Browse Prior Art Database

ISOLATED HIGH VOLTAGE CONTROLLER

IP.com Disclosure Number: IPCOM000025352D
Original Publication Date: 1984-Dec-31
Included in the Prior Art Database: 2004-Apr-04
Document File: 4 page(s) / 114K

Publishing Venue

Xerox Disclosure Journal

Abstract

Opto-isolators may be used as isolation elements in high voltage controllers. Prior art teaches the use of an light emitting diode (LED) light dependent resistor (LDR) isolator wherein the LED is energized by an analog voltage. The use of two or four isolators in a totem pole or bridge configuration enables any output voltage between the positive and negative high voltage rails to be obtained. However, in the systems described only one element in the totem pole or two elements in the bridge are active at any time.

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Page 1 of 4

XEROX DISCLOSURE JOURNAL

ISOLATED HIGH VOLTAGE CONTROLLER Proposed Classification Raymond W. Huggins U.S. C1. 250/210

Int. CI. HOlj 40/14

'IN

FIG. I

FREE RUNNING

CLOCK BUFFER

-PULSE WIDTH M

vc MODULATOR

DIFFERENTIAL AMPLIFIER

PULSE WIDTH MODULATOR

VOUT

II

INVERTING BUFFER

INVERTING I_ +A VFEF.

BUFFER

Volume 9 Number 6 November/December 1984 363

F/G 2

1BUFFER

h

D

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ISOLATED HIGH VOLTAGE CONTROLLER (Cont'd)

Opto-isolators may be used as isolation elements in high voltage controllers. Prior art teaches the use of an light emitting diode (LED) light dependent resistor (LDR) isolator wherein the LED is energized by an analog voltage. The use of two or four isolators in a totem pole or bridge configuration enables any output voltage between the positive and negative high voltage rails to be obtained. However, in the systems described only one element in the totem pole or two elements in the bridge are active at any time.

In this proposal, pulse width modulation of the LED rather than analog excitation is used. In the totem pole configuration, the duty cycle of the first LED is arranged to equal one minus the duty cycle of the second LED. Thus, both opto-isolators are always active. The advantages of this mode of operation of the active elements in a totem pole or bridge configuration will be immediately apparent to those skilled in the electronics art.

If the LED in an LED-LDR opto-isolator is excited by a pulse train of duty cycle D and frequency f, provided that the time constant of the LDR is long compared to l/f, the conductance of the LDR will equal

G = DGo

Where G equals the conductance of the LDR when D=1.0 and is determined by the value of ?he on current through the LED. In a totem pole configuration as shown in Figure 1, where D1 + D2 = (O( D < 11, and where Go is the same for each LDR, it can be shown (2) that the unloaded output voltage is

'out = 'HVD

and that the output impedanc...