d2.6: status report - europa

FP7-ICT-2011-8 – Collaborative Project (STREP)

PowerSWIPE (Grant agreement 318529)

Objective ICT-2011.3.1 – Very advanced nanoelectronic components: design,

engineering, technology and manufacturability

PowerSWIPE (Project no. 318529)

“POWER SoC With Integrated PassivEs”

D2.6: Status Report

“Analysis and optimisation of

the integrated passives” Dissemination level:

Responsible Beneficiary

Centro de Electrónica Industrial, Universidad Politécnica de Madrid

D2.6 Analysis and optimization of Integrated Passives, April 2016

2

Summary

Name Analysis and optimisation of the integrated passives

Status Due Month 41 Date 01-March-2016

Author(s) Marcelo Silva, Pedro Alou, José A. Cobos and Jesús Ángel Oliver

Editor(s) Jesús Ángel Oliver

DoW Report on Analysis and optimisation of the integrated passives

Dissemination

Level Public

Nature Report

Document history

V Date Author Description

1 30/04/2016 Jesús A. Oliver Final Version


3

Contents

1. Objectives .................................................................................................................. 5

2. New Coupled Inductor Structures ............................................................................... 6

2.1. Coupled Inductor Strategies ................................................................................. 7 2.1.1. Coupled Inductors Structure A ..................................................................... 7

2.1.1. Coupled Inductors Structure B ...................................................................... 8

3. Comparsion of single phase vs two phase converters ................................................ 10

4. Analysis of Non-linear effects .................................................................................. 12

5. Conclusions .............................................................................................................. 16


4

List of figures

Figure 1 Simplified Schematic of the HF DC-DC Covnerter with coupled indutors ........ 6

Figure 2 Simulated current waveforms in the HF DC-DC Covnerter with coupled

indutors: output current (blue), phase 1 current (red), phase 2 current (green) ...................... 6

Figure 3 Proposed Coupled Inductor Structure A ......................................................................... 7

Figure 4 Coupled Inductor Design for 100MHz DC-DC Converter (ITV 2B) ...................... 8

Figure 5 Proposed Coupled Inductor Structure B ......................................................................... 8

Figure 6 Coupled Inductor Design for 100MHz DC-DC Converter Structure B (ITV 2C)

......................................................................................................................................................................................... 9

Figure 7 Coupled Inductor Design for 100MHz DC-DC Converter (ITV 2A) ................... 10

Figure 8 Measured Results of the variation of the Inductance with the bias current 12

Figure 9 Steady state simulation of the effect of the variation of the inductance at

maximum load (500mA). Comparison with ideal inductance. ......................................................... 13

Figure 10 Steady state simulation of the effect of the variation of the inductance at

typical load (270mA). Comparison with ideal inductance................................................................. 13

Figure 11 Start-up simulation of the variation of the inductance with the bias

current. Comparison with ideal inductance. ............................................................................................ 14

Figure 12 Start-up simulation of the effect of the variation of the inductance with the

bias current with soft start. ............................................................................................................................. 14

Figure 13 Start-up simulation detail of the effect of the variation of the inductance

with the bias current with soft start ............................................................................................................ 15

List of tables

No se encuentran elementos de tabla de ilustraciones.

TABLE I 100MHz Coupled Inductor Structure A Design Optimization Results ............... 7

TABLE II 100MHz Coupled Inductor Structure B Design Optimization Results ............. 9

TABLE III 200MHz DC-DC Converter Inductor Design Optimization Results ............... 10

TABLE IV Comparison of Single-Phase vs Two-phases coupled inductores .................. 11


5

1. Objectives

This document summarizes the main results of the work done on the virtual optimization

of integrated passives based on the theoretical models and finite element analysis simulations.

The results have been validated with measurements performed on the magnetic components

designed with this tool and fabricated by the parters for both the high frequency DC-DC

converter (100MHz and 200MHz) and the low frequency (10MHz) converter.

The optimization of the passive components has been done using both analytical and finite

element analysis tools. Measurements on the samples have served to validate the validity of

the analysis and to proposed improvements to take into account effects not previously

considered, especially the variation of the inductance with the current.

