Direct Computer DC Backup System
ECE 345 Final Project Proposal
By: Tom Galos
June 23, 1999
I propose to build a dc-dc boost converter that takes power from a 12 V battery, and
converts it to a higher dc level consistent with the dc bus level inside a conventional
desktop PC. The purpose is to provide a direct dc power backup. If the ac power is lost,
the battery system will take over automatically. This maintains the system without the
need for an inverter.
All computer power supplies use a rectifier-dc link-dc/dc converter architecture for
power conversion. This means that there is an internal dc bus, at a level of 150 V to 170
V or so, that follows the rectifier. In conventional backup systems, an inverter is built to
take the 12 V battery power and create 120 V rms ac power. This almost a waste since
the first step inside the computer is to rectify the ac input.
The idea behind the direct dc backup system is to provide a 150 V dc jack on the back of
the computer. A diode is connected with the anode to the jack and the cathode to the
internal dc bus to prevent backfeed or other problems. An external dc-dc converter can
take power from the battery and develop 150 V directly. If the ac power then goes away,
this dc power takes over automatically with no special effort. However, the drawback is
that this works for the PC itself, but not the display. Nonetheless, this is enough to
provide complete protection against short-term disturbances.
A block diagram of the initial design is shown below.
12 V Battery #1
Figure 1: Block diagram of the converter to be designed.
For the boost converter to be designed the magnitude of the output voltage is greater than
that of the input voltage. The input current and output voltages are fixed source values,
while input voltage and output current are determined by the switching of the converter.
Also the input power and output power must be equal for no power to be lost in the
The converter will have a 12 V battery input source. Switch #1 carries the input current
when on and when it is off, switch #2 is on, and switch #1 must block the output voltage.
Therefore, switch #1 will be a forward-conducting forward-blocking device, a FET.
Switch #2 can be a diode since it only needs to carry and block the output current
depending when it is on or off. For the FET a gate drive circuit will be necessary and a
PWM IC will be used to implement the gate control. The inductor and capacitor act as
interfaces to transfer the current and voltage respectively. These devices are just transfer
sources so power flows through the converter without loss. However, some loss will be
there due to the switching of the device. The load will be modeled as a resistor to
produce the fixed output voltage at a certain power rating.
Input voltage range: 12 V 2%
Output voltage range: 150 V 0.2%
Output load range: 5 W to 20 W
In order to validate these performance specifications and proper operation, a series of
tests will be performed. These tests are:
1. Verify that current is always in continuous mode.
2. Verify that the output voltage is within the specified range.
3. Verify that output ripple meets output specification.
4. Verify the load range.
5. Verify the output voltage agrees with the fixed voltage value.
6. Verify that the output power resistor is ± 10% of the rated value.
7. Verify that the chips are within temperature limits.
6/28 – Design and specify best converter layout and begin construction.
7/5 – Run initial test to see how converter performs.
7/12 – Finish testing and make final adjustments.
7/19 – Demo.
7/26 – Oral report.
The cost was calculated at 25$/hour for labor. A total of 100 hours will be required to
complete the design. Using the equation below the total labor costs were calculated to be
TotalLabor hour * 2.5 * totalhours
The table below shows the parts required and their estimated costs. The last row of the
table contains the grand total cost of the project.
Part Estimated Cost
Pulse-Width-Modulator Chip $10.00
Power FET $5.00
Power Diode $3.00
Total = $48.00
Grand Total = Total Labor + Total parts = $6298
The parts listed here will be purchased by the designer because it is my desire to keep the
finished product. All other necessary products for the design of this project will either be
purchased by the designer or borrowed from the parts lab if necessary.