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Old 27th April 2012, 06:24 PM   #1
akis is offline akis  United Kingdom
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Default Load and device protection

I am building (slowly) a device that will produce a sinusoidal of fixed frequency 200 KHz at 50 V peak, into a load of approximately 15 Ohms. The supply will be a +/- 6V so I have used a transformer to raise the output to the required 50 V.

I am attaching the schematic as I have already built it and tested it.

Under certain conditions it is possible that the load may drop considerably, from 2000 Ohms to anything, even a short. When this happens I want the device to "throttle".

Currently I am feeding the prototype from my bench PSU which automatically employs current limiting, and this protects the device and the load from such conditions.

What can I do to employ some type of similar protection but without using my regulated and current limiting bench PSU?

As a first step I thought of using the voltage that gets developed across R10 and R11 - but I am not sure what to do when this voltage reaches a cerain threshold.

I'd be grateful for any help.

I have designed the output stage to be something like this
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File Type: jpg output stage.JPG (135.2 KB, 241 views)
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Old 28th April 2012, 07:41 AM   #2
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You need circuitry like this VI Limiters in Amplifiers ......fig 1

It collapses the voltage across the biasing circuit , in your case the 4 diodes , to limit the output current .
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Old 28th April 2012, 08:07 AM   #3
Mooly is offline Mooly  United Kingdom
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Just thinking aloud

Efficiency is a major problem. Perhaps reduce those 0.22's.

Is distortion an issue ? Reduce the bias to Class B

Do you need a split supply ?

Use a low value resistor (say 0.01ohm) in the power supply to sense current using an opamp and use that for limiting.

Can use the transformer winding inductance be used to your advantage if it is running at a fixed frequency and you can tune the winding for greater efficiency with cap.

Resistors across B and E for output transistors to stop cross conduction.
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Old 28th April 2012, 02:40 PM   #4
akis is offline akis  United Kingdom
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Quote:
Originally Posted by epicyclic View Post
You need circuitry like this VI Limiters in Amplifiers ......fig 1

It collapses the voltage across the biasing circuit , in your case the 4 diodes , to limit the output current .
Very good. This "VI" is what I was also trying to do but could not get right. I think I have it now, works on the simulator, will also go build it now. I am not sure I understand the purpose of 82K resistors, I left them out. My new schematic now looks like this.
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Old 28th April 2012, 02:49 PM   #5
AndrewT is offline AndrewT  Scotland
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You have an I limiter.
It takes virtually no account of the heat developed across the output device during a limiting incident.

A VI limiter does take account of Power Dissipation.
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Old 28th April 2012, 02:52 PM   #6
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Be aware that this is not a VI limiter, it is only an I limiter.
If you need to protect your devices from exceeding the Safe Operating Area you need a VI limiter.

A VI limiter not only takes the current info but also a sample of the Vce and does the limiting such that the allowed I gets lower with increasing Vce.
An I-only limter does noting, for example, to protect against loads that are inductive or capacitive like many speakers especially with complex xovers.

Edit: Andrew, we xposted....

Edit2: the 82 k resistor referred to above, in fig 1, is essential. It makes it a VI limiter. Don't leave it out.

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Old 28th April 2012, 02:54 PM   #7
AndrewT is offline AndrewT  Scotland
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That's OK, we corroborate each other's information. Then doubters are more likely to listen.
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Old 28th April 2012, 06:38 PM   #8
akis is offline akis  United Kingdom
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Quote:
Originally Posted by Mooly View Post
Just thinking aloud

Efficiency is a major problem. Perhaps reduce those 0.22's.

Is distortion an issue ? Reduce the bias to Class B

Do you need a split supply ?

Use a low value resistor (say 0.01ohm) in the power supply to sense current using an opamp and use that for limiting.

Can use the transformer winding inductance be used to your advantage if it is running at a fixed frequency and you can tune the winding for greater efficiency with cap.

Resistors across B and E for output transistors to stop cross conduction.
I did not think the 0.22s would be a problem as they are less than 1/63 of the expected load of 13-14 Ohms. The prototype is with 0.15R.

I am not sure if distortion will be an issue, but I have erred on the side of caution and have provided as much bias as necessary so that it looks good on the oscilloscope.

The previous stages use a split supply, and I just used that. In reality it will be 2* 6V batteries. Would a single supply make anything better ?

I had originally tried to build something similar to your idea : sense the voltage drop on the 0.22R (which is too low to be of any use) and then amplify it using a MC33072. I had referred the sense voltage to ground and the output was also referred to the ground, and had no idea what to do next.

The op-amp approach is better for a steeper protection scheme, ie a protection that will trigger suddenly , not gradually. Here is a half-finished schematic :
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File Type: jpg push-pull-VI-opamp.JPG (217.9 KB, 184 views)
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Old 28th April 2012, 07:25 PM   #9
akis is offline akis  United Kingdom
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Could you please explain to me what error condition the 82K resistor is meant to catch? With the current limiting in place there is no possible combination of Vce + Ic that will even go anywhere near the limits of the output transistors ?

Sorry for being slow
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Old 28th April 2012, 07:51 PM   #10
cbdb is offline cbdb  Canada
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Quote:
I did not think the 0.22s would be a problem as they are less than 1/63 of the expected load of 13-14 Ohms.
The load is actual 13-14 ohms divided by the transformer ratio squared, so closer to .1 ohms. The load (15 ohms at 50v) draws 167 watts peak. So at 6 volts you need 27 amps peak! So the .22 ohm resistor will drop over 6 volts, or the full supply voltage. Try .03 ohms.
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