For the design, simplified analytical equations are used to have a physical understanding

of the critical parameters and then finite element analysis simulations are performed to obtain

high accuracy on the calculations.

The document is divided in the following sections:

1) Evaluation fo new coupled inductors for High Frequency DC-DC Converter

(100MHz-200MHz).

2) Comparison of single phase and coupled inductors designs for High Frequency DC-

DC Converter (100MHz-200MHz).

3) Analysis of Non-Linear Effects on the low frequency DC-DC Converter (10MHz)


6

2. New Coupled Inductor Structures

New coupled inductor estructures have been analyzed due to its interest, especially for the

high frequency buck converter (100MHz, 200Mhz) design within this project. The

advantages of using coupled inductors, in a two phase converter, is that the value of the

output inductor can be considerably reduced due to the frequency doubling effect and the

reduced volt.sec applied. The penalty is that a tightly coupled transformer is needed and the

individual ripple of each phase is doubled, increasing RMS valued of the current through the

MOSFETs and the transformer.

Figure 1 shows the simplified schematic of the HF DC-DC converter with coupled

inductors. Simulations results of the current through the output inductor, and the phase

currents are shown in Figure 2. It can be noticed how the output current ripple (blue

waveform) is significantly lower than the individual phase current ripple due to the cancelling

effect, and the output current frequency is double the switching frequency. In other words,

the output inductor can be made significantly lower (in the order of 8 times) compared to a

single buck converter for the same output voltage ripple. Nevertheless, the individual phase

currents will have a higher RMS value and it will penalize conduction losses in the switches.

Figure 1 Simplified Schematic of the HF DC-DC Covnerter with coupled indutors

Figure 2 Simulated current waveforms in the HF DC-DC Covnerter with coupled indutors: output current (blue),

phase 1 current (red), phase 2 current (green)

L1

L0

Lout

k


7

2.1. Coupled Inductor Strategies

In order to fabricate the coupled inductors using Tyndall racetrack technology, two

different structures have been designed and fabricated to see the benefits/limitations of them.

In both cases the number of conductor layers is limited to two (top and botton).

2.1.1. Coupled Inductors Structure A

Option A consists on a tightly coupled transformer and one individual inductor per phase

(see Figure 3). In this case, the number or turns of the inductors and the number of turns of

each winding in the transformer are the same, and the thichness of the windings are also the

same. The thickness of all the cores are also kept constant for manufacturability.

a) Output filter for a Two-phase

converter with Coupled inductors

(Structure A)

b) Magnetic structure schematic

Figure 3 Proposed Coupled Inductor Structure A

The analysis and optimization of this structure is made using analytical equations

described in D2.2 and D2.5 and validated by means of Finite Element Analysis. The

optimization of this structure for the high frequency buck converter (100MHz) developed

within POWERSWIPE project is shown in Figure 3 and the optimization results shown in the

next table:

TABLE I 100MHz Coupled Inductor Structure A Design Optimization Results

PowerSWIPE ITVs

L (nH) Core Thickness

Core Length

Copper width

Copper Thickness

DCR

(Ohm) Device

Footprint

ITV 2B 47 Coupled (k=0.4)

1.6 µm 1.78 mm 50.62 15 μm 0.3425 2 mm2

Lo1

Lo2

Xfr

Option A

L Tx L


8

Figure 4 Coupled Inductor Design for 100MHz DC-DC Converter (ITV 2B)

The estimated efficiency of this structure in nominal conditions (Vout = 1.2V and Iout =

270mA) is 90,2%. Accurate simulations results of the Power Stage in CADENCE showed an

efficiency of 90,4%. The combined efficiency of this solution will be 80%.

