1. When the delay time is greater than the rise time then impedance mismatches between the line and either end will create reflections. 2. When matched to the line at one end only, it is best to match the source. ( e.g. CMOS 33 ohms 1ns drivers to 200 ohm cable, add the difference to source to eliminate overshoot. ) 3. When delay is less than rise time, the delay will further attenuate signals with 1/4 wave cyclic ripple. If you seriously want to match impedances to maximize rise time you would have a very low impedance (=driver or load) 3 ohm transmission line which would radiate more drive current and cause more crosstalk issues. https://ufile.io/2g1og0o3 Falstad simulation in freq domain A 50 Ohm line is pretty easy to choose with aq trace width/Gap to ground plane of 2/1 for FR4 and easier with multi layer thin laminate Gap.
Short track and impedance matching
Started by ●April 25, 2023
Reply by ●April 25, 20232023-04-25
Reply by ●April 25, 20232023-04-25
On 25-04-2023 17:56, Ricky wrote:> On Tuesday, April 25, 2023 at 11:24:11 AM UTC-4, Klaus Vestergaard Kragelund wrote: >> On 25-04-2023 16:31, John Larkin wrote: >>> On Tue, 25 Apr 2023 13:31:13 +0200, Klaus Vestergaard Kragelund >>> <klau...@hotmail.com> wrote: >>> >>>> Hi >>>> >>>> We have a GaN FET that is dumping voltage into a capacitor. The GaN >>>> drain is connected to 50V, and the capacitor is connected on the source >>>> of the FET. Sort of like this: >>>> >>>> https://www.infineon.com/export/sites/default/_images/product/power/GateDriver/circuit_diagram/Single-High-Side-Driver_Diagram.jpg_1153011823.jpg >>>> >>>> "To load" is connected to a small 50pF capacitor to ground. >>>> >>>> We need to pump current into it very fast for reasons I cannot disclose. >>>> So we turn the high side FET on in about 500ps, and charge the >>>> capacitor. Pretty basic :-) >>> >>> Where do you get a gate driver that can turn on a GaN fet in 500 ps? I >>> need that! A little inductance in series with the gate could help >>> some. The gate resistor will slow things down. >>> >> LMG1020 >> >> https://www.ti.com/product/LMG1020 >> >> 2 ohms gate resistor >>> Actually, I need to drive a capacitive load, an optical thing, to >>> maybe 25 volts in 1 ns. Up and down! At 100 MHz! >> It's going to have a lot of charge losses, depending on the parasitics >> of course ;-) >>> >>> I can't disclose the application. Even I'm not allowed to know the >>> application. Something related to war, maybe. >>> >>> I like to use the small EPC parts, the tiny 4-ball things, for stuff >>> like this, but I;ve never seen, in real life, the sorts of speeds I >>> see in Spice. >>> >>> >>>> >>>> We need maximum current into the capacitor, so we are looking to >>>> reducing parasitics and losses. >>>> >>>> One guy on the team is worried about loss and reflections, treating the >>>> line from the supply, through the FET and going to the capacitor to >>>> ground as a transmission line. Wanting to use best possible PCB material >>>> for low loss (Rogers 4350B), and using ADS to simulate in order to >>>> optimize the design. He also wants to do it with matched 50ohms >>>> impedance all the way. >>> >>> The PCB dielectric won't matter. The part and trace parasitics sure >>> will. >>> >>>> >>>> For a 500ps pulse, the equivalent bandwidth is 700MHz. Propagation speed >>>> is 150E6 m/s in FR4, so wavelength is 21cm. Normal rule is that >>>> transmission line impedance comes into play at 1/6 of the wavelength, so >>>> that's 35mm. So we just need to keep the distance from the decoupling >>>> caps down to the FET and capacitor path less than 35mm. >>>> >>>> The path length is about 10mm. >>> >>> Envision copper pours, lots of paralleled caps, and very short traces. >>> >>> Inductive peaking of the load could help. >>> >>> >>> >>>> >>>> About losses, if I compare standard FR4 to Rogers 4350B (good material), >>>> I get 0.06dB/in loss for FR4, and 0.01dB/in for 4350B. So really no loss >>>> at 10mm path length >>>> >>>> https://www.intel.com/content/www/us/en/docs/programmable/683624/current/loss-tangent-and-dissipation-factor.html >>>> >>>> Additionally, if I use Saturn calculator, for a 2mm wide trace of 10mm. >>>> I get 1.5mOhms. Skin depth is 2.5um (7%) on a 35um copper trace, so I >>>> expect ac resistance to be about 20mOhm (1.5mOhm/7%). Still very low >>>> resistance. >>>> >>>> I am used to do SMPS design, in which we use wide traces, big ground >>>> plane and place components tight so reflections matter less (the return >>>> time is less than the rising edge of the waveform). Also, I never match >>>> impedances for traces. >>>> >>>> What would your take be? >>> >>> 50 ohms and 50 pF is a time constant of 2.5 ns. So forget making >>> anything 50 ohms. >>> >>> Your "50 ohm" guy