# Half Bridge inverter output problem

Thread Starter

#### EthanWu0131

Joined Jan 16, 2024
19
Hi, I have been working on a tesla coil design, and it involves a half bridge inverter. My goal is to use a LC as the load to both act as the primary coil of the tesla coil and also achieve soft switching at the same time, but the simulation result shows the power dissapation is too high with LC as the load.

However when I use a resistor as the load instead, I get a square wave(which I believe is the ideal output) but it has some really wierd charachteristics, as you can see from the picture below, the positive voltage is very high in contrast to the low negative voltage. Also I find that by increasing the load resistance the negative voltage increases and the positive decreases, hope this clue helps.

I believe the positive and negative voltage are each supplied by the two 1m capacitors, I find that their voltages match(as seen from the picture below), the red line is C4(name is in the circuit) and the green line is C3.

Below is the circuit of the half bridge, input voltage is 320V with a switching frequency of around 154Hz, also I'm using two IGBTs as the switch(AFGB40T65RQDN), the LC is tuned to the same frequency as the half bridge.
(the square wave I got was just swapping the LC with a 5k resistor)

Can anyone explain what causes the high positive and low negative when I use the resistor as the load, and what I can do to make the positive and negative voltage be the same without increasing the load resistance.
Also what are some methods I can use to get rid of the high spikes.
Lastly how can I use the LC as the load without creating excessive heat that will destroy the circuit.

Thanks in advanced!

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#### ronsimpson

Joined Oct 7, 2019
3,038
With out comment.

Thread Starter

#### EthanWu0131

Joined Jan 16, 2024
19
I redid the circuit like you said and changed a few values and it now creates an ideal square wave, but when I add the LC as the load the power dissapation problem happens, are my arrangements correct or am I missing something?
(Below is my circuit with LC as the load, the second picture is the fixed output square wave)

ps. I wish to have the LC resonate with the half bridge switching so that it can achieve soft switching, I'm not sure if I'm doing it right though, help is much appreciated!

Last edited:

#### ronsimpson

Joined Oct 7, 2019
3,038
or does that also causes the other problems?
In spice it works, but in real life it kills the transistor.

Please post you SPICE files.

Thread Starter

#### EthanWu0131

Joined Jan 16, 2024
19
In spice it works, but in real life it kills the transistor.

Please post you SPICE files.
I edited my last response, the square wave problem is fixed, but when I use LC as the load it overheats. I have the circuits and the spice simulation posted on the edited response.

#### ronsimpson

Joined Oct 7, 2019
3,038
The LC is open at low frequencies because of C.
The LC is open at high frequencies because of L.
At resonant frequencies, the LC is a short. The currents will be very high. Move the frequency up or down 20%.

Thread Starter

#### EthanWu0131

Joined Jan 16, 2024
19
The LC is open at low frequencies because of C.
The LC is open at high frequencies because of L.
At resonant frequencies, the LC is a short. The currents will be very high. Move the frequency up or down 20%.
But for a tesla coil they need to be in resonance, it possible to increase the impedence of the two capacitors, and leave the LC at near 0 impedence, that way the current won't go over the roof.

#### MisterBill2

Joined Jan 23, 2018
18,538
With the same major components you could make this work with a split supply,
In addition, you have a series resonant circuit which presents a very low impedance at resonance. And probably there is no secondary winding yet to change that . So really the circuit IS operating as designed, although the result is not as desired. You ARE getting the high current at resonance and also the high voltage and thus the transistors fry. It might actually work as intended with a secondary to reflect some impedance back to the primary side.
OR!!
For a TESLA COIL to produce the high voltage it seems that a PARALLEL resonant circuit will have the high circulating current to induce the high voltage in the secondary.

Did you create this circuit from a bare sheet? or is this a variation of a published design that you could share with us?? And that 320 volts is about what I would expect from a voltage double fed with 120 volts mains power, under load.. (Did I guess that right??)

#### ronsimpson

Joined Oct 7, 2019
3,038
To save parts I build power supplies like this.
I removed C21. Changed C3,4 to a smaller value.
(C3 is in parallel with C4) series with C21. Changed to C3 parallel with C4.

#### MisterBill2

Joined Jan 23, 2018
18,538
Now the question is: What is the purpose of this circuit??? With no capacitor to provide resonance it will simply be reversing the polarity of the voltage across the inductance. If that inductance is to be the primary of an actual Tesla coil then it needs to be present .
The Tesla coils that I have seen that were electronically driven often had the primary coil tuned to resonate at the same frequency as the secondary. At least one of them had a push-pull driver/oscillator arrangement.

