Switch mode power supply using transformer flux sensing for...

Details Switch mode power supply using transformer flux sensing for... Switch mode power supply using transformer flux sensing for...

2000-12-19

2002-05-28

Vu, Bao Q. (Department: 2838)

Electric power conversion systems

Current conversion

Including d.c.-a.c.-d.c. converter

C363S021100, C363S021150, C363S021180, C363S041000

Reexamination Certificate

active

06396718

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to switch mode power supplies (SMPS) and, more particularly, to forward mode SMPS that use transformer saturation sensing to control the maximum duty cycle of the SMPS switching elements.
BACKGROUND OF THE INVENTION
Switch mode power supplies (SMPS) are commonly used to provide DC power to integrated circuits.
FIG. 1
illustrates an SMPS
100
in accordance with the prior art. SMPS
100
is coupled to an AC input voltage source
102
, and includes a full-wave rectifier
104
, bulk capacitor
106
, output transformer
108
, power supply controller
110
, a switch (e.g., a high-power transistor
112
, such as a MOSFET), and secondary circuit
114
that includes a filter capacitor
130
.
Voltage source
102
provides an input AC voltage between low line and high line values (e.g., between 90 Vac and 264 Vac). Rectifier
104
rectifies the input AC voltage. The output of rectifier
104
is smoothed by bulk capacitor
106
, and the resulting input voltage is applied to the primary winding of transformer
108
. Bulk capacitor
106
also enables SMPS
100
to maintain the output voltage, VOUT, within regulation for a minimum period of time after the AC power is switched off to SMPS
100
. This minimum period of time is commonly referred to as the “hold up time.”
Transistor
112
is used as a power switch, which is controlled by controller
110
. Controller
110
includes a pulse width modulator
116
(PWM), which is driven by a clock
118
. A pulse width modulated signal produced by PWM
116
activates and deactivates transistor
112
in response to power supply loading and other control parameters.
Regulation of VOUT is accomplished by varying the on-to-off duty cycle of transistor
112
. Essentially, VOUT can be increased or decreased by increasing or decreasing, respectively, the duty cycle of the pulse width modulated signal provided by controller
110
.
During steady state operations, when the load applied to the SMPS remains the same, the decrease in the transformer flux when transistor
112
is off must equal the increase in flux when the transistor
112
is on. Otherwise a net change in flux per cycle will occur, and the flux will continue to build up to saturation. Accordingly, if the duty cycle of the pulse width modulated signal is too long, transformer
108
will become saturated. Saturation increases the risk that the power transistor
112
will be damaged by high currents. Therefore, it is desirable to maintain the transformer operation within the transformer's non-saturation region.
During transient state operations, when the load applied to the SMPS suddenly increases, a longer duty cycle is required to provide the energy necessary to maintain a constant VOUT. This longer duty cycle during transient state operations also must be limited to avoid transformer saturation.
Some prior art systems attempt to avoid transformer saturation by setting a fixed, maximum duty cycle that will avoid saturation during both steady state and transient state operations. Unfortunately, however, these prior art systems have relatively high transistor conduction losses, and require a large bulk capacitor (e.g., capacitor
106
) to meet hold up time requirements during power off.
What is needed is an SMPS that efficiently avoids transformer saturation and its associated transistor damage during both steady state and transient state operations. Further needed is an SMPS having a lower transistor conduction loss, and a relatively small bulk capacitor.


REFERENCES:
patent: 5301095 (1994-04-01), Teramoto et al.
patent: 5313381 (1994-05-01), Balakrishman
patent: 5408402 (1995-04-01), Nonnenmacher
patent: 5701238 (1997-12-01), Weidinger et al.
patent: 5949661 (1999-09-01), Minkkinen
patent: 5959851 (1999-09-01), Shutts
patent: 6091616 (2001-06-01), Jacobs et al.
patent: 6243278 (2001-06-01), Jacobs
“Unitrode Low Power Pulse Width Modulator”, Unitrode Application Information, pp. 3-94 to 3-99, Mar. 1997.

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