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Old 27th December 2005, 04:26 PM   #1
Bensen is offline Bensen  Belgium
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Default Q about Total power dissipation

Hi guys,

I'm using MJE15030 and MJE15031 in an amp design. The power dissipation with a simulation is about 5.5W. The Vce is max120Volts, Ic = +-45mA. When I take a look at the Safe Operation Area graph from the datasheet (see link), I can pull about 55mA.
http://pdf.alldatasheet.com/datashee.../MJE15030.html

But the first page says that:
*Total Power Dissipation @ TC = 25deg -->50W
*Total Power Dissipation @ TA = 25deg -->2W

Can anybody explane me how to work with this, which for example can I take as PD?
If I can work with 50W, than I would have much reserve.

Are my simulated values to close to the SOA curve?

Thanks
Ben
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Old 27th December 2005, 04:43 PM   #2
lineup is offline lineup  Sweden
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50 Watt
is at 25 degrees and WITH infinite heatsink
( 2 Watt without heatsink )

I would say you should not go above 25 Watt, even with a large heatsink.
Not for contunious operation.

Say you have a heatsink with temp increase 1.0 C/Watt
MJE15030/31 has an increase 2.5 C/Watt.
Isolation brick adds 0.5 C/Watt

At 20 Watt and room temp, inside amp case = 25 C
this means the the internal temp in MJE15030 will be:
25 + 20 ( 1.0 + 0.5 + 2.5 ) = 105 C

Now even if max internal, junction, temp is 150 C
105 is a bit too much for best operation.

A larger heatsink may take it down a bit.
I think a junction operation temp above 80 C is not what we want.
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Old 27th December 2005, 05:46 PM   #3
Bensen is offline Bensen  Belgium
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Thanks Lineup,

But how does it come that with Vce=120V, I only can pull +-50mA throught this BJT, and not 0.4A, regarding the datasheet?
50/120 = 0.4A

I know there excists secondary breakdown, is it that?

Power dissipation isn't the problem in my case then, but current. What do you think, am I to close to the SOA curve?

The MJE340/350 devices have a lower PD, but they can handle more current with the same voltages as the MJE150xx's do.

Ben
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Old 27th December 2005, 06:23 PM   #4
AndrewT is online now AndrewT  Scotland
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Hi Bensen,
plug the numbers into that modified spreadsheet you designed.

It shows the driver SOAR and how the load with phase angle pulls the junction towards failure as temp rises or load impedance falls.

Remember that the temperature derating applies to the current read from the SOAR graph at each instananeous operating voltage.

As a guide I would use a 50W driver at 2W to 3W maximum bias dissipation on a 5Cdeg/W sink and keep the peak loading below the permitted DC SOAR for the Tc generated by that previous bias/sink combination.
A 20W driver would be happier running at 1W bias.
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Old 28th December 2005, 07:51 AM   #5
Bensen is offline Bensen  Belgium
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Hi Andrew

I know that with bipolars it is easy to calculate the current throught the drivers, but with FET's this is not that easy. That wy I measure the current and voltages throught and over the drivers and set these values out in the SOA curve from the datsheet.

I am just curious wy a MJE15030 can't deliver 0.4A with a Vce of 120V, while 10V Vce and 5A possible is.

To me it looks that those drivers aren't good fot the job.

MJE340/350 seem to better according the datasheet.
What do you think Andrew? Vce=120V max and Ic=38mA max.

Ben
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Old 28th December 2005, 10:00 AM   #6
AndrewT is online now AndrewT  Scotland
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Hi Bensen,
I should have realised you had a FET output stage connected, oops.

340; 100mA @120V & Tc=25degC (only 12W due to SOAR reduction and second breakdown)
15030; 55mA @120V & Tc=25 etc.

The difference is partly due to the higher voltage rating of the 340 which survives better than 15030 but also due to the smaller die area for the lower P dissipation.
Now apply derating for SAME power on the SAME heatsink giving a higher Tc on the 340 than 15030. eg. 340 Tc=90egC & 15030 Tc=40degC have the same Ic @120V but the 15030 has higher Ic at lower voltages. You need to apply both SOAR and Temp derating to compare specs.

Why are you comparing at such a high voltage?

