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Old 13th April 2012, 12:55 PM   #1
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Default Car amp protection circuits

Hey all,

I'm hoping to pick your knowledgeable brains...

I'm putting together my own car amp, pretty run of the mill stuff, two channel 75W into 4R each. I'm using an AVR micro for generating the PWM waveform and protection circuits.

So I've got most of it prototyped and working very well, but I need to sort out what protection circuits/sensors are needed. From studying a bunch of pioneer and alpine schematics it seems the following are common:

- Basic fused battery input with reverse voltage protection diodes.
- temp sensing of the heatsink in several places near important devices
- temp sensing of SMPS transformer (just basic thermistors)
- Amp rail voltage monitoring (presumably for an over voltage condition)
- Input voltage monitoring (check within pre-defined range ~10-15V)
- Over current in the amp output
- DC offset on the output.

All these are very straightforward to implement within the microcontroller. I'm just wondering if I've over looked something critical?

Any thoughts would be great. Thanks.
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Old 13th April 2012, 01:05 PM   #2
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Check some way to monitor the possibility of the saturation of the magnetics in case of short overload. It may be done checking for the waveform, in must be a linear ramp, or a ramp on a step depending of the topology, if it deviates from it, there is a risk of short saturation and destroy of semiconductors in the power stage. Only a suggestion.

Regards.
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Old 13th April 2012, 01:13 PM   #3
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Thanks, I understand what you're saying as it's talked about it quite a bit of detail in the texts I've read (A. Pressman and K Billings). However I'm yet to see a practical implementation at this low a voltage and high current. Current mode control is anything but straightforward at this voltage/current.

I think the basic concept taken by OEMs is if it does going into saturation it will probably kill the mosfets but pretty quickly the fuse will blow. Obviously the rate that this happens must be low enough that they can get away with it. If an OEM can get away with it, then it should be fine for my own personal project...
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Old 13th April 2012, 01:45 PM   #4
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I have the Pressman book too, I donīt know about your circuit, but an easy way to check for current is a live resistor in source of the FET and a comparator (like LM393), and forward this info the the uPC and then implement a overcurrent protection circuit. Or a current trafo.

Which circuit are you using, Half Bridge, Full, Push Pull, Flyback?
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Old 13th April 2012, 02:02 PM   #5
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Just the normal push-pull.

My concern for a resistor in the source is the very high peak currents will require a very low resistance, and effectively zero inductance (any inductance in the source is extremely bad.) For that reason also a current transformer could be problematic.

I just haven't seen anyone monitor cycle by cycle current in a medium power car amp before, so I kind of came to the conclusion it was more trouble than the benifits. I'd love to have a look at one if you know of any?
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