fast. > > The cheapest uP on Digikey is 25 cents in quantity. It has a 10-bit > ADC. > >
Pay a little more and get the ATTiny10 for around 30 cents, so you can program it in nice "modern" C or C++ with a free regularly-maintained compiler and not yucky 8051 assembly-derivative
"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message 
news:250fvdh2vs36qmvs0f586fk03e38koc0lk@4ax.com...
> I don't think you'd need to sample that fast. >
If you're okay with exploding transistors and shitty control loops, sure, sample just voltage. :-) Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
On Sat, 24 Nov 2018 14:45:35 -0600, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

>"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message >news:250fvdh2vs36qmvs0f586fk03e38koc0lk@4ax.com... >> I don't think you'd need to sample that fast. >> > >If you're okay with exploding transistors and shitty control loops, sure, >sample just voltage. :-) > >Tim
If I know the voltage and control the gate drive, I know the currents. -- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
For a boost converter, it would be wise to add a current sense resistor in the source of the FET, and use a microcontroller that has a fast comparator.

That way control is current mode, easier compensator design and has over current protection. Does require a good comparator to keep sense voltage low, both due to power loss and gate drive negative feedback 

Using microcontroller allows for a lot of extra features, the sky is almost the limit 

Cheers

Klaus 
On 11/24/2018 04:58 PM, Klaus Kragelund wrote:
> For a boost converter, it would be wise to add a current sense resistor in the source of the FET, and use a microcontroller that has a fast comparator. > > That way control is current mode, easier compensator design and has over current protection. Does require a good comparator to keep sense voltage low, both due to power loss and gate drive negative feedback > > Using microcontroller allows for a lot of extra features, the sky is almost the limit > > Cheers > > Klaus >
Don't forget that current-mode control has a subharmonic instability problem at duty cycles greater than 50%: <https://www.digikey.com/en/articles/techzone/2015/jan/the-role-of-slope-compensation-in-current-mode-controlled-voltage-regulators>
On 11/24/2018 04:04 PM, John Larkin wrote:
> On Sat, 24 Nov 2018 14:45:35 -0600, "Tim Williams" > <tiwill@seventransistorlabs.com> wrote: > >> "John Larkin" <jjlarkin@highlandtechnology.com> wrote in message >> news:250fvdh2vs36qmvs0f586fk03e38koc0lk@4ax.com... >>> I don't think you'd need to sample that fast. >>> >> >> If you're okay with exploding transistors and shitty control loops, sure, >> sample just voltage. :-) >> >> Tim > > If I know the voltage and control the gate drive, I know the currents. > >
A modern 8-bit RISC processor running at 16 or 20 MHz (external clock or crystal) is remarkably speedy even at doing software-based floating point calculations. The control loop doesn't have to be "shitty" you can do real DSP math, it's not your granpa's 8 bit.
On Sunday, November 25, 2018 at 12:23:47 AM UTC+1, bitrex wrote:
> On 11/24/2018 04:04 PM, John Larkin wrote: > > On Sat, 24 Nov 2018 14:45:35 -0600, "Tim Williams" > > <tiwill@seventransistorlabs.com> wrote: > > > >> "John Larkin" <jjlarkin@highlandtechnology.com> wrote in message > >> news:250fvdh2vs36qmvs0f586fk03e38koc0lk@4ax.com... > >>> I don't think you'd need to sample that fast. > >>> > >> > >> If you're okay with exploding transistors and shitty control loops, sure, > >> sample just voltage. :-) > >> > >> Tim > > > > If I know the voltage and control the gate drive, I know the currents. > > > > > > A modern 8-bit RISC processor running at 16 or 20 MHz (external clock or > crystal) is remarkably speedy even at doing software-based floating > point calculations. The control loop doesn't have to be "shitty" you can > do real DSP math, it's not your granpa's 8 bit.
It's true an 8 bit can be fast, but not at floating point math. A single floating point operation takes up to 1000 times longer than a fractional I9Q23 calculation Using floating point can be tricky if you do not do the scaling work, otherwise saturation can occur Cheers Klaus
On Sunday, November 25, 2018 at 8:04:52 AM UTC+11, John Larkin wrote:
> On Sat, 24 Nov 2018 14:45:35 -0600, "Tim Williams" > <tiwill@seventransistorlabs.com> wrote: > > >"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message > >news:250fvdh2vs36qmvs0f586fk03e38koc0lk@4ax.com... > >> I don't think you'd need to sample that fast. > >> > > > >If you're okay with exploding transistors and shitty control loops, sure, > >sample just voltage. :-) > > If I know the voltage and control the gate drive, I know the currents.
Another of John Larkin's over-confident assertions. -- Bill Sloman, Sydney
On 11/24/2018 07:27 PM, Klaus Kragelund wrote:
> On Sunday, November 25, 2018 at 12:23:47 AM UTC+1, bitrex wrote: >> On 11/24/2018 04:04 PM, John Larkin wrote: >>> On Sat, 24 Nov 2018 14:45:35 -0600, "Tim Williams" >>> <tiwill@seventransistorlabs.com> wrote: >>> >>>> "John Larkin" <jjlarkin@highlandtechnology.com> wrote in message >>>> news:250fvdh2vs36qmvs0f586fk03e38koc0lk@4ax.com... >>>>> I don't think you'd need to sample that fast. >>>>> >>>> >>>> If you're okay with exploding transistors and shitty control loops, sure, >>>> sample just voltage. :-) >>>> >>>> Tim >>> >>> If I know the voltage and control the gate drive, I know the currents. >>> >>> >> >> A modern 8-bit RISC processor running at 16 or 20 MHz (external clock or >> crystal) is remarkably speedy even at doing software-based floating >> point calculations. The control loop doesn't have to be "shitty" you can >> do real DSP math, it's not your granpa's 8 bit. > > It's true an 8 bit can be fast, but not at floating point math. A single floating point operation takes up to 1000 times longer than a fractional I9Q23 calculation > > Using floating point can be tricky if you do not do the scaling work, otherwise saturation can occur > > Cheers > > Klaus >
an AVR with hardware multiplier can do a 4 byte single precision "float"-type multiply in about 200 clocks, a core without a hardware multiplier takes around twice that. Plenty of compute power to do e.g. a real time FFT even at 8 MHz: <https://www.youtube.com/watch?v=tCmaOb-VAEo>