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Richie Bernett's SSTC Power Stage

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Shayne

Sat Nov 14 2009, 07:37PM

Registered Member #2467 Joined: Sat Nov 14 2009, 07:22PM
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Posts: 2

Hi Everybody,

I've been researching SSTCs for a while now and found this forum to be a great resource. This is my first post here and I look forward to contributing in some manner once I've gained some more experience:) I was wondering if any of the seasoned experts here would guide me through the power stage of Richie Bernett's SSTC schematic. I understand all of the theory presented on his website, however details regarding the power stage of the schematic seem to be lacking. I would also be highly obliged if any of the veterans would be so kind as to point me in the right direction if a quick explanation isn't possible.

Here's the schematic diagram for those interested :

Thanks Again! :)

Mads Barnkob

Sun Nov 15 2009, 09:19AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

This is a standard full bridge configuration of 4x STW15NB50 MOSFETs (500V 15A 0.33ohm)

To aid the internal reverse blocking diode in the MOSFETs, for each MOSFET two ultra fast rectifiers are added in parallel between drain and source.

Each half of the full bridge is decoupled with a 470nF capacitor.

The MMC is the 4x 220nF capacitors in series with the primary winding.

There is no electrolytic capacitor between the rails, there is only half wave rectification through the two 1N5804 diodes in the top.

The resistor across the rails discharges the capacitors across the rails when power is removed, making it safer to work with after power have been removed.

The BZXC15 zener diodes mounted back to back is to clamp the gate voltage below the ratings for the MOSFET.

Gates fra driven from 16 turn Gate Driver Transformers.

The only thing I do not understand would be the schottky diode in series with the drain on all MOSFETs, since its only rated 40 - 60V

uzzors2k

Sun Nov 15 2009, 10:54AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

The Schottky diode in series with the drain prevents the internal diode from conducting under any conditions. Without it, if enough were to current pass through the ultra fast rectifiers their forward voltage would match the internal diode's, and it would start conducting too. Stopping it from conducting again takes a long time and leads to losses. The Schottky diode is clamped by the ultra fast rectifiers in anti parallel, so they'll never see more than a few reverse volts.

raff

Sun Nov 15 2009, 02:41PM

Registered Member #2315 Joined: Tue Aug 25 2009, 02:35AM
Location: Leyte, PH
Posts: 161

Mads Barnkob wrote ...

The only thing I do not understand would be the schottky diode in series with the drain on all MOSFETs, since its only rated 40 - 60V

same here :) but I have tried it in actual (for a SB diode to have 40V rating pass more than 220VDC) and did NOT explode... :)

I wonder if its STILL necessary with CT feeddback SSTCs? and also for the FR diodes across the MOSFETs if they are still need with CT feddback w/c IS always in tune,..

GeordieBoy

Sun Nov 15 2009, 06:28PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

As the "designer" of that circuit I should probably chip in here, but as others have stated it is just a simple H-bridge circuit. Most power electronics text books will describe the operation of this arrangement describing it either as an H-bridge or a full-bridge. It simply consists of 4 switches (the MOSFETs) with a load tied between them. They are connected in such a way that when the MOSFETs are switched on alternately in pairs the supply voltage is applied with alternating polarity across the load.

As for the schottky blocking diodes and FREDs they are used to block the slooooooow internal body-drain diode of the MOSFET and provide an alternative route for the free-wheeling load current through much faster external diodes designed for the job.

The schottky diode's only need to be rated for 20V or so because their reverse voltage is limited by the forward voltage-drop of the fast recovery epitaxial diodes. The voltage across these may be a few volts plus a few more volts forward-recovery spike before they realise they're meant to be turned on!

As for whether this arrangement of diodes is necessary, IT DEPENDS ENTIRELY ON HOW IT'S TUNED! I've you drive the resonator by a link coupled primary and drive at the resonant frequency of the secondary or higher, then the net impedance seen by the inverter is always inductive. In this case you can dispence with the fancy arrangement of bypass schottkys and FREDs for the freewheel diodes. However if there's a chance you might end up driving the coil just below it's resonant frequency where the load might appear capacitive then you'd better bypass the slow body-drain diodes and provide an external path.

