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Registered Member #1733
Joined: Thu Oct 02 2008, 03:17PM
Location: Hamilton, ON, Canada
Posts: 100
So I've built the typical astable 555 flyback driver. Actually I built 2 on one board, and while testing one side a thought came to mind.
The 2 primaries are fed from seperate IRFP250's. The primaries share a common +12V. Modulation of the 555 is nearly identical, but small frequency changes are expected from the source. It's my thought that the differing modulation may not work properly sharing a common +12V.
Scope shows identical forms from both MOSFETS. The two paths I see from running the complete circuit are 1) primaries increase voltages across the FETs and they die, or 2) they cancel each other and nothing happens. I've already blown 2 timers and 2 FETs mucking with it, I'd like to avoid killing more components.
If anyone has any thoughts, I'd like to hear them...
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The primaries are coupled through the core, and if both FETs switch at the same time, big currents will flow and damage your transistors.
The solution is to use a single pulse source and use inverting and noninverting drivers to power the FET gates. Or use a dedicated SMPS controll IC (I have good experience with the SG3525 controller).
Registered Member #1733
Joined: Thu Oct 02 2008, 03:17PM
Location: Hamilton, ON, Canada
Posts: 100
Dr. Kilovolt wrote ...
The primaries are coupled through the core, and if both FETs switch at the same time, big currents will flow and damage your transistors.
The solution is to use a single pulse source and use inverting and noninverting drivers to power the FET gates. Or use a dedicated SMPS controll IC (I have good experience with the SG3525 controller).
My bad, I wasn't clear. This board drives 2 flybacks, not 2 primaries on a single core. The +12V and the 0V are common (a single 12V wall-wart.) 555 Timers Pin 5 is unique +V for modulation, common to the 0V common. The common +12V is connected to both primary coils, the common of the primaries to individual FETs.
My goal is this: Audio-modulated flybacks, single drive board, driving left/right audio channels. I can get a single channel to work, I'm worried about running both channels together. At the end of the day, 555's are 50 cents, FETS are $3. If they blowup, meh, I have more. If I blowup flybacks, meh, $5 for brand-new AC or DC xfmr... I stocked up last time at the store.
If I can get this to work as desired, the next step would be a quad-driver with passive crossover before the driver (Linkwitz-Riley 2nd Order to keep it simple)
I only have the PCB designed, if I can get a schematic done up today, I'll post it.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Yea, just use a ucc37322 for one transistor and ucc37321 for the other for an inverting(ucc37321) and non inverting(ucc37322). Or, you could add a dc blocking capacitor to the ucc37322 output, and hook the two drivers to a gate drive transformer, one driver per side of the primary, and have two outputs to connect to the mosfets for added protection to the driver.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
If you want two independent modulators, add good filtering to your DC power supply (big capacitors across it), because two 555 oscillators powered from a single poorly regulated supply will really tend to get synchronized.
Registered Member #1733
Joined: Thu Oct 02 2008, 03:17PM
Location: Hamilton, ON, Canada
Posts: 100
Here's my PCB. Should be just as easy to read as a schematic.
I've got some big caps, I'll throw them on then fire up both sides...
As a side note - I can only pull about a 10mm arc without it distorting. I'd like to pull 20 to 30mm arcs. I "could" build a different driver (Maz) but I'd like to use the current one. Am I SOL?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Do you have a good filtering cap and a ceramic/foil decoupling cap across the 555?
If you want more power you can run half-bridge, and if you want a clear sound and a lot of power at the same time you can use frequency modulation, but things get a bit more complex.
Or you can simply increase the power by reducing primary turns, and use transistors with less Rds (e.g. IRFP150).
Registered Member #1733
Joined: Thu Oct 02 2008, 03:17PM
Location: Hamilton, ON, Canada
Posts: 100
You guys must find it frustrating when non-EE people try to do EE related projects. I'm a 30 year-old computer engineer who has an affinity to all technology. I get most of it, and what I don't understand I can learn quickly by searching here and reading datasheets for the parts I'm using, and generally reading as much as I can. SO....
Filtering Cap I understand. No there is not one in place yet, bu there will be tonight. Decoupling Cap - lost. You can tell me, or I can research it. Half-bridge - understand what it is, what it does, never built one. Less turns, IRFP150 I'll try that too.
All of this (yes the frustrations too) is fun. I enjoy building these things (Boost Converter, SGTC, Flyback drivers) and I'd love to build an AMSSTC, but I know that my skill level is lacking for that. Point? I'm thinking of going to night school for an EE Cert. Learn stuff and be less of a scrounger and more of a contributor. Classes start in January, so I only have a few weeks to decide.
Thanks for the guidance so far guys... It's appreciated.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Zenador wrote ...
You guys must find it frustrating when non-EE people try to do EE related projects. I'm a 30 year-old computer engineer who has an affinity to all technology. I get most of it, and what I don't understand I can learn quickly by searching here and reading datasheets for the parts I'm using, and generally reading as much as I can. SO....
Filtering Cap I understand. No there is not one in place yet, bu there will be tonight. Decoupling Cap - lost. You can tell me, or I can research it. Half-bridge - understand what it is, what it does, never built one. Less turns, IRFP150 I'll try that too. All of this (yes the frustrations too) is fun. I enjoy building these things (Boost Converter, SGTC, Flyback drivers) and I'd love to build an AMSSTC, but I know that my skill level is lacking for that. Point? I'm thinking of going to night school for an EE Cert. Learn stuff and be less of a scrounger and more of a contributor. Classes start in January, so I only have a few weeks to decide.
Thanks for the guidance so far guys... It's appreciated.
Z
Basically, a decoupling cap is used to filter out any noise caused anywhere in the circuit. Just the inductance of the circuit can be enough to cause some noise, like with a sstc. I am sure this circuit would not have noise like a sstc because of the lower frequency, but they are always a good idea, in most any oscillator circuit. It is basically a noise shunt, in other words. The decoupling capacitor is usually quite low capacitance, and just give it a safety margin, just like you would for a filter cap(i guess).
You can just use that basic schematic, but take off the interrupter and attach pin 3's straight to the positive supply,and tie the input pins of the two ucc's together and attach that straight to the 555. Or you can just use this schematic which is pretty much the same thing, but since the mosfets are running at a much lower frequency that a sstc, you can just use a less powerful mosfet driver (tc4422) and a stepup gate drive transformer, because the tc4422 will only put 12 volts into the gate driver transformer, and that is right on the edge of what the mosfets need to switch, so you want the gdt(gate drive transformer) to step the voltage up to make sure that the losses like winding resistance and core, don't drop the voltage below 12 volts, and you want more than 12 volts on the mosfet gate anyway.
Registered Member #1733
Joined: Thu Oct 02 2008, 03:17PM
Location: Hamilton, ON, Canada
Posts: 100
Thanks guys... I think I have other issues.
I put the cap across the 12V supply, and that cleaned up noise. Caps across the FET/+12V primary just blew out the 555's.
The other problem - the flyback is singing, not the arc. Hard to tell if it's the primary (self wound) or the secondary. I'm going to assume the secondary. I can't see potted coils making sound unless they are arcing. So that leaves me with winding my own secondary coils. Woo hoo!!
On the plus side, running both coils together posed no problems - as long as the arc pulled is the same length. Adjusting one seems to impact the other. I'm getting there... Lol.
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Another Flyback and driver question...