is driven by convention and not thinking. That >>> happens a lot. >>> >>> >> Yeah, my opinion also. Just wanted to hear other input before getting >> into the nitty gritty with him. He's a good guy, just a little too >> focused on impedance matching > > I agree. I've only ever seen impedance matching in signal propagation, not power distribution. In power distribution, it's more a matter of reducing losses and resonances. However, one of the classic mistakes in PDS is assuming you need to treat each power cap as a discrete device, connected to the chip with wires. So people focus on "loop impedance" and such. In reality, the power planes form a transmission line, supplying current to/from the capacitor with distance from the chip being a very small factor. In fact, power planes form resonant structures based on the distances to the edges where reflections occur. >Yes, and placement of the capacitor does not need to be right at the IC. Feranec has some great videos on the subject: https://www.youtube.com/watch?v=XumNc480qYo&t=0s> But yeah, there's no real math to show treating your 2 x 10 mm connection needs to be analyzed as a transmission line. The point of a transmission line would be to match impedances. What is the impedance of your capacitor? Wouldn't you want to match that to the transmission line, when doing such an analysis? I can't imagine why 50 ohms would be the magic number. > > This is a case where the loop inductance would be significant, and that means the path from the decoupling cap on the FET drain to the load capacitor. After all, that's where the power is going to come from... that, and the power planes. > > Why 10 mm? Is that as close as you can get them?Yes, that is the total path, including some components in series to limit peak currents to protect the FET. I wrote to EPC, and even though the peak currents are for sub ns, they recommended to not have the peak current more than 2-3 times the rated current of the FET
Reply by ●April 25, 20232023-04-25
On Tue, 25 Apr 2023 17:24:04 +0200, Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:>On 25-04-2023 16:31, John Larkin wrote: >> On Tue, 25 Apr 2023 13:31:13 +0200, Klaus Vestergaard Kragelund >> <klauskvik@hotmail.com> wrote: >> >>> Hi >>> >>> We have a GaN FET that is dumping voltage into a capacitor. The GaN >>> drain is connected to 50V, and the capacitor is connected on the source >>> of the FET. Sort of like this: >>> >>> https://www.infineon.com/export/sites/default/_images/product/power/GateDriver/circuit_diagram/Single-High-Side-Driver_Diagram.jpg_1153011823.jpg >>> >>> "To load" is connected to a small 50pF capacitor to ground. >>> >>> We need to pump current into it very fast for reasons I cannot disclose. >>> So we turn the high side FET on in about 500ps, and charge the >>> capacitor. Pretty basic :-) >> >> Where do you get a gate driver that can turn on a GaN fet in 500 ps? I >> need that! A little inductance in series with the gate could help >> some. The gate resistor will slow things down. >> > >LMG1020 > >https://www.ti.com/product/LMG1020 > >2 ohms gate resistorNice part. Do you even need a GaN fet?
Reply by ●April 25, 20232023-04-25
On Tue, 25 Apr 2023 17:55:29 -0700, John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:>On Tue, 25 Apr 2023 17:24:04 +0200, Klaus Vestergaard Kragelund ><klauskvik@hotmail.com> wrote: > >>On 25-04-2023 16:31, John Larkin wrote: >>> On Tue, 25 Apr 2023 13:31:13 +0200, Klaus Vestergaard Kragelund >>> <klauskvik@hotmail.com> wrote: >>> >>>> Hi >>>> >>>> We have a GaN FET that is dumping voltage into a capacitor. The GaN >>>> drain is connected to 50V, and the capacitor is connected on the source >>>> of the FET. Sort of like this: >>>> >>>> https://www.infineon.com/export/sites/default/_images/product/power/GateDriver/circuit_diagram/Single-High-Side-Driver_Diagram.jpg_1153011823.jpg >>>> >>>> "To load" is connected to a small 50pF capacitor to ground. >>>> >>>> We need to pump current into it very fast for reasons I cannot disclose. >>>> So we turn the high side FET on in about 500ps, and charge the >>>> capacitor. Pretty basic :-) >>> >>> Where do you get a gate driver that can turn on a GaN fet in 500 ps? I >>> need that! A little inductance in series with the gate could help >>> some. The gate resistor will slow things down. >>> >> >>LMG1020 >> >>https://www.ti.com/product/LMG1020 >> >>2 ohms gate resistor > >Nice part. Do you even need a GaN fet?Shame it's a single 5-volt driver, not one of those bootstrap half-bridge drivers. One might drive a GaN fet half bridge with two of those. One would be grounded, but the other would have to float on the source of the upper fet. Getting power up there would be no big deal, but the logic drive would be interesting.
