1 Period Oscillation through 1:1 Transformer - Page 1
Forum section: electronics / projects

author: Time
author_rank: Frequent Contributor
timestamp: February 14, 2014, 06:16:44 pm
content: Right now I am trying to design a circuit that discharges a series LC circuit through a 1:1 transformer. The idea is to just discharge one period cycle than stop.I was going to use a MOSFET and a diode in parallel with the series LC circuit and transformer primary.My question is related to the properties of the transformer. My knowledge on this subject lacks. I know I need to design the L and C so that they have the characteristic ringing frequency I need, in this case its ~2.5 MHz (400 ns period). The load on the 2ndary is a reactive 50 Ohm load.I was thinking of just doing the calculations based an initially charge cap in series with an an inductance and the xfrmr primary. I know I need to have a parallel magnetizing inductance at the primary. What other characteristics of the transformer are going to be involved in the circuit analysis? What do I need to know about the load on the 2ndary so that I can choose the right values of L and C on the primary to get the right oscillating conditions? Does just a purely reactive 50 Ohm load @ 2.5 MHz tell you enough?I could really use some good literature or conversation related to this issue.

author: dannyf
author_rank: Super Contributor
timestamp: February 14, 2014, 06:30:50 pm
content: Having a schematic would help.

author: dannyf
author_rank: Super Contributor
timestamp: February 14, 2014, 10:58:22 pm
content: QuoteHow does this thing work?

author: johansen
author_rank: Super Contributor
timestamp: February 14, 2014, 11:01:28 pm
content: you'll need another switch to short out the transformer, otherwise it will just ring down. you'll also need something to charge up the capacitor, depending on the repetition rate, this can be a resistor, or something more complex.

author: Time
author_rank: Frequent Contributor
timestamp: February 14, 2014, 11:49:58 pm
content: I didnt show the supply that charges the cap. It was just assumed. Sorry, I realize I left a lot of labels out.Just assume the cap is initially charged to some voltage and the Mosfet triggers for 200 ns. The load is an LC resonant circuit. I am wondering - do I even need the inductor on the primary side?

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: February 15, 2014, 05:06:36 am
content: Imperfect transformers differ from ideal ones in that flux leaks out of the magnetic circuit -- it interacts with one side of the transformer but not the other. So the rise and fall of the flux with changing current tends to act as a lossy inductor as seen from the electrical circuit. But generally, you just throw a resistor and inductor in series with the primary side of an 'ideal' transformer to simulate what happens with a 'real' one.

author: ejeffrey
author_rank: Super Contributor
timestamp: February 15, 2014, 05:53:23 am
content: Assuming the transformer is ideal and 1:1, it will act just like it wasn't there and the resistor were directly connected to the LC. Now you have a standard LCR circuit. You just need to pick L, C, R, so that you are approximately critically damped, or whatever behavior you actually want.Is the idea that you want to discharge the capacitor quickly, or to generate a certain shape of pulse? Is it ok if it rings a little bit as long as it is damping quickly? Do you need the ringing period to be exactly 200 ns so you cut it off at a zero crossing?

author: Time
author_rank: Frequent Contributor
timestamp: February 15, 2014, 10:05:18 pm
content: I just need the 2.5 MHz 1 period oscillation through the load. 400 ns total period, the circuit would have to be under damped. The idea was to turn the mosfet on for the first 200 ns so it can conduct current through the FET and than switch it off after. Than the circuit continues to ring through the diode for the half period of current in the reverse direction. Than the oscillation stops at one period since the MOSFET is no longer conducting. Than I would wait awhile, like 1 ms, and do it again. During that waiting time the power supply would recharge the capacitor. The charging resistance would be some what high. I am essentially creating these 2.5 MHz 1 period under damped oscillations as pulses.Does this make sense?Do I only need to discharge the capacitor into the primary and just take out the inductor?

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: February 15, 2014, 10:22:44 pm
content: Can you sketch out what you might want the current though the load to look like?

author: Time
author_rank: Frequent Contributor
timestamp: February 16, 2014, 12:01:15 am
content: Actually, I figured this out. It turns out the person asking me to solve this problem was not providing me enough information at first.

author: dannyf
author_rank: Super Contributor
timestamp: February 16, 2014, 01:11:19 pm
content: Sometimes, asking the simplest questions can help you understand it a lot better than just jumping into answering it.

author: ejeffrey
author_rank: Super Contributor
timestamp: February 16, 2014, 08:51:30 pm
content: OK, so it sounds like you have some resonator that you want to periodically excite with a single-cycle pulse, then let it ring-down naturally?Then I have a few notes:1) Resonant circuits can usually be directly excited by a square wave directly -- possibly directly from whatever pulse generator you are using as the trigger, or possibly with a buffer/level translator to get the voltage right. The resonance of ZL will reject the odd harmonics present in the square wave.2) This circuit may not work as drawn if the load impedance ZL is an LC resonator. The problem is that the oscillating voltage from the resonator will couple back through the transformer and turn on the catch diode. You need active switching for both phases to stop this.3) If you do need the LC resonator to generate a single cycle sine wave, you just need to work out the equations for an RLC resonator to be underdamped and at the right frequency. In this case, 'R' is the impedance of the load on resonance. The relevant equations are:damping ratio = R/(4*pi*f*L) -- choose a damping ratio (0.1 is a good start) to pick L.2*pi*f0 = 1/sqrt(LC) -- use to compute the inductance.The damping ratio of 0.1 basically means that you only discharge 10% of the capacitor each cycle. You may need to adjust this based on reasonable component values.

author: Time
author_rank: Frequent Contributor
timestamp: February 18, 2014, 02:16:48 am
content: Thanks everyone for your responses. What I didnt know was that the capacitor on the primary was to be chosen to ring with an inductor on the 2ndary side where the timing characteristics are determined by the respective L and C. There is no inductance on the primary side.Knowing that now, does the placement of the diode and switch make sense?