Heatsink for WMHP Series High Power Resistors

WMHP-HSSeries

09.17

Application Notes

Steady State Thermal Calculations

Type

P Max @ TF =25°C

(W)

TE Max

(°C)

RθEF

(°C/W)

WMHP20

20

125

6.25

WMHP35

35

3.57

WMHP50

50

2.50

WMHP100

100

175

1.50

The maximum value of

Pfor TF >25°C is a function of TF and is shown in the WMHP datasheet de-ratinggraph.

The value of

RθFH depends on the thermal interface material. Forthermal paste atypical figure is around 1°C/W.

For forced air coolingthe value of

RθHA is shown as a function of airvelocity in the Forced AirCoolinggraph. For

natural convection

RθHA is slightly non-linear, as shown in the Natural Convection graph. The standard figure at P

= 20W is given, and for lowerpowers you can directly read the heatsink temperature rise

TH – TA.

Example: WMHP20 is dissipating5W at an ambient air temperature of30°C. The WMHP-HS is PCB mounted and cooled bynatural

convection. Is the maximum resistor element temperature acceptable?

TA = 30°C, and TH – TA = 50°C at P= 5W. Therefore TH = 80°C. AssumingRθFH = 1°C/W, TF = 85°C.

Therefore

TE = 85°C + 6.25°C/W x 5W = 116°C. This is acceptable as it is below 150°C.

Dynamic Thermal Calculations

In some applications such as inrush currentcontrol, the

dissipation of poweris of limited duration. If the

duration is between about30s and 10 minutes then

the dynamictemperature rise graph may be used to

determine the highestvalueof

TH achieved atthe end

of the loading. The derivation of

TF and TE is then the

same as for the steady state case.

If poweris reapplied beforethe heatsink has fully

cooled, then cumulativeheatingmustbe accounted

for. This can generally be ignored if the mean power

dissipation is below 10% of the maximum permissible

steady state dissipation.

For durations below about30s, the bestguide is the

Pulse Performance graph on the WMHP datasheet.

TE = Temperature of resistor element

P = Power dissipated

RθEF = Thermal resistance element to flange of WMHP

TF = Temperature of resistor flange

RθFH = Thermal resistance flange to heatsink ofthermal interface

TH = Temperature ofheatsink mountingsurface

RθHA = Thermal resistance heatsinkto ambient of WMHP-HS

TA = Temperature of ambient air

A simple thermal model of apowerresistoron a

heatsink is shown, in which temperature rise may

be calculated from the product of powerdissipated

and thermal impedance. Maximum

P, TE and RθEF

values forWMHP are given below.

Figure 1. Thermal model of WMHP and WMHP-HS

RθEF

RθFH

RθHA

TE

TF

TH

TA

P

Natural Convection: τ = 2.4 minutes

Forced Air 2.5m/s: τ = 50 seconds

Heatsink for WMHP Series High Power Resistors

WMHP-HS Series

09.17

General Note

TT Electronics reserves the right to make changes in product specification without notice or liability.

All information is subject to TT Electronics’ own data and is considered accurate at time of going to print.

© TT Electronics plc

www.ttelectronics.com/resistors

BI Technologies IRC Welwyn

Application Notes

Type

P Max @ T

F =25°C

(W)

T

E Max

(°C)

R

θEF

(°C/W)

WMHP20

20

150

6.25

WMHP35

35

3.57

WMHP50

50

2.50

WMHP100

100

175

1.50

T

E = Temperature of resistor element

P = Power dissipated

R

θEF = Thermal resistance element to flange of WMHP

T

F = Temperature of resistor flange

R

θFH = Thermal resistance flange to heatsink of thermal

interface

T

H = Temperature of heatsink mounting surface

R

θHA = Thermal resistance heatsink to ambient of WMHP-HS

T

A = Temperature of ambient air

Heatsink for WMHP Series High Power Resistors

WMHP-HSSeries

09.17

Application Notes

Steady State Thermal Calculations

Type

P Max @ TF =25°C

(W)

TE Max

(°C)

RθEF

(°C/W)

WMHP20

20

125

6.25

WMHP35

35

3.57

WMHP50

50

2.50

WMHP100

100

175

1.50

The maximum value of

Pfor TF >25°C is a function of TF and is shown in the WMHP datasheet de-ratinggraph.

The value of

RθFH depends on the thermal interface material. Forthermal paste atypical figure is around 1°C/W.

For forced air coolingthe value of

RθHA is shown as a function of airvelocity in the Forced AirCoolinggraph. For

natural convection

RθHA is slightly non-linear, as shown in the Natural Convection graph. The standard figure at P

= 20W is given, and for lowerpowers you can directly read the heatsink temperature rise

TH – TA.

Example: WMHP20 is dissipating5W at an ambient air temperature of30°C. The WMHP-HS is PCB mounted and cooled bynatural

convection. Is the maximum resistor element temperature acceptable?

TA = 30°C, and TH – TA = 50°C at P= 5W. Therefore TH = 80°C. AssumingRθFH = 1°C/W, TF = 85°C.

Therefore

TE = 85°C + 6.25°C/W x 5W = 116°C. This is acceptable as it is below 150°C.

Dynamic Thermal Calculations

In some applications such as inrush currentcontrol, the

dissipation of poweris of limited duration. If the

duration is between about30s and 10 minutes then

the dynamictemperature rise graph may be used to

determine the highestvalueof

TH achieved atthe end

of the loading. The derivation of

TF and TE is then the

same as for the steady state case.

If poweris reapplied beforethe heatsink has fully

cooled, then cumulativeheatingmustbe accounted

for. This can generally be ignored if the mean power

dissipation is below 10% of the maximum permissible

steady state dissipation.

For durations below about30s, the bestguide is the

Pulse Performance graph on the WMHP datasheet.