#### ronsimpson

Joined Oct 7, 2019
3,038
What is the purpose of this circuit??? With no capacitor to provide resonance it will simply be reversing the polarity of the voltage across the inductance.
There are three capacitor that cause the inductance to resonant. C3,4,21

Thread Starter

#### EthanWu0131

Joined Jan 16, 2024
19
With the same major components you could make this work with a split supply,
In addition, you have a series resonant circuit which presents a very low impedance at resonance. And probably there is no secondary winding yet to change that . So really the circuit IS operating as designed, although the result is not as desired. You ARE getting the high current at resonance and also the high voltage and thus the transistors fry. It might actually work as intended with a secondary to reflect some impedance back to the primary side.
OR!!
For a TESLA COIL to produce the high voltage it seems that a PARALLEL resonant circuit will have the high circulating current to induce the high voltage in the secondary.

Did you create this circuit from a bare sheet? or is this a variation of a published design that you could share with us?? And that 320 volts is about what I would expect from a voltage double fed with 120 volts mains power, under load.. (Did I guess that right??)
Thanks for the tip on the secondary winding, I'll try adding it and come back to you.
This is the circuit that I based my half bridge on :
I changed a few components and values to fit my needs, the biggest difference is I added the capacitor to form a LC, making it a drsstc.
And the 320V is the rectified voltage after a voltage step up from 110VAC source(not in the US).

#### MisterBill2

Joined Jan 23, 2018
18,538
OK about the 320 volts. I am relieved to know it is not from a mains fed doubled circuit.
as for the simulation working but in real life failing, understand that in SPICE parts never fail. ten amps thru a small transistor works well in simulation , just like 10 watts in a half watt resistor does not overheat in SPICE.
Really, you have made some serious changes in that driver circuit relative to the one in post #12. In that circuit the transistor drives are thru a capacitor, so they are a spike, not a square wave. And the 1M series base resistor for X1 in the first post assures a lack of adequate base drive.
So there is quite a bit of difference between the reference circuit and your posted circuit.
Is there any explanation provided for the operation of the circuit shown in post #12? Your posted circuits appear to be different in some important aspects.

Thread Starter

#### EthanWu0131

Joined Jan 16, 2024
19
OK about the 320 volts. I am relieved to know it is not from a mains fed doubled circuit.
as for the simulation working but in real life failing, understand that in SPICE parts never fail. ten amps thru a small transistor works well in simulation , just like 10 watts in a half watt resistor does not overheat in SPICE.
Really, you have made some serious changes in that driver circuit relative to the one in post #12. In that circuit the transistor drives are thru a capacitor, so they are a spike, not a square wave. And the 1M series base resistor for X1 in the first post assures a lack of adequate base drive.
So there is quite a bit of difference between the reference circuit and your posted circuit.
Is there any explanation provided for the operation of the circuit shown in post #12? Your posted circuits appear to be different in some important aspects.
Here: https://hackaday.com/2021/10/31/a-builders-guide-for-the-perfect-solid-state-tesla-coil/
There is a video that talks about the operation.

Also you mentioned that the circuit I based off from produces spikes not square waves, and since mine produce square waves, would this effect the operation of the Tesla coil, if so how can I turn them into spikes?

#### ronsimpson

Joined Oct 7, 2019
3,038
Also you mentioned that the circuit I based off from produces spikes not square waves, and since mine produce square waves, would this effect the operation of the Tesla coil, if so how can I turn them into spikes?
Try reducing the duty cycle. Then you will have less energy. Right now it looks like one of the MOSFETs is on 95% of the time. I know it will take some effort but increasing the off time to 50% or 25% will reduce the power.

#### MisterBill2

Joined Jan 23, 2018
18,538
somewhere between spikes and square waves is a pulse width that will provide just the power that the Tesla coil will work best with. That may require a bit of experimenting..

Thread Starter

#### EthanWu0131

Joined Jan 16, 2024
19
So I'm kinda new to this and I'm not sure how to set it up, below is the half bridge with LC, I already used a resistor as the load and found out that the best impedence is about 10ohms, and I want the LC to have that value, I added the secondary winding(the capacitor is to simulate the earth), my quenstion is about how to set up the coupling to make the impedence 10 ohms(since the impedence comes form the secondary winding).

if you guys need more imformation plz let me know.

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