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Transmitting Multiple Signals Via a Single Pulse Transformer

IP.com Disclosure Number: IPCOM000107417D
Original Publication Date: 1992-Feb-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 3 page(s) / 145K

Publishing Venue

IBM

Related People

Najm, EM: AUTHOR

Abstract

Disclosed is a circuit that can transmit multiple signals via a single pulse transformer. This circuit is used to transmit control, protection and synchronization information across the primary to secondary isolation barrier in a power supply. Signals are separated by voltage differences.

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This is the abbreviated version, containing approximately 49% of the total text.

Transmitting Multiple Signals Via a Single Pulse Transformer

       Disclosed is a circuit that can transmit multiple signals
via a single pulse transformer.  This circuit is used to transmit
control, protection and synchronization information across the
primary to secondary isolation barrier in a power supply.  Signals
are separated by voltage differences.

      Most power supplies have DC outputs which are isolated from the
AC power line.  In design of these power supplies, there is a need to
send information from the output or secondary side of the power
supply back to the primary.  This may be done for control purposes,
overload protection or synchronization and must be done in such a way
that safety isolation is maintained.  Pulse transformers are often
used for these purposes.

      In today's competitive business environment, it is important to
minimize the cost of power supplies.  The figure shows how a single
pulse transformer can be used to transmit control, synchronization
and protection information.  Transmitting these three signals is
accomplished using a single secondary winding, where its output
signals are distinguished from one another by voltage levels.  The
circuit in the figure shows how this technique is applied in a
non-reset FLYBACK regulator.  One can easily modify the circuit for
use with other power supply topologies or for other applications
outside the power supply field.

      The circuit shown in the figure has been simplified somewhat
for clarity.  Several components which would be needed in a practical
power supply have been omitted, such as leakage resistors.  Some
other essential circuits are also not shown.  These are the clamp and
snubber circuits which are needed with any high voltage switching
device, the primary OVERCURRENT protection and the start circuit.
These circuits are well known and do not affect the pulse transformer
circuit.

      The pulse transformer can be driven from any of three DC
secondary voltages (V1, V2 or V3).  These voltages are generated by
secondary windings of T1 (not shown).  V3 is greater than V2, which
is greater than V1.  Transistors Q7, Q8 and Q9 connect these voltages
to the primary of the pulse transformer.  Only one transistor is
turned on at a time, depending on which signal is to be transmitted.
Diodes CR7, CR8 and CR9 maintain isolation among the three voltages.
(The details of the drive circuits for Q7, Q8 and Q9 are not
essential to this new approach).  The voltage appearing on the output
winding of the pulse transformer depends on which transistor is
driving it.  The lowest output is for a control signal and the
highest output is for a shutdown signal.

      Three circuits are connected to the secondary of the pulse
transformer.  One (Q2, C1, R2 and R3) tries to respond to any output
from the pulse transformer.  R2, R3 and C1 establish a short delay
time before the turn on of Q2.  The need for this delay will be
ex...