On 02/08/2019 01:21, Jan Panteltje wrote:> On a sunny day (Fri, 2 Aug 2019 01:08:38 +1000) it happened Chris Jones > <lugnut808@spam.yahoo.com> wrote in <_fD0F.380989$4d7.71297@fx30.am4>: > >> On 01/08/2019 19:14, Winfield Hill wrote: >>> Chris Jones wrote... >>>> >>>> If I am not restricted to using standard opamps, then my preferred >>>> approach is to make the gate capacitance of the MOSFET be part of the >>>> dominant pole for loop compensation. >>>> >>>> One way to do this is to use a transconductance amplifier as the error >>>> amplifier, and feed the error current into the gate. It works nicely on >>>> chips, where there is no incentive to use standards op-amp designs. >>>> >>>> On circuit boards rather than chips, the LT1228 is an option. >>>> >>>> Here is an electronic load where I used the LT1228 to control >>>> a MOSFET - it is a constant voltage load rather than a current >>>> source, but the problems of driving a MOSFET gate are similar: >>>> https://bitbucket.org/chrisgj198/projects/src/master/eload/kicad/eload1a.pdf >>> >>> The more I studied your circuit, the higher my >>> eyebrows went. It's a pleasure to see someone >>> who likes using discrete MOSFETs in analog designs. >> Hrm...thank you, though they discontinued the BSS83 about as soon as I >> ordered the PCBs. I suspect a conspiracy. Luckily I already had stock. >> Next time I could use a JFET for Q1, but I'd better pick one that I >> don't like, in case they cancel that one too. >> >>> Examining your LT1228 driving an IXTN60N50L2 (a >>> $40 735-watt 500V SOT-227 monster MOSFET w/ bolts), >>> with R100 = a DNL source resistor,** >> There are two source pins, well, bolts, on that MOSFET and I have no >> idea why I added the R100 DNL between them - perhaps I was afraid of >> some effect with the terminal inductance and wanted a placeholder for a >> way to de-Q it, can't remember. >> >>> my eyebrows >>> peaked, when I saw a 100nF in series with 1R, >>> from gate to ground. Those must be compensation >>> components, but we're talking a slow loop here! >> Yep, it's for testing solar panels on the other end of many metres of >> cable. Sometimes we wanted to step through the IV curve, then sometimes >> we wanted to sit on the maximum power point until the PV module reached >> thermal equilibrium. The solar cells get a bit cooler when loaded >> correctly rather than being left open circuit. >> >> I did some simulations in LTSpice (also in that repository but no idea >> which one is the right version!) and changed stuff until the loop seemed >> not to oscillate or ring much. For comparison, most Keithley >> Sourcemeters in voltage source mode don't like the capacitance of big >> solar cells, and can oscillate unless you add external resistors and >> capacitors. >> >> I like the isolated mounting base on those MOSFETs, it's worth the extra >> money to me. I wish someone made a copper water block just the right >> size for the SOT-227 though. I made a couple, but the machining and >> brazing work involved was considerable. Sadly drilling mounting holes >> for the MOSFET into an existing CPU water block is likely to make a >> sprinkler instead. >> >>> And then I see five 10n caps in parallel from >>> drain to source, plus another 1uF (!) in parallel, >>> plus another 1uF in series with 5R6, plus after >>> tiepoint, two more 1uF in parallel, sheesh! >> I have designed a few electronic loads for solar panels in the past, and >> this is the first one that I haven't had oscillation problems with, yet. >> Oh no, that isn't true, whilst the electronic load part didn't oscillate >> this time, the LM337 regulators did, because I used MLCC capacitors on >> them. And the LM317s ring like a bell, though not quite oscillating they >> do amplify the oscillation from the LM337s. In the end I bodged big >> tantalums on there and that fixed it. >> >> The tie point P5 was for an option to insert a power supply or battery >> or charged supercapacitor, if I ever needed to test right down to zero >> volts across the solar module. >> >>> ** Further examination shows low-value 0R01, 0R1, >>> etc., current-sensing source resistors, selected >>> with Si4048DY MOSFET switches to the neg return. >>> These are innocent-looking 50-cent 30-volt MOSFETs >>> in so-8, but with only 8.5m-ohm of Ron at 25C. >> >> I put two of them in anti-parallel, in series with the 10mOhm shunt, to >> use the substrate diodes in the hope that it might limit the voltage and >> make it harder to damage something if my code or the processor goes >> silly, or if someone hooks up the solar module backwards with the >> control power off. >> >> The ADC for the current shunts likes to work at an inconvenient common >> mode voltage, so the microcontroller is running off +1.6V and -0.9V. >> That was a nuisance, particularly in the PCB layout. Also that micro >> doesn't provide a nice (not polling) way to know when its UART has >> finished sending, so disabling the TX at the right time after sending >> over RS485 is inconvenient. A more convenient stand-alone ADC and then a >> free choice of microcontroller might have been more sensible. >> >> In the end I only needed to use the electronic load up to a fraction of >> its intended power rating, so I don't know its true limitation. Still, >> the whole thing basically worked, and fitted into a little weatherproof >> box outside, and was able to use an existing supply of rather hot >> glycol/water for cooling. >> > > How about switching 8 power resistors of values > 1, 2, 4 ,8, 16, 32, 64, 128 R > with the correct decreasing power rating > by some 8 power MOSFETS driven by a simple micro to get 256 steps? > Or perhaps an R2R ladder of power resistors? > No oscilililiations, big piece of iron heatsink..... like this perhaps: > http://panteltje.com/pub/big_3kg_heatsink_IMG_3745.GIFThat can work, though I wanted more steps, or at least more in certain parts of the IV curve. Another thing you can do is use just one resistor and switching MOSFET in series with it, and PWM the MOSFET to get the average conductance that you want. Then you need to put an enormous electrolytic capacitor in parallel with the load and solar panel, so that the solar panel doesn't see any ripple in the current and voltage. Both of the above are a lot easier to design and get a nice IV curve, if someone has told you beforehand quite an accurate estimate of what the current and voltage from the solar panel will be. Otherwise choosing the power resistor values is a bit tricky.
