Power supply device with reference signal generating circuit for power saving mode

A power supply device having a transformer for generating at least one output voltage supplied to an electronic apparatus with power saving mode includes: a switching circuit connected to the transformer, a reference signal generator for generating a reference signal having a first high frequency or a second low frequency according to whether the electronic apparatus is in a normal operation mode or a power saving mode, and a PWM control circuit for generating a PWM signal to control the switching operation of the switching circuit according to the reference signal and a feedback signal from the output voltage.

CLAIM OF PRIORITY 
This application makes reference to, incorporates the same herein, and 
claims all benefits accruing under 35 U.S.C. .sctn.119 from an application 
for POWER SUPPLY DEVICE WITH REFERENCE SIGNAL GENERATING CIRCUIT FOR POWER 
SAVING MODE earlier filed in the Korean Industrial Property Office on Dec. 
1, 1997 and there duly assigned Serial No. 35235/1997. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns a switching mode power supply device, and 
more particularly a power supply device with normal operation and power 
saving modes. 
2. Description of the Related Art 
The switching mode power supply device generally employs as the reference 
signal an oscillation signal with a frequency as high as possible, e.g., 
100 KHz, in order to increase the transformation efficiency. In the 
power-off or sleep mode, such an apparatus cuts off all the power sources 
supplying the load except the power source supplying the circuits for 
controlling the power saving mode, thus minimizing the power consumption. 
An earlier switching mode power supply device which connected between a 
rectifier circuit and a load with a power saving mode, includes an 
oscillator, a pulse width modulation (PWM) control circuit, switching 
circuit and a transformer with multiple secondary coils. The transformer 
has at least one output voltage delivered to the load, a part of which is 
fed back to a feedback circuit connected to a comparator. 
The oscillator generates an oscillation signal of a high frequency which 
serves as a reference signal for the PWM control circuit to generate a PWM 
signal to control the on/off operation of the switching circuit. The feed 
back circuit feeds a part of the output voltage of the transformer back to 
the comparator to detect changes in the output voltage depending on the 
rising and falling of the feedback voltage. 
The comparator compares the output feedback signal with an inherent 
reference signal of the feedback circuit to provide the resultant signal 
for the PWM control circuit to maintain the output voltage of the 
transformer at a given value corresponding to the normal operation mode or 
power saving mode. Namely, the PWM control circuit generates a PWM signal 
to modify the on-duty interval in response to the oscillation signal and 
the feedback signal of the feedback circuit, thereby controlling the 
on/off operation of the switching circuit 23 to control the output voltage 
of the transformer. 
Hence, the switching circuit carries out the switching operation according 
to the same oscillation signal at both the normal operation mode and the 
power saving mode, thus increasing the power consumption for the switching 
operation which reduces the advantage of the power saving mode. 
The following patents each discloses features in common with the present 
invention but do not teach or suggest the specifically recited power 
supply device with a reference signal generating circuit for the power 
saving mode of the present invention: U.S. Pat. No. 5,737,616 to Watanabe, 
entitled Power Supply Circuit With Power Saving Capability, U.S. Pat. No. 
5,542,035 Kikinis et al., entitled Timer-Controlled Computer System 
Shutdown And Startup, U.S. Pat. No. 5,745,375 to Reinhardt et al., 
entitled Apparatus And Method For Controlling Power Usage, U.S. Pat. No. 
5,628,001 to Cepuran, entitled Power Saving Method And Apparatus For 
Changing The Frequency of A Clock In Response To A Start Signal, U.S. Pat. 
No. 5,684,681 to Huh, entitled Drive Circuit Of Switching Element For 
Switching Mode Power Supply Device, U.S. Pat. No. 5,812,386 to Youn, 
entitled Power Supply Control Method And Corresponding Circuit, U.S. Pat. 
