Environment-Friendly Vertical Air-Cooling Heatsink:
Heatpipe-Based Heatsink “POWER KICKER®”
by Takeshi Takahashi * , Kazuhiro Konishi * , Junji Sotani * ,Takayuki Kikuchi * 2 , Satoru Isogai * 3 and Yuichi Kimura * 3

A variety of heatsinks of high efficiency has been used in electric power devices
for power conversion to maintain their performance. Recently, these heatsinks are required to have small footprint and environment-friendliness —such as small ozone
depletion and reduced environmental impact— as well as high heat-dissipating performance.
Accordingly, the authors have developed and put into practical use, jointly with a certain company, “POWER KICKER®” —an environment-friendly vertical air-cooling heatsink— for heat dissipation of high heat-generating devices to replace conventional heatsinks that use CFCs and
CFC substitutes. This paper reports on the performance and practical applications of the newheatsink.

ABSTRACT1.

INTRODUCTION

In recent years, electric power devices for power conversion have much improved in performance due to thedevelopment of electronics, making these high-capacity
power devices easily available. Thus, high-capacity semiconductor devices such as GTO (Gate Turn-Off) thyristors, IGBTs (Insulated Gate Bipolar Transistor) and power
diodes as shown in Photo 1 are widely used in electricrailway substations.
Normally these devices for substations have been using
natural air-cooling heatsinks in order to reduce the costof maintenance. In the electric railway substations, inparticular, boiling-type heatsinks using CFCs or CFC substitutes have generally been employed for performance
maintenance because of the large heat the power conversion devices generate. But as the CFC control regulation
comes into force and awareness of preventing the global
warming increases in recent years, use of environmentfriendly heatsinks is extensively required.
Furukawa Electric has long established a track recordof supplying POWER KICKER® —a registered trademarkof Furukawa Electric for heatpipe-based heatsinks forhigh-heat generating semiconductor devices— to various
fields as heatsinks for electric power conversion devices.
In addition the company has recently developed, jointly* Thermal and Antenna Solutions Dept., Electronic
Components Div., Electronics and Automotive Systems Co.

* 2 Okano Electric Wire Co., Ltd.
* 3 Ecology and Energy Lab., R&D Div.

Photo 1

Flat-type thyristor and power module for power
conversion.

with a certain company, a new series of heatpipe-based
vertical air-cooling heatsinks that feature environmentfriendliness and small footprint to be applied to powerdevices such as circuit breakers in electric railway substations. This paper reports on the features, application
examples and recent trends of this vertical air-coolingheatsink POWER KICKER.

2.

CONVENTIONAL HEATSINKS FOR
POWER CONVERSION DEVICES IN
ELECTRIC RAILWAY SUBSTATIONSTransmission and reception system of electric powerin Japanese railways is such that the primary power for
motor driving is supplied to electric vehicles on the rail
track together with the auxiliary power for lighting and thelike, and conversely, the recovered power generated by
the driving motors onboard is received from the vehicles.
And varied power conversion accompanied by the transmission and reception of electric power is controlled at

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Environment-Friendly Vertical Air-Cooling Heatsink: Heatpipe-Based Heatsink “POWER KICKER®”

the electric railway substations, where power control systems are installed to provide stable supplies of electric
power to the vehicles by globally controlling the AC and
DC transformation subsystems. See Figure 1.
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Figure 1������������ ��� ������� ������ �� ���������� ��������� ������������

Power control system at electric railway substationsin Japan.

In the substations where high-capacity power conversion devices are used in various systems, high-performance heatsinks are needed to maintain the performanceof the power devices, so that, conventionally, a heatsink
based on the CFC boiling-tank method has been largely
used as shown in Figure 2. With this heatsink, the conversion elements are immersed and cooled in the insulating
medium contained in the boiling tank, where large quantities of CFC or CFC substitute are used. Other coolingmethods including those based on individual boiling-tankheatsinks or heatpipe-based heatsinks are also used.
Table 1 shows a comparison of these cooling methods.

3.

