"The Wire AMP" Class A/AB Power Amplifier based on the LME49830 with Lateral Mosfets - Page 213 - diyAudio
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Old 29th March 2013, 01:06 PM   #2121
AndrewT is offline AndrewT  Scotland
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If one "starts up" with a very low bias current to enable the SMPS to start up normally, then I would be very tempted to use a timer to activate the next stage of output bias.
The highest stage of output bias could be activated with a manual Summer/Winter switch, or could be done with a temp sensor and switched to Summer setting if the sink gets too hot. The comparator for the automated S/W switch over should have a high hysteresis Feed Back to prevent frequent back and forth switching.
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Old 29th March 2013, 01:34 PM   #2122
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Andrew,

I have a schematic that does all of that and quite a bit more. Including latching circuits for momentary switches, 555 timers for delays, hysterises on the comparators etc was all calculated. The layout was a bit too unwieldy by the time I added the rest of the functions that I wanted I could do it all for similar cost and have a more compact with a dedicated microprocessor. So I shelved that half done layout and will proceed on a hybrid where I replace parts of that into a microprocessor. I will ensure that the microprocessor and any remote comms are in deep sleep state unless necessary, they will make no difference to EMI/RFI near the amps.

I'll share what I come up with


Chris
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Old 7th April 2013, 10:37 AM   #2123
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Question Measuring & Setting Bias

Hi, another daft but important question... BIAS

Measuring: Multimeter in series with the Output V+ power supply & set to Ampere. As the trim pot is adjusted the current draw increases / decreases.

Setting: Bias required 360mA - MM in series, power on, trim pot set to minimum. Adjust trim pot until a stable 360mA is shown on MM...

Question: I take it that when it's said a bias of 360mA, that this is per rail? So we're getting a +/- 360mA (720mA) current draw from the supply.

Or... 360mA total, and we set 180mA per supply rail?
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Old 7th April 2013, 01:55 PM   #2124
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Quote:
Originally Posted by BuildMeSomething View Post
Hi, another daft but important question... BIAS

Measuring: Multimeter in series with the Output V+ power supply & set to Ampere. As the trim pot is adjusted the current draw increases / decreases.

Setting: Bias required 360mA - MM in series, power on, trim pot set to minimum. Adjust trim pot until a stable 360mA is shown on MM...
All the above is correct and exactly describes the bias procedure. Remember that if you mounted the pots the way the silk layer indicates to, then minimum bias correlates to the pot being turned fully clockwise (all the way "up")

Quote:
Originally Posted by BuildMeSomething View Post
Question: I take it that when it's said a bias of 360mA, that this is per rail? So we're getting a +/- 360mA (720mA) current draw from the supply.

Or... 360mA total, and we set 180mA per supply rail?
This is a commonly misunderstood area. Current doesn't add in series, it's the same throughout the loop. It flows (in the conventional sense) from the V+ terminal, through the N channel fet, then through the P channel fet, and back into the V- terminal completing the loop. The value you read on the multimeter is the current through this loop, and if you set it to say 360mA, then you'll measure the same value if you connect the DVMM between the V- supply and the P fet. The total current drawn from the supply is 360mA.

If you want to know the power dissipated by the output stage, then you need to consider the current through the loop (360mA) and the total voltage differential which is your |V+| + |V-|. If you have +/- 20V rails, then your total voltage would be 40V and your total dissipation in the output stage would be P=V*I or 40V * 360ma = 14.4W.

Regards,
Owen
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Old 7th April 2013, 03:48 PM   #2125
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Question Understanding it...

Brilliant, understood.

So using the DPS-600 +/-45/55 I get 90V * 0.360A = 32.4W

A higher Bias would equate to higher power dissipation & a need larger heatsinks.


In terms of Power Supply requirements, it should be capable of sustaining the current drawn by biasing, plus the required draw for music & head room?

Using a regulated power supply capable of 6 Amp [+/- 45V], as the bias increases, this decreases the amount left for powering the load & head room?

If bias is set to 1A and the remaining potential current draw is split 50/50, giving 2.5A to drive the load [8Ohm]. I should get 50W output?

However, at full output, the +/-45V power rails would cause a larger drain than the allocated 2.5A, the amp/supply would distort, clip, overheat, blow-up?

Paul
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Old 7th April 2013, 04:42 PM   #2126
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Quote:
Originally Posted by BuildMeSomething View Post
Brilliant, understood.

So using the DPS-600 +/-45/55 I get 90V * 0.360A = 32.4W

A higher Bias would equate to higher power dissipation & a need larger heatsinks.
Exactly correct.

Quote:
Originally Posted by BuildMeSomething View Post
In terms of Power Supply requirements, it should be capable of sustaining the current drawn by biasing, plus the required draw for music & head room?

Using a regulated power supply capable of 6 Amp [+/- 45V], as the bias increases, this decreases the amount left for powering the load & head room?
Not really... the bias current always contributes to the output current up until the point where the current delivered to the load exceeds the bias current, then the surplus required has to be delivered from the supply.