No. 5,650,924 to Huh, entitled Electric Power Supply Device For A Monitor, 
U.S. Pat. No. 5,726,871 to Choi, entitled Power Supply Circuit With Power 
Saving Mode For Video Display Appliance, U.S. Pat. No. 5,657,257 to Lee, 
entitled Power-Supply Controller Of Computer, U.S. Pat. No. 5,808,881 to 
Lee, entitled Power-Supply Controller Of Computer, U.S. Pat. No. 5,646,572 
to Masleid, entitled Power Management System For Integrated Circuits, and 
U.S. Pat. No. 5,737,616 to Watanabe, entitled Power Supply Circuit With 
Power Saving Capability. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a switching mode power 
supply device having a reference signal generator which generates a 
reference signal to reduce the frequency of the on/off operation of the 
switching circuit in the power saving mode. 
According to an embodiment of the present invention, a power supply device 
having a transformer for generating at least an output voltage supplied to 
an electronic apparatus with a power saving mode comprises a switching 
circuit connected to the transformer, a reference signal generator for 
generating a reference signal having a first high frequency or a second 
low frequency according to the electronic apparatus being respectively in 
the normal operation mode or power saving mode, and a PWM control circuit 
for generating a PWM signal to control the switching operation of the 
switching circuit according to the reference signal and the feedback 
signal from the output voltage. 
Preferably, the reference voltage generator further includes an oscillator 
for generating the first high frequency oscillation signal, a mode 
detection circuit for detecting the operation mode of the electronic 
apparatus by receiving the feedback signal from the output voltage so as 
to generate a mode detection signal, a controller for generating a first 
or a second control signal according to whether or not the mode detection 
signal is activated, and a frequency divider for transferring the first 
high frequency oscillation signal to the PWM control circuit upon 
receiving the first control signal or frequency-dividing the oscillation 
signal to generate a frequency divided reference signal supplied to the 
PWM control circuit upon receiving the second control signal. 
Thus, the inventive power supply device divides the frequency of the 
oscillation signal to generate a frequency reduced oscillation signal 
supplied to the PWM control circuit in the power saving mode detected by 
the voltage supplied from the transformer to the load. The PWM control 
circuit generates the PWM signal to adjust the on-duty interval of the 
switching circuit in response to the frequency reduced signal. Then, the 
switching circuit performs the switching operation according to the PWM 
signal generated by the reference signal having a lower frequency than the 
oscillation signal. 
The present invention will now be described more specifically with 
reference to the drawings attached only by way of example.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Schematically shown in FIG. 1 is the structure of a switching mode power 
supply device 20, which is connected between a rectifier circuit 10 and a 
load 30 with power saving mode, and comprises an oscillator (OSC) 21, a 
pulse width modulation (PWM) control circuit 22, a switching circuit 23, 
and a transformer 24 with multiple secondary coils. The transformer 24 has 
at least one output voltage delivered to the load 30, a part of which is 
fed back to a feedback circuit 25 connected to a comparator 26. 
OSC 21 generates an oscillation signal (fosc) of a high frequency, e.g., 
100 KHz, which serves as a reference signal for the PWM control circuit 22 
to generate a PWM signal to control the on/off operation of the switching 
circuit 23. The feedback circuit 25 feeds a part of the output voltage of 
the transformer 24 back to the comparator 26 to detect changes in the 
output voltage depending on the rising and falling of the feedback voltage 
Vf. 
The comparator 26 compares the output feedback signal fb1 with an inherent 
reference signal Vref of the feedback circuit 25 to provide the resultant 
signal for the PWM control circuit to maintain the output voltage of the 
transformer at a given value corresponding to the normal operation mode or 
power saving mode. Namely, the PWM control circuit 22 generates a PWM 
signal to modify the on-duty interval in response to the oscillation 
signal fosc and the feedback signal fb2 of the feedback circuit 25, 
thereby controlling the on/off operation of the switching circuit 23 to 
control the output voltage of the transformer 24. 
Hence, the switching circuit 23 carries out the switching operation 
according to the same oscillation signal fosc at both the normal operation 
mode and the power saving mode, thus increasing the power consumption Pd 
for the switching operation, which reduces the advantage of the power 
saving mode. 
More specifically describing the drawbacks of the device of FIG. 1 with 
reference to FIGS. 2 and 3, there is graphically shown in FIG. 2 the 
relationship between the feedback voltage Vf and the primary coil current 
If of the transformer 24 at the normal operation mode interval Tn and the 
power saving mode interval Tps of the power supply device as shown in FIG. 