APPLICATION OF HEATPIPE-BASEDHEATSINKS TO ELECTRIC RAILWAYS

Application of heatpipe-based heatsinks has early beenstudied for their non-power-driven operation and ease ofmaintenance, and POWER KICKER products —air-cooling, heatpipe-based heatsinks using water— are alreadyin practical use in some substations.
Conventional heatpipe-based heatsinks have beenused in such a configuration that two heatsink units arearranged on the both sides of each power conversion
element such as GTO sandwiching it to receive the generated heat, which is then transferred via the heatpipes
embedded in the unit to be dissipated by the heat-radiating fins provided at the radiating section. Owing to such astructure of individual cooling mechanism, the heatpipebased heatsinks feature, besides environment-friendliness, high redundancy in terms of reliability and ease ofmaintenance, so that replacement of the power element
is facilitated unlike heatsinks of boiling-tank type.
It is to be noted that conventional heatpipe-basedheatsinks were basically installed in a horizontal position
with an inclination angle of 3~5 degrees to the horizontal
plane, in consideration of efficient heat dissipation fromthe fins taking advantage of natural convection. Photo 2
shows a horizontal-type product with a heat-dissipating
length of 1000 mm and a thermal resistance of 0.06 K/Wagainst a heat generating rate of 500 W. The one shownin Photo 3 is 1500 mm in length to provide a thermal
resistance of 0.04 K/W.

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Figure 2

Schematic of CFC boiling-tank cooling system
conventionally used.
Table 1

Horizontal-type heatpipe-based heatsinks have a longtrack record thanks to their environment-friendliness,ease of maintenance and long-term reliability. Becauseof their long fin sections, however, they inevitably needconsiderable space for installation. Thus these heatsinks
have the disadvantage that it is sometimes difficult to usethem to replace existing heatsinks of other type —theComparison of heatsinks for electric railway substations.

Boiling-tank heatsink

Individual boiling-tank heatsink������ ����Heatpipe-based heatsink��������

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Structure

DEVELOPMENT OF HEATPIPE-BASEDVERTICAL AIR-COOLING HEATSINK��������� ����������� ���������� ��������������

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Size

Large

Large

Small

Weight

Heavy

Heavy

Light

CFC or CFC substitute

CFC or CFC substitute

CFC substitute or water

Low

Low

High (due to separated cooling)

Working fluid
Reliability

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Environment-Friendly Vertical Air-Cooling Heatsink: Heatpipe-Based Heatsink “POWER KICKER®”(a) Power conversion equipment where POWER KICKER is
installed

compact heatsink with heatpipes in a vertical position.
We have developed and put into practical use the newheatsink through multifaceted studies and evaluations
with regard to the following design problems.
(1) To reduce the thermal resistance of heatpipe itself.
(2) To reduce the thermal resistance of radiating fin
section.
In particular, heatpipe-based heatsinks tend to have alarge ratio between the thermal resistance of the radiating fin section and that of the whole heatsink. To solve
this problem, we devised countermeasures to makethe shape and pitch of the fins suitable for effective aircooling in a limited space of installation, thus succeedingin developing a heatsink that achieves the developmental
objectives. See Photo 4.
Figure 3 shows schematics of the cooling mechanism
for heatpipe-based vertical air-cooling heatsink (hereinafter called HP-VAH). The slanted fins developed here
are provided at the radiating section to generate drafts
enabling efficient natural convection, resulting in the cooling performance that is virtually equivalent to that of thehorizontal-type heatpipe-based heatsinks conventionallyused. This has realized a reduction in the footprint to one
half or less that of earlier Furukawa products.

(b) Horizontal POWER KICKER (Radiator length: 1000 mm)
Photo 2 Horizontal POWER KICKER and its applications.

Photo 4Fin configuration of HP-VAH.
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Horizontal POWER KICKER (Radiator length:1500 mm)mainstream was boiling-tank type conventionally— or that
it is occasionally impossible to install them because of thelimited open space available.
Accordingly we have developed, jointly with a certain
company, a new heatsink based on the following concept.
(1) To provide thermal performance equivalent to thatof horizontal-type heatpipe-based heatsink.
(2) To minimize the installation space, it should be a

Figure 3

Cooling mechanism for HP-VAH.

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Appearance of three standard models of HP-VAH.

Table 2
ThermalModel load
(W)

5.

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Evaluation method of thermal characteristics of
HP-VAH.

Standard specifications of HP-VAH.

