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

 

IMG_7659.JPG

 

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

 

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 transfo, just a single PCB trace on each side of the board.

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

 

Planar transormer 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 secundary winding

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).

 

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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.

 

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