Switching power supply system for primary and secondary loads with less switching loss

A power supply system to be built into a television receiver, for powering both the main electric circuitry of the TV set and a remote control receiver circuit. The power supply system comprises a transformer having a primary winding coupled to a direct current power supply via a switching transistor, and secondary windings coupled to the main TV circuitry and to the remote control receiver circuit via rectifying and smoothing circuits. Duration modulated pulses for on-off control of the switching transistor are reduced in recurrence rate when the main TV circuitry is not being powered, so that the remote control receiver circuit is powered with less switching loss than if the switching frequency is left the same as when the main TV circuitry is being powered.

BACKGROUND OF THE INVENTION 
This invention relates to power supply systems, and more specifically to 
those of the switching type for powering both a primary and a secondary 
load, the secondary load being significantly less in power requirement 
than the first. The switching power supply system according to the 
invention is particularly well suited for use in or with remotely 
controllable television receivers, among other applications. 
A greater majority of television sets today are remotely controllable. Such 
TV sets have an inbuilt remote control receiver circuit for receiving 
signals from the remote control unit. The remote control receiver circuit 
must be powered even when the main electric circuitry of the TV set is 
not. The power supply systems of remotely controllable TV sets are 
therefore required to possess a circuit for powering the main circuitry 
(primary load) and another for powering the remote control receiver 
circuit (secondary load). The provision of two totally independent power 
supply circuits would be undesirable, however, because the resulting 
system would be too bulky and costly. 
A solution to this problem is found for example in Japanese Unexamined 
Patent Publication No. 4-308465, in which the two power supply circuits 
are combined so as to share some components. More specifically, this prior 
art power supply system comprises a switching transistor for on-off 
control of a unidirectional supply voltage, and a transformer having a 
primary winding connected in series with the switching element, a 
secondary winding for powering the main TV set circuitry, and a tertiary 
winding for powering the remote control receiver circuit. The switching 
transistor is controlled so as to make constant the supply voltage applied 
to the main TV set circuitry. 
Customarily, the switching frequency of such TV set power supply systems 
has been as high as from 40 to 100 kilohertz, with a view to the reduction 
of the size of the transformer and the suppression of noise in the audio 
frequency range. However, the constant switching of the direct current 
power supply at such high frequencies inevitably involves considerable 
power losses. The trouble with the prior art switching power supply system 
is that such high switching frequencies have been maintained not only when 
the main TV set circuitry is being powered but also when it is not, that 
is, when only the remote control receiver circuit is. Naturally, TV sets 
are held switched on for a much shorter period of time than off. 
SUMMARY OF THE INVENTION 
The present invention seeks to enhance the efficiency of switching power 
supply systems of the kind defined, particularly when only the secondary 
load is being powered. 
Briefly, the invention may be summarized as a switching power supply system 
for a primary and a secondary load, the primary load being greater in 
power requirement than the secondary load. The system comprises a 
transformer having a primary winding connected across a direct current 
power supply via a switching element, and secondary winding means having 
first output means for providing a first supply voltage, and second output 
means for providing a second supply voltage, the first supply voltage 
being greater than the second supply voltage. The first output means of 
the transformer secondary means is connected to a first rectifying and 
smoothing circuit, to which the primary load is intended to be connected, 
whereas the second output means of the transformer secondary means is 
connected to a second rectifying and smoothing circuit, to which the 
secondary load is to be connected. Also included are load state signal 
means for providing a load state signal indicative of whether the primary 
load is being powered or unpowered while the secondary load is held 
powered, and pulse generator means connected between the switching element 
and the load state signal means for generating a train of pulses at a 
first recurrence rate for opening and closing the switching element when 
the primary load is being powered, and at a second recurrence rate, less 
than the first recurrence rate, for opening and closing the switching 
element when the primary load is not being powered. 
Thus, when the primary load is not being powered, the secondary load can be 
powered with less switching loss than if the switching element is opened 
and closed with the same frequency as when the primary load is being 
powered. 
As an additional advantage, the reduction of the switching frequency during 
the powering of only the secondary load makes it possible to make longer 
the durations of the switching pulses than if, as has been the case 
heretofore, their recurrence rate is held the same as when the primary 
load is being powered. Such longer duration pulses are more stable and 
positive than the conventional very short duration pulses that have been 
required for powering the secondary load of very small power requirement. 