op-amp-controlled MOSFET cucrrent-source
Started by ●July 31, 2019
Reply by ●August 1, 20192019-08-01
Reply by ●August 1, 20192019-08-01
Winfield Hill wrote...> Winfield Hill wrote... >>> On July 31, 2019, Winfield Hill wrote: >> >>https://www.dropbox.com/s/nednxac2bykld8q/opamp-MOSFET_current-source.pdf?dl=1 >> >>>>>> In the formulas, I assume the opamp's output >>>>>> signal is controlled by Vin-Vs and R2 C2. >>>>>> >>>>>> still checking the math ... but one formula >>>>>> ... gives us Zin = Vg/i3 at the MOSFET's gate. >>>>>> >>>>>> Zin = R1 (1 + fT / f) + 1 / s C1. >> >> Example circuit, 50mA full scale, running at 10mA. >> Q1: IXTP02N120, 1.2kV, 33W Ciss 104pF, gm n=6.2, > fixed gm error: so gm = 65mS at 10mA, fT = 99MHz. >> Circuit, R1 = 250 ohms, goal 10MHz BW. >> >> Q1 gate Zin = 250*10 + j153 ohms from Ciss(10MHz). >> Latter term non-material. Given high Zin, chose >> R3 = 270, isolates opamp. Opamp = LT1360, 50MHz. >> Chose R2 1.5k, C2 10pF for our desired 10MHz BW. >> Opamp Zout 80/5 = 16 ohms. Safe margins all-round. > > The above is a small-signal model. I processed it > with SPICE, response is flat to 20MHz, with 1 dB of > peaking at 65MHz, pulse response is about 5ns, with > 10% overshoot. Something may be wrong, this is way > way too good. OTOH, large-signal results, including > full OFF to ON, and back, will be far slower.Dunno if anybody's following this aspect, but there's a goal and path to follow. Today I was able to bench verify a 10ns IXTP02N120 step response (5mA to 10mA), but only if Vds >= 35 volts. My simple model ignores Coss = 10pF and Crss = 2pF at 35V. But at 5 volts, Coss = 70pF and Crss = 50pF, which slows the current switching to 20 to 30ns or longer. Actually, my plan was to cascode the "signal" MOSFET with a HV power part, running off the op-amp's Vcc supply rail, so Vds may well be under 5 to 10 volts. That needs to be a low-capacitance low-voltage part. With the right parts we can keep the simple model. -- Thanks, - Win
Reply by ●August 1, 20192019-08-01
On Thursday, August 1, 2019 at 8:26:21 PM UTC-4, Winfield Hill wrote:> Winfield Hill wrote... > > Winfield Hill wrote... > >>> On July 31, 2019, Winfield Hill wrote: > >> > >>https://www.dropbox.com/s/nednxac2bykld8q/opamp-MOSFET_current-source.pdf?dl=1 > >> > >>>>>> In the formulas, I assume the opamp's output > >>>>>> signal is controlled by Vin-Vs and R2 C2. > >>>>>> > >>>>>> still checking the math ... but one formula > >>>>>> ... gives us Zin = Vg/i3 at the MOSFET's gate. > >>>>>> > >>>>>> Zin = R1 (1 + fT / f) + 1 / s C1. > >> > >> Example circuit, 50mA full scale, running at 10mA. > >> Q1: IXTP02N120, 1.2kV, 33W Ciss 104pF, gm n=6.2, > > fixed gm error: so gm = 65mS at 10mA, fT = 99MHz. > >> Circuit, R1 = 250 ohms, goal 10MHz BW. > >> > >> Q1 gate Zin = 250*10 + j153 ohms from Ciss(10MHz). > >> Latter term non-material. Given high Zin, chose > >> R3 = 270, isolates opamp. Opamp = LT1360, 50MHz. > >> Chose R2 1.5k, C2 10pF for our desired 10MHz BW. > >> Opamp Zout 80/5 = 16 ohms. Safe margins all-round. > > > > The above is a small-signal model. I processed it > > with SPICE, response is flat to 20MHz, with 1 dB of > > peaking at 65MHz, pulse response is about 5ns, with > > 10% overshoot. Something may be wrong, this is way > > way too good. OTOH, large-signal results, including > > full OFF to ON, and back, will be far slower. > > Dunno if anybody's following this aspect, but there's > a goal and path to follow. Today I was able to bench > verify a 10ns IXTP02N120 step response (5mA to 10mA),5-10 mA? what's the load? I imagine current sources for driving coils, TECs or laser diodes on the low end. George H.> but only if Vds >= 35 volts. My simple model ignores > Coss = 10pF and Crss = 2pF at 35V. But at 5 volts, > Coss = 70pF and Crss = 50pF, which slows the current > switching to 20 to 30ns or longer. > > Actually, my plan was to cascode the "signal" MOSFET > with a HV power part, running off the op-amp's Vcc > supply rail, so Vds may well be under 5 to 10 volts. > That needs to be a low-capacitance low-voltage part. > With the right parts we can keep the simple model. > > > -- > Thanks, > - Win
Reply by ●August 1, 20192019-08-01
George Herold wrote...> > 5-10 mA? what's the load? I imagine current sources > for driving coils, TECs or laser diodes on the low end.For micro-step charge pulses, piezo actuator scanning in charge mode. Precision pulses of current X time. 50mA or 250mA FS, etc. Low currents are the hardest. -- Thanks, - Win
