It cannot!
You can only set a bare minimum bias and hope thermal runaway will not happen.
Hi!
What is the divine panel with a + -63V power supply at only 8R load? My toroid transformer I want to use 630W power and 2 panels in the end.
Regards.
What is the divine panel with a + -63V power supply at only 8R load? My toroid transformer I want to use 630W power and 2 panels in the end.
Regards.
An externally hosted image should be here but it was not working when we last tested it.
Hello,
I need to replace amplifier LME49830 + mosfets. 49830 production ended. I want use APEX HV350. But I don‘t know what is the quality of this amplifier. Comparable with LME49830 (plus 4xpair IRFP240/9240)? I can‘t find anywhere parameters HV350 (THD, slew rate ….).
I have transformer 2x43VAC (i.e. cca 2x60VDC supply-out). I need 200W to PA sat speakers (150-18000Hz).
I don’t want low-end. Can I use HV350?
I need to replace amplifier LME49830 + mosfets. 49830 production ended. I want use APEX HV350. But I don‘t know what is the quality of this amplifier. Comparable with LME49830 (plus 4xpair IRFP240/9240)? I can‘t find anywhere parameters HV350 (THD, slew rate ….).
I have transformer 2x43VAC (i.e. cca 2x60VDC supply-out). I need 200W to PA sat speakers (150-18000Hz).
I don’t want low-end. Can I use HV350?
This amplifier has quite big 0R47 resistors at each output MOSFET's. These source resistors helps to stabilize the current vs temperature. Even more vertical Mosfets has lower drain current dependency vs temperature compared for example with BJT's.
Do not be afraid of big source resistors here. 4 resistors when 4 output mosfets are in parallel gives about only 0.12 Ohm.
If i had to choose between LME and HV350, the choice would be simple, LME is definitely better.
HV350 isn't the best but it's simple enough to be worth giving it a try seeing as LME is not an option anymore.
The source resistors help at better current sharing between the paralleled output devices and with a more precisely biasing, but without the VBE multiplier in thermal contact with the output devices, thermal runaway is still a risk even with vertical mosfets which have an inversion point on their ID versurs Vgs with rising temperature, meaning that at some point giving a fixed bias, increasing the temperature will lower ID, but that inversion point is rather high, too high to rely on it. At low ID as it is with quiescent current, the curve is positive and quite steep.
Dear MarianB,
Yes, source resistors of course helps for better current sharing.
Also stabilizes current. How? Very simple: if Drain current rises because of risen temperature, then more voltage drop (Vrs) is on the source resistor. This additional voltage drop (Vrs) is extracted from Bias voltage (gate-source Vgs) and respectively reduces drain current.
The bigger these resistors are - the better stabilization is produced if the Vbe multiplier in thermal contact with output heatsink it is not used.
The heatsink size is not an issue here but the principle of thermal runaway and how it can be avoided. Sure, the bigger the heatsink is, the better cooling for the power devices, but the positive thermal feedback still happens because the heatsink cannot maintain the junction temperature at a steady 25*C, no matter how large it is, there is still some resistance in heat transfer between the junction and the heatsink, and between the heatsink and the ambient, thus junction temperature cannot stay at 25*C, so ID will increase with rising temperature, and with it, the power dissipated by the mosfets, and thus, the temperature, and so on...
If i have a look at the Intersil datasheet for IRFP240, a see fig. 7, "transfer characteristics", and i look at the 4Vgs line, at 25*C i can estimate an ID of about 0,14, maybe 0,15A. Going up the line to 150*C, for the same bias i can see about 1,3A ( or something like that ), so an increase of ID of almost 1000 percent.
With 0.47 Ohm source resistor, we have a biasing drop on it of a bit more than 800 percent for the same conditions, the difference is significant and can be mitigated by larger source resistors, but then you can waste too much power on them, in stead of just using the Vbe multiplier the correct way. A BD139-16 is quite enough for just about any VBE multiplier, it has much more voltage and current handling capability then you would ever need, it has high enough Hfe, it can be easily mounted on the heatsink, and is very cheap. I cannot see any reason not to use it...