The transformer in use may be reduced in size to an extent necessary for 
powering the primary load at the relatively high first switching 
frequency. The reduction of this switching frequency to the second is when 
the primary load is not powered will present no inconvenience at all 
because, at least in the typical intended application of the invention, 
the power consumption of the secondary load is sufficiently small for such 
a small transformer being switched at the reduced frequency. 
The above and other features and advantages of this invention and the 
manner of realizing them will become more apparent, and the invention 
itself will best be understood, from a study of the following description 
and appended claims, with reference had to the attached drawings showing 
some preferable embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention will now be described in detail as embodied in the switching 
power supply system shown in FIGS. 1-8 as adapted for use in or with a 
remotely controllable television set. With reference first to FIG. 1 a 
pair of commercial alternating current supply terminals 1 are shown 
coupled to a direct current power supply 2 of any known or suitable 
construction having rectifying and smoothing circuit means. Connected 
between the pair of output terminals of the power supply 2, that is, a 
direct current power supply 8 and a ground terminal 4, is a serial circuit 
of a primary winding 6 of a transformer 5 and a switching element herein 
shown as a transistor 7. The transformer 5 is shown to additionally 
comprise a secondary winding 8, a tertiary winding 9 and a quaternary 
winding 10. 
The transformer secondary 8 and tertiary 9 are both coupled to an output 
circuit 11 for powering a primary and a secondary load according to the 
invention. The primary load in this case is the main electric circuitry of 
the TV set, and the secondary load a remote control receiver circuit built 
into the TV set. It is therefore understood that the secondary load is to 
be constantly held powered whereas the primary load is to be switched on 
and off at the will of the user. The output circuit 11 is shown in detail 
in FIG. 2, to which reference will be had presently. Suffice it to say for 
the moment that the output circuit 11 includes a light emitting diode 
(LED) 12 which gives off light with an intensity depending upon the 
magnitude of the voltage being applied to the primary load, for constant 
voltage control of the primary load. 
For on-off control of the switching transistor 7 there is provided a 
duration-modulated pulse generator circuit 13 which is connected to the 
base of the switching transistor via a driver circuit 15. The pulse 
generator circuit 13 includes a phototransistor 14 which is optically 
coupled to the LED 12 of the output circuit 11 for constant voltage 
control of the primary load. The pulse generator circuit 13 will be later 
described in detail with reference to FIG. 8. 
Coupled to an input of the pulse generator circuit 13, a comparator 16 
functions to detect whether only the secondary load, or both primary and 
secondary loads, are being powered, and to vary the recurrence rate of the 
output pulses of the pulse generator circuit accordingly. This power 
supply system is so constructed that when only the secondary load is being 
powered, the voltage across the transformer secondary 8 drops to one fifth 
of that when both primary and secondary loads are powered. It is therefore 
possible for the comparator 16 to determine from such a change in 
transformer output voltage whether only the secondary load or both primary 
and secondary loads are being powered. 
The transformer quaternary 10 serves to power the pulse generator circuit 
13, switch driver circuit 15 and comparator circuit 16. A capacitor 18 is 
connected across the transformer quaternary 10 via a diode 17, and its 
output is coupled via a voltage regulator circuit 19a to the supply 
terminals of the pulse generator circuit 15, switch driver circuit 15 and 
comparator 16. The number of turns of the transformer quaternary 10 should 
be so determined that the voltage regulator circuit 19a can provide an 
output voltage of required magnitude even when the quaternary output 
voltage is at its lowest level. A starting resistor 19 is connected 
between the supply terminal 3 and the capacitor 18. 
Connected in parallel with the capacitor 18 is a serial connection of two 
voltage dividing resistors 20 and 21. The connection between these 
resistors is coupled to one input of the comparator 16, the other input of 
which is coupled to a constant voltage source herein shown as a Zener 
diode 22. It is understood that the transformer quaternary 10 is so 
polarized that the diode 17 conducts during the nonconducting periods of 
the switching transistor 7, and that the output circuit 11 is so designed 
that the transformer quaternary 10 provides a voltage proportionate with 
the transformer secondary voltage during such periods. Thus the comparator 
16 can provide the desired output indicative of the magnitude of the 
transformer secondary voltage and, in consequence, of whether both primary 
and secondary loads or only the secondary load is being powered. The 
recurrence rate of the output pulses of the pulse generator circuit 13 is 
to change in response to the output signal of the comparator 16. 
