Playing with a switching circuit, yet I seem to have made the observation that these things are fantastic for linear. Infineon SPA07N60C3, but everyone has their line of SuperJunction MOSFETs. Circuit, for posterity: https://www.seventransistorlabs.com/Images/High%20Voltage%20Bridge.pdf Intended load is high impedance, capacitive. It can easily source 5A peak though. Since the load is capacitive, it operates in hard switching. The transistors start singing as soon as they get into the Miller plateau. Which in this circuit, I've intentionally exaggerated (27pF D-G), to help keep the transistor voltages matched. A word about SuperJunction transistors: Coss tanks by two decades, over the 5 to 20V range. Very nonlinear, brutal. This is fantastic for switching converters, because it "cushions" the switching edge, doing a better job of snubbing than an external network ever could. By pushing all the Miller effect to the low voltage end, switching loss can be very low. With stacked transistors, that works against me, because they'll probably be mismatched in the low-capacitance region. So the switching times, and voltages, probably won't be matched, forcing much more voltage across just one over-performing transistor. So I increase Miller capacitance, so the rise is slower, and more linear. And to protect against accidental turn-on or damage, due to opposite side hard-switching or output sparks, I put zener diodes on G-S. (Back-to-back pairs, since the drive is transformer coupled.) I think between the zeners and the Miller cap, I've got a particularly nasty loop that makes a wonderful oscillator. In the 200 to 400MHz region, depending on which transistor you ask. (Ferrite beads on the gate leads solves the oscillation, more or less.) I'm definitely going to try an RF amplifier with these, soon. I can't do very much power, because of thermal limitations, and bandwidth won't be fantastic because of the high load resistance versus Coss (note that the load resistance has to be high, i.e. the supply voltage high, and because of power limits, the current relatively low, to stay in the low-Coss range). The useful frequency range seemingly should be worthy of vacuum tubes, though! Assuming lead parasitics don't trash it first, which is likely. :) Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
These transistors sure do scream...
Started by ●February 20, 2017
Reply by ●February 20, 20172017-02-20
On 20/02/2017 10:31, Tim Williams wrote:> Circuit, for posterity: > https://www.seventransistorlabs.com/Images/High%20Voltage%20Bridge.pdfJust checking ... is the transformer phasing different for top three stages vs lower three stages? piglet
Reply by ●February 20, 20172017-02-20
On 20.2.17 15:43, piglet wrote:> On 20/02/2017 10:31, Tim Williams wrote: >> Circuit, for posterity: >> https://www.seventransistorlabs.com/Images/High%20Voltage%20Bridge.pdf > > Just checking ... is the transformer phasing different for top three > stages vs lower three stages? > > piglet >It is better to be - if you do not want the whole string being a dead short from +1500 to ground. -- -TV
Reply by ●February 20, 20172017-02-20
Tim Williams wrote...> >Circuit, for posterity: >https://www.seventransistorlabs.com/Images/High%20Voltage%20Bridge.pdfTell us about your gate transformers. What risetime and falltime are you observing? -- Thanks, - Win
Reply by ●February 20, 20172017-02-20
On Mon, 20 Feb 2017 04:31:07 -0600, "Tim Williams" <tiwill@seventransistorlabs.com> wrote:>Playing with a switching circuit, yet I seem to have made the observation >that these things are fantastic for linear. > >Infineon SPA07N60C3, but everyone has their line of SuperJunction MOSFETs. > >Circuit, for posterity: >https://www.seventransistorlabs.com/Images/High%20Voltage%20Bridge.pdf > >Intended load is high impedance, capacitive. It can easily source 5A peak >though. > >Since the load is capacitive, it operates in hard switching. The >transistors start singing as soon as they get into the Miller plateau. >Which in this circuit, I've intentionally exaggerated (27pF D-G), to help >keep the transistor voltages matched. > >A word about SuperJunction transistors: Coss tanks by two decades, over the >5 to 20V range. Very nonlinear, brutal. This is fantastic for switching >converters, because it "cushions" the switching edge, doing a better job of >snubbing