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Improved Turn Off Gate Drive Circuit

IP.com Disclosure Number: IPCOM000121398D
Original Publication Date: 1991-Aug-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 1 page(s) / 63K

Publishing Venue

IBM

Related People

Hitchcock, LJ: AUTHOR [+2]

Abstract

In designing Fullbridge converters that use high input voltage, gate drive transformers are often used for driving the Power MOSFETs and to maintain primary to secondary isolation. However, gate drive transformers pose problems in turning the Power MOSFET on and off at fast speeds. Typical gate drive transformers have a high leakage inductance. This acts as a high impedance to the gate of the Power MOSFET. When the Power MOSFET is turning off the current from the gate to drain capacitance charges the gate to source capacitance, thus turning the device back on. To prevent this action from happening a low impedance is needed between the gate to source of the Power MOSFET during turn-off.

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Improved Turn Off Gate Drive Circuit

      In designing Fullbridge converters that use high input voltage,
gate drive transformers are often used for driving the Power MOSFETs
and to maintain primary to secondary isolation.  However, gate drive
transformers pose problems in turning the Power MOSFET on and off at
fast speeds. Typical gate drive transformers have a high leakage
inductance.  This acts as a high impedance to the gate of the Power
MOSFET.  When the Power MOSFET is turning off the current from the
gate to drain capacitance charges the gate to source capacitance,
thus turning the device back on.  To prevent this action from
happening a low impedance is needed between the gate to source of the
Power MOSFET during turn-off.

      Shown in the figure is the new circuit developed to solve the
turn-off problem.

      Components R1 and CR1 control the rate of turn-on and turn-off.
During turn-on the gate is charge through R1 and during turn-off the
gate is discharged through CR1 until VDS begins to rise.

      Component R3 maintains a maximum impedance across the gate to
source of Q2.

      Component Q2 is the main switch device.

      Components CR2, CR3, Q1, CR4 and R2 forms the turn-off clamp
across the gate to source of Q2.  CR2 is used as variable capacitor
as a function of the drain to source voltage VDS.  Parasitic
capacitance of CR2 and CGS of Q1 form a capacitor divider network.
When Q2 starts to turn-off, VDS begins to rise. ...