Reference may now be had to FIG. 2 for a closer study of the output circuit 
11. The transformer secondary 8 is shown tapped and thereby divided into 
two divisions 8a and 8b, although two separate windings could be provided 
instead of one such tapped winding. The tapped transformer secondary 8 has 
a first terminal 23a for providing a relatively high voltage of, say, 112 
volts, a second terminal 23b for providing a lower voltage of, say, 60 
volts, and a ground terminal 23c 
The first terminal 23a of the transformer secondary 8 is coupled to an 
output terminal 27 via a first rectifying and smoothing circuit 26 
comprising a rectifying diode 24 and smoothing capacitor 25. The ground 
terminal 23c of the transformer secondary 8 is connected to another output 
terminal 28. Connected between the pair of output terminals 27 28 is the 
primary load 30 which in this embodiment is understood to be the main 
electric circuitry of the television set. The power requirement of the 
primary load 30 may be approximately 90 watts. An internal power switch 29 
of the TV set is connected between output terminal 27 and primary load 30. 
Less in the number of turns than the transformer secondary 8, the 
transformer tertiary 9 is intended to power a secondary load which takes 
the form of a television remote control receiver circuit 34 in this 
embodiment. One extremity of the transformer tertiary 9 is coupled to the 
remote control receiver circuit 34 via a second rectifying and smoothing 
circuit 33, and the other extremity of the transformer tertiary grounded. 
The second rectifying and smoothing circuit 33 comprises a diode 31 and a 
capacitor 32. The power requirement of the secondary load 34 is 
significantly less than that of the primary load 30, being one watt or so 
in the illustrated embodiment. 
The noted remote control receiver circuit 34 is itself of conventional 
design capable of controlling various operations of the TV set in response 
to signals transmitted from a remote control unit, not shown, for that TV 
set. It is among the standard functions of the receiver circuit 34 to make 
on-off control of the TV set power switch 29. In connection with this 
particular function, the receiver circuit 34 is equipped to put out what 
may be termed a primary load state signal on a terminal 42 which, just 
like the output from the FIG. 1 comparator 16, indicates whether the TV 
set circuitry is powered or unpowered. The primary load state signal is 
high when the TV set circuitry is powered, that is, when the power switch 
29 is closed, and low when the TV set circuitry is not powered. 
For powering the remote control receiver circuit 34 when the TV set 
circuitry is unpowered, the second terminal 23b of the transformer a 
secondary 8 is connected to a third rectifying and smoothing circuit 37 
comprising a diode 35 and a capacitor 36. The output line 38 of this 
circuit 37 is coupled to the remote control receiver circuit 34 via a 
switching transistor 39 of pnp type. Another transistor 40 of npn type is 
connected between the base of the transistor 39 and the ground for 
operating this transistor 39 only when the TV set circuitry is unpowered. 
The transistor 40 has its base coupled to still another transistor 41 and 
thence to the terminal 42 of the remote control receiver circuit 34. There 
are connected between the output line 38 of the rectifying and smoothing 
circuit 37 and the base of the transistor 40 a resistor 43 for a 
adjustment of the supply voltage applied to the remote control receiver 
circuit 34 and a Zener diode 44 for voltage stabilization. 
The primary load state signal from the terminal 42 is high as aforesaid 
when the primary load 30 is powered, and low when it is not. Consequently, 
during the operation of the primary load 30, the transistor 41 is 
conductive whereas the transistors 39 and 40 are not, so that the third 
rectifying and smoothing circuit 37 is disconnected from the remote 
control receiver circuit 34. Then, however, the remote control receiver 
circuit 34 can be powered from the second rectifying and smoothing circuit 
33. 
The reference numeral 50 generally denotes a voltage control signal forming 
circuit for controlling the phototransistor 14, FIG. 1, of the duration 
modulated pulse generator circuit 13 according to the voltage between the 
pair of output terminals 27 and 28. The output of the circuit 50 is 
connected to the LED 12 which has been set forth with reference to FIG. 1. 
The voltage control signal forming circuit 50 comprises two output voltage 
detecting resistors 45 and 46 connected in series with each other between 
the pair of output terminals 27 and 28. The connection between these 
resistors 45 and 46 is coupled to the base of an amplifying transistor 48. 
