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Driving 'Fiddy's Flybacks' in flyback mode.

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Shrad

Fri Nov 28 2014, 06:42PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

you want the shortest current path between switches, diodes and caps, and no loops, so use star or "T" topology in the switching zone for power

ground planes are cheap and spare your etch solution, and you'll eventually need to make more than one or two prototypes

switching ground plane and command ground plane can be separate and connected via a wire passing through a ferrite bead, or choke

this is theory, as you'll easily understand ;)

Ash Small

Fri Nov 28 2014, 08:45PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Shrad wrote ...

you want the shortest current path between switches, diodes and caps, and no loops, so use star or "T" topology in the switching zone for power

ground planes are cheap and spare your etch solution, and you'll eventually need to make more than one or two prototypes

switching ground plane and command ground plane can be separate and connected via a wire passing through a ferrite bead, or choke

this is theory, as you'll easily understand ;)

Thanks for the pointers, Shrad.

I'm going to divide this up into three boards now, driver, switch and Schottkys, and snubber.

At the moment it's looking like the connections to the primary will be on the 'snubber' board.

The 'driver' board and the 'snubber' board will be ~ half the size of the 'switch' board, and will be mounted above the 'switch' board. The heatsinks will be mounted on the underside of the 'switch' board, along with the MOSFET and Schottkys.

I will switch to 3D CAD once I finalise the layout of each board.

hen918

Sat Nov 29 2014, 06:11PM

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

Ash Small wrote ...

hen918 wrote ...

You will need a low pass filter/smoother/snubber between the power components and the driver. I made that mistake last year - had a load of SMPS boards made; ended up having to cut the power trace to the driver and then bridge it with a 100uH inductor. I also put a little electrolytic on the driver side to form an LC low pass filter. Works now: I was getting basically random noise coming from the H-Bridge before!

So the capacitor is in series with the inductor?

What value capacitor should I start with, assuming I use a 100uF inductor?

No, the cap is between the load side of the inductor and ground. I used a 33uf capactior, but the bigger the more smoothed, as I'm sure you'll guess! Because the load drawn by the driver circuitry isn't massive I only used a small one.

Ash Small

Sat Nov 29 2014, 07:31PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

hen918 wrote ...

No, the cap is between the load side of the inductor and ground. I used a 33uf capactior, but the bigger the more smoothed, as I'm sure you'll guess! Because the load drawn by the driver circuitry isn't massive I only used a small one.

Ok, I already have plenty of capacitance between driver +ve rails and ground for precisely this reason, so I just need to add inductors/ferrite beads to the feed to the driver board, and between the driver groundplane and switch groundplane?

hen918

Sun Nov 30 2014, 10:59AM

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

Ash Small wrote ...

Yep, that's it

Rod-on

Thu Dec 18 2014, 11:29PM

Registered Member #6541 Joined: Sun Sept 09 2012, 05:01AM
Location: Melbourne Australia
Posts: 16

any updates on this?

Ash Small

Fri Dec 19 2014, 10:58AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Rod-on wrote ...

any updates on this?

Should be soon, just got a bit side tracked with helicopter blades and laminar flow, and going to collect a transformer today for another project. Should see some more progress with this over Christmas, though

dexter

Fri Dec 26 2014, 12:59PM

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

Ash Small wrote ...

1416076925 3414 FT1630 Pnp Turn Off Circuit

a bit unrelated but this circuit has any usefulness or even works with bipolar gate signal from a GDT?

Ash Small

Fri Dec 26 2014, 02:44PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

dexter wrote ...

Ash Small wrote ...

a bit unrelated but this circuit has any usefulness or even works with bipolar gate signal from a GDT?

I think the idea of this circuit is that it doesn't discharge through the driver chip, thus speeding up 'switch off'.

I'm not sure if there is any advantage to be gained using this with a GDT, and I think a GDT would saturate due to DC component, unless maybe if it was gapped, but the main advantage of this circuit (without GDT) is that discharge isn't limitued by driver chip current limit.

I do intend to try this, and I'm leaving provision to add it later.

Dr. Dark Current

Fri Dec 26 2014, 03:05PM

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

This circuit, with an additional diode in series with the input and a resistor from the PNP transistor base to ground, is very commonly used for gate driving using a GDT driven in "single ended" mode (duty<50%). The advantage is that you can drive the GDT primary using just a single transistor.

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