UCC28610
SLUS888C–JANUARY 2009–REVISED SEPTEMBER 2009 ......................................................................................................................................... www.ti.com
Cascode Bias and Start-Up
The UCC28610 uses a cascode drive and bias to control the high voltage power MOSFET and provide initial
bias at start-up. Thus, the external high voltage power MOSFET provides the start-up function in addition to the
power switching function during converter operation. The cascode architecture utilizes a low voltage switch
operating between ground and the source of a high voltage MOSFET (HVMOSFET) configured in a common
gate configuration, as shown in Figure 21. There are some key points to note.
1. The gate of the external HVMOSFET is held at a DC voltage.
2. The HVMOSFET is driven through the source, not the gate.
3. The entire primary winding current passes through the internal low voltage Driver MOSFET (both DRV and
GND pins).
Bulk
Bulk
Primary
Winding
Primary
Winding
External
External
HVMOSFET
HVMOSFET
+
Gate Bias
14VDC
Gate Bias
14VDC
V
th
_
Cascoded
MOSFET
Pair
Internal
Internal
Low Voltage
Low Voltage
DRIVER
ON
DRIVER
OFF
PWM
PWM
Control
Control
( a )
( b )
Figure 21. Cascoded Architecture
16
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