SM74611 Datasheet, PDF (5/11 Pages) Texas Instruments – Smart Bypass Diode

SM74611
www.ti.com
APPLICATION INFORMATION
SNVS903 – DECEMBER 2012
The SM74611 is designed for use as a bypass diode in photovoltaic modules. The SM74611 utilizes a charge
pump to drive an N-channel FET to provide a resistive path for the bypass current to flow. Please refer to
Figure 5 and Figure 6 for operational description below
From t₀ to t₁:
When cells in the solar panels are shaded, the FET Q1 is off and the bypass current will flow through the body
diode of the FET as shown on Figure 5. This current will produce a voltage drop (V_F) across ANODE and
CATHODE terminal of the bypass diode. During this time, the charge pump circuitry is active and charging
capacitor C1 to a higher voltage.
At t₁:
Once the voltage on the capacitor reaches its predetermined voltage level, the charge pump is disabled and the
capacitor voltage is used to drive the FET through the FET driver stage.
From t₁ to t₂ :
When the FET is active, it provides a low resistive path for the bypass current to flow thus minimizing the power
dissipation across ANODE and CATHODE. Since the FET is active, the voltage across the ANODE and
CATHODE is too low to operate the charge pump. During this time, the stored charge on C1 is used to supply
the controller as well as drive the FET.
At t₂:
When the voltage on the capacitor C1 reaches its predetermined lower level, the FET driver shuts off the FET.
The bypass current will then begin to flow through the body diode of the FET, causing the FET body diode
voltage drop of approximately 0.6V to appear across ANODE and CATHODE. The charge pump circuitry is re-
activated and begins charging the capacitor C1. This cycle repeats until the shade on the panel is removed and
the string current begins to flow through the PV cells instead of the body diode of the FET.
The key factor to minimizing the power dissipation on the device is to keep the FET on at a high duty cycle. The
average forward voltage drop will then be reduced to a much lower voltage than for a Schottky or regular P-N
junction diode.
C1
Charge
Pump
CATHODE
Q1
Controller
and FET
Driver
ANODE
Figure 5. SM74611 Block Diagram
V ^IBYPASS
F
Copyright © 2012, Texas Instruments Incorporated
5
Product Folder Links: SM74611