ALl the best.
If i have a look at the Intersil datasheet for IRFP240, a see fig. 7, "transfer characteristics", and i look at the 4Vgs line, at 25*C i can estimate an ID of about 0,14, maybe 0,15A. Going up the line to 150*C, for the same bias i can see about 1,3A ( or something like that ), so an increase of ID of almost 1000 percent.
With 0.47 Ohm source resistor, we have a biasing drop on it of a bit more than 800 percent for the same conditions, the difference is significant and can be mitigated by larger source resistors, but then you can waste too much power on them, in stead of just using the Vbe multiplier the correct way. A BD139-16 is quite enough for just about any VBE multiplier, it has much more voltage and current handling capability then you would ever need, it has high enough Hfe, it can be easily mounted on the heatsink, and is very cheap. I cannot see any reason not to use it...
ALl the best.
That is just as bad, "killing" performance to avoid thermal runaway.
Even slight variations of temperature inside the amp case, will strongly influence the quiescent current of the output devices.
Why would you do that?
I'm sorry but i cannot see any reason to use such poor solutions as you do, electronics can be quite fun and simple if used the right way.
Even slight variations of temperature inside the amp case, will strongly influence the quiescent current of the output devices.
Why would you do that?
I'm sorry but i cannot see any reason to use such poor solutions as you do, electronics can be quite fun and simple if used the right way.
You do not understand this circuit. There is no poor solutions, HV23 is high perfomance amplifier with stable bias... Tested.
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Dear MarianB, thank You that you want to discuss technically. I appreciate that.
Thank You that You have looked at datasheet and calculated possible situation. I will not recalculate now.
First of all it is recommended to adjust bias not to a cold amplifier (room temperature), but when it was in working condition about an hour or so, then readjust. If amplifier was adjusted at working temperature about 50 degrees the procentage difference would be not so significant or even negative (NTC). The datasheet does not show 50 degrees or other temperature You need to measure Yourself current and Vgs. Even more wire-wound type resistor have more resistance when it warms up. When current increases the Power Supply unit's voltage drops (sags) - that also reduces drain current vs the same Vgs (see typical output characteristics in data sheet, Vds vs Ids they are rising when current is relatively small).
Because thermal compensation will be too negative (I have tested it) and if bias current is relatively small 100 mA or less - it will be huge crossover distortion when amplifier warms up or gets hot. The situation is "overcompensation".
And as far as I remember here is submitted a protection circuit with thermal shutdown - and named as mandatory. So once more - do not be afraid here of the thermal runaway, Mr. Apexaudio knows what he is doing
Yes I see, that VAS transistors that are in CFP loop have thermal contact to the heatsink. So even more I do not see any problems with thermal runaway.
Have You measured the slew rate and how this amp acts when clipping does ocour? and THD vs frequency? By the way these topology amps has poorer PSRR, have you noticed that it hums at idle - if power supply is unregulated/unfiltered? (sorry if it was posted, but I overlooked it).
Guitar.mod, agreed, the simple Vbe multiplier tends to overcompensate, but that is not an BD139 issue, but the simplicity of the biasing circuit, a single BE junction cannot accurately control 2xVgs, but there are options to improve it, there are quite a large number of different Vbe multiplier circuits with custom compensation, so it is quite doable.
I'm sorry but there isn't much to understand on such a simple design.
I think you and me have different definitions of high performance, and good electronics. A simple CFP VAS cannot compensate for the crudeness of the other stages, but whatever...
All the best.
I'm sorry but there isn't much to understand on such a simple design.
I think you and me have different definitions of high performance, and good electronics. A simple CFP VAS cannot compensate for the crudeness of the other stages, but whatever...
All the best.
It is kind of late after all this work but I would like to remind you that ADCOM GFA5500 uses five IRFP240 and five IRFP9240 MOSFETs in its power stage with 0R68 ohm source resistors. The supply is +/-60V and amplifier is commercially rated as 200W to 8 ohms. It is Class AB biassed with large heatsinks.
You can compare your circuit with the attached circuit diagram.
You can compare your circuit with the attached circuit diagram.