Author Topic: High Voltage Half Bridge Drive  (Read 720 times)

Offline iraquois

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High Voltage Half Bridge Drive
« on: June 19, 2020, 07:43:37 AM »
I am working on a project to produce high voltage output. Since I supply this circuit directly from mains I have some problems. Here is my representative circuit:
https://imgur.com/t9LxmNl

I am controlling transistors from a PWM generator. Duty cycle is %25. I checked for dead time. It is safe. Now, here I used safety bulb in series with the main (220VAC). When I plug it in Lamp goes fully on after 1-2 second later it goes almost off (just a little bit blinking). I thought first light is because capacitors are charging. So my questions are;

1) After I start pwm generator, I can see arcs but bulb also goes almost fully on. Is this a sign of a failure or short in the circut ?

2) I am checking voltage across C3 after I start pwm. Depending on my distance adjustment on flyback's secondary cables to have arc, it shows up to 1800V. Is this normal isn't it too much ?

3)I will later add voltage feedback and current feedback. Other then these additions, what kind of improvements should I do?

Offline Weston

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Re: High Voltage Half Bridge Drive
« Reply #1 on: June 19, 2020, 10:55:00 AM »
If you flyback is producing sparks its probably working (congrats!). The lamp will limit the total power drawn from the AC line to whatever its power rating is, and at that point little power is being delivered to your circuit. Do you expect your circuit to draw significantly less power than the rating of the lamp?

Are you measuring the voltage with a digital volt meter? High voltage stuff generates a lot of electrical noise which can cause incorrect readings on volt meters. Your transistors would fail way before 1800V.


Offline iraquois

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Re: High Voltage Half Bridge Drive
« Reply #2 on: June 19, 2020, 11:24:30 AM »
If you flyback is producing sparks its probably working (congrats!). The lamp will limit the total power drawn from the AC line to whatever its power rating is, and at that point little power is being delivered to your circuit. Do you expect your circuit to draw significantly less power than the rating of the lamp?

Are you measuring the voltage with a digital volt meter? High voltage stuff generates a lot of electrical noise which can cause incorrect readings on volt meters. Your transistors would fail way before 1800V.

Thanks a lot for your answer. Actually I am going to remove lamp. I built this circuit before. It was also working with lamp safely but when I remove the lamp, and start the pwm like %5 duty, Something blowed up , and one transistor died which is rated 1000V :D one input of primary cable burned.(I noticed later, it was very thin cable. I am not sure if it was the reason. Also now I seperated rectifier side from the main circuit) I don't want this to happen again it was scary :)). Do you think using an NTC is enough?


I tried again. The time It goes ~1800V is when I put arc cables far from each other.
« Last Edit: June 19, 2020, 12:16:16 PM by iraquois »

Offline davekni

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Re: High Voltage Half Bridge Drive
« Reply #3 on: June 19, 2020, 08:55:11 PM »
1800V is a confused meter.  It is not real.  All my cheap meters get confused by arcs or even just high-frequency power signals such as your 50kHz.

Lamps make great fuse alternatives for testing line-powered electronics.  I'd suggest finding a higher wattage lamp rather than removing it.
David Knierim

Offline iraquois

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Re: High Voltage Half Bridge Drive
« Reply #4 on: June 21, 2020, 08:34:49 AM »
1800V is a confused meter.  It is not real.  All my cheap meters get confused by arcs or even just high-frequency power signals such as your 50kHz.


Lamps make great fuse alternatives for testing line-powered electronics.  I'd suggest finding a higher wattage lamp rather than removing it.

Thank you for the reply. Eventually I am going to need to remove the bulb from the circuit. Could you suggest or give an idea for me to test if it is safe using it without bulb? Replacing bulb with NTC is eno─▒gh? What kind of safety circuitary is needed for this type of circuit?

Offline davekni

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Re: High Voltage Half Bridge Drive
« Reply #5 on: June 21, 2020, 08:26:42 PM »
An NTC is good to limit inrush current charging the electrolytic cap after the bridge rectifier.  If removing the lamp, replace it with a fuse.  (You could rely on just the breaker in the circuit feeding the outlet you plug into.  But I'd suggest being a bit more cautious than that.)
David Knierim

Offline iraquois

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Re: High Voltage Half Bridge Drive
« Reply #6 on: June 22, 2020, 08:39:51 PM »
An NTC is good to limit inrush current charging the electrolytic cap after the bridge rectifier.  If removing the lamp, replace it with a fuse.  (You could rely on just the breaker in the circuit feeding the outlet you plug into.  But I'd suggest being a bit more cautious than that.)
Okay. I will try. If I won't blow myself up then I will share my experience :)

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Re: High Voltage Half Bridge Drive
« Reply #6 on: June 22, 2020, 08:39:51 PM »

 


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