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Old 19th November 2004, 09:49 PM   #11
Eva is offline Eva  Spain
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Your layout is pretty inadequate

1- The output of the transformer forms a big loop antenna and the control IC is inside this loop

2- The control IC is inside a big ground loop as the small signal parts are grounded between the transformer and the output capacitors

3- The primary and the secondary side share the same ground, so you will have to use balanced inputs in the amplifier to get rid of alternator whine [et all] if you use such a circuit in a car

4- You are taking feedback from the output in a topology that is not suitable for regulation [output inductors would be required for that]

To make this prototype work, remove feedback and see how to rewire the transformer and separate the grounds on the attached picture....

Altough I suggest a complete redesign :

- Output capacitors and rectifiers should be near the transformer

- Use independent grounds for primary and secondary side, couple them with something like a 10nF capacitor and a 1K resistor in paralell [using feedback then would require an optocoupler or differential techniques]

- Use two independent grounds for the control IC : A signal ground and a power ground. Route both to a common point

- Place the control circuit away from the transformer, try to place some filter capacitors inbetween for shielding

I recommend placing the control circuit in a daughter PCB soldered vertically over the main PCB, this causes some inmunity to magnetic fields radiated by tracks in paralell to the daugther PCB
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Old 20th November 2004, 04:42 AM   #12
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Default New board

Here is the new circuit board.
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Old 20th November 2004, 04:43 AM   #13
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Here is the new schematic
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Old 20th November 2004, 04:56 AM   #14
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Default summary of changes

Hey,
I took your advice and redid the circuit board. Here is a basic summary of the changes:
1. There are now 2 grounds: Primary Gnd "PGND" and Secondary GND "SGND"
2. I placed the rectifier diode and caps right next to output of xformer.
3. A Mosfet with a higher breakdown voltage has been chosen
VBRdss = 55V
Package = T0-247
rds(on) = 12mOHM
Id @ 25C = 72 Amps
Qg = 86.7nC
Pd = 130Watts
4. Control circuit is as far away from xformer as i can get it...

Eva-- In your last post you said: "couple [SGND and PGND] with a 10nF and 1K resistor in parallel" why would I want need to couple the grounds if i went through the trouble of seperating them?
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Old 20th November 2004, 05:01 AM   #15
Eva is offline Eva  Spain
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The secondary side layout is now much better but you are still making some mistakes in the primary side

Never assume a ground plane to have the same potential on all its points. Never ground signal devices in a ground plane that is carryng current from power devices, this will allways cause voltage gradients across the plane and errors in the signal

All the signal components have to be grounded in a single track that connects to the ground plane in a single point. This ensures that all the signal components are grounded to the same potential [assuming negligible signal currents]

Also, the drains from the mosfets must be routed following the shortest path straight to the ground of the input filter capacitors, and this involves using wire bridges or double sided PCB

So your current layout still involves two big current loops in the primary side and ground loops on the IC. Try to fix them and post the new layout

And be aware that ground planes are no longer effective when PCB traces are dividing them in several isolated sub-planes. In these circumstances, they only effective in terms of reducing copper related pollution when etching the boards

EDIT: Coupling grounds at RF reduces potential EMI problems related to the capacitance between the windings of the transformer. If you don't couple them at RF, there will be AC potentials between them [and the metal case, remebmer to AC couple it to the primary ground] and this may cause RF currents to flow throug audio signal lines. Try to couple them through the shortest path. The resistor in paralell with the capacitor is optional and adds some DC coupling to prevent excessive DC potentials. Some designers use strings of back-to-back clamping diodes to allow for a limited swing, this allows to pass remote and shutdown signals without optocouplers

Also, I don't know how much power you are expecting to transfer reliably, but I've used a pair of IRFZ44V TO-220 devices with suitable heatsinking and 18V gate drive to transfer up to 1KW continuous at 14.4V, so you may be a bit on the overkill side

A simple trick to reduce Rds-on is to get 24..28V by rectifying the push-pull sides of the primary, regulate it to 20V using a 7820 and power the control IC with that voltage. This produces about 18V Vgs drive. An additional diode from +12V to the output of the 7820 ensures proper startup [use also a diode from intput to output of the 7820 to prevent reverse bias at startup]
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Old 20th November 2004, 11:36 PM   #16
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Default Board Image -- Top Layer

Here is the top layer of the circuit board:
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Old 20th November 2004, 11:38 PM   #17
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Default Board Image -- Bottom Layer

Here is the bottom layer of the board
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Old 20th November 2004, 11:39 PM   #18
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Default Summary Of Changes

Summary Of Changes:

1. Its now 2 layers.
2. I put the driver transistors near the mosfets
3. Controller IC has its own ground: "IC-GND" and this trace connects to Primary GND "PGND" at one point like.
4. For heatsinking purposes I moved rectifiers to the walls.
5. Xformer Secondary ground connects straight to cap gnds

What do you think?
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Old 21st November 2004, 12:11 AM   #19
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Eva--

in that resistor in parallel with capacitor circuit that connects both ground planes what do you suggest I use as a standard cap value?

Thanks,
-=Randy
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Old 21st November 2004, 05:40 PM   #20
Lauris is offline Lauris  Lithuania
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Default need help on designing new SMPS.

planing control
TL494(mb TL4941), using Half-Bridge Push-Pull Converter [Vin=260V-240V, Vout=(in calculations 50V actualy for 4xamp ), Iuot=10A(I think this would be enought), f= 100KHz].
now there is few problems:
1: what R & C to use on TL494 pin 5&6 to get 100KHz
2: how to do feed back(this is my first designing SMPS )?
3: what C to use in converter?

Lauris
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