Reply by ●April 26, 20232023-04-26
On 26-04-2023 03:05, John Larkin wrote:> On Tue, 25 Apr 2023 17:55:29 -0700, John Larkin > <jlarkin@highlandSNIPMEtechnology.com> wrote: > >> On Tue, 25 Apr 2023 17:24:04 +0200, Klaus Vestergaard Kragelund >> <klauskvik@hotmail.com> wrote: >> >>> On 25-04-2023 16:31, John Larkin wrote: >>>> On Tue, 25 Apr 2023 13:31:13 +0200, Klaus Vestergaard Kragelund >>>> <klauskvik@hotmail.com> wrote: >>>> >>>>> Hi >>>>> >>>>> We have a GaN FET that is dumping voltage into a capacitor. The GaN >>>>> drain is connected to 50V, and the capacitor is connected on the source >>>>> of the FET. Sort of like this: >>>>> >>>>> https://www.infineon.com/export/sites/default/_images/product/power/GateDriver/circuit_diagram/Single-High-Side-Driver_Diagram.jpg_1153011823.jpg >>>>> >>>>> "To load" is connected to a small 50pF capacitor to ground. >>>>> >>>>> We need to pump current into it very fast for reasons I cannot disclose. >>>>> So we turn the high side FET on in about 500ps, and charge the >>>>> capacitor. Pretty basic :-) >>>> >>>> Where do you get a gate driver that can turn on a GaN fet in 500 ps? I >>>> need that! A little inductance in series with the gate could help >>>> some. The gate resistor will slow things down. >>>> >>> >>> LMG1020 >>> >>> https://www.ti.com/product/LMG1020 >>> >>> 2 ohms gate resistor >> >> Nice part. Do you even need a GaN fet? > > Shame it's a single 5-volt driver, not one of those bootstrap > half-bridge drivers. > > One might drive a GaN fet half bridge with two of those. One would be > grounded, but the other would have to float on the source of the upper > fet. Getting power up there would be no big deal, but the logic drive > would be interesting. >In our design we use a digital opto to do the levelshift. Downside is that the switch node goes to several components so has some capacitance. One option is to use the LMG1210 which is a high side driver, has a little slower switching speed however.
Reply by ●April 26, 20232023-04-26
On 26-04-2023 02:55, John Larkin wrote:> On Tue, 25 Apr 2023 17:24:04 +0200, Klaus Vestergaard Kragelund > <klauskvik@hotmail.com> wrote: > >> On 25-04-2023 16:31, John Larkin wrote: >>> On Tue, 25 Apr 2023 13:31:13 +0200, Klaus Vestergaard Kragelund >>> <klauskvik@hotmail.com> wrote: >>> >>>> Hi >>>> >>>> We have a GaN FET that is dumping voltage into a capacitor. The GaN >>>> drain is connected to 50V, and the capacitor is connected on the source >>>> of the FET. Sort of like this: >>>> >>>> https://www.infineon.com/export/sites/default/_images/product/power/GateDriver/circuit_diagram/Single-High-Side-Driver_Diagram.jpg_1153011823.jpg >>>> >>>> "To load" is connected to a small 50pF capacitor to ground. >>>> >>>> We need to pump current into it very fast for reasons I cannot disclose. >>>> So we turn the high side FET on in about 500ps, and charge the >>>> capacitor. Pretty basic :-) >>> >>> Where do you get a gate driver that can turn on a GaN fet in 500 ps? I >>> need that! A little inductance in series with the gate could help >>> some. The gate resistor will slow things down. >>> >> >> LMG1020 >> >> https://www.ti.com/product/LMG1020 >> >> 2 ohms gate resistor > > Nice part. Do you even need a GaN fet? >GaNs are several times faster than standard FETs
Reply by ●April 26, 20232023-04-26
