LM7480-Q1: Potential risk with using higher CdVdT

Roy Chou1

Part Number: LM7480-Q1
Other Parts Discussed in Thread: LM25066, LM74800-Q1

Hi Team,

I use datasheet to calculate the CdVdT. The value is around 145nF. But customer only have 470nF in customer factory. Can you let me know if there is any potential risk that we place much higher cap for CdVdT?

Roy

over 4 years ago

0 Praveen GD over 4 years ago

TI__Guru 57737 points

Hi Roy,

The 470nF cap at gate is very high. This will cause the gate voltage and output voltage to ramp up very slowly. In case there is load current (along with output cap charge current) flowing through the FETs during startup, the FETs may be stressed beyond its SOA limit.

The circuit has to be tested during startup, shutdown and all fault conditions.

0 Roy Chou1 over 4 years ago in reply to Praveen GD

TI__Genius 16190 points

Hi Praveen,

So larger cap at gate will cause over SOA potential issue, is it correct?

Can you let me know how to select inrush current?

I follow the datasheet in the beginning, inrush current = 250mA. The calculation result, CdVdt = 145nF. It may be a little large. Our condition is below.

Cout = 660uF

Iout = 7A

Roy

0 Praveen GD over 4 years ago in reply to Roy Chou1

TI__Guru 57737 points

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.

0 Roy Chou1 over 4 years ago in reply to Praveen GD

TI__Genius 16190 points

Hi Praveen,

Thanks for information. But I still have confusion about inrush current parameter. In datasheet, the parameter is 250mA. I have no idea how to define this parameter. May you give me some comments?

Roy

0 Praveen GD over 4 years ago in reply to Roy Chou1

TI__Guru 57737 points

Hi Roy,

During dvdt based startup, the slew rate of output voltage will be equal to that of gate voltage.

Igate = Cdvdt * dVgate/dt

Iinrush = Cout * dVout/dt

Since, dVgate/dt = dVout/dt

Igate/ Cdvdt = Iinrush/ Cout

Igate = 55uA , Cout, Cdvdt are known. now calculate Iinrush

Using this Iinrush calculate the stress on the FET and SOA margin. If there is no enough margin, change the Cdvdt and calculate Iinrush and SOA margin once again until the SOA margin is met.

0 Roy Chou1 over 4 years ago in reply to Praveen GD

TI__Genius 16190 points

Hi Praveen,

Thanks for information. Can you help check my design flow?

Formula.

Design flow

1. IHGATE = 55uA, Cout = 660uF(typ), Design IINRUSH is 2A, and we can get CdVdT = 18.15nF(typ)

2. Use IINRUSH = 2A to get dT and and also set Vin = 12V, dT = 3.96ms.

3. Below is MOSFET SOA chart, We use worst case.(right line) VDS = 60V, when current is 2A MOSFET can accept dT> 10ms. 4ms duration is in SOA spec.

Regards,

Roy

0 Praveen GD over 4 years ago in reply to Roy Chou1

TI__Guru 57737 points

Hi Roy,

You need to consider the below points while calculations,

The total current flowing through the FET will be = Iinrush + Iload (if there is load during startup). Therefore you need to verify the SOA for Iinrush + Iload
The SOA has to be verified at Vin(max) and not 60V (VDS(max) of the FET)
During startup the power stress on the FET reduces as the Vout starts raising. This is because the voltage across the FET (Vin-Vout) reduces as Vout starts ramping up to Vin. Follow step 1 and 2 specified in ''FET SOA stress calculation"
Now the SOA has to be derated for temperature before calculating SOA margin. Follow step 3 specified in ''FET SOA stress calculation" .

0 Roy Chou1 over 4 years ago in reply to Praveen GD

TI__Genius 16190 points

Hi Praveen,

Is there any approximate calculation can be referred? I understand you points, but we are hard to get. We are seeking a easy calculation to customer to judge the CdVdT. As my step below.

1. For startup, there is no Iload because Vout wasn't built up at that time.

2. I use VDS(max) because I want to check in worst case.

I still don't know how to calculate the 3/4 you mentioned above. Can you help calculate it for my reference? our MOSFET https://www.panjit.com.tw/upload/datasheet/PJQ5462A-AU.pdf

You can assume any parameters in formula.

Roy

0 Praveen GD over 4 years ago in reply to Roy Chou1

TI__Guru 57737 points

Hi Roy,

For your easy understanding, I have modified the LM25066 Hotswap calculator to suit the LM74800-Q1 startup FET SOA margin calculations. Please find the attachment below.

LM74800_Cdvdt.XLSX

With a Vin(max) = 13.2V , Cout = 660uF and Cdvdt = 22nF, the FET selected has a SOA margin of 3.55. This is a good SOA margin.

For more understanding on how to use the design calculator, Please refer to the training videos below,

https://training.ti.com/node/1133677

https://training.ti.com/node/1133673

https://training.ti.com/node/1133664

You can also deep dive into how each calculation is being made in calculator to understand steps 3-4 in my above message.