LM3478: Review Boost Converter Design for Charging Battery

Travis Duncan-Haynes

Part Number: LM3478
Other Parts Discussed in Thread: LM51772

Tool/software:

Hello,

This is a follow up question to the forum: LM3478: Use boost converter for charging battery with BMS - Power management forum - Power management - TI E2E support forums

I designed a LM3478 Boost Converter with the WeBench. Below are the main specs:

- The design has a Vin of 4-5V and output 7V/2A.

- Charge a LiPO4 battery represented as BATT with resistance of R <= 85mOhms

In simulating the design in Pspice for TI, no errors were found. However, Vout = 3.4702V, I_in = 40.83A, and Iout = -40.83A.

Schematic and transient response are attached below:

LM3478_Schematic and Simulations.pdf

Please advise on the corrective steps I must make to ensure accurate functionality of 7V/2A output.

Thank you,

Travis.

9 months ago

0 Brigitte 9 months ago

TI__Guru 65470 points

Hello Travis,

If you apply a battery to the output of a boost converter, you fix the voltage to the battery voltage on the output. If the battery voltage on the output is smaller than the input voltage, there is nothing limiting the current from the input, as there is a direct path from VIN through the inductor and the diode to VOUT. You need to make sure that the voltage on the battery at the output is above the input voltage, otherwise the converter cannot work.

Best regards,
Brigitte

0 Travis Duncan-Haynes 9 months ago in reply to Brigitte

Prodigy 30 points

Hello Brigitte,

Merry Christmas! Thank you for your advice. So how should I model a battery (Talentcell 6V 6Ah LiFePO4 Battery Pack) in Pspice then? Do we just use a DC Voltage Source or is there another way to model this? Do we use a resistor to model the internal resistance?

Let me know.

Thank you.

Travis

0 Brigitte 9 months ago in reply to Travis Duncan-Haynes

TI__Guru 65470 points

Hello Travis,

I will move your question to the Spice for TI team. They might know.

Best would be asking the company that creates the battery.

Nevertheless, if you want to stick to your input voltage range and battery voltage range, have a look at LM51772. You will need to use a buck-boost converter.

Best regards,

Brigitte

0 Travis Duncan-Haynes 9 months ago in reply to Brigitte

Prodigy 30 points

Hello Brigitte,

Ok, that would be helpful, however, I chose the LM3478 SOP to solder by hand to a PCB. Just to confirm, adding a battery on the output end of the DC-DC converter will regulate the output voltage to 7V/2A?

0 Shaheen Makandar 9 months ago in reply to Travis Duncan-Haynes

TI__Expert 3110 points

Hi Travis,

As mentioned by Brigitte we have moved this post to Spice for TI team. However, we suggest you check with the company that manufactures this battery if they can provide you with Pspice library file for the battery.

Brigitte has already answered the query regarding adding a battery on the output end of the DC-DC converter.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1455319/lm3478-review-boost-converter-design-for-charging-battery/5582965#

For any additional query related to LM3478, Please raise a new e2e if this post is locked.

Regards

Shaheen

0 David Larkin 9 months ago in reply to Shaheen Makandar

TI__Mastermind 20460 points

Hi Travis,

I looked at the properties for the simple voltage source. It did not have any user variables for loading resistance.

The other voltage sources didn't look promising.

You could always check with Cadence at pspice.com. Any of these sources will originate with them.

David

0 Travis Duncan-Haynes 8 months ago in reply to David Larkin

Prodigy 30 points

Hello David,

Thank you. I will reach out to Pspice. I attempted to model the battery with the Model Application. However, this feature seems limited as seen below:

0 David Larkin 8 months ago in reply to Travis Duncan-Haynes

TI__Mastermind 20460 points

I think that's a limitation of PSpice for TI. Cadence can work with you on upgrading to the full version SPB. That won't be free though.

David