TE = Temperature of resistor element

P = Power dissipated

RθEF = Thermal resistance element to flange of WMHP

TF = Temperature of resistor flange

RθFH = Thermal resistance flange to heatsink ofthermal interface

TH = Temperature ofheatsink mountingsurface

RθHA = Thermal resistance heatsinkto ambient of WMHP-HS

TA = Temperature of ambient air

A simple thermal model of apowerresistoron a

heatsink is shown, in which temperature rise may

be calculated from the product of powerdissipated

and thermal impedance. Maximum

P, TE and RθEF

values forWMHP are given below.

Figure 1. Thermal model of WMHP and WMHP-HS

RθEF

RθFH

RθHA

TE

TF

TH

TA

P

Natural Convection: τ = 2.4 minutes

Forced Air 2.5m/s: τ = 50 seconds

Steady State Thermal Calculations

A simple thermal model of a power resistor on

a heatsink is shown, in which temperature rise

may be calculated from the product of power

dissipated and thermal impedance. Maximum

P,

T

E and RθEF values for WMHP are given below.

The maximum value of

P for T

F >25°C is a function of TF and is shown in the WMHP datasheet de-rating graph.

The value of

R

θFH depends on the thermal interface material. For thermal paste a typical figure is around 1°C/W.

For forced air cooling the value of

R

θHA is shown as a function of air velocity in the Forced Air Cooling graph. For

natural convection

R

θHA is slightly non-linear, as shown in the Natural Convection graph. The standard figure at P

= 20W is given, and for lower powers you can directly read the heatsink temperature rise

T

H – TA.

Example: WMHP20 is dissipating 5W at an ambient air temperature of 30°C. The WMHP-HS is PCB mounted

and cooled by natural convection. Is the maximum resistor element temperature acceptable?

T

A = 30°C, and TH – TA = 50°C at P = 5W. Therefore TH = 80°C. Assuming RθFH = 1°C/W, TF = 85°C.

Therefore

T

E = 85°C + 6.25°C/W x 5W = 116°C. This is acceptable as it is below 150°C.

Dynamic Thermal Calculations

In some applications such as inrush current control,

the dissipation of power is of limited duration. If the

duration is between about 30s and 10 minutes then

the dynamic temperature rise graph may be used to

determine the highest value of

T

H achieved at the end of

the loading. The derivation of

T

F and TE is then the same

as for the steady state case.

If power is reapplied before the heatsink has fully

cooled, then cumulative heating must be accounted

for. This can generally be ignored if the mean power

dissipation is below 10% of the maximum permissible

steady state dissipation.

For durations below about 30s, the best guide is the

Pulse Performance graph on the WMHP datasheet.

Heatsink for WMHP Series High Power Resistors

WMHP-HS Series

09.17

Application Notes

Steady State Thermal Calculations

Type

P Max @ TF =25°C

(W)

TE Max

(°C)

RθEF

(°C/W)

WMHP20

20

125

6.25

WMHP35

35

3.57

WMHP50

50

2.50

WMHP100

100

175

1.50

The maximum value of

P for TF >25°C is a function of TF and is shown in the WMHP datasheet de-rating graph.

The value of

RθFH depends on the thermal interface material. For thermal paste a typical figure is around 1°C/W.

For forced air cooling the value of

RθHA is shown as a function of air velocity in the Forced Air Cooling graph. For

natural convection

RθHA is slightly non-linear, as shown in the Natural Convection graph. The standard figure at P

= 20W is given, and for lower powers you can directly read the heatsink temperature rise

TH – TA.

Example: WMHP20 is dissipating 5W at an ambient air temperature of 30°C. The WMHP-HS is PCB mounted and cooled by natural

convection. Is the maximum resistor element temperature acceptable?

TA = 30°C, and TH – TA = 50°C at P = 5W. Therefore TH = 80°C. Assuming RθFH = 1°C/W, TF = 85°C.

Therefore

TE = 85°C + 6.25°C/W x 5W = 116°C. This is acceptable as it is below 150°C.

Dynamic Thermal Calculations

In some applications such as inrush current control, the

dissipation of power is of limited duration. If the

duration is between about 30s and 10 minutes then

the dynamic temperature rise graph may be used to

determine the highest value of

TH achieved at the end

of the loading. The derivation of

TF and TE is then the

same as for the steady state case.

If power is reapplied before the heatsink has fully

cooled, then cumulative heating must be accounted

for. This can generally be ignored if the mean power

dissipation is below 10% of the maximum permissible

steady state dissipation.

For durations below about 30s, the best guide is the

Pulse Performance graph on the WMHP datasheet.

TE = Temperature of resistor element

P = Power dissipated

RθEF = Thermal resistance element to flange of WMHP

TF = Temperature of resistor flange

RθFH = Thermal resistance flange to heatsink of thermal interface

TH = Temperature of heatsink mounting surface

RθHA = Thermal resistance heatsink to ambient of WMHP-HS

TA = Temperature of ambient air

A simple thermal model of a power resistor on a

heatsink is shown, in which temperature rise may

be calculated from the product of power dissipated

and thermal impedance. Maximum

P, TE and RθEF

values for WMHP are given below.

Figure 1. Thermal model of WMHP and WMHP-HS

RθEF

RθFH

RθHA

TE

TF

TH

TA

P

Natural Convection: τ = 2.4 minutes

Forced Air 2.5m/s: τ = 50 seconds

Heatsink for WMHP Series

High Power Resistors

WMHP-HS Series

01.22

General Note

TT Electronics reserves the right to make changes in product specification without notice or liability.

All information is subject to TT Electronics’ own data and is considered accurate at time of going to print.

© TT Electronics plc

www.ttelectronics.com/resistors

BI Technologies IRC Welwyn

OBSOLETE