Mosfet switching to transformer - Page 1
Forum section: electronics / beginners

author: yonyi
author_rank: Newbie
timestamp: December 02, 2024, 07:20:25 pm
content: I want to obtain a big signal with the help of a mosfet and a transformer.If I disconnect the transformer the signal Va has the values that I want (max value VCC) however when I connect the transformer Va it's almost zero, Why is the transformer messing up my signal? How can I solve this problem?

author: Harry_22
author_rank: Frequent Contributor
timestamp: December 02, 2024, 08:06:43 pm
content: You will not get a big signal because the transformer impedance is too low and not matched with MOSFET stage output.The input impedance of your transformer is Rds=(n1/n2)**2*R

author: yonyi
author_rank: Newbie
timestamp: December 02, 2024, 08:33:47 pm
content: Hmm.. then I don't understand this circuit. I was trying to simplify an inductive adder in order to understand its working

author: PGPG
author_rank: Frequent Contributor
timestamp: December 02, 2024, 08:43:30 pm
content: Here VC is a source of energy.In your circuit from VCC you will not get more current than I=VCC/100k so no more power than P=VCC*I= VCC²/100k (when whole power is dissipated in 100k resistor and 0 in rest of circuit).If VCC=5V it is only 0.00025W.

author: yonyi
author_rank: Newbie
timestamp: December 02, 2024, 09:12:15 pm
content: Ok, I can understand that the capacitor is the energy source but how is it charged? when the mosfet is activated the capacitor is discharged through the transformer, but when the mosfet is off I don't see how is it charged.

author: inse
author_rank: Super Contributor
timestamp: December 02, 2024, 09:31:14 pm
content: Other sources say the capacitor Vc is part of a precharged capacitor bank.Why don’t you start with something simpler?

author: yonyi
author_rank: Newbie
timestamp: December 03, 2024, 07:24:44 am
content: I totally agree with you, I started with the mosfet and transformer circuit without the capacitor bank but I didn't manage to obtain a signal with the first circuit of the post.To conclude this, any idea to adapt the stage output of the mostfet with the transformer?Thanks for your answers to my newbie post.

author: inse
author_rank: Super Contributor
timestamp: December 03, 2024, 09:46:03 am
content: Sorry, no further ideas, I just wonder whether nuclear physics instrumentation is a good starting point for a beginner…I‘d assume all the involved components are selected or designed for very specific parameters.

author: PGPG
author_rank: Frequent Contributor
timestamp: December 03, 2024, 10:45:51 am
content: Quote from: yonyi on December 03, 2024, 07:24:44 amJust add that capacitor. It will be slowly loaded by your 100k resistor so you will be able to use its voltage when transistor is switched on.

author: CaptDon
author_rank: Super Contributor
timestamp: December 03, 2024, 02:01:18 pm
content: First off, the physics example is simplified and incomplete. It doesn't show where the charge energy even came from. Your circuit would fail terribly even in simulation. How much energy did you think would transfer through a 100K resistor in series with a small voltage. There are better topologies for pulsing an inductor than that very poor physics example. If you put the physics example on a simulation it wouldn't work because no source of initial charge energy is shown for the VC capacitor. So many poor illustrations in books when taken 'as shown' without reading or understanding the associated text explaination.

author: yonyi
author_rank: Newbie
timestamp: December 03, 2024, 03:00:34 pm
content: Quote from: CaptDon on December 03, 2024, 02:01:18 pmThe 100k resistor was an example value in order to understand the circuit. You are right and I did not fully understand how it works, with respect to the topologies you mention, can you recommend where I can find out about these?

author: CaptDon
author_rank: Super Contributor
timestamp: December 03, 2024, 04:00:17 pm
content: The circuit as shown consists of two things, 1. It is a switch mode power supply 2. The individual power supplies are stacked in series for higher voltage but not more current than any one supply can produce. The drive pulse would have to be applied simultaneously to each of the supplies so that their outputs would be additive. What you are looking at is a series string of switched mode power supplies (SMPS) and about a dozen topologies exist depending on the need. There are flyback, half bridge, full bridge, switch forward, and modes like PWM, continuous, discontinuous. There is a world of SMPS design information available including complete evaluation circuit boards and schematics from the various semiconductor manufacturers. There is even design information from the magnetics manufacturers.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 05, 2024, 10:09:20 am
content: Your circuit will only pull current current though the primary on positive half cycles minus Vgson of the MOSFET.You can look at a transfomer as an energy transfer device.R1 is limiting the energy that can be stored in the primary to very low values.You need to drive the primary with a sinusoid which the single MOSFET cannot do.The solution is to use a linear amplifier (with a gain to suit) that can suply the necessary current to the transformer.At least Iload/n. A supply voltage that can accomodate input voltage swing is required.The amplifier can be single supply and its output capacitively coupled to the primary or directly connected with a dual supply amplfier.A TPA6211 or similar could be used for low power applications.