1. In this case, the energy stored in the primary coil is 
1/2LI.sup.2.sub.f. As shown in FIG. 2, the power consumption becomes low 
in the power saving mode, so that the primary coil current If is 
relatively small and the feedback voltage Vf is lower than the reference 
voltage Vref. The power supply device feeds back a part of the output 
voltage Vf supplied from the secondary coil of the transformer 24 to 
distinguish the normal operation mode interval Tn and the power saving 
mode interval Tps according to the variation of the feedback voltage Vf. 
In addition, the power supply device is made to supply an oscillation 
signal fosc of the same frequency to the PWM control circuit 22 at both 
the normal operation mode and the power saving mode. 
Referring to FIG. 3, the switching operation is performed at the rising and 
falling edges of the PWM signal (not shown) having the same on-duty 
intervals as the oscillation signal fosc having a period of T in response 
to it both in the normal operation mode and the power saving mode. Hence, 
the switching operation is performed at the high frequency of the 
oscillation signal, thus increasing the power consumption Pd. While the 
oscillation signal fosc of such high frequency increases the 
transformation efficiency at the normal operation mode, it decreases the 
transformation efficiency at the power saving mode because of the high 
power consumption caused by the high frequency switching operation. 
Referring to FIGS. 4 and 5, there are schematically shown a feedback 
current taken from the output voltage of the transformer of the inventive 
power supply device and the waveforms of the primary coil current 
proportional to the feedback current. 
As shown in FIG. 4, a novel reference signal generator 120 is included in 
the power supply device. Provided between a rectifier circuit 90 supplying 
a DC voltage and a load 130 having power saving mode are a PWM control 
circuit 102, a switching circuit 104 and a transformer 106. At least a 
part of the output voltage of the transformer 106 supplied to the load 130 
is fed back via a feedback circuit 108 to a comparator 110, which compares 
the feedback voltage Vf with an inherent reference voltage Vref to supply 
the resultant signal fb3 to the PWM control circuit 102. In addition, the 
reference signal generator 120 includes an oscillator OSC 122, a mode 
detector 124 and a frequency divider 128. 
OSC 122 generates a first frequency oscillation signal fosc. The mode 
detector 124 receives a feedback signal fb2 from the feedback circuit to 
detect voltage variations to generate a mode detection signal D.sub.-- mod 
supplied to the controller 126. The controller 126 generates a first or 
second control signal Cnt1 or Cnt2 according to whether or not the mode 
detection signal D.sub.-- mod is activated. Consequently, the frequency 
divider 128 transfers the first high frequency oscillation signal fosc of 
the oscillator to the PWM control circuit 102 upon receiving the first 
control signal Cnt1, or divides the frequency of the oscillation signal to 
generate a frequency divided reference signal fdiv supplied to the PWM 
control circuit upon receiving the second control signal Cnt2. 
The PWM control circuit 102 generates a PWM signal (not shown) to control 
the on/off operation of the switching circuit 104 according to the 
reference signal fosc or fdiv and the feedback signal fb3 from the 
comparator. The switching circuit 104 performs the switching operation 
between the PWM control circuit 102 and the transformer 106 in response to 
the PWM signal. The transformer 106 is provided with multiple secondary 
coils (not shown) to provide the voltages required for the load 130 
according to the switching operation. Thus, the reference signal generator 
120 generates the first frequency reference signal fosc in the normal 
operation mode of the load 130, and the second frequency reference signal 
fdiv having a lower frequency in the power saving mode. 
Namely, when the power supply device is in the normal operation mode, and 
the DC voltage is supplied from the rectifier circuit 90, OSC 122 
generates the oscillation signal fosc supplied to the PWM control circuit 
102, which turns on or off the switching circuit 104. In response to the 
switching operation, the transformer 106 stores the energy into the 
inductor or secondary coil to supply the voltage necessary for the load. 
Meanwhile, the feedback circuit 108 detects the voltage variations 
according to the rising and falling of the output voltage Vf fed back from 
the transformer 106, transferring the detected variations to the 
comparator 110, which compares the feedback voltage Vf with the reference 
voltage Vref to detect the stable voltage according to the normal 
operation mode or power saving mode, and supplies the resultant signal fb3 
to the PWM control circuit 102. 