than an external network ever could. By pushing all the Miller >effect to the low voltage end, switching loss can be very low. > >With stacked transistors, that works against me, because they'll probably be >mismatched in the low-capacitance region. So the switching times, and >voltages, probably won't be matched, forcing much more voltage across just >one over-performing transistor. > >So I increase Miller capacitance, so the rise is slower, and more linear. > >And to protect against accidental turn-on or damage, due to opposite side >hard-switching or output sparks, I put zener diodes on G-S. (Back-to-back >pairs, since the drive is transformer coupled.) > >I think between the zeners and the Miller cap, I've got a particularly nasty >loop that makes a wonderful oscillator. In the 200 to 400MHz region, >depending on which transistor you ask. > >(Ferrite beads on the gate leads solves the oscillation, more or less.) > >I'm definitely going to try an RF amplifier with these, soon. I can't do >very much power, because of thermal limitations, and bandwidth won't be >fantastic because of the high load resistance versus Coss (note that the >load resistance has to be high, i.e. the supply voltage high, and because of >power limits, the current relatively low, to stay in the low-Coss range). >The useful frequency range seemingly should be worthy of vacuum tubes, >though! Assuming lead parasitics don't trash it first, which is likely. :) > >TimWhat sort of switching speed do you need? -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●February 21, 20172017-02-21
"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message news:k74mac1bqfdmcima5nipujklu8445fej5o@4ax.com...> What sort of switching speed do you need?Pokey -- around a hundred nanoseconds. Intended application: gate drive V_cm testing. A fairly large swing, at reasonably high dV/dt (a few kV/us), and essentially capacitive load. It's doing about 7kV/us, which is nice, but I was hoping for >10. Maybe I'll drop the Miller caps to 10pF and see how she sings. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Reply by ●February 21, 20172017-02-21
"Winfield Hill" <hill@rowland.harvard.edu> wrote in message news:o8etdc0te3@drn.newsguy.com...> Tell us about your gate transformers. > What risetime and falltime are you observing?As Piglet noted, yes, the phasing is inverse for half of the windings. ;) The transformer impedance is pretty crappy (kind of intentionally), so the response is, uh, what was it... think the small-signal calculation was 3uH leakage per secondary, 33 ohms ESR and 2nF equivalent Cg. So whatever that works out to, as a LPF. Drive is a pair of TC4420, so the drive resistance is lowish, but not scary low. LL + R_G dominates. Gate waveform something like 200ns (full rise, about half being Miller plateau), output depends on supply voltage, but it's also about 200ns at the full 1500V supply (imagine that: constant drive current yields constant dV/dt? :) ). Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Reply by ●February 21, 20172017-02-21
On Tue, 21 Feb 2017 01:07:40 -0600, "Tim Williams" <tiwill@seventransistorlabs.com> wrote:>"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message >news:k74mac1bqfdmcima5nipujklu8445fej5o@4ax.com... >> What sort of switching speed do you need? > >Pokey -- around a hundred nanoseconds. > >Intended application: gate drive V_cm testing. A fairly large swing, at >reasonably high dV/dt (a few kV/us), and essentially capacitive load. > >It's doing about 7kV/us, which is nice, but I was hoping for >10. Maybe >I'll drop the Miller caps to 10pF and see how she sings. > >TimAnother way to get insane edge speeds is avalanche transistors. A modest stack of the Zetex SOT23 parts could switch a couple of KV in a couple of ns. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●February 21, 20172017-02-21
Tim Williams wrote...> >Circuit, for posterity: >https://www.seventransistorlabs.com/Images/High%20Voltage%20Bridge.pdfYou should have two sets of drivers and gate transformers, one each for high and low sides. Then you can create an adjustable deadtime. Another attractive idea: pos/neg pulsing to turn a set of MOSFETs on or off, with the gate capacitance storing the state. Combine fast on/off times with low switching frequencies. -- Thanks, - Win
Reply by ●February 21, 20172017-02-21