On Wed, 26 Apr 2023 14:46:53 +0200, Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:>On 26-04-2023 03:05, John Larkin wrote: >> On Tue, 25 Apr 2023 17:55:29 -0700, John Larkin >> <jlarkin@highlandSNIPMEtechnology.com> wrote: >> >>> On Tue, 25 Apr 2023 17:24:04 +0200, Klaus Vestergaard Kragelund >>> <klauskvik@hotmail.com> wrote: >>> >>>> On 25-04-2023 16:31, John Larkin wrote: >>>>> On Tue, 25 Apr 2023 13:31:13 +0200, Klaus Vestergaard Kragelund >>>>> <klauskvik@hotmail.com> wrote: >>>>> >>>>>> Hi >>>>>> >>>>>> We have a GaN FET that is dumping voltage into a capacitor. The GaN >>>>>> drain is connected to 50V, and the capacitor is connected on the source >>>>>> of the FET. Sort of like this: >>>>>> >>>>>> https://www.infineon.com/export/sites/default/_images/product/power/GateDriver/circuit_diagram/Single-High-Side-Driver_Diagram.jpg_1153011823.jpg >>>>>> >>>>>> "To load" is connected to a small 50pF capacitor to ground. >>>>>> >>>>>> We need to pump current into it very fast for reasons I cannot disclose. >>>>>> So we turn the high side FET on in about 500ps, and charge the >>>>>> capacitor. Pretty basic :-) >>>>> >>>>> Where do you get a gate driver that can turn on a GaN fet in 500 ps? I >>>>> need that! A little inductance in series with the gate could help >>>>> some. The gate resistor will slow things down. >>>>> >>>> >>>> LMG1020 >>>> >>>> https://www.ti.com/product/LMG1020 >>>> >>>> 2 ohms gate resistor >>> >>> Nice part. Do you even need a GaN fet? >> >> Shame it's a single 5-volt driver, not one of those bootstrap >> half-bridge drivers. >> >> One might drive a GaN fet half bridge with two of those. One would be >> grounded, but the other would have to float on the source of the upper >> fet. Getting power up there would be no big deal, but the logic drive >> would be interesting. >> > >In our design we use a digital opto to do the levelshift. Downside is >that the switch node goes to several components so has some capacitance. > >One option is to use the LMG1210 which is a high side driver, has a >little slower switching speed however.Nice but slow. I did my own output stage for our digital delay generators, a half-bridge with GaN fets. I did it as a throwaway mouse-bite PCB subassembly because the tiny EPC parts are so fragile and hard to rework. https://www.dropbox.com/sh/h2mvej0ss02u4pr/AAD5nYsNsGq0pRhwMZSzQHOca?dl=0 I'd like to do something faster and higher voltage.
Reply by ●April 26, 20232023-04-26
On Wed, 26 Apr 2023 14:47:26 +0200, Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:>On 26-04-2023 02:55, John Larkin wrote: >> On Tue, 25 Apr 2023 17:24:04 +0200, Klaus Vestergaard Kragelund >> <klauskvik@hotmail.com> wrote: >> >>> On 25-04-2023 16:31, John Larkin wrote: >>>> On Tue, 25 Apr 2023 13:31:13 +0200, Klaus Vestergaard Kragelund >>>> <klauskvik@hotmail.com> wrote: >>>> >>>>> Hi >>>>> >>>>> We have a GaN FET that is dumping voltage into a capacitor. The GaN >>>>> drain is connected to 50V, and the capacitor is connected on the source >>>>> of the FET. Sort of like this: >>>>> >>>>> https://www.infineon.com/export/sites/default/_images/product/power/GateDriver/circuit_diagram/Single-High-Side-Driver_Diagram.jpg_1153011823.jpg >>>>> >>>>> "To load" is connected to a small 50pF capacitor to ground. >>>>> >>>>> We need to pump current into it very fast for reasons I cannot disclose. >>>>> So we turn the high side FET on in about 500ps, and charge the >>>>> capacitor. Pretty basic :-) >>>> >>>> Where do you get a gate driver that can turn on a GaN fet in 500 ps? I >>>> need that! A little inductance in series with the gate could help >>>> some. The gate resistor will slow things down. >>>> >>> >>> LMG1020 >>> >>> https://www.ti.com/product/LMG1020 >>> >>> 2 ohms gate resistor >> >> Nice part. Do you even need a GaN fet? >> >GaNs are several times faster than standard FETsI meant, can you just use the TI driver? Maybe you need more voltage into your cap than 5 volts.
