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# SOA calculations using ADM1270 for 40A inrush current

I have an application where I am controlling Inrush current of 40Amps; To control this I am using ADM1270 IC. This can be understood by following block diagram.

I have issue in Timing parameters to calculate capacitor values.

My spec are mentioned below

1. Vdc min.=16V
2. Vdc max=32
3. Vdc nom=28V
4. Idc max =3.33A@16V
5. Inrush current by EMI Filter and Converter = 40A
6. Output capacitor= 100-500uF(After Filter and Converter)
7. Amb Temp.=71°C
8. Power Good threshold=16
9. Voltage Foldback=15.2
10. UV=10% of 16V
11. OV=10% of 32
12. RPOWERUP = don’t know (static load resistance during system power up)?
13. P channel Mosfet Used to suit design is SPB80P06P G

Vds=-60V;Ids=-80A;Rdson=23mΩ @ 25°C and 36mΩ @ 150°C;θja=60K/W and θjc=0.6K/W

I did not get following things

1. MOSFET SOA Analysis – Short Circuit
• Graph of Op voltage Vs Power Dissipation
• Vout_wcp
• How it is going to protect by Short ckt operation
• How it is protecting converter and filter by 40A Inrush Current.
• In spread sheet and Datasheet Inrush Value is not considered.
• But I have to protect my converter and filter by 40A inrush current.
• Kindly help me for these calculations in detail
attachments.zip
• Hi Nitesh,

Can you share the design schematics?

What are the design targets?

- Max allowable inrush current? i.e. ADM1270 current limit.

- What is the operating current after in-rush?

Is Idc, the max current that the system power supply can provide to the load?

- Can the power good signal be used to enable the downstream load after power-up?

• Hello Fil,

Block Schematic i have sent you in earlier mail.

What are the design targets?

- Max allowable inrush current=40A i.e. ADM1270 current limit.

- What is the operating current after in-rush=max 3.33A

Is Idc, the max current that the system power supply can provide to the load=Yes

- Can the power good signal be used to enable the downstream load after power-up=No.Why It is necessary.?

All details is given in earlier mail.

• Hi Nitesh,

I am coordinating with the team in Europe who are just on holiday. Let me get back to you next week. Thanks.

• Hi Nitesh,

1. Vdc: Is this the voltage input of the DC-DC converter?
2. Idc MAX: If the current at input of DC-DC is held at 3.33A maximum, how is 40A inrush possible?

3. Is the inrush caused by the 500uF cap at the input of DC-DC?
4.  Why is there a need to put 500uF at the input of DC-DC?  What is DC-DC application?

5. What did you mean by "RPOWERUP" and "Vout_wcp"?

Thanks!

• Hi Nitesh,

Looking at your given circuit, it seems that the large 500uF capacitor is the one that caused the 40A inrush current when VIN=16V assuming that your supply has a rise time of ~400us. Basically DC-DC converters have built-in over-current protection, so it is safe from inrush currents. How about your filter? Do you have something in your EMI filter that needs protection from the 40A inrush current? If you really need the ADM1270, then below are the answers to your questions regarding the inrush current protection and the short circuit protection.

The ADM1270 has an internal high bandwidth, current sense amplifier that is used to detect severe overcurrent that is indicative of a short circuit. This circuit ensures that ADM1270 can detect an overcurrent event of approximately 200% of the normal current limit and control within approximately 2us. Therefore, we recommend that you set the current limit of the ADM1270 slightly higher than your maximum load of 3.33A. 4A or 5A will do and this way, your circuit will also be protected from inrush currents greater than 200% the value that you set your current limit.

I have also checked the MOSFET that you have used and I think it will work with your circuit.

Best Regards,

Alex

• This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

Thank you,