DN-43
Design Note
Simple Techniques to Generate a
Negative Voltage Bias Supply
from a Positive Input Voltage
by Bill Andreycak
Developing a low power negative supply voltage
from a positive input supply can be accomplished
using some very common PWM control ICs. Typical
applications include generating a negative five
through twelve volt (-5V to -12V) supply for analog
function ICs (OP amps), RS-232 communication
circuits, and MOSFET or IGBT gate drives at power
levels below a few Watts.
Many PWM ICs contain a high current totem pole
output which goes high during the device’s ON-time
or pulse width. The exact pulse width is modulated
to regulate a converter’s output voltage during
changes in input voltage and output current. The
IC’s totem pole output can also be used as the main
switch in low power applications. One example of
this is a Flyback converter configured as shown in
Figure 1.
Figure 1: Low Power Negative Output Voltage +5V
to -5V Inverter
IC’s lower totem pole transistor thus saving one
component. Reverse recovery characteristics are
not a concern since current also flows through the
MOS transistor channel in parallel with the body di-
ode.
The circuit can be designed for either continuous or
discontinuous inductor current operation, depending
on the application. Discontinuous mode is generally
preferred at lower power levels to minimize inductor
size. Continuous inductor current operation is more
applicable with higher load currents. High frequency
switching and surface mount packaging options
minimize overall size.
Conventional duty cycle control ( voltage mode ) is
less complex to implement as the control technique
than current mode control. The principal difficulty is
sensing the inductor current which is not referenced
to the IC’s return connection, -Vout instead of
ground. Adding a current sense transformer is pos-
sible, but will increase cost and complexity. Overcur-
rent protection is obtained by using the IC’s internal
maximum current limit at the switch.
Note that the BiCMOS UCC3803 device used in the
example circuit of Figure 1. has a maximum low im-
pedance input supply voltage rating of 12VDC. This
limits it’s applications to less than negative seven
volts outputs (-7V max) with a +5V DC input. Higher
voltage outputs, for example -12V to -15V, can be
obtained by using a UC3843 with a higher maxi-
mum input voltage ( 30VDC ). Here too there are
some limitations. These fully bipolar ICs draw higher
supply current and have higher undervoltage lock-
out (UVLO) thresholds, but are acceptable choices
for some applications.
The IC datasheet should be referred to for addi-
tional information. Application Notes within the IC
databook contain general information about the de-
sign of the Flyback converter, and others. For fur-
ther assistance contact a Unitrode Field Application
Engineer or the factory.
With this arrangement, the inductor (L) charges
when the IC output is high and discharges or flies-
back when the IC output goes low. Energy stored
during the inductors charging time is transferred to
the output capacitor (Cout) during the flyback, or
OFF portion of the cycle. When a BiCMOS PWM
control IC is used, the external diode can be re-
placed by the MOS channel and body diode of the
8/93
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