Short track and impedance matching

Klaus Vestergaard Kragelund · 2023-04-25T11:31:13+00:00

HiWe 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:"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 :-)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.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.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 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?RegardsKlaus