Hello, everyone.
I'm trying to build a high-efficiency AC MOSFET relay, configured as the classic back-to-back n-channel pair, such as the one shown here.
JDT actually drew an excellent driver for this configuration a few years ago, which can be found at the end of this thread.
The basic circuit works because most (if not all) MOSFETS have a inherent reverse diode built into them. So when current flows in one of the MOSFETS in the allowed on-direction (from positive Drain to negative source), the other MOSFET will let it through its body diode (from positive source to negative drain) independently of the voltage present at its gate. I hope I'm making sense here... This behavior is reversed as the polarity changes.
The issue here is that even though a MOSFET's on-resistance is usually quite low (about 0.55 ohms in the IRF740, for instance) most MOSFET's body diodes are not very efficient. The body diode present in the IRF740 has a voltage drop of about 2V, for instance. That means that the MOSFETs will have a tendency to heat up when used in this configuration, even if we're running them well below their rated current.
Consider this example, if we were to run 8 amps through the IRF740's body diode, the 2V voltage drop would force it to dissipate about 16 watts of heat, which in my book is quite a lot... (feel free to correct me if I'm wrong or just saying something stupid)
If I were to add an external normal or schottky diode in parallel with each of the MOSFET's body diode then maybe things would improve quite a bit, since they would be relieving the MOSFET's body diodes from some current.
Considering that a MOSFET can behave like a diode when properly configured (like shown in this video) and that their equivalent resistance is very low, my question here is this:
Is there a way to add a couple (or more) additional MOSFETs to the basic back-to-back AC relay circuit, so that they could behave as the external diodes I just described?
Maybe this way the circuit would become far more efficient and would generate less heat.
Thank you all for your patience and your time.
I'm trying to build a high-efficiency AC MOSFET relay, configured as the classic back-to-back n-channel pair, such as the one shown here.
JDT actually drew an excellent driver for this configuration a few years ago, which can be found at the end of this thread.
The basic circuit works because most (if not all) MOSFETS have a inherent reverse diode built into them. So when current flows in one of the MOSFETS in the allowed on-direction (from positive Drain to negative source), the other MOSFET will let it through its body diode (from positive source to negative drain) independently of the voltage present at its gate. I hope I'm making sense here... This behavior is reversed as the polarity changes.
The issue here is that even though a MOSFET's on-resistance is usually quite low (about 0.55 ohms in the IRF740, for instance) most MOSFET's body diodes are not very efficient. The body diode present in the IRF740 has a voltage drop of about 2V, for instance. That means that the MOSFETs will have a tendency to heat up when used in this configuration, even if we're running them well below their rated current.
Consider this example, if we were to run 8 amps through the IRF740's body diode, the 2V voltage drop would force it to dissipate about 16 watts of heat, which in my book is quite a lot... (feel free to correct me if I'm wrong or just saying something stupid)
If I were to add an external normal or schottky diode in parallel with each of the MOSFET's body diode then maybe things would improve quite a bit, since they would be relieving the MOSFET's body diodes from some current.
Considering that a MOSFET can behave like a diode when properly configured (like shown in this video) and that their equivalent resistance is very low, my question here is this:
Is there a way to add a couple (or more) additional MOSFETs to the basic back-to-back AC relay circuit, so that they could behave as the external diodes I just described?
Maybe this way the circuit would become far more efficient and would generate less heat.
Thank you all for your patience and your time.