2 hours ago
Hi Guys
We use power mosfets in various applications in tube amps and in hifi amps. These are LINEAR applications, so we need robust devices that can withstand heat with a modest heat sink arrangement. As TUTs detail, mosfets are generally designed for switching applications where the device is fully 'on' while conducting current, or fully 'off' while withstanding voltage. In both states, the heat dissipated by the mosfet is ideally zero.
The most common packages for mosfets are:
TO-220, good for up to 50W
TO-3P, TO-247, good for up to 150W
TO-264, good for up to 300W
There are many other cases that are minor variations of the above and generally identical in power dissipation.
You will see mosfets with massive power ratings over 1kW, but have the TO-247 or TO-264 case, and this tells you right away that the devices are for switching only. A look at the safe-operating-area graph displays plots of various pulse times, but NOT 100ms, which is considered by some designers to be equal to DC operation - which it is not. When looking at one of these graphs, the scale is log on both axis, current on the y-axis and voltage on the x-axis. You will see that the vertical space between the 1ms line and 10ms line is the same as for the 100us line and the 1ms line. From this you can use a scale (ruler) to see where the 100ms line would be, or further, where the DC line would be. At DC, the 1kW+ device is now less than a couple of hundred watts.
Another thing that is assumed for those super-high power ratings is water cooling.
In general, because of the optimisation for switching, a given mosfet will have the smallest die that can handle the rated current and voltage. For linear and DC applications the mosfet will be 'on' all the time, conducting varying current depending on the load. For this we have to go to larger die sizes and to safely achieve that we go to larger package sizes. The default for linear use of mosfets is to use the largest package size that is reasonable, meaning up to TO-247. There are large modules with screw terminal connections, but these are for switching use in very high-power inverters.
Whether we are building an amp from scratch or modifying an amp, the physical arrangement often limits us to using the chassis as a heat sink, or maybe to the use of a real but small-ish heat sink. A fan may be necessary in some applications.
ON-resistance will limit mosfet performance at high currents and low voltages. This will be relevant for audio power amps, or for a low-voltage regulator, where a regular bipolar-junction-transistor would be a better choice.
Many mosfets have built-in gate protection zener diodes. However, it is safer to use external gate zeners to avoid having to replace the mosfet should its own zener diodes be damaged. Generally, a single 12V zener oriented correctly from source to gate will provide protection against static discharge. A gate-stop resistor should be placed directly at the mosfet gate.
Mosfets come in two forms, as n-channel and p-channel, complementary materials with complementary characteristics, similar to NPN and PNP BJTs. P-channel mosfets are inherently linear-rated since the process to manufacture them corresponds to linear requirements. Most applications we encounter use only n-channel devices, which are easier to produce with voltage ratings up to 4kV and current ratings exceeding hundreds of amperes, neither extreme applying to what we do.
Despite their impressive ratings, mosfets can be rather delicate when it comes to heat management. Use large-package devices. Assure free air flow over the mounting area. Add a fan if possible. Add a heat sink with the fins oriented correctly. These recommendations apply to all of the uses below.
Power Supply Regulator
To use a mosfet as the pass element of a supply regulator it must be able to withstand the incoming voltage and the maximum current load. This product could well exceed the capability of a single device, warranting the use of parallel devices. Of course, there are ways to alleviate this situation by limiting load current until the regulator output voltage has risen to its rated value, and thus reducing the voltage across the device before load current begins.
In a high-voltage regulator, the ON-resistance is not an issue and the device may exhibit a few ohms; current ratings will likely far exceed the load current; voltage rating 50V or higher than the unloaded input value is sufficient. It is often the case that a much higher-voltage rate device is not much more expensive than the adequate device.
Active Hum Filter
The same caveats apply here as for the power supply regulator. Under normal circumstances, the active hum filter application is a bit easier on the mosfet as the voltage across the mosfet will settle to be equal to the gate-to-source voltage required to turn the device 'on'. Output voltage inherently rises slowly, which will control the load current until maximum output is reached. Large package sizes are preferred for best performance. In a tube amp the use of a TO-220 here is a mistake and will impair tone, for factors imposed by the small die within the small case.
Power Scaler
In Power Scaling applications for tube amplifiers, we need high voltage devices with moderate current capabilities. Excess voltage rating is not too important as long as there is at least 50V above the highest voltage in the circuit that the mosfet is used in. Because there are high ambient temperatures created by the tubes themselves, the larger mosfet package sizes should be used exclusively.
