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ZVS - info about it

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Whiplash

Mon Feb 16 2015, 06:58PM

Registered Member #11363 Joined: Mon Mar 18 2013, 04:29PM
Location: Brazil
Posts: 19

Hi all, im trying to get alot of information for a good build.
what i know for an good zvs:
Colored Schem

mosfets need an RDS(on) 120mOhm or less, this is only for heat issues? if i get an mosfet like 11n90c = 1ohm ? the driver dont work? or heat alot?
zenner diodes are not critical
diodes of ultra fast recovery = mur460 or better. im using mospec 12sc40 = 40v 12A.
inductor are not critical
resistor for gate source on mosfet 10k are the original values, if i put 4.7k or 50k what happens?

sorry for newbie questions and my bad english, but im afraid of get the things bad.
I have an 2x Power supply 12v 70A atx from an dell server :)

Antonio

Tue Feb 17 2015, 12:00AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

The on resistance of the mosfets corresponds to their current capability. You may need a lot of current in the mosfets.
The diodes shall support relatively high voltage, and not much current. 40 V may be not enough.
The resistors in parallel with the zeners are not critical. They waste power and limit the gate voltage below the zener voltage if too small, and are there more for safety, to guarantee that the gate charges are drained. The circuit probably works without them, but I don't recommend trying.
This circuit is not safe. It may enter an unstable mode that drains much more current than in normal operation and be destroyed.
When first powering the circuit, put a 1 ohm resistor in series with the power supply, to avoid disasters.

hen918

Tue Feb 17 2015, 07:18PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

I agree with Antonio, I usually use a current shunt to help stop the "latch up" problem. On big lead acid batteries the MOSFETs can easily explode under those conditions. Also, If you are powering it from a Switched Mode Power Supply without a series resistor, you will probably need a small(ish) capacitor to stop the SMPS thinking it has shorted out and current limiting until the circuit gets latched up. (I think it just gives it a slower start as the capacitor charges and creates a buffer between the power supply and ZVS circuit.)

Ken Williams

Wed Feb 18 2015, 03:04AM

Registered Member #8247 Joined: Thu Nov 22 2012, 03:14AM
Location:
Posts: 18

hen918 wrote ...

I agree with Antonio, I usually use a current shunt to help stop the "latch up" problem. On big lead acid batteries the MOSFETs can easily explode under those conditions. Also, If you are powering it from a Switched Mode Power Supply without a series resistor, you will probably need a small(ish) capacitor to stop the SMPS thinking it has shorted out and current limiting until the circuit gets latched up. (I think it just gives it a slower start as the capacitor charges and creates a buffer between the power supply and ZVS circuit.)

So on the positive lead of my power supply, which does give an inrush current of about 15A but immediately settles down to 7.5A once running a half second later, I should put a cap in series to stop this. Can you recommend a uF value for the cap?

Ken Williams

Wed Feb 18 2015, 03:16AM

Registered Member #8247 Joined: Thu Nov 22 2012, 03:14AM
Location:
Posts: 18

I have a ZVS outputting to a center tapped transformer (not a flyback). Does anyone know if I'm suppose to see a sine wave going into the transformer? I thought it was suppose to oscillate nicely via a sine wave. I get a proper oscillation on the gates as shown in the first attachment, but when I put the scope on the center tap of the transformer (and therefore from/after the powersupply and inductor) I see no sine wave at all. Just what you see in second and third (which is a zoomed out view showing more cycles) attachments. Now it works simply because the waveform is still bouncing up and down, but its not a nice sine wave. Does anyone have an idea how to improve my wave form and make it a sine wave? If I remove or double the inductor, or if I remove or double the .68uF cap bank in parallel with the transformer nothing much changes. Its essentially the same ugly spiking wave.

Antonio

Wed Feb 18 2015, 03:44PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

You should see something as a full-wave rectified sinusoid at the center tap, and complementary half-wave sinusoids at the two drains (at quite high voltage). The difference between the two voltages at the drains is a sinusoid.

dexter

Wed Feb 18 2015, 03:48PM

Registered Member #42796 Joined: Mon Jan 13 2014, 06:34PM
Location:
Posts: 195

Ken Williams wrote ...

but when I put the scope on the center tap of the transformer (and therefore from/after the powersupply and inductor) I see no sine wave at all.

putting the scope on the center tap would show you the inductor response to the power drawn from the power supply which is not continuous

to see the sine wave put the scope across the whole coil or in the secondary side of the transformer

GrantX

Tue Mar 03 2015, 12:16PM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

I've played around with a couple of ZVS oscillators over the years and have found a few good tips for getting a reliable driver with a lower chance of latching up:
- Short thick connections and a compact overall layout.
- A hefty DC supply. For example, a 12V 15A supply might seem good enough, but load it to the full 15A and scope the output. If you see any ripple that dips below 10V, there is a risk your MOSFETs will latch up. Basically, you just need to ensure that the gates will always charge up beyond 10V (12V or more is better) so they are fully on.
- MOSFETs with a very low on state resistance. This reduces the voltage drop across them while they are conducting. A large voltage drop can interfere with the diode feedback, which leads to latch up.
- A small air gap in the ferrite core of the coupling/step-up transformer seems to help. It seems to help lessen the changes in frequency as the loading of the transformer changes, as well as lessen heat in the FETs. I could be wrong though, this is only my own experience.
- Also, remember that your FETs should be rated for around 4x the input voltage, due to the resonant rise across the LC tank.

Whiplash

Tue Mar 03 2015, 10:56PM

Registered Member #11363 Joined: Mon Mar 18 2013, 04:29PM
Location: Brazil
Posts: 19

tank to all ppl for the infos.
i have made my zvs now, its working great
here a vid:

mosfets are IRF3205 (irfp250 out of stock).
lm7812 for gates.
diodes are mospec 10sn40 (40v 10A) need uf 4007 but are hard to find.
primary on flyback are 10+10 (safe for mosfets) and have 3x 0.22uf 300v caps.
powersupply = 110 -> 24v transformer from an old nobreak 1200kva + 100A bridge and an total of 20.000Uf 50v caps.
sparks come from 1cm and can be stretched to 4cm.
i can make an small tesla with this?

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