7 Detailed Description

7.1 Overview

The SN6501 is a transformer driver designed for low-cost, small form-factor, isolated DC-DC converters utilizing

the push-pull topology. The device includes an oscillator that feeds a gate-drive circuit. The gate-drive,

comprising a frequency divider and a break-before-make (BBM) logic, provides two complementary output

signals which alternately turn the two output transistors on and off.

The output frequency of the oscillator is divided down by an asynchronous divider that provides two

complementary output signals with a 50% duty cycle. A subsequent break-before-make logic inserts a dead-time

between the high-pulses of the two signals. The resulting output signals, present the gate-drive signals for the

output transistors. As shown in the functional block diagram, before either one of the gates can assume logic

high, there must be a short time period during which both signals are low and both transistors are high-

impedance. This short period, known as break-before-make time, is required to avoid shorting out both ends of

the primary.

7.2 Functional Block Diagram

OSC

Q

Q

SN6501

Gate

Drive

D1

V

CC

D2

GND

GND

7.3 Feature Description

7.3.1 Push-Pull Converter

Push-pull converters require transformers with center-taps to transfer power from the primary to the secondary

(see Figure 7-1).

C

CR1

CR2

Q1

Q2

VIN

VOUT

RL

C

CR1

CR2

Q1

Q2

VIN

VOUT

RL

Figure 7-1. Switching Cycles of a Push-Pull Converter

When Q1 conducts, VIN drives a current through the lower half of the primary to ground, thus creating a negative

voltage potential at the lower primary end with regards to the VIN potential at the center-tap.

At the same time the voltage across the upper half of the primary is such that the upper primary end is positive

with regards to the center-tap in order to maintain the previously established current flow through Q2, which now

has turned high-impedance. The two voltage sources, each of which equaling VIN, appear in series and cause a

voltage potential at the open end of the primary of 2×VIN with regards to ground.

Per dot convention the same voltage polarities that occur at the primary also occur at the secondary. The

positive potential of the upper secondary end therefore forward biases diode CR1. The secondary current

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SN6501

SLLSEA0I – FEBRUARY 2012 – REVISED JANUARY 2021

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