Audio Power Amplifier
A pure solid-state power amplifier works at voltages to accommodate the peak voltage required by the speaker.For example, an 8-ohm load driven to 100Wrms needs a peak power of 200W, corresponding to 40Vpk at 5Apk. This requires +/-40Vdc for ideal loss-less nonexistent devices. Most output stages are voltage followers which place the turn-on voltage of the output device in series with the load, requiring that the loaded supply voltage be higher than this sum. Using mosfets, we should have at least 5-6V more than the peak load voltage, so +/-46V as a minimum loaded value. Generally, designs aim for +/-55V to +/-65V.
Waste heat is based on the signal shape and the load characteristics. For a resistive load and sine wave, the ideal maximum waste heat is 25% of the load power. Real world conditions drive this to 35% or so. if one biases the amp for class-A operation then waste heat approaches 100% or even higher.
Because of the nature of mosfets compared to BJTs, the latter perform much better with less complicated circuitry and/or compensation. Mosfets benefit more from feed-forward techniques and complex frequency compensation, unless the goal is to experience the mosfet's unique distortion characteristics.
This is the only application where mosfets require back-to-back zener gate protection. The usual practice is to have a series-pair of zeners for the upper circuit half (n-mosfets) and a second zener pair for the lower circuit half (p-mosfets).
Tube Selection
In a tube power amp with multiple tubes, individual tubes and/or pairs of tubes can be turned 'on' and 'off' using cathode switching. This is generally achieved using mechanical switches, but TUT shows that mosfets can be used here quite easily. This is one application where a small package like TO-220 is ample, since this is a pure switching application.
Cathode Follower Replacement
A few tube circuits use a cathode-follower directly tied to a common-cathode gain stage. The CF can be replaced by a mosfet source-follower if the triode is to be reconfigured for gain. In this case, the mosfet can be a TO-220 of nearly any current rating as long as the voltage rating is sufficient for the unloaded supply voltage. it is typical that the circuit resistances stay the same and the mosfet will conduct 1mA or so, and does not require a heat sink.
Mains Current Limit
Back-to-back n-channel mosfets can be used to control mains voltage, either as a switch, or linearly to control current. the latter is useful when a DC mains-blocker is used. The mosfets must be able to withstand the peak mains voltage, preferably with some margin if the unit is used where the mains can become excessive.
We use power mosfets in various applications in tube amps and in hifi amps. These are LINEAR applications, so we need robust devices that can withstand heat with a modest heat sink arrangement. As TUTs detail, mosfets are generally designed for switching applications where the device is fully 'on' while conducting current, or fully 'off' while withstanding voltage. In both states, the heat dissipated by the mosfet is ideally zero.
The most common packages for mosfets are:
TO-220, good for up to 50W
TO-3P, TO-247, good for up to 150W
TO-264, good for up to 300W
There are many other cases that are minor variations of the above and generally identical in power dissipation.
You will see mosfets with massive power ratings over 1kW, but have the TO-247 or TO-264 case, and this tells you right away that the devices are for switching only. A look at the safe-operating-area graph displays plots of various pulse times, but NOT 100ms, which is considered by some designers to be equal to DC operation - which it is not. When looking at one of these graphs, the scale is log on both axis, current on the y-axis and voltage on the x-axis. You will see that the vertical space between the 1ms line and 10ms line is the same as for the 100us line and the 1ms line. From this you can use a scale (ruler) to see where the 100ms line would be, or further, where the DC line would be. At DC, the 1kW+ device is now less than a couple of hundred watts.
Another thing that is assumed for those super-high power ratings is water cooling.
In general, because of the optimisation for switching, a given mosfet will have the smallest die that can handle the rated current and voltage. For linear and DC applications the mosfet will be 'on' all the time, conducting varying current depending on the load. For this we have to go to larger die sizes and to safely achieve that we go to larger package sizes. The default for linear use of mosfets is to use the largest package size that is reasonable, meaning up to TO-247. There are large modules with screw terminal connections, but these are for switching use in very high-power inverters.
Whether we are building an amp from scratch or modifying an amp, the physical arrangement often limits us to using the chassis as a heat sink, or maybe to the use of a real but small-ish heat sink. A fan may be necessary in some applications.
ON-resistance will limit mosfet performance at high currents and low voltages. This will be relevant for audio power amps, or for a low-voltage regulator, where a regular bipolar-junction-transistor would be a better choice.
Many mosfets have built-in gate protection zener diodes. However, it is safer to use external gate zeners to avoid having to replace the mosfet should its own zener diodes be damaged. Generally, a single 12V zener oriented correctly from source to gate will provide protection against static discharge. A gate-stop resistor should be placed directly at the mosfet gate.