(Block) × (Fin) × Total (mm)Thermalresistance(K/W per pair)A

300(140 × 110 × 68) × (120 × 242) × 850

0.12

B

500

(140 × 110 × 68) × (120 × 242) × 1155

0.072

C

800

(140 × 110 × 68) × (120 × 242) × 1580

0.044

Dimensions

BASIC COOLING CHARACTERISTICS
OF HP-VAH

There are three standard models of HP-VAHs as shown in
Table 2 and Photo 5.
Basically, the heatsinks are used in pairs like horizontaltype heatsinks, i.e. two heatsinks are arragnged on theboth sides of one heat-generating element. Based on thisconfiguration, the standard models can cope with thermalloads of 300 W, 500 W and 800 W, respectively.
With regard to their basic thermal characteristics, thestandard models of the heatsinks are type-tested toobtain: 1) steady-state thermal resistance characteristics,2) transient-state thermal resistance characteristics and 3)
low-temperature start-up characteristics.
Below will be outlined the thermal evaluation methodof the basic cooling characteristics for the heatsinks. The
characteristics were evaluated in accordance with practical applications following the procedures and methods
described below. See Figure 4.
(1) Assembly of the power conversion element with theheatsink and installation method
A metal block with a heater is used to simulate thepower conversion element, and the block and twoheatsinks are assembled to make a test assemblyin a vertical position, and a specified quantity ofheat Q is applied to the heater block.
(2) Temperature measurement and calculation of temperature increase at the block: ∆ T
Using the thermocouples installed on the heater
block, temperatures at the inlet of airflow and theblock are measured to calculate the temperature
increase at the block: ∆ T = Heater block temperature - Airflow inlet temperature.
(3) Definition and calculation of thermal resistance: R
The thermal resistance is defined as���������������
�where, Rc is the contact thermal resistance at theinterface between the heatsink and the heater
block, to be obtained experimentally.
(4) Manner of thermal load applicationa) Thermal resistance behavior when a thermalload Q is input in a stepwise manner is referred
to as steady-state thermal resistance characteristics, and the stabilized value is defined as
steady-state thermal resistance.
b) Transient behavior of thermal resistance during
a short period —30 or 60 sec— after a thermalload Q 2 is input to the stabilized state with aheat input of Q1 is referred to as transient-state
thermal resistance characteristics, and the value
measured after a specified time is defined astransient-state thermal resistance.
In practical applications for power conversion devices,heatsinks are usually used under the combined conditions of steady-state and transient-state thermal inputs,
so that the characteristics of both the steady-state andtransient-state thermal resistances become essential.
Figure 5 shows the steady-state and transient-state thermal resistances of standard models of HP-VAHs.

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Environment-Friendly Vertical Air-Cooling Heatsink: Heatpipe-Based Heatsink “POWER KICKER®”

Besides the steady-state and transient-state thermal
resistances, start-up characteristics at low temperatures
are a crucial performance indicator. This is because there
is a possibility that an alarming phenomena may occur
whereby the water used as a working fluid in the heatpipes freezes at ambient temperatures of 0°C and below
rendering them unable to work. It is thus necessary toevaluate the low-temperature characteristics of the heatpipe for confirmation if they normally work at low ambient
temperatures during winter.

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that, conventional heatpipe-based heatsinks found it difficult to start up smoothly, because water freezes at thesetemperatures. As a result of many improvements made inheatpipes, however, recent products of HP-VAHs can startup at temperatures lower than -5°C, e.g. -15°C.

6.

APPLICATION EXAMPLES OF HP-VAH

Photo 6 shows a power controller for DC substations
for electric railways in which silicon rectifiers and circuit breakers are used together with HP-VAHs. To cool
the power elements, C-type standard model of POWER
KICKER with 800 W rating is used. The heatsink consistsof a nickel-plated copper block, six heatpipes 15.88 mm
in diameter and 1600 mm in length and 74 heat-radiating
fins. The heatpipe is tin-plated for corrosion proof, andthe fins are treated with black alumite aimed at long-term
use and corrosion proof.

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Photo 6

�����Power controller for electric railways using POWER
KICKER.

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Figure 5

Thermal resistance of HP-VAH.

Figure 6 shows the test result of low-temperature start
up characteristics of C-type standard model with 800 W
rating, whereby the thermal performance was evaluatedwhile the ambient temperature was kept at -5°C. It has
been confirmed that the standard models using water
as the working fluid can start up smoothly at an ambient
temperature of -5°C or higher. At temperatures lower than����������� ���� ���

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Figure 6

Low-temperature start-up characteristics of HP-VAH.