Connected between the output terminals 27 and 28 via another resistor 49, 
a Zener diode 47 applies a reference voltage to the emitter of the 
transistor 48. The collector voltage of the amplifying transistor 48 
decreases with an increase in output voltage in the illustrated 
embodiment. 
The collector of the amplifying transistor 48, or the output of the voltage 
control signal forming circuit 50, is coupled as aforesaid to the cathode 
of the LED 12 which is optically coupled to the phototransistor 14. The 
anode of the LED 12 is coupled to the output of the rectifying and 
smoothing circuit 33 via a current limiting resistor 51. 
The transistor 40, which functions to decrease the supply voltage the pair 
of output terminals 27 and 28 when the TV set circuitry is unpowered, has 
its collector coupled to the switching transistor 39 via a resistor 52 on 
one hand and, on the other hand, to the cathode of the LED 12 via a diode 
53. The emitter of the transistor 40 is grounded. 
As diagramed in detail in FIG. 3, the duration modulated pulse generator 
circuit 13 of FIG. 1 broadly comprises a voltage comparator 55, an 
interpulse space control circuit 56, a pulse duration control circuit 57, 
and a reference voltage source 58, for generating duration modulated 
pulses to be applied to the switch driver circuit 15. 
More specifically, the interpulse space control circuit 56 comprises a 
capacitor C.sub.1, four resistors R.sub.1 -R.sub.4, two transistors 
Q.sub.10 and Q.sub.11, and three diodes D.sub.1 -D.sub.3. The capacitor 
C.sub.1 is connected between a supply terminal 59 and the ground via the 
transistor Q.sub.11 and diode D.sub.1. The resistor R.sub.1 is connected 
in parallel with the capacitor C.sub.1 to provide a discharge circuit. The 
transistor Q.sub.10 is also connected in parallel with the capacitor 
C.sub.1 via the resistor R.sub.4 in order to switch the time of discharge. 
The capacitor C.sub.1 is also coupled to the negative input of the 
comparator 55, the positive input of which is coupled to the reference 
voltage source 58 via the resistor R.sub.3, and the output of which is 
coupled to the switch driver circuit 15. The transistor Q.sub.11 has its 
base coupled to the reference voltage source 58 via the resistor R.sub.3 
on one hand and, on the other hand, to the output of the comparator 55 via 
a serial connection of the resistor R.sub.2 and diode D.sub.2. This serial 
circuit functions to make the transistor Q.sub.11 nonconductive when the 
comparator 55 goes low, thereby terminating the charging of the capacitor 
C.sub.1. 
The pulse duration control circuit 57 comprises a capacitor C.sub.2, two 
transistors Q.sub.12 and Q.sub.13, a NOT circuit 60, and the noted 
phototransistor 14. The capacitor C.sub.2 has one terminal coupled to the 
supply terminal 59 via the phototransistor 14, and another terminal 
grounded. Said one terminal of the capacitor C.sub.2 is additionally 
coupled to the base of the transistor Q.sub.12 which is connected between 
the positive input of the comparator 55 and the ground. Connected in 
parallel with the capacitor C.sub.2, the transistor Q.sub.13 has its base 
coupled to the output of the comparator 55 via the NOT circuit 60. 
In the operation of the FIG. 3 duration modulated pulse generator circuit 
13, the reference voltage Vr of, say, 6.3 volts will be applied from its 
source 58 to the positive input of the comparator 55. The reference 
voltage will also cause conduction through the transistor Q.sub.11 of the 
interpulse space control circuit 56. Consequently, the capacitor C.sub.1 
will be charged by the voltage +V from the supply terminal 59, to an 
extent corresponding to the value (e.g. five volts) obtained by 
subtracting the sum of the base to emitter voltage V.sub.BE of the 
transistor Q.sub.11 and the voltage Vf of the diode D.sub.1 from the 
reference voltage Vr. The capacitor C.sub.1 will be charged rapidly to 
that value because no resistor is included in its charging circuit. The 
charging voltage is also applied to the negative input of the comparator 
55. 
FIG. 4(A) indicates by the solid line the waveform of the voltage V.sub.1 
to the positive input of the comparator 55 and by the broken line the 
sawtooth waveform of the voltage V.sub.2 to the negative input of the 
comparator. Since the input voltage V.sub.1 has a maximum amplitude of 6.3 
volts, and the input voltage V.sub.2 that of 5.0 volts, in this 
embodiment, the comparator 55 will be high when the capacitor C.sub.1 is 
fully charged. No error in operation will take place due to noise because 
the difference between the two input voltages is 1.3 volts. 
