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Blog GaN Point of Load converter 48V to 1V 50A - part 1: Design Overview
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  • Author Author: Jan Cumps
  • Date Created: 10 Feb 2017 10:02 AM Date Created
  • Views 4980 views
  • Likes 9 likes
  • Comments 24 comments
  • gallium_nitride
  • step_down
  • dc_dc_converter
  • lmg5200
  • experimenting_with_magnetic_components
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  • energy_power
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GaN Point of Load converter 48V to 1V 50A - part 1: Design Overview

Jan Cumps
Jan Cumps
10 Feb 2017

I'm reviewing a Gallium Nitrate step-down converter for Point of Load (PoL) high power conversion

 

image

 

This design is intended to deliver low voltage (0.5 - 1.5V) at a power hungry point of load. It can source 50A. With a switching frequency of 600 kHz, the footprint can stay small.

 

 

A 48V DC power bus is common in industrial environments. That voltage often needs to be converted to other levels.

For high-performance processors, FPGAs and application specific ICs, this can be as low as 0.5V or 1V,  with a current draw of many amps.

The evaluation kit here (LMG5200POLEVM-10), that I got for review from TI almost a year ago, is intended for such applications - located close to the low voltage load.

 

GaN Power Stages

 

This isn't the simplest circuit. It consists of a half-bridge Buck converter, a transormer (a funny one) an output rectifier with filtering.

 

Primary Side

 

image

source: Using the LMG5200POLEVM-10 48V to Point of Load EVM

 

The half-bridge on the primary side is a GAN device that I've extensively reviewed in previous blogs: LMG5200. Check the blogs for details.

For this design, it's essential to understand that it is used as a typical Buck and that it switches at high frequencies.

We'll visit this component again when we check the PWM controller (it's a smart one with i2c interface) that is the brain of the converter.

 

Transformer

 

It's built from PCB traces, with a ferrite core over it. SO no wires in this transformer , just a single PCB trace on each side of the board.

edit: This is a 10 layer board. The Transformer has a 5:1 ratio. Check the comments below.

 

Planar transformer surrounded by a ferrite core

(two half cores that connect trough rectangular openings in the PCB)

10 layer board with 5 primary windings and 1 secondary winding

image

Layers 1,4,5,7,10 dedicated to the secondary side (each one turn in parallel) and the other to the primary side (one turn each in series).

 

 

The stacking goes S PP SS P S PP S (S=secondary P= primary).

 

image

source: Using the LMG5200POLEVM-10 48V to Point of Load EVM

It's crazy that you can transfer 75 Watt with this construct, isn't it?

 

Secondary side

 

This is a GaN transistor based rectifier and an LC filter to smoothen the output.

The gates of these transistors (in reality Q3 and Q4 in Figure 1 each are constructed with two discrete GaN FETs) are driven with the same signal that drives Q1 and Q2 on the primary side.

 

image

Core of the power circuit. Additional decoupling and filter caps not shown here.

source: Using the LMG5200POLEVM-10 48V to Point of Load EVM

 

 

 

There's no heat sink required. When the output current exceeds 20A, active cooling (a fan) is needed.

 

In the next blogs, I'll check out additional circuits on the development kit: the step-down controller and the on-board 10A pulsing(!) test load.

I hope that someone with sound electromagnetic knowledge chimes in to discuss the PCB transfo (the core is ER18-3.2-10-3F45-S from Ferroxcube).

 

Blog Posts
part 1: Design Overview
part 2: Current Doubler
part 3: Dynamic Test Load
Related Blog
Checking Out GaN Half-Bridge Power Stage: Texas Instruments LMG5200 - Part 1: Preview
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Top Comments

  • Jan Cumps
    Jan Cumps over 8 years ago in reply to jc2048 +5
    Jon Clift wrote: ... Can't quite get my head around the control side for the moment - does the input have to operate at the point between continuous and discontinous operation? Perhaps I should read the…
  • alberto@ti
    alberto@ti over 8 years ago in reply to Jan Cumps +5
    Hi Jan, since I have some data at temperature, I would like to share it with you so that you can have a reference point. The thermal images below have been taken running at 75V input, 50A output current…
  • Jan Cumps
    Jan Cumps over 8 years ago in reply to jc2048 +4
    These are posts that I fitted on the board, Jon. There's loads of copper going to that point. I just mounted the ones from my breadboard for convenience. What the connections really look like: With copper…
  • Jan Cumps
    Jan Cumps over 6 years ago in reply to Jan Cumps

    Jan Cumps  wrote:

     

    ...

    The planar transformer is build on a 10 layer board, with layers 1,4,5,7,10 dedicated to the secondary side (each one turn in parallel) and the other to the primary side (one turn each in series).

    The stacking goes S PP SS P S PP S (S=secondary P= primary).

    Please reference to the picture below.

    image

     

    https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/1001/48pol-xfrm-layers.bmp

     

    The gerbers for the evaluation kit I am using aren't published, but there's a similar design that uses the same planar transformer. Those gerbers are available:

    PMP4497 LMG5200 48V to 1V/40A Single Stage Converter Reference Design | TI.com

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  • Jan Cumps
    Jan Cumps over 8 years ago

    Part 2 posted : GaN Point of Load converter 48V to 1V 50A - part 2: Current Doubler

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  • Jan Cumps
    Jan Cumps over 8 years ago in reply to jc2048

    You were right, Jon. We have an answer from alberto@ti.

    (source: my question on e2e@ti)

    The planar transformer is build on a 10 layer board, with layers 1,4,5,7,10 dedicated to the secondary side (each one turn in parallel) and the other to the primary side (one turn each in series).

    The stacking goes S PP SS P S PP S (S=secondary P= primary).

    Please reference to the picture below.

    image

     

    https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/1001/48pol-xfrm-layers.bmp

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  • jc2048
    jc2048 over 8 years ago in reply to Jan Cumps

    I realised after I wrote it that I was slightly wrong. The top and bottom traces constitute a single winding (they go once round the core). So the internal part would need to be five turns, however it was done.

     

    Maybe it explains the odd areas, halfway round the single turn, connected by 5 vias, that don't seem to connect to anything else.

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  • Jan Cumps
    Jan Cumps over 8 years ago in reply to jc2048

    I've raised a request on the ti e2e forum to ask how the transfo is constructed...

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