Webench current limit with external switches?

Olli Mannisto

Other Parts Discussed in Thread: LM5022, LM5122

I'm trying to simulate a white LED supply circuit with Webench. I'm shooting for 24V to 55V @ 5.6A output max. So far so good but Webench starts complaining LM5022 is limited to 15A..

As far as I can tell, LM5022 is a pure external switch design that has no provision at all to determine inductor current beyond what you feed into the current sense from switch source. So why this limitation? Can we have it lifted/modified?

Also the datasheet has no mention of any current limit beyond the cycle-by-cycle CMC limit.

over 9 years ago

0 Howard Chen-WEBENCH Design Center over 9 years ago

TI__Expert 6365 points

Olli,

The goal of WEBENCH is to created designs will always function properly and safely.

Looking at your design requirement of 300+ Watts, the LM5022 is not capable to support that type of power. I would suggest you take a look of the LM5122 which is a synchronous multiphase capability controller. Here is a link for a 2 phase EVM design as the starting point..

0 Jeff Perry - WEBENCH Design Center over 9 years ago in reply to Howard Chen-WEBENCH Design Center

TI__Intellectual 1685 points

Olli,
To expand on this, the losses in a MOSFET are not only due to DC losses which depend in the RdsOn of the FET, but also AC losses. These are a function of the controller drive current, Vin, FET Qg and switching frequency. All of these affect the rise and fall time as a percentage of the switching time period. If this is large, then there is more power dissipation and overheating of the FET.
More information can be found here: www.ti.com/.../slyt664.pdf
Regards,
Jeff

0 Olli Mannisto over 9 years ago in reply to Howard Chen-WEBENCH Design Center

Expert 1750 points

Hey,

As I pointed out, LM5022 is an external switch converter so there should be no problem as long as the external components won't croak under the load. I've been considering using two FETs in parallel and I intend to stick a heat sink and/or thermally couple them to the metal chassis. I'm planning to use NXP BUK7Y38-100E FET that's quite a manly component, especially if you can provide enough cooling. For Diode I'm thinking FSV20100V.

One of the issues is that you can't customize FET values and for high-performance FETs the selection in Webench is quite limited. I don't know if Webench considers rdson increasing with junction temperature, this can get beyond 2x of +25C Rdson. MLCC capacitance loss with bias voltage is another unknown. So far I've made a custom part that has capacitance matching actual cap at 55V bias.

0 Olli Mannisto over 9 years ago in reply to Jeff Perry - WEBENCH Design Center

Expert 1750 points

I have actually used the formulas provided in the datasheet to estimate switching losses and RDSON dominates by a fair margin.

0 Howard Chen-WEBENCH Design Center over 9 years ago in reply to Olli Mannisto

TI__Expert 6365 points

Hi Olli,

I'm sorry WEBENCH is not offering a solution. Thank you for using the tool and your suggestions of adding custom FET feature. We will look into it.

If you would like more support, please send me your email contact and I will put you in contact with a local AFE. Here is my email: howard.chen@ti.com

0 Olli Mannisto over 9 years ago in reply to Howard Chen-WEBENCH Design Center

Expert 1750 points

Handling the MLCC DC bias issue would be neat too but since it's heavily dependant on the geometry etc..

0 Howard Chen-WEBENCH Design Center over 9 years ago in reply to Olli Mannisto

TI__Expert 6365 points

Yes, we will investigate the feasibility of adding it into WEBENCH.

0 Olli Mannisto over 9 years ago in reply to Howard Chen-WEBENCH Design Center

Expert 1750 points

So in other words you should create a custom MLCC with the correct biased capacitance right now.

TDK provides Spice models for variable capacitance for what it's worth. From what I've found out is that on "high" capacitance ranges the manufacturers seem to have quite a bit of variation for same size/capacitance/voltage/dielectric. Murata claims much higher capacitances for their 1210 50V 10uF caps at 24V than Samsung / Taiyo Yuden and so on.

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Webench current limit with external switches?