Introduction
One day some PCBs and insol an undetermined cause of a PCB of a S-SUB charge went wrong. Lacquer was corrupt the whole side, just the output stage, and right hand only. Like the rest had gone quite well thought of attaching a simple output stage, as he needed an amplifier to make subjective evaluations of sound, along with the monitor.
The result was the S-MOSFET SUB version not can be nothing simpler than an output stage push-pull common-drain MOSFET.
The circuit used is almost the same as in the S-SUB, but with some modification resulting from the output stage employed. Also modified some elements of gain and frequency compensation.
On the right you can see the new circuit.
Topology
Although a priori it easy, an output stage with two transistors, "fat" and a few resistors, in practice it is not so, in fact I had to make a separate PCB because it also incorporates a limitation current.
The first is that the output impedance of the voltage gain stage is very high, and the input capacitance of the mosfet as well. It is not really that of the power BJT (1500-3000pF), but if there is a different feature: as you do not need a driver transistor, the EGV deal directly with that capability. We need this capability in conjunction with high output impedance of the EGV form a pole (lowpass filter behavior) that creates a lag of 90 degrees. If we add the gap resulting from the frequency compensation, we have another 90 º to 180 º invert the output phase. At that time negative feedback ceases to be "negative" and passes to be positive, so that the amplifier becomes unstable and tends to the power rails, and in proper condition ranges (the most normal).
This avoids degenerating the pole, adding a resistor in series with the gates of the output MOS transistors. At audio frequencies this technique has no deleterious effect on the dynamics.
Moreover, the temperature coefficient of the MOSFET is initially negative, charge carriers that use does not generate heat (one of the reasons they do not suffer from secondary breakdown). As I have understood today, there are differences between the temperature coefficient of voltage of strangulation and transconductance. The transconductance decreases with temperature, so that reason can place them in parallel and ideally dispense step allows thermal compensation.
In recent times, practically the only application that uses MOSFET transistors in linear region is the audio and radio, which still survive without rival many vacuum tubes. In all other linear applications with this operation has been replaced by the PWM, more efficient, linear and much higher for low frequencies below 1 kHz and applications requiring high accuracy (0.1% min) and motor control , servos ...
The need to optimize them for switching and a low channel resistance has brought new forms of manufacture: V-MOS-FET Trench, T-MOS-FET ... HEX, and the result is that the voltage bottleneck decreases with increasing temperature, unlike the classic FETs Hitachi, which did not require any thermal compensation. The required temperature compensation is finally adjusted by a VBE multiplier, but degenerated to a diode that is not in contact with the radiator.
HEXFETs The models are of International Rectifier, the famous IRFxxxx. At first, I I used version of IRF640 and IRF9640, two models of 150W TO-220, that will output to 25W more than enough, but there are more suitable as IRFP9240 IRFP240 and also 150W, but in capsule TO-3P.
The difference between these two models is that the TO-3P support more power continuously because the body heat resistant silicon and the radiator is substantially lower. Goes from 1.5 to 1.07, 30% lower. This implies that when the radiator of 0.5 º C / W at 50 ° C in the TO-3P, the silicon transistor will be at 103 º C, while the TO-220 will be 125 º C. The limit is 150 º C, and the colder the transistor will be more linear.
Therefore, it is possible to operate with IRF540 and IRF9540, IRF640 and IRF9640, but I can only recommend and IRFP9240 IRFP240 with powers equal to or more than 80W.Por Finally, the assembly is much simpler because the transistors are mounted on the plate and are attached to the radiator, instead of mounting on the radiator and use cables to connect them to the plate
Subject: ESS.
Student: Pedro Jose Contreras Urbina
Source: http://www.pcpaudio.com/pcpfiles/proyectos_amplificadores/ssub-mosfet/Ssub_mosfet.html
0 comments:
Post a Comment