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Old 8th December 2005, 11:08 AM   #21
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Problem solved, a softstart circuit did it.
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Old 8th December 2005, 12:31 PM   #22
Eva is offline Eva  Spain
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Flyback and Boost converters may latch-up at start-up when power supply impedance is too high, this happens because the error amplifier is asking for a too high peak current level that cannot be reached or for a too high duty cycle that doesn't allow the output voltage to rise.

As you have found out, the problem is reduced to a much lesser extent by adding a soft-start feature, that may be as simple as clamping the output of the error amplifier to a slowly rising voltage ramp. Further protection is achieved if a undervoltage lockout feature is added in such a way that when the supply voltage gets too low the soft-start voltage ramp is restarted from zero, thus resetting the PSU.

Concerning your design, I think that you should put the transformer, the switches, the current sense resistors and the input and output filter capacitors in a thightly laid-out PCB with short tracks, because with such long wires and such wide current-loop areas, your point-to-point prototype is likely to radiate EMI like hell. Don't you have an oscilloscope to check waveforms and ringing issues?
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Old 8th December 2005, 01:19 PM   #23
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No i don't have a scope I think emi is kept to a minimum as i no longer disturb tv and the audio out from the amp is quiet with no signal, juse faint hiss but thats normal for a smp like this and the puters soundcard out is not the best either.

When i box the thing i gonna use thick wire on the input part, and shortest possible wires to minimize excess inductance.

Now i have made a circuit that drives the controller from the output so if the smps gets overloaded it just shuts down and requires a manual restart.
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Old 8th December 2005, 05:03 PM   #24
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Audio signals are quite hard to distort in an audible way with EMI, whose most usual symptom is just an increased noise floor. Also, TV and FM radio are somewhat hard to distort since they employ high frequencies well above 50Mhz. However, AM radio is easily distorted because it operates well below 50Mhz, in the frequency range where most SMPS radiate, so I would recommend that kind of testing.
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Old 8th December 2005, 05:27 PM   #25
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I am going to mount the thing in a metal box as well as shorten all wires as mutch as possible.

I suppose that the performanve could only be better from where i stand now if i shorten wires and add thickness to the ones in the primary part.

Which box material would be best, a steel case from a computer psu or a aluminum box ?
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Old 8th December 2005, 06:38 PM   #26
Eva is offline Eva  Spain
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Ferrous materials are preferable since they provide some magnetic shielding.
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Old 8th December 2005, 09:07 PM   #27
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Ok. But as i meant this to be used in car, shielding would be contained by the car chassis.

I think that a real car amp emits more EMI if run like my diy version tho. Most car amps i have seen inside doesent even have a input filter, only some small coils on the output on the amp rails so it wont hiss from the smps switching.
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Old 8th December 2005, 11:48 PM   #28
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Remember that car-audio amplifiers employ push-pull power supplies that generate much less EMI than a flyback to start with. They require little filtering and little attention to stray magnetic fields due to the toroid core. Manufacturers use that approach beacuase they know its simplicity and advantages quite well, even at the expense of bigger size.
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Old 9th December 2005, 02:37 AM   #29
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Default Car Amp SPMS

Hi Tekko,

Al always, Eva is right. And the most common PWM chip you will fing in almost every car amp employing a DC-DC Converter is the TL494. Same is also true for almost any AT or ATX PSU box.

While the '494 is, by far, the most widely used chip, there are much better ones, like the SG3525 (voltage-mode operation) or the UC1846/2846/3846 (current-mode operation), because these two chips have true totem-pole outputs, enabling them to drive almost any MOSFET or power BJT (with speed-up turn-off caps for the BJTs) directly, without the need for a separate driver circuit.

Another thing to take into consideration is that you are looking for +/-35v @ 2.5A = 175W. Assuming a 65% efficiency rating, make this something like 270W. OK, so at low line of 11V, 270W will result in the primary MOSFET drain current of just under 25A. Remember for a flyback, the PEAK drain current for the primary switching MOSFET is something like 3.2X the max rated drain current. EVA, correct me on this if I'm wrong.

So for a 25A drain current, you will be peaking out at just shy of 80A at full power. There are few MOSFETs that can handle that kind of drain current and still keep it together, even with excellent heatsinking.

Using the push-pull topology, like EVA suggests, with voltage-mode, or perhaps even current-mode control GREATLY reduces your PEAK drain currents, cuts your EMI by at least half, and just works out nicely. You could use the TL494, but if you can get an SG3525, or a UC1846, or even an MC33025, you would have some great beginnings for a really good supply. Try Rod Elliot's pages or search some of the Power Supply threads here for some great examples.

Hope this helps!

Regards,

Steve
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Old 9th December 2005, 03:32 AM   #30
Eva is offline Eva  Spain
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The primary peak current at full load in a flyback converter depends on a lot of things:

- Turn ratios.
- Primary inductance
- Continuous or discontinuous mode.

These parameters may be tailored to reduce the primary-side peak current, that is inherently high, at the expense of increasing the secondary side peak current, that is inherently low.

It's quite simple to make a working flyback, but it's quite hard to make it optimum.

Tekko: Have you done the corresponding math in order to choose optimum primary inductance, turn ratios and operating frequency? I have written a small program that helps me to calculate flybacks...
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