Mosfets come in two forms, as n-channel and p-channel, complementary materials with complementary characteristics, similar to NPN and PNP BJTs. P-channel mosfets are inherently linear-rated since the process to manufacture them corresponds to linear requirements. Most applications we encounter use only n-channel devices, which are easier to produce with voltage ratings up to 4kV and current ratings exceeding hundreds of amperes, neither extreme applying to what we do.
Despite their impressive ratings, mosfets can be rather delicate when it comes to heat management. Use large-package devices. Assure free air flow over the mounting area. Add a fan if possible. Add a heat sink with the fins oriented correctly. These recommendations apply to all of the uses below.
Power Supply Regulator
To use a mosfet as the pass element of a supply regulator it must be able to withstand the incoming voltage and the maximum current load. This product could well exceed the capability of a single device, warranting the use of parallel devices. Of course, there are ways to alleviate this situation by limiting load current until the regulator output voltage has risen to its rated value, and thus reducing the voltage across the device before load current begins.
In a high-voltage regulator, the ON-resistance is not an issue and the device may exhibit a few ohms; current ratings will likely far exceed the load current; voltage rating 50V or higher than the unloaded input value is sufficient. It is often the case that a much higher-voltage rate device is not much more expensive than the adequate device.
Active Hum Filter
The same caveats apply here as for the power supply regulator. Under normal circumstances, the active hum filter application is a bit easier on the mosfet as the voltage across the mosfet will settle to be equal to the gate-to-source voltage required to turn the device 'on'. Output voltage inherently rises slowly, which will control the load current until maximum output is reached. Large package sizes are preferred for best performance. In a tube amp the use of a TO-220 here is a mistake and will impair tone, for factors imposed by the small die within the small case.
Power Scaler
In Power Scaling applications for tube amplifiers, we need high voltage devices with moderate current capabilities. Excess voltage rating is not too important as long as there is at least 50V above the highest voltage in the circuit that the mosfet is used in. Because there are high ambient temperatures created by the tubes themselves, the larger mosfet package sizes should be used exclusively.
Audio Power Amplifier
A pure solid-state power amplifier works at voltages to accommodate the peak voltage required by the speaker.For example, an 8-ohm load driven to 100Wrms needs a peak power of 200W, corresponding to 40Vpk at 5Apk. This requires +/-40Vdc for ideal loss-less nonexistent devices. Most output stages are voltage followers which place the turn-on voltage of the output device in series with the load, requiring that the loaded supply voltage be higher than this sum. Using mosfets, we should have at least 5-6V more than the peak load voltage, so +/-46V as a minimum loaded value. Generally, designs aim for +/-55V to +/-65V.
Waste heat is based on the signal shape and the load characteristics. For a resistive load and sine wave, the ideal maximum waste heat is 25% of the load power. Real world conditions drive this to 35% or so. if one biases the amp for class-A operation then waste heat approaches 100% or even higher.
Because of the nature of mosfets compared to BJTs, the latter perform much better with less complicated circuitry and/or compensation. Mosfets benefit more from feed-forward techniques and complex frequency compensation, unless the goal is to experience the mosfet's unique distortion characteristics.
This is the only application where mosfets require back-to-back zener gate protection. The usual practice is to have a series-pair of zeners for the upper circuit half (n-mosfets) and a second zener pair for the lower circuit half (p-mosfets).
Tube Selection
In a tube power amp with multiple tubes, individual tubes and/or pairs of tubes can be turned 'on' and 'off' using cathode switching. This is generally achieved using mechanical switches, but TUT shows that mosfets can be used here quite easily. This is one application where a small package like TO-220 is ample, since this is a pure switching application.
Cathode Follower Replacement
A few tube circuits use a cathode-follower directly tied to a common-cathode gain stage. The CF can be replaced by a mosfet source-follower if the triode is to be reconfigured for gain. In this case, the mosfet can be a TO-220 of nearly any current rating as long as the voltage rating is sufficient for the unloaded supply voltage. it is typical that the circuit resistances stay the same and the mosfet will conduct 1mA or so, and does not require a heat sink.
Mains Current Limit
Back-to-back n-channel mosfets can be used to control mains voltage, either as a switch, or linearly to control current. the latter is useful when a DC mains-blocker is used. The mosfets must be able to withstand the peak mains voltage, preferably with some margin if the unit is used where the mains can become excessive.