The CR time constant determined by the capacitance of the capacitor C.sub.2 
of the pulse duration control circuit 57 and the resistance of the 
phototransistor 14 is set higher than that determined by the capacitance 
of the capacitor C.sub.1 and the resistance of its charging circuit, so 
much so that, as indicated in FIG. 4(B), the voltage Vc.sub.2 of the 
capacitor C.sub.2 will rise gradually during the t.sub.0 -t.sub.1 
conducting period Ton of the switching transistor 7. During the conducting 
period Ton the comparator 55 is high, as in FIG. 4(C). The NOT circuit 60 
is therefore low, holding the transistor Q.sub.13 nonconductive, so that 
no discharge circuit is formed for the capacitor C.sub.2. 
The transistor Q.sub.12 will become nonconductive when the voltage Vc.sub.2 
of the capacitor C.sub.2 rises to 0.7 or 0.8 volt. The reference voltage 
V.sub.1 to the positive input of the comparator 55 will then drop and 
become less than the voltage V.sub.2 to the negative input of the 
comparator, with s the result that the comparator will go low at time 
t.sub.1, as in FIG. 4(C). Thereupon the diode D.sub.2 will become 
conductive. Thus, during the nonconducting period Toff of the switching 
transistor 7, as from time t.sub.1 to time t.sub.2 in FIG. 4, the voltage 
V.sub.1 to the positive input of the comparator 55 will be fixed at 
approximately three volts according to the equation 
EQU V.sub.1 ={(Vr-Vf)R.sub.2 /(R.sub.2 +R.sub.3)}+Vf 
where Vf stands for the forward voltage of each of the diodes D.sub.1 and 
D.sub.2. 
Also, when the comparator 55 goes low as at the time t.sub.1, the NOT 
circuit 60 will go high thereby causing conduction through the transistor 
Q.sub.13. Consequently, the capacitor C.sub.2 will discharge rapidly as in 
FIG. 4(B). 
Since the voltage V.sub.1 to the positive input of the comparator 55 is 
less than the voltage V2 to its negative input during the t.sub.1 -t.sub.2 
nonconducting period Toff of the switching transistor 7, the transistor 
Q.sub.11 and diode D.sub.1 will be reverse biased and so nonconductive. 
The capacitor C.sub.1 will therefore be not charged; instead, this 
capacitor will discharge through both resistors R.sub.1 and R.sub.4 when 
the primary load 30 is being powered, causing a gradual decrease in the 
voltage V.sub.2 to the negative input of the comparator 55, as in FIG. 
4(A). It is thus seen that during each nonconducting period of the 
switching transistor 7, the voltage V.sub.1 to the positive input of the 
comparator 55 remains the same whereas the voltage V.sub.2 to its negative 
input drops at a constant rate until it becomes the same in magnitude as 
the voltage V.sub.1 at the time t.sub.2. The duration of each 
nonconducting period Toff is therefore constant. 
If the primary load 30 is not powered, on the other hand, the capacitor 
C.sub.1 will discharge at a reduced rate only through the resistor R.sub.1 
when the transistor Q.sub.10 is nonconductive. The voltage V.sub.2 to the 
negative input of the comparator 55 will then drop at a correspondingly 
reduced rate. Each nonconducting period Toff will therefore be longer than 
that when the primary load is being powered, resulting in a decrease in 
the switching frequency of the transistor 7. 
The comparator 55 will go high when the input voltage V.sub.2 becomes less 
than the other input voltage V.sub.1, as at time t.sub.2 in FIG. 4. The 
procedure of the t.sub.0 -t.sub.2 period will be repeated thereafter. The 
FIG. 4(C) output Vout of the comparator 55, or of the duration modulated 
pulse generator circuit 13, is applied to the switch driver circuit 15, 
thereby causing the same to switch the transistor 7 accordingly. 
The phototransistor 14 will decrease in resistance, and the charging 
current of the capacitor C.sub.2 will increase, if the voltage between the 
pair of output terminals 27 and 30 exceeds the desired value of 112 volts. 
As the capacitor C.sub.2 will then be charged at a higher rate, the 
transistor Q.sub.12 will become conductive in a shorter period of time 
from the moment the comparator 55 goes high. Then the durations Ton of the 
output pulses of the comparator 55, or of the pulse generator circuit 13, 
will become shorter, as indicated by the broken line in FIG. 4(C). The 
interpulse space Toff will nevertheless remain the same, so that the 
supply voltage will return to the desired value through the decrease in 
the duty ratio of the pulse generator output pulses. 