The PWM control circuit 102 adjusts the on-duty interval according to the 
oscillation signal fosc and the resultant signal fb3 to generate the PWM 
signal. Then, the switching circuit 104 performs the switching operation 
to make the output voltage of the transformer 106 constant in response to 
the PWM signal. In addition, when the power saving mode is demanded by an 
external device, e.g., a circuit to control the power saving mode, DPMS 
control circuit, etc., all the power sources are cut off except the power 
source to maintain the power saving mode. 
Hence, the feedback voltage Vf has a short on-duty interval and thus is 
lowered considerably in the power saving mode as compared with that in the 
normal operation mode. The mode detection circuit 124 compares the 
feedback voltage fb2 with the reference voltage Vref to detect the 
operational mode to generate the mode detection signal D.sub.-- mod, in 
response to which the controller 126 supplies the first or second control 
signal Cnt1 or Cnt2 to the frequency divider 128. If the frequency divider 
128 receives the second control signal Cnt2, it divides the first 
frequency oscillation signal fosc to generate the second frequency 
reference signal fdiv supplied to the PWM control circuit 102, which then 
generates the PWM signal to adjust the on-duty interval of the switching 
circuit 104. Consequently, the switching circuit 104 performs the 
switching operations at a lower frequency in response to the second 
frequency reference signal fdiv which has a lower frequency than the first 
frequency oscillation signal fosc, thereby considerably reducing the power 
consumption in the power saving mode. 
Additionally describing the present invention with reference to FIG. 5, the 
power supply device detects the operational mode depending on the 
variation of the feedback voltage Vf, and executes the normal operation 
mode or power saving mode according as the mode detection signal D.sub.-- 
mod causes the first or the second frequency reference signal fosc or 
fdiv. The mode detection signal D.sub.-- mod is activated or not according 
to the result that the mode detection circuit 124 compares the feedback 
voltage Vf with the reference voltage Vref. Namely, the controller 126 
generates the first or second control signal Cnt1 or Cnt2 supplied to the 
frequency divider 128 according to whether or not the mode detection 
signal D.sub.-- mod is activated. Thus, as shown in FIG. 5, the frequency 
divider 128 generates the first frequency oscillation signal fosc in the 
normal operation mode interval Tn while dividing the first frequency 
oscillation signal fosc to generate the second lower frequency reference 
signal fdiv in the power saving mode interval Tps. 
Referring to FIG. 6 for illustrating the power consumption Pd1 and Pd2 
according to the first and the second reference signal fosc and fdiv, the 
on-duty interval of the PWM signal (not shown) generated by the PWM 
control circuit 102 is adjusted to have the same value as the first 
frequency oscillation signal in the normal operation mode Tn. However, in 
the power saving mode Tps, the second frequency reference signal fdiv 
obtained by dividing the first frequency oscillation signal makes the 
on-duty interval of the PWM signal of the PWM control circuit 102 shorter 
than the first frequency oscillation signal, so that the power consumption 
due to the on/off operation of the switching circuit 104 becomes Pd2. 
The power consumption Pd2 occurs at the rising and falling edges of the PWM 
signal. The switching circuit is turned on at the rising edge, making the 
first power consumption Pd2.sub.-- on, and turned off at the falling edge, 
making the second power consumption Pd2.sub.-- off. Thus, the frequency of 
the switching operations is reduced by the second frequency reference 
signal fdiv in the power saving mode Tps. For example, assuming that the 
first frequency oscillation signal have a frequency of 100 KHz and be 
divided by 4 to generate the second frequency reference signal of 25 KHz, 
the frequency of the switching operations in the power saving mode is 
reduced to are quarter of that in the normal operation mode. Hence, the 
frequency of effecting the power consumption in the inventive device is 
considerably reduced compared to that of the device shown in FIG. 3. 
It should be understood that the present invention is not limited to the 
particular embodiment disclosed herein as the best mode contemplated for 
carrying out the present invention, but rather that the present invention 
is not limited to the specific embodiments described in this specification 
except as defined in the appended claims.