2.1.1. Coupled Inductors Structure B

In Option B, the coupling transformer is connected to each phase of the buck converter,

the filter inductor is connected to the middle point of the transformer (see Figure 5). In this

case, the number or turns of the inductors and the number of turns of each winding in the

transformer do not need to be the same. Due to manufacturability, the thichness of the

windings are the same for the transformer and the inductor and the thickness of all the cores

are also kept constant.

a) Output filter for a Two-phase

converter with Coupled inductors

(Structure B)

b) Magnetic structure schematic

Figure 5 Proposed Coupled Inductor Structure B

Taking these facts into consideration and all the restrictions of the desing, the optimization

of the transformer and the inductor using this structure was done using the analytical

equations and Finite Elemenent Analysis Simulations. The results of this optimization are

show in the following table, and a picture of the designed magnetic components is shown in

Figure 6, where the transformer efficiency has been calculated to be 90% and the inductor

efficiency close to 95%, providing a total efficiency for the magnetic components of 85,6%.

90,4%

90.25%

Xfr

Lo Option B


9

In this operating conditions, the power stage efficiency is 90,4% and the total effieciency,

power converter and magnetic components, will be 77%.

TABLE II 100MHz Coupled Inductor Structure B Design Optimization Results

PowerSWIPE ITVs


Core Length

Copper width

Copper Thickness

DCR

(Ohm) Device

Footprint

ITV 2C 35 nH Coupled

k=0.8

1.6 µm 1.83 mm 75.71 μm 15 μm 0.155 2 mm2

20 nH 1.6 µm 0.78 mm 97 μm 35 μm 0.053 2 mm2

Figure 6 Coupled Inductor Design for 100MHz DC-DC Converter Structure B (ITV 2C)

90,4%

90.25%94.8%

85.6%


10

3. Comparsion of single phase vs two phase converters

The solutions based on coupled inductors have been compared with the solution of a

single phase converter operating at twice the switching frequency (200MHz). The

optimization results for the inductor at 200MHz are shown in TABLE III, and the picture of

the fabricated inductor is shown in Figure 7.

The estimated efficiency of the power stage, based on CADENCE simulations, is 87,4%

and the efficiency of the inductor is 95,5%. The overall efficiency will be 83%.

TABLE III 200MHz DC-DC Converter Inductor Design Optimization Results

PowerSWIPE ITVs


Core Length

Copper width

Copper Thickness

DCR

(Ohm) Device

Footprint

ITV 2A 33 1.2 µm 1.22 mm 72.2 μm 35 μm 0.084 2 mm2

Figure 7 Coupled Inductor Design for 100MHz DC-DC Converter (ITV 2A)

If we compare the results of the single phase converter, operating at 200MHz, with the

results of the coupled inductor converters operating at 100MHz (see TABLE IV), it can be

noticed that the benefit in the efficiency of the power stage due to the decrease of the

switching frequency from 200MHz, in the single phase converte, to 100MHz, for the coupled

inductor converter, is only 3%, from 87% efficiency to 90% efficiency. Meanwhile the

magnetic components are penalized in a bigger proportion due to the addition of the

transformer. The efficiency of the magnetic components in the case of the single phase

converter is 95,5% but in the case of the coupled inductors the efficiency drops to 90% in the

case of the structure A and 85,6% for the structure B.

87,4%

200 MHz

33nH95,5 %


11

As a consequency, for this specifications, the best option will be single phase converter

running at 200 MHz.

TABLE IV Comparison of Single-Phase vs Two-phases coupled inductores

Inductor

design Freq.

(MHz) L

(nH) Coupling

factor Efficiency

(magnetics)

Efficiency

(IC)

Total

efficiency

ITV2a Single

phase

200 33 -- 95,5 % 87,4% 83%

ITV2b Coupled

(str. A)

100 45 ~0.4 90% 90,4% 81%

ITV2c Coupled

+Lout

(str. B)

100 35+21 >0.8 85.6%

(90.25%·94.8%)

90,4% 77%


12

4. Analysis of Non-linear effects

The effect of the variation of the permeability of the core with the magnetic field was not

taken into account in the first stages of the design, nevertheless it can be critical in the

operation of the magnetic component and the conveter. The optimization of the magnetic

components was done avoiding the saturation of the core, but its influence on the transients

was not analyzed.