The operation of the FIG. 3 pulse generator circuit 13 having been set 
forth, that of the FIGS. 1 and 2 power supply system will now be 
described. The primary load state signal on the remote control receiver 
circuit output terminal 42, FIG. 2, is high when the primary load 30 is 
being powered with the power switch 29 closed. The transistors 39 and 40 
and diode 53 are then nonconductive. The LED 12 will therefore emit light 
with an intensity in proportion with the output from the control signal 
forming circuit 50, and the phototransistor 14, FIG. 3, of the pulse 
generator circuit 13 will conduct to an extent determined by the light 
intensity. 
For example, if the supply voltage for the primary load 30 exceeds the 
desired magnitude, the resistance of the phototransistor 14 will drop 
correspondingly. Then the pulse generator circuit 13 will produce pulses 
of reduced durations, as has been set forth with reference to FIGS. 3 and 
4. Regulated supply voltages will thus be obtained from the output 
terminals 27 and 54. 
The transformer secondary 8 will develop a sufficiently high voltage for 
powering the primary load 30, causing the 112 volts supply voltage to 
appear between the output terminals 27 and 28, when the power switch 29 is 
closed by the remote control receiver circuit 34. The diode 24 of the 
rectifying and smoothing circuit 26 is conductive during the nonconducting 
periods of the switching transistor 7, so that a regulated voltage is 
impressed from the capacitor 25 to the transformer secondary 8. As a 
result, the transformer quaternary 10 develops a voltage having a 
magnitude determined by the ratio of the numbers of turns of the 
transformer secondary 8 and quaternary 10. Therefore, when the output 
voltage of the rectifying and smoothing circuit 26, or of the capacitor 
25, is high, the comparator 16, FIG. 1, goes high to indicate the high 
output voltage. 
Since the output line 16a of the comparator 16 is coupled to the base of 
the transistor Q.sub.10, FIG. 3, of the pulse generator circuit 13, this 
transistor is conductive when the primary load 30 is being powered. With 
the resistor R.sub.4 thus connected in parallel with the capacitor 
C.sub.1, the frequency of the sawtooth or triangular wave shown in FIG. 4 
will become higher. The pulse generator circuit 13 will then produce 
pulses at a high recurrence rate of 40 to 100 kHz. The switching of the 
transistor 7 at such high frequencies will lead to somewhat greater 
switching losses. This disadvantage, however, is more than amply offset by 
the reduction of power losses at the transformer 5, which can therefore be 
made less in size than heretofore. 
When the power switch 29 is opened, the primary load state signal on the 
remote control receiver circuit terminal 42 will go low to indicate that 
the primary load 30 is not powered. Thereupon the transistors 39 and 40 
will both become conductive. The conduction of the transistor 40 will 
result in the grounding of the cathode of the LED 12 via the diode 53, so 
that the LED will emit light of more or less full intensity substantially 
independently of the transistor 48 of the control signal forming circuit 
50, thereby intercepting the feedback control of the transformer output 
voltages. Then the phototransistor 14 will also become substantially fully 
conductive. 
Thus, as indicated in FIG. 4(C), the durations of the output pulses of the 
pulse generator circuit 13 will become shorter, as from t.sub.3 to 
t.sub.4. The output voltage of the rectifying and smoothing circuit 26, 
FIG. 2, will then drop to one fifth of that (e.g. 60 volts) when the 
primary load 30 is being powered, reducing hazard to humans when the TV 
set is out of operation. 
It might be contemplated to connect an on-off switch to the transformer 
secondary terminal 23a, and to open the switch when the primary load 30 is 
not powered, in order to prevent the application of 112 volts between the 
output terminals 27 and 28. This alternative is undesirable because such a 
switch is very expensive. 
As the output voltage of the rectifying and smoothing circuit 26, connected 
between the opposite extremity terminals 23a and 23c of the transformer 
secondary 8, drops as above when the primary load 30 is switched off, so 
does the output voltage of the second rectifying and smoothing circuit 33 
connected between the opposite extremity terminals of the transformer 
tertiary 9. However, in this case, the third rectifying and smoothing 
circuit 37, connected between the transformer secondary terminals 23b and 
23c, becomes coupled to the remote control receiver circuit 34 via the 
transistor 39. The receiver circuit 34 can thus be held powered pending 
the reclosure of the power switch 29. 
