Need to Trigger MOSFET at 48V for a Source switching circuit, can you help. THe primary source should be feeding the load and the failure of primary should switch the auxilary source.
Hi Suhas,
LM7480-Q1 would be right device to achieve Priority Power Muxing. You need to use 2x LM7480-Q1 to drive back-to-back FETs in each power path to realize Priortiy Power Muxing as shown below.
For more understanding on this circuit, please refer to section 'Design # 6: Reverse Battery Protection With Priority Power MUXing' of Six System Architectures With Robust Reverse Battery Protection Using an Ideal Diode Controller Application note.
Hi Suhas,
1. The Orientation of Diode FET (T2, T6, T4, T8) is wrong. Reveres the FET Drain and Source connections, i.e. connect Source of the FET to the input. The correct FET connections are as shown below.
2. If the internal disconnect switch between VSNS and SW is not used then short them together and connect to VS pin.
3. The OV pin of LM7480-Q1 is connected to GND which means U2 is always ON (related to OV). Please check if this connection is okay for you.
Hi Suhas,
With high output capacitance, the current flow during startup will be very high. This can cause FET SOA violation and damage to the FET.
Here, you will have to use dvdt based startup for inrush current control. Select Cdvdt such that the inrush current during startup (calculated using the below formulae) is within FET SOA.
Hi Suhas,
30A inrush is Huge. You need to reduce the inrush current which the FET can handle with VDS of Vin across it. Please follow the below steps to check if the FET selected has good SOA margin for the selected Cdvdt. I would recommend to start with a Cdvdt of around 100nF.
FET SOA stress calculation
A more detailed explanation of SOA calculation is specified in section 2.3 Understanding MOSFET’s Stress Limitations of Robust Hot Swap Design App note.
For your understanding below is a step by step procedure (extracted from the app note),
1. Calculate the power stress on the FET during startup. Refer to section ‘2.3.3 Checking SOA for Non-Square Power Pulses’ of the app note.
Derive t2. Approximation here is that the FET is stressed for a power loss = P2 = PMAX for t2 duration.
2. Now, estimate SOA the FET can handle for t2 duration. This value will be at room temperature. Refer to section ‘2.3.2 Checking SOA for Intermediate Time Intervals’ of the app note.
Remember that the SOA value in the table below needs to be taken at VDS = VIN(max) in the SOA curve of the FET.
3. Now, De-rate the SOA calculated in the above step for the Max operating junction/ case temperature of the FET. Refer to section ‘2.3.1 MOSFET SOA Curve and Thermal Model’ of the app note.
This value can be taken as the temp. of the FET at max load condition,
4.You can consider that the FET has strong SOA if
SOA(TC) > 1.2 x (P2 / VDS)
Where,
SOA(Tc) is from step 3
P2 is from step 1
VDS = VIN(max)
The factor 1.2 is taken to include a 20% margin.
