Interesting Balun Circuit Question - Page 1
Forum section: electronics / rf-microwave

author: caillou
author_rank: Newbie
timestamp: July 12, 2023, 11:02:17 pm
content: Hello all! I'm a student building a switch mode push-pull amplifier at around 13 MHz as a hobby circuit, and it should deliver > 500 watts. I want really low distortion at the output (<3% THD). I'm using two single ended amplifiers operating 180 degrees out of phase, which help with second harmonic cancellation at the output. However, I need a balun to go to a grounded output (50 ohms for now). Many balun topologies (for example the guanella current balun) lead to asymmetry and thus nullifies the purpose of using a push pull to cancel 2nd harmonics.I've tried using a voltage balun or a center tapped transformer but the input DC voltage I want the amplifier to operate at is several hundred volts, which means to get to 1 kW, I want a drain impedance that is relatively high, but that means I need a pretty large inductor in series with a capacitor (labeled Ls and Cs on the schematic) at each branch of the drain nodes of each single ended amplifier for narrowband filtering. At these frequencies, I want to use material 67 for the ferrite core since it's the least lossy at 13 MHz, but it's pretty difficult to achieve high inductance due to its low relative permeability. I could also add a series LC at the output, but the amount of current the inductor has to handle is relatively high. So I'm left with two options that are both suboptimal.So I was wondering if there is some balun topology that could simultaneously perform filtering, impedance matching, and maintain symmetry to keep 2nd harmonic relatively suppressed.I've attached my schematic on LTSpice below! Appreciate it

author: RFDx
author_rank: Regular Contributor
timestamp: July 13, 2023, 02:58:50 am
content: Looks like a (push-pull) Class-E amplifier. The max. allowed DS-voltage of the Wolfspeed MOSFETs doesn't seem to be high enough for a power supply of several hundred Volts. The drain voltage approaches almost 4x the applied DC voltage. Are L1/L2 or L4/L5 the DC chokes?A LC-balun can do (some) filtering and simultaneous matching. A simulation could be helpful to determine how much harmonic suppression you can expect from it.Another approach would be to design the amplifiers for 25 Ohm load impedance to be able to use a low loss 1:1 balun made of ferrite loaded coaxial cable. For additional filtering add a (symmetrical) lowpass filter behind the Ls/Cs resonant circuit or the balun instead of raising the Q-factor of Ls/Cs. With Q=4...5 the inductance is quite low as is the AC-voltage Ls/Cs must handle.Ls/Cs are a source for (phase) error. They would have to be identical in both arms. If put behind the balun, there would be only one Ls/Cs resonant circuit which can be easily augmented with additional filter elements to suppress the harmonics.

author: caillou
author_rank: Newbie
timestamp: July 13, 2023, 06:47:37 pm
content: It's a variation of the class E, but with the second harmonic shorted to reduce the peak voltage across the transistors, so in reality the transistors only experience about 2.2x the input voltage, which is why I selected 650 V SiC devices. L1 and L2 are DC chokes while L4 and L5 are part of the network with C2 to perform a second harmonic short. For the even harmonics, the center looks like an open so you can split the capacitor C2 into two.I would like a 50 Ohm output since I have a directional coupler that is 50 ohm matched and can be used for power measurements.I have two designs so far that I would like to improve (see schem).1) an LC balun + low pass impedance matching filter + two arms that have Ls/Cs-this is a lot of passive components and I'd like to somehow synthesize the output network to reduce part count. Also, for a reason unknown to me at the moment, putting Ls/Cs after the balun causes my circuit to break down completely (L4 and L5 see over several kA)2) a 1:1 voltage balun + low pass impedance matching filter + single output Ls/Cs-Ls/Cs at the output have to see 5 A RMS, which can get pretty lossy.-I could move the low pass impedance matching filter to the output as well, but then like the output Ls/Cs, the current through the series inductor would be really big. Also, THD goes up (due to more asymmetry?) if I do this (to about 4.5%)The THD of both designs are pretty similar (~3.7%). If I can't synthesize the first network to something simpler, I'm leaning toward going with the latter option. Otherwise, tuning will be a nightmare. I'm not too familiar with coax baluns. Could I make a voltage coax balun? What core should I use for 6.78 MHz/13.56 MHz? Will interwinding capacitance affect my results significantly at these frequencies?

author: mtwieg
author_rank: Frequent Contributor
timestamp: July 16, 2023, 06:51:19 pm
content: Quote from: caillou on July 13, 2023, 06:47:37 pmI'm guessing your circuit is something like a voltage-mode class D (drain voltage approximately a square wave, drain current is approximately sinusoidal). Or maybe your switching your FETs with duty cycle much less than 50%?It's unclear which harmonics are giving you trouble. Can you attach the actual simulation files?I've built a lot of switchmode class D amplifiers from 5MHz up to 150MHz (usually current-mode above 10MHz), maybe some general observations will help:For voltage-mode types, it's helpful to couple the drain chokes together on the same core, especially if you're using a ferrite core (since coupling them together means the flux will swing around zero rather than some nonzero bias). You can add another winding on the same core for your output, so there you get a balun for free.At high frequencies I observed that using a simple transformer as a balun gave rise to even harmonics, even if the transformer itself was perfectly symmetric, and everything on the balanced side was perfectly symmetric. The reason was the parasitic capacitance between the primary and secondary of the transformer. Even if that capacitance is also distributed symmetrically, once you ground one side of the secondary (like what you're doing) the balanced primary side will "see" a bit of the unbalanced secondary side. This can be mitigated somewhat by adding a bit of capacitance to one side of the primary (not perfect though). At >50MHz this was generally my main source of even harmonics.If odd harmonics are your main problem, consider using a current-mode class D circuit instead. The drain-to-drain voltage waveform is close to sinusoidal, which means your starting odd harmonics are lower than VMCD. The peak drain voltage will be higher, so you will need to adjust your impedance transformation to get the desired output power. But in general CMCD has better power efficiency than VMCD as frequency increases since it switches at near zero voltage (13MHz is sort of in the transition region where both can work well, if you choose your FETs well).

author: RFDx
author_rank: Regular Contributor
timestamp: July 21, 2023, 08:11:58 am
content: Quote from: caillou on July 13, 2023, 06:47:37 pmDoes shorting the second harmonic to ground have an impact on the efficiency of the power amplifier?Quote from: caillou on July 13, 2023, 06:47:37 pmA few ideas:- Remove the matching network C13/C14/L22/L23 and use the LC-balun to match to 50 Ohm output impedance.- Instead of using 2 DC-chockes use only one choke connected to C2.- Use air coils for L4/L5 implemented as air coils so you don't have to bother with saturation.According to C13/C14/L22/L23, the differential load for the amplifier is 1kOhm. This is unusually high.After making changes, always check if the efficiency changes.Quote from: caillou on July 13, 2023, 06:47:37 pmHigh currents and voltages are normal with high output power, especially if there are high-Q resonant circuits involved. Moving the balun to the left isn't such a good idea with the amplifier needing a 1kOhm load.Quote from: caillou on July 13, 2023, 06:47:37 pmThere are DC-blocking broadband (voltage) baluns made with coax but you are, depending on the impedance of the coax used, constrained to very specific port impedances.NiZn ferrite material with an initial permability of ~800 (e.g. Fair-Rite 43, Würth 3W800, Epcos-TDK K10) would be suitable for 6.78/13.56MHz. Make sure the flux density doesn't exceed ~44 Gauß @ 13.56MHz or ~70 Gauß @ 6.78MHz.