Contrary to popular beleif CT feedback is not always perfectly in tune! The CT usually senses the current-zero crossing and some time passes before the power devices actually switch. During this time the current has moved to the free-wheel diodes and this is where the potential problem lies.

In short: Sometimes the MOSFET's own body-drain diodes are sufficient, other times they are entirely inadequate.

-Richie,

Thomas

Sun Nov 15 2009, 11:56PM

Registered Member #120 Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83

Why do you have two 1000V 3A diodes in series?

teravolt

Mon Nov 16 2009, 04:38AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

GeordieBoy in your opinion if I wanted to controle a full bridge power out by controling gate pulse width would the mur1660 be nesisary? The object would to amplitude modulate a tesla.

do you think that the mur1660 may play a role when an arc may detune or ground srike may occure. In a DRSSTC largely a antiparalell doide is not used but I would think that thay would be a must in a SSTC.

GeordieBoy

Mon Nov 16 2009, 02:50PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

> GeordieBoy in your opinion if I wanted to controle a full bridge power out by controling gate pulse width would the mur1660 be nesisary? The object would to amplitude modulate a tesla.

Yes, when you decrease the duty ratio of the gate-drive signals you introduce dead-time where no MOSFETs are conducting. The resonant sinusoidal load current doesn't stop however, it circulates through the free-wheel diodes during the times when all MOSFETs are off. It is when you next try to turn on a MOSFET that the problems can start. It is particularly the commutation of current from a slow free-wheel diode to a fast switch on the opposite side of the bridge leg that causes the problem.

With excessive dead-time added to reduce the power and implement audio modulation, you are almost guaranteed that the MOSFETs will turn on well after the current has gone through zero and commutated to the free-wheel diode across the opposite device. This is exactly the situation where the MOSFET body-drain diodes are too slow, and need to be bypassed by fast recovery external devices like the MUR1660.

However, I would say that if your ultimate aim is to achieve amplitude modulation you might want to look at implementing it another way. You can keep the duty ratio fixed at full, and modulate the frequency of the drive instead. This Frequency Modulation causes indirect AM of the current drawn by the resonator and still achieves audio modulation of the spark. The neat thing is that if you do this by detuning the driver above the natural resonant frequency of the coil, it essentially works up and down the high-side slope of the resonant peak where the load always appears inductive. This type of load is much easier on a MOSFET inverter, and you can surely get away with using the internal body-diodes in this case.

In short, I would say that the external bypass diodes are required if you want to build a MOSFET driver that is robust enough to withstand any manual mistuning, or errors in automatic tuning without blowing up. If the load seen by the inverter is always net inductive the body-diodes should be sufficient as they never see forced-reverse recovery in this mode. (That is why most power electronics applications don't require the body-diodes to be bypasses by external discrete diodes. The most common exception being DC motor control where forced-reverse recovery is a problem.)

Fast antiparallel diodes are not normally required in a DRSSTC that uses IGBTs. This is because most IGBTs are co-packaged with a second die comprising of a fast-recovery diode. The switchign properties of this free-wheel diode are usually chosen to match and complement those of the main IGBT. The important point here is that IGBTs contain a proper fast diode that has been chosen specifically to do the free-wheel task well - Conversely MOSFETs contain a crappy slow diode that you get for free, that is occasionally just adequate to do free-wheel duty in some applications!

-Richie,

Shayne

Wed Nov 18 2009, 05:06AM

Registered Member #2467 Joined: Sat Nov 14 2009, 07:22PM
Location:
Posts: 2

Wow that is a lot of helpful information added to the thread.Thanks guys for being such a supportive and helpful group. Elated to see Richie reply to my thread as well!!

My primary goal is to build an audio modulated tesla coil. I'll be using the TL494 to achieve the modulation and PLL so as to ensure that the resonant frequency is tracked as it goes down after start up of the tesla coil.

I'm however planning to use full wave rectifier instead of the two 1N5408 on the top. As far as i've read, it doesn't adversely affect the length of the spark as i'm aiming for a spark length of about 14" to 18". Any advice regarding that?

I'll be posting up my final schematic as soon as i'm done perfecting it and perhaps you could provide me with more input regarding the same.

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