I find that the worst case is, very close to, reactive load at the output zero crossing (Vo=0V) with Ipk (into resistive load) at the same time. You will need to assume a maximum frequency and FET capacitance to estimate the Iq + extra current from the driver.

With 70Vdc rails the voltage, I would use, is 70V. By the time the reactive load has maximised the voltage across the output devices the load current has fallen to zero (=zero power).

A 200W amp driving a peak current into 8r has Ipk=7A and using Vee=70V gives Ppk=490W (this needs 3 or 4 devices) and then estimate the driver high frequency current.

If you add protection zeners across gate to source you also need to look at currents through the zeners when protection is activated.
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Old 28th December 2005, 05:46 PM   #7
Bensen is offline Bensen  Belgium
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*Why are you comparing at such a high voltage?

Well, my Power supply is +-75V, the two drivers are emitter followers. So, when one is passing current the other one isn't. It is just like output devices.
In theory there should be 150V over one device, but practicly it is about 115V in the simulation.
The max current throught a device is almost the same when the output is delivering max power, as when the amp is idling.


*If you add protection zeners across gate to source you also need to look at currents through the zeners when protection is activated.

I have to do first some research about what is happening when this occurs.
At first look, I would say the current is flowing throught the speaker in combination with the resistor between the two drivers and also all the source resistors.

Simulation with short between gates and sources of mosfet's.
Three point from the signal, Ic throught the drivers and Vce.
+-1.45A with Vce=1V
+-0.5A with Vce=60V
+- 0A with Vce=135V
--> Pd= 30W
This is out of the SOA. Damn!

So, when the protection is activated, the drivers are giving about (1.45A - current throught resistor between the drivers), this has to flow througt the zeners and diodes. I guess everything has gone up in smoke then. Not?
To be exactly --> 160mA throught each zener in simulation.

At the moment I've put the MJE15034/35 devices as drivers, this works fine for the SOA.

Ben
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Old 29th December 2005, 07:44 AM   #8
AndrewT is online now AndrewT  Scotland
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Hi Bensen,
Quote:
In theory there should be 150V over one device, but in practice it is about 115V in the simulation.
Yes, my Sugden P128 uses 120V devices for +-73Vdc rails, so it appears JESugden agrees with your sym.
Quote:
The max current through a device is almost the same when the output is delivering max power, as when the amp is idling.
I think you will find that the driver Ic will vary only slightly for output current upto about half maximum. When output current rises above this limit the driver current then starts to rise substantially. I think this is normally how the EF behaves.
So for low to medium current output the other driver never switches off. Some EF topologies do switch the drivers off but not necessarily at the same time as the output devices switch off.
Quote:
when one is passing current the other one isn't. It is just like output devices.
Often the drivers can stay in classA until very high outputs are delivered.
Quote:
when the protection is activated, the drivers are giving about 1.45A
Put in protection on the VAS just like many designers do (Leach). This removes the drive from the EF and gives some current limit. A base stopper and a gate stopper will also reduce the currents when protection is triggered.
The same applies when IV protection is added to the output.
Most DIY designs put the gate to source Zeners across the driver bases, but I think they would be better located on the gate and source pins AFTER the gate resistor. Or even gate pin to far side of source resistor to include the Rs volt drop in the zener reference trigger.
Finally for your +-75V rails I suggest you check the driver SOAR at Vce= 65V to 75V and not at 100V where the power will be well down. See the 60degree phase angle plot and note how far below SOAR the device powers are when Vce>1.3*Vrail.
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Old 29th December 2005, 10:02 AM   #9
Bensen is offline Bensen  Belgium
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Quote:
Originally posted by AndrewT
Some EF topologies do switch the drivers off but not necessarily at the same time as the output devices switch off. Often the drivers can stay in classA until very high outputs are delivered.

Finally for your +-75V rails I suggest you check the driver SOAR at Vce= 65V to 75V and not at 100V where the power will be well down. See the 60degree phase angle plot and note how far below SOAR the device powers are when Vce>1.3*Vrail.

Hi Andrew,

I made a mistake, my drivers are always in Class A. The idle current is 37mA. Also when Vce is more than 100V, at full output power the Vce is even 140V.
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