SMT PLANAR TRANSFORMER
AND INDUCTOR
For use with Linear Technology’s LT1681 and LTC1698
Notes from Tables
For PA0191: Basic Insulated Planar Transformer
1.To determine if the transformer is suitable for your application it
is necessary to ensure that the temperature rise of the compo-
nent (ambient plus temp. rise) does not exceed its operating
temperature. To determine the temperature rise of the compo-
nent it is necessary to calculate the total power losses (copper
and core) in the application.
Total Copper Losses
CopperTotal (W) = sum of the losses in each winding
The losses in each winding can be calculated by:
CopperWindingX (W) = .001 * DCR(mΩ) * (Irms
2
)
Core Losses
To calculate the core loss, use the following formula:
CoreLoss (W) = 2.5E-8 * (Freq kHz)
1.7
* (∆B/2000)
2.5
where:
∆B/2000
= 1.01 * (Voltage Min * Dutycycle Max/Freq kHz)
Temperature Rise
TempRise (°C) = .603 * CopperTotal (W) * (50.6 * CoreLoss
(W)/CopperTotal (W) + 88.1)
For P1977: Planar Inductor
2.The rated current as listed is either the saturation current or the
heating current depending on which value is lower.
3.The saturation current is the current which causes the induc-
tance to drop by 10% at the stated ambient temperatures (25°C,
125°C). This current is determined by placing the component in
the specified ambient environment and applying a short duration
pulse current (to eliminate self-heating effects) to the compo-
nent.
4.The heating current is the dc current which causes the tempera-
ture of the part to increase by approximately 40°C. This current
is determined by mounting the component on a PCB with a .25"
wide (2 ounce equivalent copper traces) and applying the
current to the device for 30 minutes.
5.In high volt*time applications additional heating in the compo-
nent can occur due to core losses in the inductor which may
neccessitate derating the current in order to limit the tempera-
ture rise of the component. In order to determine the approxi-
mate total losses (or temperature rise) for a given application
both copper and core losses should be taken into account.
Total Copper Losses
CopperTotal (W) = .001* DCR(mΩ) * (Irms
2
)
where:
Irms = (IDC
2
+ (∆I/2)
2
)
.5
Core Losses
To calculate the core loss, use the following formula:
CoreLoss (W) = 2.5E-8 * (Freq kHz)
1.7
* (.0635 *
∆I)
2.5
Temperature Rise
TempRise (°C) = .603 * CopperTotal (W) * (50.6 * CoreLoss
(W)/CopperTotal (W) + 88.1)
APPLICATION NOTES
The
PA0191
transformer and
P1977
inductor were designed for
use with Linear Technology's LT1681 and LTC1698 IC's and are
featured in Linear Technology's Design note 261 which details the
solution for a low cost, discrete component alternative to telecom
power modules.
The
PA0191
transformer and
P1977
inductor were designed for
(but not limited to) the following application:
Topology
– Two-switch forward
Frequency
– 230 kHz
Pri./Sec. Isolation
– Basic Insulation (1500 Vdc)
Input Voltage
– 36-72 v telecom input
Output Voltage(s)
– 3.3 v / 20 A output (with an auxilary
primary side control winding - 12 v/100 mA)
2.50
2.00
P1977 Ind. vs. Current
Ind (µH)
1.50
1.00
0.50
0.00
0
5
Current (A)
25
°
C
100
°
C
10
15
20
25
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P553.A (7/02)
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