AN3432 datasheet(20/24 Pages) STMICROELECTRONICS

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Optimized bypass diode for a given solar panel or junction box

AN3432

20/24

Doc ID 019041 Rev 1

Optimized bypass diode for a given solar panel or

junction box

This section describes a method to choose the optimized bypass diode through an

application example with a 400 W photovoltaic panel. Its short circuit current (Isc) is 6.4 A

and a its total open circuit voltage (Voc) is 85.3 V in STC conditions. This solar panel needs

3 bypass diodes placed in a junction box having Rth(j-a) = 30 °C/W.

Step 1: Determine VRRM minimum value

The highest open circuit voltage value for the solar panel (Voc) is for the -40 °C ambient

temperature, that is Voc (-40 °C) = 107 V, with a deviation tolerance of 20% applied. Then

the maximum reverse voltage applied to each bypass diode is 43 V.

So the VRRM minimum value is 43 V @ -40 °C. In the product range the nearest VRRM value

is 45 V.

Step 2: Diode characteristics versus application constraints

The bypass diode must be compatible with the thermal test. In general its maximum junction

temperature is 175 °C.

Then the thermal losses can be calculated as:

The junction box is able to dissipate 3.33 W when the junction temperature of the diode is

175 °C.

To meet the test requirements (Tj < Tjmax) the following condition applies:

Pcond(Tjmax) < Pthermal (Tjmax)

With this condition the maximum forward voltage at Tjmax and 1.25 · Isc can be calculated

using Equation 18:

The STPS3045CG is compliant with the thermal test requirements since its

VFmax (8 A, 175 °C) = 0.31 V

The forward voltage (VF) of a Schottky diode is temperature dependent. VF decreases

linearly with temperature and can be calculated using the following:

Then:

VFmax (8 A, 175 °C) = -0.09 + 0.4 = 0.310 V:

Thermal

) =

) = 3.33 W

175 °C

(

(175 °C - 75 °C)

) <

(1.25 ·

(8 A,

(175 - 75)

max

175 °C

1.25 · 6.4

0.417 V

) =

)

(8 A,

max

125 °C

175 °C

125 °C

(175 - 125 °C) +

with

= -1.8 mV/°C

125 -25

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