Soft start of a switching power supply system

Details Soft start of a switching power supply system Soft start of a switching power supply system

2002-08-15

2004-01-27

Vu, Bao Q. (Department: 2838)

Electricity: power supply or regulation systems

Output level responsive

Using a three or more terminal semiconductive device as the...

C323S901000, C363S049000

Reexamination Certificate

active

06683442

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to switching power supplies, and more particularly to soft start during switch on of a plurality of power supplies connected in a system.
2. Background Information
Switching power supplies are often designed to produce one DC output voltage from a DC input source of power. When it is desired to have power supplied at a plurality of different DC voltages, a plurality of single voltage switching power supplies are assembled into a system, with each desired voltage supplied by a different switching power supply. The plurality of single voltage switching power supplies are then connected to the source of input DC power, and each supplies power at its designated output voltage.
A system designer assembles as many single voltage switching power supplies as he needs to supply power to the loads in the system. Often the loads are computer chips, that is the loads are a group of integrated circuits. The integrated circuits are often connected to the same system ground, and the integrated circuits are also connected together by signal leads between the integrated circuits. Thus, when voltage is applied by one of the switching power supplies to a particular integrated circuit, this voltage may appear across other of the integrated circuits.
“Soft start” of power supplies is a term used to describe bringing the output voltage of the power supply up gradually, usually bringing the voltage up as a linear function of time. When several switching poser supplies are connected in a system, it may be desired to bring up the various voltages in an orderly sequence, so that one voltage begins coming up before a different voltage becomes coming up. To bring up the various voltages up at different times requires that the different individual switching power supplies bring up their voltages at different times.
Since the integrated circuits being supplied by the different switching power supplies are all connected together, voltages different from normal operating voltages may develop between the integrated circuits during a soft start operation. Uncontrolled voltage differences caused by soft start of different switching power supplies may injure the integrated circuits. System designers use a variety of strategies to protect their integrated circuits during a soft start operation.
Turning now to
FIG. 1
, a typical arrangement
100
of a single switching power DC to DC converter power supply is shown. Power source
102
supplies DC power to the switching power supply
104
through connection
103
. Typically, a switching power supply uses Field Effect Transistors (FETs) as switches, and the FETs are switched synchronously by the switching power supply
104
. Such a switching power supply is often referred to as a Synch FET power supply. Synch FET power supply
104
supplies power to a load
106
through connection
105
. The load is often an integrated circuit.
The Synch FET power supply receives power at an input voltage and input current through connection
103
, and supplies power through connection
105
at an output voltage and output current, where the output voltage is typically different from the input voltage.
Turning now to
FIG. 2A
system
200
is a design using three single voltage synch FET power supplies
202
,
203
,
204
to supply power at three different voltages. For example, synch FET power supply
202
supplies power at 1.5 Volts to bus
202
B, synch FET power supply
203
supplies power at 2.5 Volts to bus
203
B, synch FET power supply
204
supplies power at 3.3 Volts to bus
204
B. Typically, each synch FET power supply may supply from around 100 Watts to around 500 Watts of power. Input power supply line
206
supplies DC power to each of the synch FET power supplies
202
,
203
,
204
at a convenient input voltage.
Diode
210
and diode
212
are specified by a system designer in order to protect integrated circuits supplied by power by bus
202
B,
203
B,
204
B during soft start of the power supplies.
Turning now to
FIG. 2B
, graphs
220
show voltage on bus
202
B,
203
B,
204
B during soft start of the synch FET power supplies
202
,
203
,
204
. First, the low voltage power supply is enabled at time
231
, graph
222
rises linearly with time during portion
222
A, and then stabilizes at the desired output voltage 1.5 Volt. Diode
210
pulls up bus
203
B, and diode
212
pulls up bus
204
B so that bus
203
B and bus
204
B follow bus
202
B as synch FET power supply
202
is turned on using soft start. Both diodes
210
and
212
have an internal voltage drop, and so bus
203
B voltage remains less than bus
202
B voltage, and bus
204
B voltage remains less than bus
203
B voltage, as shown in graph
200
, between time
231
and time
232
.
At time
232
synch FET power supply
203
is turned on using soft start, and the voltage on bus
203
B begins to rise as shown by graph
224
. During segment
224
A of graph
224
the voltage rises linearly and then stabilizes at the desired output voltage of 2.5 Volt.
Diode
210
prevents a current flow into bus
202
B from
203
B, and so bus
202
B is not affected by activation of synch FET power supply
203
, as shown by graph
222
. However, diode
212
pulls up bus
204
B so that the voltage on bus
204
B follows that of bus
203
B, but remains less by the voltage drop across diode
212
, as shown by graph
226
.
At time
223
synch FET power supply
204
is turned on by soft start. The voltage on bus
204
B begins to rise linearly as shown by graph
226
at section
226
A, and then stabilizes at the desired output voltage of 3.3 Volt. Diode
212
prevents current flow from bus
204
B to bus
203
B, and so the voltage of bus
203
B is not affected by the rise of voltage applied by synch FET power supply
204
to bus
204
B.
The use of diodes
210
and
212
by a system designer who combines a plurality of single voltage synch FET power supplies to supply a plurality of voltages to an integrated circuits as loads work by pulling up the un-activated power buses, and so the diodes prevent unwanted large voltage differences to develop across the loads, typically integrated circuit loads (not shown in FIG.
2
A).
However, the diodes
210
,
212
cause a problem in the un-activated synch FET power supplies, and the problem is called the “back bias” problem.
The back bias problem is illustrated in
FIGS. 3A
,
3
B,
3
C.
FIG. 3A
illustrates a synch FET DC to DC converter power supply
300
. Input power and current are supplied on bus
302
at an input voltage V
in
. Output power and current are supplied by the DC/DC converter
304
on output bus
305
at a desired output voltage V
out
. A sense voltage line
308
, shown as directly connected to output bus
305
is used by DC/DC converter
304
to monitor and control the output voltage on output bus
305
. A reference voltage is supplied on line
306
to DC/DC converter
304
. The DC/DC converter
304
regulates the output voltage to match the reference voltage on line
306
.
FIG. 3B
is a graph showing operation of synch FET DC to DC converter power supply
300
when the output voltage is pulled up above the reference voltage, as occurs through diodes
210
and
212
before the higher voltage synch FET power supplies are turned on.
Back bias voltage
310
is applied to the synch FET power supply, for example by a diode such as diode
210
,
212
. At time
312
the synch FET power supply
304
is turned off and the back bias voltage
310
has no effect. At time
314
synch FET power supply
304
is turned on and the reference voltage, shown as the dotted line graph
316
, is less than the output voltage. The output voltage of synch FET
304
is driven down during segment
310
A of graph
310
until time
318
, and at time
318
the reference voltage is caused by control circuits (not shown) to rise linearly in order to implement soft start. Segment
310
B of graph
310
shows the output voltage rising with the rise in reference voltage, as the reference voltage rises linearly with time. At time
3

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