I am working on a design for the power distribution circuit of an upcoming EV project. The power source is a LiFePO4 pack that will operate in the 58V-80V range. I am using multiple dc-dc step-down regulators to support 24V, 12V, and 5V subsystems. I am planning to use p-channel mosfets as high-side switches to control each voltage subsystem so that I don't have to employ expensive/bulky physical switches with high power ratings. ( I'm intentionally doing high-side switching because many components throughout the overall design are chassis-grounded.) I have included in-line fuses for some added protection, however all of my dc-dc converters feature built-in over-current and over-voltage protection. My LiFePO4 BMS also features over-current and over/under-voltage prevention.
I have already tested each of my dc-dc converters with their respective loads, so I have indicated on the diagram what the max current draw for each part of the circuit should be. However I have not yet attempted any other calculations. Disclaimer: I have used BJTs and mosfets fairly regularly for both low-side and high-side switching, but only with regulated 12V supplies and 3V/5V micro controllers. This is my first time contemplating using FETs with voltage this high, or voltage that varies so widely over time.
So I'm hoping to get some general guidance on the direction I'm headed, and also on a few specific questions:
1) Given the fact that my battery pack's voltage will range from 58-80V over time, how do I make sure I am selecting a mosfet for PMOS1-PMOS3 that will function properly across this entire range? And once a proper mosfet is selected, what is the method for determining proper values for R1-R3 (again with respect to the wide voltage supply range)?
2) Would the diodes D1-D4 that I've included in my diagram be necessary to protect the mosfets in this design? Are dc-dc step-down converters the type of load that would create a need for such protection? (I have not used diodes to protect my mosfets in the past, but until now I have only switched simple 12V lighting circuits.)
3) I have learned that diodes are also commonly used to protect the gates of mosfets in some cases, but I don't fully understand what creates that requirement. Once a particular mosfet is selected, what is the math behind determining if gate protection is necessary?
Thanks in advance for any assistance.
I have already tested each of my dc-dc converters with their respective loads, so I have indicated on the diagram what the max current draw for each part of the circuit should be. However I have not yet attempted any other calculations. Disclaimer: I have used BJTs and mosfets fairly regularly for both low-side and high-side switching, but only with regulated 12V supplies and 3V/5V micro controllers. This is my first time contemplating using FETs with voltage this high, or voltage that varies so widely over time.
So I'm hoping to get some general guidance on the direction I'm headed, and also on a few specific questions:
1) Given the fact that my battery pack's voltage will range from 58-80V over time, how do I make sure I am selecting a mosfet for PMOS1-PMOS3 that will function properly across this entire range? And once a proper mosfet is selected, what is the method for determining proper values for R1-R3 (again with respect to the wide voltage supply range)?
2) Would the diodes D1-D4 that I've included in my diagram be necessary to protect the mosfets in this design? Are dc-dc step-down converters the type of load that would create a need for such protection? (I have not used diodes to protect my mosfets in the past, but until now I have only switched simple 12V lighting circuits.)
3) I have learned that diodes are also commonly used to protect the gates of mosfets in some cases, but I don't fully understand what creates that requirement. Once a particular mosfet is selected, what is the math behind determining if gate protection is necessary?
Thanks in advance for any assistance.