This also defines the point the amplifier leaves class A, as one of the output devices will stop conducting when twice the bias current is exceeded (for push-pull class AB). In this case, the amplifier will run in class A until the load requires more than 720mA peak. For an 8 ohm load, that means roughly 2 watts RMS.

It also means that the load on the power supply up until the 2W point is constant. The current either travels thought the transistors as bias current, or into the load as output power.

Quote:
Originally Posted by BuildMeSomething View Post
If bias is set to 1A and the remaining potential current draw is split 50/50, giving 2.5A to drive the load [8Ohm]. I should get 50W output?

However, at full output, the +/-45V power rails would cause a larger drain than the allocated 2.5A, the amp/supply would distort, clip, overheat, blow-up?

Paul
Again, not really. If you bias at 1A, the output stage will now operate in class A up to 2A peak of output current. For an 8 ohm load, that's roughly 16W without leaving class A.

If you want to know what your power supply limited output power would be, then you need to look at your peak output voltage, and your load, and figure out what your peak output current will be.

If you have 45V rails on the fets, and 55V rails on the front end, you should be able to swing within a few volts peak of the rails. Let's be conservative and say you can output 41V peak. With an 8 ohm load, that gives a peak output current of 5.125A. Your supply has to be able to provide that level of current, otherwise your rails will sag and your output will voltage clip earlier than the estimated 41V.

The above scenario would have a power output of 112 watts RMS into an 8 ohm load.

Note that this is regardless of the bias current. If you set the bias current to 2.5A, then your output power capability remains the same, as long as your supply can provide the needed peak output current.

In the case of the DPS600, you cannot bias at super high levels, as the continuous power draw from the supply will cause it to heat up significantly and shut down (or explode) due to thermal overload. Even though the supply is rated at 6A output current, it cannot do this continuously without getting too hot.

Regards,
Owen
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Old 7th April 2013, 05:11 PM   #2127
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Got you, Thanks
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Old 8th April 2013, 10:41 AM   #2128
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Quote:
Originally Posted by opc View Post
............In the case of the DPS600, you cannot bias at super high levels, as the continuous power draw from the supply will cause it to heat up significantly and shut down (or explode) due to thermal overload. Even though the supply is rated at 6A output current, it cannot do this continuously without getting too hot.......
I thought the SMPS was rated at a continuous 600W.
From +-45V that would equate to just over 6A continuous.

If we were to take the continuous 6A as the current limit of the DPS600 then it should be able to hold the regulated +-50Vdc at upto 6A continuous.

Or is the DPS600 rated at lower than 600W continuous?
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Old 8th April 2013, 10:45 AM   #2129
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by opc View Post
...................If you have 45V rails on the fets, and 55V rails on the front end, you should be able to swing within a few volts peak of the rails. Let's be conservative and say you can output 41V peak. With an 8 ohm load, that gives a peak output current of 5.125A. Your supply has to be able to provide that level of current, otherwise your rails will sag and your output will voltage clip earlier than the estimated 41V..............
peak current demand of a speaker is worse than the resistively modeled current shown above.
A single speaker driver is likely to demand at least 50% more than this on fast transients.
A passive crossover speaker is likely to demand at least 100% more and can be upto 5times the resistive current demand for fast transients.
I design for a peak current demand of 3times the resistive model, i.e. 8ohms requires 15Apk.
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Old 8th April 2013, 11:12 AM   #2130
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Andrew,

I expect the thermal limit of those SMPS is dependent on the mounting arrangement for thermal reasons.

Roberto posted this on the DPS600 thread:

Quote:
Originally Posted by AP2 View Post
Hi, in this all..dps-600 break at >82 in side of Power-Unit. just a info
From the datasheet that I have there are also the following hard protection limits in place:

- Output to a short circuit
- 14Amps drawn for longer than 100ms

The heat spreader provided on the DPS600 has mounting holes. Can be mounted to a heat sink, floor of enclosure or separate aluminium sheet. Roberto recommended 70mm wide x 5mm thick alu sheet for mounting 2 x DPS600. For mine I am using two separate 50mm wide x 3mm thick alu sheets spaced off the floor with plenty of ventilation holes above and below.

I think you'd come against a point where the thermal interface between the heat bar on the DPS600 and the alu sheets is limiting the thermal dissipation of the whole unit, this would become the limit for the continuous current rating. Though we don't have enough data available to do this other than by testing at different bias settings and measuring temperatures observed. This is no different to the limitation for a linear regulated power supply continuous rating, is it?

I expect that Owen was just using some intuition with the continuous current limitation. With 6A bias wouldn't the output stage heatsinking make the DPS600 a false economy in saved space? Remember that these PSU's have a fixed output voltage so you're dissipating for 45 or 55V (depending on the version purchased). So somewhere upwards of 400W idle thermal load??
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