Handbook for Sound Engineers

690 Chapter 19

source tied to the negative output and its drain tied to
the switching node.

The circuit tricks the resulting 300 kHz buck regu-
lator into performing as an inverting-topology switcher
by connecting the IC’s GND pin to the negative output
voltage instead of circuit ground. This switching regu-
lator’s efficiency of about 88% exceeds that of compa-
rable asynchronous-rectifier supplies by 4%.

19.7 Converters

A converter changes low-voltage dc to high-voltage dc.
Basically, a dc-to-dc converter consists of a dc source of
potential (generally a battery) applied to a pair of
switching transistors. The transistors convert the applied
dc voltage to a high-frequency ac voltage. The ac volt-
age is then transformed to a high voltage that is rectified
to dc again and filtered in the conventional manner.
Power supplies of this nature are often used for a source
of high voltage, where the usual ac line voltage is not
available.

19.8 Inverters

An inverter converts direct current to alternating cur-
rent. Inverters are used in applications where the pri-
mary source of power is direct current. Because direct
current cannot be transformed, it is converted to alter-
nating current so that alternating current output from the

inverter may be applied to a transformer to supply the

desired voltage.

An inverter operates much like the switching circuit

and transformer section of a converter. In Fig. 19-25, R 1

and R 2 assure that the oscillator (switch) will start. T 1 is

a saturable base-drive transformer that determines the

drive current to turn on Q 1 or Q 2. T 2 is a non-saturable

transformer; therefore, collector current through Q 1 and

Q 2 is dependent upon load. Base resistors Rb are current

limiting resistors. By adding a rectifier and filter

section, this inverter can be changed to a converter.

19.9 Ultra Capacitor (UPS)

A backup power supply for medical computers manu-

factured by Ram Technologies utilizes ultra capacitor

technology. The model 8000 Ultra UPS module con-

tains the charge and discharge circuitry to ensure high-

efficiency energy transfer, Fig. 19-26. The proprietary

patent pending module is designed to directly interface

with RAM Technologies line of ATX/SFX medical-

grade power supplies. The unit can be modified by Ram

Technologies to operate with other sensitive and/or life-

threatening devices. The module may be expanded by

adding additional ultra capacitor modules. The base

module contains 8000 J of energy; expansion modules

also contain 8000 J of energy. Any number of additional

modules can be added to increase load capabilities. Fig.

19-27 shows a typical installation in a computer.

The module’s input voltage is +12 Vdc and has an

efficiency of >90%. Charge time is 2 minutes for each

8kJ module.

The maximum output current is 30 A at 12 Vdc. Run

time is

(19-23)

Figure 19-24. The inverting topology requires that the syn-
chronous switch be in series with the output. Courtesy
Maxim Integrated Products.

Input 4.5–35 V

0–5 V

1.5 A

MAX797

Figure 19-25. Two-transistor, two-transformer, push-pull

inverter that uses a resistive voltage-divider network to pro-

vide starting bias.

Rb

R 1

R 2 Vdc

Rb

Rb

Rb

T 1 T 2

RFB

RLoad

Run time number^ of^ modulesu^133

dc load

= -------------------------------------------------------------------