Reply by ●April 26, 20232023-04-26
On Wed, 26 Apr 2023 08:41:42 -0700, John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:>On Wed, 26 Apr 2023 14:47:26 +0200, Klaus Vestergaard Kragelund ><klauskvik@hotmail.com> wrote: > >>On 26-04-2023 02:55, John Larkin wrote: >>> On Tue, 25 Apr 2023 17:24:04 +0200, Klaus Vestergaard Kragelund >>> <klauskvik@hotmail.com> wrote: >>> >>>> On 25-04-2023 16:31, John Larkin wrote: >>>>> On Tue, 25 Apr 2023 13:31:13 +0200, Klaus Vestergaard Kragelund >>>>> <klauskvik@hotmail.com> wrote: >>>>> >>>>>> Hi >>>>>> >>>>>> We have a GaN FET that is dumping voltage into a capacitor. The GaN >>>>>> drain is connected to 50V, and the capacitor is connected on the source >>>>>> of the FET. Sort of like this: >>>>>> >>>>>> https://www.infineon.com/export/sites/default/_images/product/power/GateDriver/circuit_diagram/Single-High-Side-Driver_Diagram.jpg_1153011823.jpg >>>>>> >>>>>> "To load" is connected to a small 50pF capacitor to ground. >>>>>> >>>>>> We need to pump current into it very fast for reasons I cannot disclose. >>>>>> So we turn the high side FET on in about 500ps, and charge the >>>>>> capacitor. Pretty basic :-) >>>>> >>>>> Where do you get a gate driver that can turn on a GaN fet in 500 ps? I >>>>> need that! A little inductance in series with the gate could help >>>>> some. The gate resistor will slow things down. >>>>> >>>> >>>> LMG1020 >>>> >>>> https://www.ti.com/product/LMG1020 >>>> >>>> 2 ohms gate resistor >>> >>> Nice part. Do you even need a GaN fet? >>> >>GaNs are several times faster than standard FETs > >I meant, can you just use the TI driver? Maybe you need more voltage >into your cap than 5 volts. > >Maybe you only need to use the lower, the pull-down, drain. Maybe it's good for more than 5 volts!
Reply by ●April 26, 20232023-04-26
On 26-04-2023 17:41, John Larkin wrote:> On Wed, 26 Apr 2023 14:47:26 +0200, Klaus Vestergaard Kragelund > <klauskvik@hotmail.com> wrote: > >> On 26-04-2023 02:55, John Larkin wrote: >>> On Tue, 25 Apr 2023 17:24:04 +0200, Klaus Vestergaard Kragelund >>> <klauskvik@hotmail.com> wrote: >>> >>>> On 25-04-2023 16:31, John Larkin wrote: >>>>> On Tue, 25 Apr 2023 13:31:13 +0200, Klaus Vestergaard Kragelund >>>>> <klauskvik@hotmail.com> wrote: >>>>> >>>>>> Hi >>>>>> >>>>>> We have a GaN FET that is dumping voltage into a capacitor. The GaN >>>>>> drain is connected to 50V, and the capacitor is connected on the source >>>>>> of the FET. Sort of like this: >>>>>> >>>>>> https://www.infineon.com/export/sites/default/_images/product/power/GateDriver/circuit_diagram/Single-High-Side-Driver_Diagram.jpg_1153011823.jpg >>>>>> >>>>>> "To load" is connected to a small 50pF capacitor to ground. >>>>>> >>>>>> We need to pump current into it very fast for reasons I cannot disclose. >>>>>> So we turn the high side FET on in about 500ps, and charge the >>>>>> capacitor. Pretty basic :-) >>>>> >>>>> Where do you get a gate driver that can turn on a GaN fet in 500 ps? I >>>>> need that! A little inductance in series with the gate could help >>>>> some. The gate resistor will slow things down. >>>>> >>>> >>>> LMG1020 >>>> >>>> https://www.ti.com/product/LMG1020 >>>> >>>> 2 ohms gate resistor >>> >>> Nice part. Do you even need a GaN fet? >>> >> GaNs are several times faster than standard FETs > > I meant, can you just use the TI driver? Maybe you need more voltage > into your cap than 5 volts. >I see what you mean. I need up to 50V into the capacitor.
