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Steve Ward's Mini SSTC Questions

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Doritofingers

Sat May 01 2010, 08:23PM

Registered Member #2829 Joined: Mon Apr 26 2010, 03:52PM
Location: US
Posts: 4

Since this is my first post here I will start by saying hello to all!

Ok so I decided I want to build a copy of Steve Ward's mini SSTC and I have a few questions.I added the schematic for reference.

So here are the questions:

1. Can I use 1N914 diodes for D7 & D8?
2. What are the ratings for D9
3. I have a few 305VAC 1uF MKP EMI suppression caps,can I use these for C7 & C8?(Or should I use 2 in series for .5uF?)
4. What is the power rating for R3 & R4
5. What would the max current draw from the line be?(I ask because my Variac is rated for 3.5A)
6. It is stated that C6(.1uF) should be larger to improve gate drive,can someone elaborate on this?
7. The secondary is stated as 4.5"x10",is the 10" dimension the form length or the winding length?(If it is the form length what would the winding length be?)

Well,that's all the questions I can think of for now.If anyone has any tips or suggestions feel free to let me know as this is my first SSTC.

Thanks,

Kipmans

Sat May 01 2010, 09:32PM

Registered Member #91 Joined: Thu Feb 09 2006, 03:03PM
Location: The Netherlands
Posts: 45

1. They seem fine to me.

2. D9 should at least be able to handle the current draw from the TC. I think it is best to play safe and just grab a 35 A bridge rectifier (these guys) as they are commonly available and pretty cheap too.

3. Can't be sure on that without knowing the exact type of course, but I think there is a good chance that they'll work fine.

4. Simple 0,25 W rated resistors should suffice.

5. 3,5 A, I think, is a bit on the low side but it's surely enough to get some nice sparks. My Mini SSTC could draw as much as 650 W from the (230 V) line in interrupted mode at a high duty cycle and the continuous power draw would probably be even higher. I would suggest measuring the input power/current so that you can see whenever it gets too much for your variac too handle.

6. I'm not sure about that myself, either. Someone once said that it's best to use a capacitance about 10 times the combined gate capacitances of the MOSFETs. I didn't look the numbers up but I think something along the lines of a 0,33 uF cap should be good. Just make sure it's a decent quality MKP-type if possible.

7. I'd take that as the winding length.

Turkey9

Sun May 02 2010, 01:44AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

For the MOSFET gate caps (C6), the larger the cap, the more current available to drive the gates. Anything above a couple uF, in my experience, has no discernible advantage. If you drive it at a lower frequency, you'll need a bigger cap but at 200-200kHz a 1uF cap works great.

cjk2

Sun May 02 2010, 07:06AM

Registered Member #51 Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263

The gate drive cap forms a voltage divider with the gates of the fets. It also presents an impedance as it appears to "fill up" during each half of the cycle. Size it as large as you can (up to 10uf or so).

HV Enthusiast

Sun May 02 2010, 01:46PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

For that basic half-bridge SSTC design utilizing those particular MOSFETs, a 1-2uF capacitor should suffice. Also, you could also go with IRFP260 MOSFETs instead of IRFP460s which are a bit more expensive, depending on where you buy them.

Kipmans

Sun May 02 2010, 02:32PM

Registered Member #91 Joined: Thu Feb 09 2006, 03:03PM
Location: The Netherlands
Posts: 45

EastVoltResearch wrote ...

For that basic half-bridge SSTC design utilizing those particular MOSFETs, a 1-2uF capacitor should suffice. Also, you could also go with IRFP260 MOSFETs instead of IRFP460s which are a bit more expensive, depending on where you buy them.

Assuming, of course, that he doesn't want to run it from 230 V (i.e. European) mains voltage.

MRacerxdl

Sun May 02 2010, 06:30PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

About the variac, you can remeber that variac can handle about 4 times the nominal current for a few minutes, so for testing, you can use your variac, I never get more than 5A in that setup, even running at CW I get maximum of 6A... But that depends of your primary/secondary specs.
My 5A variac handled even 10A. Be in mind that when the variac is at maximum turn, its connected directly to the mains, so the variac handles nothing.

Dr. Dark Current

Sun May 02 2010, 09:41PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

MRacerxdl: But you will still smoke the brush contact when you overload it

Anyway, IRFP460's for 230-240V mains, IRFP260's for 120V mains, using P460s with 120volts will result in ~5x the dissipation and less than half maximum current output...

Doritofingers

Sun May 02 2010, 10:36PM

Registered Member #2829 Joined: Mon Apr 26 2010, 03:52PM
Location: US
Posts: 4

Thanks for the replies everyone!

First off I would like to state that I do not have any formal electronics background just a low level hobbyist so I hope my questions don't seem "stupid"

@EastVoltResearch: I am in the US so I will be running 120VAC,and I have purchased 10 IRFP250NPBF MOSFETs .Which I think are just
the lead free version of the IRFP250s

@MRacerxdl: Thanks for the info,but I used to have another identical 3.5A Variac that I figured I could run at ~4A for a short duration,it worked fine until the brush blew up...LOL

Also I'm concerned about the caps I plan on using.If anyone could give me some feedback that would be great.I've included a link to the .pdf Spec. sheet for reference

,The tech specs are on page 7 & 8.They have a PP dielectric but are intended for "across the line" EMI suppression and I fear there pulse ratings(Or other ratings like dissipation factor.) may be lacking but I don't have the knowledge to be certain. My caps are 1uF "A" type with a 22.5mm lead spacing.Which the spec. sheet gives a dV/dt of 200V/uS and I'm not sure about the dissipation factor(I believe that it has to do with heating at certain frequencies but beyond that I'm clueless.)

A few more things...I've been doing some reading of other posts about Steve's Mini SSTC and have seen comments about R3 & R4 needing to be close in value. About how close should they be? Would it be better to use 1% resistors or will 5% be ok?

Also what effect does changing the capacitance of C7 & C8 have on the circuit? The reason I ask is I'm not sure if I should just use one 1uF cap of if I should use 2 in series for .5uF .Do the caps need to be close in value? The caps I have are 20% parts and I only have 6 units. And Finally will the above caps be suitable for use as C6,If they are I plan on paralleling them for 2uF or 4uF depending on how I configure them for C7 & C8.

Thanks again!

HV Enthusiast

Sun May 02 2010, 11:32PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Dr. Kilovolt wrote ...

MRacerxdl: But you will still smoke the brush contact when you overload it

Anyway, IRFP460's for 230-240V mains, IRFP260's for 120V mains, using P460s with 120volts will result in ~5x the dissipation and less than half maximum current output...

Not really. Switching losses are the major contribution of heat dissipation for these devices in this configuration, not conduction losses. Yes, conduction losses will scale up by about a factor of 5, but conduction losses are the lesser fraction of the total losses of the device.

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