200khz pushpull boost converter

The above image is showing the state where the Q1 is turned on and the Q2 will turn off. Thus the current will flow through the center tap of the transformer and will go to the ground via the transistor Q1 while the Q2 will block the current flow on the other tap of the transformer. Exactly the opposite thing happens when the Q2 turns on and Q1 remains turned off.

Whenever the changes in the current flow occur, the transformer transfers the energy from the primary side to the secondary side. The above graph is very useful to check how this happens, at first, there used to be no voltages or current flow in the circuit.

Q1 turned on, a constant voltage first strike to the tap as the circuit is closed now. The current starts to increase and then the voltage is induced into the secondary side. In the next phase, after a time delay, the transistor Q1 turns off and Q2 is turned on. Here comes a few important things at work - transformer parasitic capacitance and the inductance forms an LC circuit that starts switching in opposite polarity.

The charge starts to flow back in the opposite direction through the other tap winding of the transformer. In this fashion, the current is constantly pushed in alternate modes by those two transistors. However, as the pulling is done by the LC circuit and the center tap of the transformer, it is called push-pull topology. Often it is described in such a way that the two transistors push the current alternately naming the convention push-pull where transistors do not pull the current.

The load waveform looks like the sawtooth, however, it is not that is shown in the above waveform. As we have learned how a push-pull converter design works, let's move on to building an actual circuit for it, and then we can analyze that on the bench. But before that, let's take a look at the schematic. Well, the below circuit is constructed on a breadboard.

The components used for testing circuits are as follows-. The schematic is pretty straight forward. Let's analyze the connection, the ULN is the Darlington pair transistor array. This Transistor array is useful as the freewheeling diodes are available inside the chipset and it does not require any additional components thus avoiding any additional complex routing on a breadboard. For the synchronous driver, we are using a simple RC timer that will synchronously turn on and off the transistors to create a push-pull effect across the Inductors.

The working of the circuit is simple. The RC networks are connected in a cross position with the base of Q1 and Q2, which turn on the alternate transistors using a feedback technique called regenerative feedback. It starts operating like this - When we apply voltage to the center tap of the transformer where the common connection between two inductors , the current will flow through the transformer.

Depending on the flux density and saturation of the polarity, negative or positive, the current first charges up C1 and R1 or C2 and R2, not both. Let's imagine C1 and R1 get the current first. The C1 and R1 provide a timer which turns on the transistor Q2. The L2 section of the transformer will induce voltage using the magnetic flux. In this situation, the C2 and R2 start to charge up and turn on the Q1.

The L1 section of the transformer then induces a voltage. The timing or the frequency is entirely dependent on the input voltage, the saturated flux of the transformer or inductor, the primary turns, cross-sectional square centimeter area of the core.

The formula of the frequency is-. The circuit is constructed in a breadboard and the power is slowly increased. The input voltage is 2. However, this circuit does not use any feedback topology, so the output voltage is not constant and nor isolated.

The frequency and the switching of the push-pull is observed in the oscilloscope-. Thus the circuit is now acting as a push-pull boost converter where the output voltage is not constant.

It is expected that this push-pull converter could provide wattage up to 2W, but we have not tested it due to the lack of feedback generation. This circuit is a simple form of the push-pull converter. However, it is always recommended to use a proper push-pull driver IC for the desired output. The circuit can be constructed in a manner where isolated or non-isolated, any topologies in push-pull conversion can be built. I selected a switching frequency of 20kHz.

By my math, I need a duty cycle of 0. Assuming an efficiency of 0. What I expected to see is a voltage of 5V, perhaps with a slight ripple, at point 2 between the inductor and the NMOS and a voltage of 12V with a ripple at point 3 between the diode and the capacitor.

Instead, what comes out is what looks like total chaos -- I get a peak voltage of 23V that oscillates around There was significant ripple in the voltage. Your boost is operating in discontinuous conduction mode or DCM inductor current goes to zero each switching cycle. The duty cycle becomes a function of load as well as the duty cycle. If you increase the load, the inductor value, or switching frequency, you'll reach a point where you'll see your regulation where you expect it - this is called CCM, or continuous conduction mode.

The inductor current doesn't fall to zero, but continuously flows. Your duty cycle formula will be valid here.

Most PFC boost converters operate from 70 to kHz. Lower frequency converters generally need larger inductors. If you want to achieve CCM at 20kHz, you'll need a much larger boost inductance value.

Try uH in your simulation and you'll see the voltage closer to 12V. Because your converter is so heavily into DCM, the switching node voltage resembles the output voltage. If you get closer to CCM, you'll see a clearer picture. For this simulation, the capacitor is sized such that the switch on-time voltage sag caused by the load isn't excessive.

In real life, there are other parameters that matter overall loop stability, ripple current and life rating that you must consider, along with proper MOSFET choice, reverse recovery and softness of the boost diode With the components values that you have selected it is indeed more suitable to run with the kHz frequency. Even at kHz I find that a more suitable output capacitor may be more like 33 or 47uF.

If you are using an ideal inductor with no equivalent series resistance specified then I would suggest that you try one of the realistic inductors from the LTSpice library such as the Coiltronics CTX That one has a DCR of 0.

That will help to reduce the initial surge of the startup current. Also note that a realistic design with an actual switching VR controller would have a soft start feature that gradually brings the PWM duty cycle up to its operating level without the huge initial surge.

Also a controller would monitor output voltage via a divider and compare it to a reference to continually adjust the PWM duty cycle thus regulating the output voltage.

I've also had problems with this circuit in LTspice. I don't think my problem was exactly the same as yours but this is the only decent result when searching for "ltspice boost converter" so I'll put my answer here. I used the generic "nmos" model. It doesn't work. I don't know why but it seems like it has a really high resistance even in the on state which is weird. Anyway, the way to fix it is to place the generic nmos, then right click it and click "Pick new transistor", then choose one from the list, e.

My filtering capacitor was way too big. This means the output voltage takes on the order of seconds to settle fine in real life, but annoying in a simulation. You said, "I wanted to build an excruciatingly simple boost converter circuit".

I wanted to do the same thing, and built many a Joule Thief in LTSpice, and I put it into the same category -- The Joule Thief is really a self-optimizing boost converter disguised as a hobbyist circuit, but I've learned a lot about boost converters from stepping the Joule Thief parameters. And because it's self-optimizing, it almost always does something and gives you a feel for how each aspect of the circuit affects things.

Here is a Joule Thief for you to mess with:. There is a table that gives formulaic recipes for the boost converter, buck converter, and inverted boost converter. If you alternate playing with these two directions, I believe that you can "teach yourself" some of that intuition which you want to get. I could not find an MC in the LTSpice library, but you can go through the exercise from the table, and then pull up a Joule Thief or any boost converter chip from the LTSPice library, and plug the components a given scenario has given you, and it should be close to what you want, and then you can tweak it.

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