GaN Point of Load converter 48V to 1V 50A - part 1: Design Overview

10 Feb 2017

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

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 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
	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).

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

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).

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

Top Comments

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jc2048 over 5 years ago

"I hope that someone with sound electromagnetic knowledge chimes in..."

That's me out of it, then. You'll have to ask one of the PWMs if it's clever theory you want.

It's confusing when you first look at it. That's partly because of the way it's drawn - it looks like a switched transformer. If you redraw the input like this, it becomes clearer:

the 1:1 transformer is simply using the ripple current in the coil of the input buck converter to induce the voltage in the secondary side which is then synchronously rectified for efficiency. What a neat idea - wish I could think of things like that. 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 datasheet...

Be careful with probing this one.
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jc2048 over 5 years ago in reply to jc2048

With regard to the transformer, the pcb trace winding is probably quite close to what you would achieve with a solid wire with a diameter that was 64% [2/pi] the width of the track (that's because of skin effect which would push the current to the outer of the wire - I'm assuming that the switching frequency is several MHz - imagine hollowing out 99% of the inside of the wire, where the current is low, and then flattening it to get the trace). So the winding is more substantial than it might seem. Are there parallel traces on multiple layers of the board to get the resistance down? It's still very impressive though.

I'd like to see the temperature measurements when it's running flat out - if the winding had a resistance of 1mOhm, then it would dissipate 2.5W at 50A, so it will get hot, though it's manageable (obviously so, because they've done it).
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Jan Cumps over 5 years ago in reply to jc2048

The Gerbers aren't published. In the user manual, section "EVM Assembly Drawing and PCB Layout", there's no mention of any additional layers. Your explanation makes sense. Checking if alberto@ti knows more ...
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Jan Cumps over 5 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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jc2048 over 5 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 over 5 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.

https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/1001/48pol-xfrm-layers.bmp
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Jan Cumps over 3 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.

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 over 3 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.

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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