Figure 8 shows the measurement results of the variation of the inductance of the 10MHz

DC-DC converter designed in this project with the output current. This converter will operate

with a maximum current of 500mA, but the peak current can go up to 750mA. It can be

noticed a drop in the inductance from 220nH (desined value) at DC to 90nH at 750nH. At

maximum DC load (500mA), the magnetizing inductance is around 150nH.

Figure 8 Measured Results of the variation of the Inductance with the bias current

Even though the variation of the inductance with the current is quite large, simulation

results in steady-state (Figure 9) show that the effect in terms of variation of the peak value of

the current is very limited. In this figure are compared the current waveforms for the

converter with non-linear inductance (based on Figure 8) with respect to the ideal case of

220nH. The peak-to-peak value of the inductor current for the ideal case is Ipp = 440mA,

and in the case of non-linear inductor the peak-to-peak value will go up to 500mA, that is a

difference of 12% that will have an effect on the conduction losses (higher than in the ideal

case).

50

500

0 250 500 750 1000

Ind

uct

ance

(n

H)

@ 1

0 M

Hz

Bias Current (mA)


13

Figure 9 Steady state simulation of the effect of the variation of the inductance at maximum load (500mA).

Comparison with ideal inductance.

At typical load (270 mA) the steady state results are shown in Figure 10, in this case the

effect is almost negligible. The difference in the peak to peak value is only 10mA over

440mA, that is 2%.

Figure 10 Steady state simulation of the effect of the variation of the inductance at typical load (270mA). Comparison

with ideal inductance.

Although this effect is small in steady state, during the start-up of the converter can be

significative. In Figure 11 it is shown the start-up of the 10MHz buck converter designed

within this project with constant duty cycle of 25%, that is the steady-state value of the duty

cycle. It can be observed how the inductor current in the case of the non-linear inductor

reaches a peak current value of 3,6A, and in the case of an ideal inductor the peak value is

around 2A. In both cases the values are not acceptable.


14

Figure 11 Start-up simulation of the variation of the inductance with the bias current. Comparison with ideal

inductance.

By applying a soft-start to the conveter, the current through the inductor can be limited as

shown in Figure 12 and Figure 13, where the detail of the intial part of the transient is shown.

In this case, even with the non-linear permeability the peak current is limited to 300mA in the

initial part of the transient.

Figure 12 Start-up simulation of the effect of the variation of the inductance with the bias current with soft start.


15

Figure 13 Start-up simulation detail of the effect of the variation of the inductance with the bias current with soft

start


16

5. Conclusions

The Tool developed within this Project for the analysis and optimization of integrated

passives has proven to be of great help for the design and optimization of integrated power

converters on Chip.

The optimization process is based on accurate analytical equations developed to predict

the inductance, coupling and losses of the magnetic components. The analytical equations

have been validated by means of finite element simulations and also measurements perfomed

on the magnetic component devices developed within this project.

Two different structures of integrated coupled inductors have been proposed, modelled,

optimized and fabricated for the high frequency DC-DC converter (200MHz). The main

advantage of these structures is that they allow to operate the converter at half of the ouput

frequency (100MHz) and so reduce the swithing losses. The penalty is that they required an

additional transformer to couple the phases that will penalizte losses.

The results of the coupled inductor designs have been compared with a single phase

converter at 200MHz. It has been shown that the efficiency benefit obtained by reducing the

switching frequency to 100MHz (around 3%) is lower than the efficiency penalty introduced

by the transformer (5%-10%). As a consequence, the best efficiency for this particular

especifications and application is to use a single phase converter at 200MHz and the

efficiency expected is 83%, that is 2% to 6% better than the coupled inductor converters.

The non-linear effect of the variation of the permeability with the biar current on the

inductors has also been analyzed for the 10MHz DC-DC converter. The variation of the

permability on the samples built was measured and the results were introduced into the

models. The simulation results show that at maximum load (500mA) the variation on the

peak to peak current between an ideal inductor and the real one is around 12% and at typical

load (270mA) is only 2%. This difference will cause higher conduction losses both on the

MOSFETs and the inductors.

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