RECENT TRENDS IN HP-VAH AND
DEVELOPMENT OF INSULATED HP-VAH

While Furukawa Electric has built up a track record of
supplying many HP-VAHs thus replacing boiling-tankheatsinks, there has been in recent years a trend toward
the use of insulated HP-VAHs that is required from theviewpoint of safety. In conventional boiling-tank heatsinks,
insulation is basically provided between the semiconductor elements and the radiating fins because these are
immersed in CFCs or CFC substitutes. Accordingly, there
is a potential need for a HP-VAH that is similarly provided
with an insulation function to assure a higher level of safety.
To provide insulation in heatpipe-based heatsinks, it is
a common practice to insert insulators made of aluminum
nitride between the power conversion element and theheatsink. This method, however, leads to an increase in
the thermal resistance thereby deteriorating heat dissipation globally, and in addition, to a potential decrease ininsulation if the insulators are damaged by any chance.
We also have a track record of insulated heatpipe-basedheatsinks in which an insulating working fluid is used toassure safety and compactness, but it was considereddifficult to use non-insulating working fluids such as waterto ensure insulation, so that no such heatsinks were inpractical use.
Against this background, we have succeeded in developing a new insulated heatpipe-based heatsink in which

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Environment-Friendly Vertical Air-Cooling Heatsink: Heatpipe-Based Heatsink “POWER KICKER®”

water is used as working fluid, thus realizing the world’s
first product. The insulating HP-VAH, an improved versionof HP-VAH, with intrinsically insulated heatpipes that use
water as working fluid thus eliminating the disadvantagesof using insulators are already applied to some substation
facilities of electric railways.

8.

FEATURES OF HP-VAH

HP-VAH has been developed based on our expertise inhorizontal-type heatpipe-based heatsinks. Its features
may be summarized as follows.
(1) Environmental consciousness
It uses water as working fluid eliminating CFCs
or CFC substitutes, making itself an environmentfriendly heatsink.
(2) Compactness and space-saving
Footprint has been reduced to one half or less thatof conventional horizontal-type heatpipe-basedheatsinks.
(3) Improvement in maintainability
Since heatpipe is a passive device for heat transmission without necessitating power source, its
maintenance is easy.
(4) Consideration on and improvement in reliability andsafetySince individual cooling method is employed forheat-generating devices, the redundancy is highand hence high reliability.
In addition, taking active advantage of the verticalconfiguration of specially shaped fins together withnew technologies, insulation function was given to
the heatpipe that uses water as working fluid, realizing a high level of safety.

9.

REFERENCES
1) Murase et al.: “Power Semiconductor Device Cooling System
Using Electrically Insulated Heat Pipe POWERKICKER-E for
Electric Railway,” Furukawa Electric Review, 84 (1989) 129-130.
(in Japanese)2) Kimura et al.: “Application of Heatpipe Heatsink to Electric
Railway Equipment,” Proc. of General Convention of IEEJ
(Industrial Application Section), 1991. (in Japanese)3) Murase et al.: “Thyristor Cooling System for Electric Railway
Vehicles Using Electrically Insulated Heat Pipe,” Furukawa
Electric Review, 91 (1992) 88-98. (in Japanese)
4) POWER KICKER Catalogue: No. S-065351.5ss5) Murase et al.: “Heatpipe, the Highway of Heat, Its Application
to Electric Equipment,” Journal of IEEJ, 111 (1991) 37-41. (inJapanese)
6) Heatpipe in Practice (Second edition), compiled by Japan
Association for Heat Pipe.

CONCLUSIONSThis paper has outlined the development of the heatpipebased vertical air-cooling heatsink POWER KICKER.
Under the current social situations where space saving
and environment-friendliness are always required for
products, the development included many solutions tocope with unconventional new requirements, but we haveeventually succeeded in offering the new product to themarketplace. At the same time, it is a great pleasure for
us development engineers that, due to the developmental
tasks of making the product more user-friendly, we could
build up proprietary technologies accompanied by the
new product. We intend to make further efforts to respondto the market needs to be able to supply better products.
The development and productization of the heatpipebased vertical air-cooling heatsink have been realized by
the advice and suggestions provided by many people. In
closing, the authors would like to take this opportunity to
express their appreciation to all of them.

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