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Non-Overlapping Push-Pull Driver for High Speed Switching of Power Field-Effect Transistors

IP.com Disclosure Number: IPCOM000040110D
Original Publication Date: 1987-Sep-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 4 page(s) / 58K

Publishing Venue

IBM

Related People

Bustamante, CM: AUTHOR [+2]

Abstract

This article describes a high speed push-pull driver for power field- effect transistors (FETs). The disclosed circuit's function is converting high impedance (25 ohms) logic-level signals to higher voltage levels (12 volts), performing a phase splitting function followed by a high-to-low impedance (0.06 ohms) conversion to drive the push and pull stages which switch a large FET. Traditional drivers suffer from three problems as follows: High transient surge currents during switching of the push-pull devices which is power consumptive and creates radiated and conducted interference problems. Inability to fine tune the circuit to meet the applications requirements (switching speed, power consumption) without modifying the circuit topology.

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Non-Overlapping Push-Pull Driver for High Speed Switching of Power Field-Effect Transistors

This article describes a high speed push-pull driver for power field- effect transistors (FETs). The disclosed circuit's function is converting high impedance (25 ohms) logic-level signals to higher voltage levels (12 volts), performing a phase splitting function followed by a high-to-low impedance (0.06 ohms) conversion to drive the push and pull stages which switch a large FET. Traditional drivers suffer from three problems as follows: High transient surge currents during switching of

the push-pull devices which is power consumptive and

creates radiated and conducted interference problems.

Inability to fine tune the circuit to meet the

applications requirements (switching speed, power

consumption) without modifying the circuit topology.

Difficult to interface to feedback circuits for

precise drive control (linear current-feedback).

(Image Omitted)

In an infrared (IR) system, switching the light-emitting diode (LED) bank as fast as possible is

important because of the receiver filtering method.

The topology of the generalized push-pull driver, FET,

and load is shown in Fig. 1. The difficulty in

switching a power FET rapidly is the FET's large

gate-to-source capacitance and gate-to- drain

capacitance which is effectively multiplied due to the

Miller effect. In order to have a drain-to-source

resistance of only .08 ohm when on, the power FET must

have a very large geometry (on impedance is direct

function of enhanced channel area). This large

geometry gives rise to a large capacitance. The

effective capacitance, as seen by the pre-driver for a

typical transmitter configuration, is about 5000

picofarads. To enhance the channel of the power FET to

turn it on, this capacitance must be charged to about 6

volts gate-to-source turn-on voltage (Vgs). Therefore,

a large transient current must be provided from the

pre-driver to switch the FET rapidly. The equations that dictate the switching speed of the FET are linearized when mapped to the charge domain. Required charge (Q) to switch power FET = 20 nanocoulombs Capacitance C = Q/V

dQ/dt = I = C dV/dt where I is current Therefore, switching speed t is dictated by the following equation (both on and off switching): t = Q (20 nanocoulombs) / I This equation implies that with a constant current source of 1 amp, the FET will switch in 20

1

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nanoseconds, with a source of .1 amp, 200 nanoseconds, etc. The existing solution is to use a chip specifically designed to be a pre-driver level shifter to switch a power FET. The circuit disclosed herein provides a high speed means to switch a power FET and non-overlapping, push-pull drive for switching symmetry. The non-overlapping, push-pull, FET driver circuit is shown in Fig. 2.

It comprises four basic sections: data is buffered; the phase splitter creates non-overlapping drive signals to switch the pre- driver; the pre-driver switches the push-pull...