Designing class E amplifier - Page 1
Forum section: electronics / projects

author: guusl
author_rank: Newbie
timestamp: July 13, 2016, 03:52:32 pm
content: Hi,I'm trying to design a class E amplifier for wireless power. I found a paper which does exactly what I want:http://www.jiep.or.jp/hp_e/journal/6/transaction6_1_32-37.pdf. I simulated the circuit mentioned in this paper (fig 1) and everything works like it should. Except when I try to build it, it doesn't seem to work properly.This class E amplifier works by putting a 240kHz square wave on the gate of the MOSFET (IRF640). The combination of a 54nF (C2) capacitor and 18uH (L2) inductor should filter the output to a pure sine wave of 240kHz. L1 is 470uH, C1 is 16nF and Vdd is 20V.I've now built it 3 times with different pcb layouts (each time minimizing inductive coupling). The MOSFET becomes extremely hot, which should not happen. And the measured waveforms do not correspond with the simulations (see scope image, 1: input, 2: output over R and 3: drain voltage). I've tried to simulate it by measuring each component and putting non-ideal values in the simulation but even then, according to the simulation, it should be ok.Does anyone have any idea what might be going on here or how I might get this to work?

author: Marco
author_rank: Super Contributor
timestamp: July 13, 2016, 04:08:44 pm
content: Clearly the time constants are way way off.The MOSFET is hot because C1 is charged when it turns on, preventing zero voltage switching.I see an inductor going directly from the drain of the MOSFET to the resistors, your PCB doesn't correspond to your schematic (C1/C2 names are swapped on the PCB from the description too, you like to make things interesting? :). Not sure what that 100 uF is doing across the resistors either, that's certainly not supposed to be there. You should put it on the supply input instead (not that it matters a lot at these frequencies).

author: guusl
author_rank: Newbie
timestamp: July 13, 2016, 06:26:30 pm
content: Thanks for your reply!I should probably clarify the pcb somewhat. The 100uF capacitor is indeed coupled to Vdd, the blocks on the pcb are all capacitors. The load is a large 10 ohm power resistor that is not on the pcb. I've adjusted some names on the pcb and now included the Eagle schematic, so schematic and pcb should now match.

author: Marco
author_rank: Super Contributor
timestamp: July 13, 2016, 07:03:42 pm
content: Oops, completely misinterpreted your PCB ... yeah, then I have no clue why the time constants are so far off.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 13, 2016, 09:48:55 pm
content: Welcome to the wonderful, weird world of RF power. You might find the below website useful:http://people.physics.anu.edu.au/~dxt103/calculators/class-e.php

author: qwaarjet
author_rank: Regular Contributor
timestamp: July 14, 2016, 04:32:50 am
content: another fun thing, if you are probing the gate of the fet your probe is adding an order of magnitude more capacitance than the fet alone, that your clock has to drive wich can cause issues. though at 250khz you might be ok.

author: Marco
author_rank: Super Contributor
timestamp: July 14, 2016, 09:01:18 am
content: The probed gate signal still has edges more than steep enough, the problem is that the circuit's oscillation when the MOSFET is off isn't even a quarter cycle while it should be a half cycle.Even if he used a 1x probe he couldn't add enough capacitance at the drain to screw up the time constant that much.

author: bktemp
author_rank: Super Contributor
timestamp: July 14, 2016, 09:55:52 am
content: I tried simulating the circuit and it works fine. If I disconnect or increase the output resistor from 10ohms to 100ohms the waveform looks pretty much identical to your measurements.My guess would be the load resistor is not connected properly or has a large parasitic inductance.

author: guusl
author_rank: Newbie
timestamp: July 16, 2016, 01:29:27 pm
content: The power resistor I was using was indeed way too inductive. I switched it with another and it is now working perfectly. Thank you all for answering!