Also, when the primary load 30 is switched off, the potential of the 
capacitor 25, FIG. 2, will drop, and so will the voltage of the 
transformer quaternary 10. Then the comparator 16, FIG. 1, will go low to 
indicate that the primary load is not powered, resulting in the 
nonconduction of the transistor Q.sub.10, FIG. 3, of the pulse generator 
circuit 13. Thereupon the frequency of the sawtooth voltage V.sub.2 to the 
negative input of the comparator 55 will drop to, say, 18 kHz, as after 
time t.sub.3 in FIG. 4(A). With the repetition rate of the duration 
modulated pulses thus reduced, the switching transistor 7 will cause less 
loss than heretofore when the primary load 30 is not powered. 
As an additional advantage, the reduction of the switching frequency when 
the primary load 30 is not powered makes possible the production of stable 
duration modulated pulses for providing the desired 12 volts supply 
voltage from the transformer secondary division 8.sub.b for powering the 
remote control receiver circuit 34. More specifically, if the desired low 
supply voltage were to be obtained with the same high switching frequency 
(short period T.sub.1) as when the primary load is being powered (shown in 
the left hand half of FIG. 4), then the durations Ton of the FIG. 4(C) 
pulses would have to be inconveniently short. In the illustrated 
embodiment, however, the low supply voltage is obtained with the lower 
switching frequency (longer period T.sub.2), as indicated at the right 
hand half of FIG. 4. The lower switching frequency is realized by 
increasing the interpulse spaces Toff rather than by decreasing the pulse 
durations Ton, so that stable pulses can be produced for positively 
driving the secondary load. 
Even though the switching frequency is reduced as above when the primary 
load is not powered, the currents flowing through the transformer primary 
6 and secondary 8 are much less in magnitude than when the primary load is 
being powered. No significant temperature rise or other problem is 
therefore to occur if the transformer is desired for operation at high 
switching frequencies. 
FIG. 5 shows an alternate power supply system according to the invention. 
The alternate system differs from that of FIGS. 1-3 in that the former has 
no rectifying and smoothing circuit 37, transistors 39, 40 and 41, Zener 
diode 44, diode 53, resistors 43, 51 and 52, and comparator 16 of the 
latter, and includes a modified duration modulated pulse generator circuit 
13a and a modified voltage control signal forming circuit 50a. 
The voltage control signal forming circuit 50a includes a differential 
amplifier 48a having one input coupled to the voltage dividing point 
between the serial connection of resistors 45 and 46, and another input to 
a reference voltage source 47a. The output of the differential amplifier 
48a is coupled to an LED 12a and thence to the ground. 
The duration modulated pulse generator circuit 13a includes a 
phototransistor 14a which is optically coupled to the LED 12a and which is 
electrically connected between a direct current supply terminal 70 and the 
ground via a resistor 72. The voltage dividing point between 
phototransistor 14a and resistor 72 is coupled via a first switch 74 to 
the negative input of a comparator 55a, to which is also coupled a 
reference voltage source 73 via a second switch 75. The positive input of 
the comparator 55a is coupled to a sawtooth generator circuit 56a and 
thence to the primary load state signal terminal 42 of the remote control 
receiver circuit 34. The switches 74 and 75 also have their control inputs 
coupled to the primary load state signal terminal 42. 
The first switch 74 is closed when the primary load state signal is high, 
indicating that the primary load 34 is being powered, whereas the second 
switch 75 is closed when the primary load state signal is low. The 
reference voltage from the source 73 is so determined that the 12 volts 
supply voltage is obtained from the output terminal 54 when the primary 
load is not being powered. The sawtooth generator circuit 56a puts out a 
sawtooth wave at a relatively high frequency when the primary load is 
being powered, and at a lower frequency when it is not. The comparator 55a 
puts out duration modulated pulses for on-off control of the switching 
transistor 7 by comparing the sawtooth wave to its positive input and the 
voltage to its negative input. 
Thus, in this alternate embodiment, too, the transistor 7 is turned on and 
off at a lower frequency when the primary load is not powered than when it 
is. Less switching loss is therefore to occur than heretofore. 
Notwithstanding the foregoing detailed disclosure, it is not desired that 
the invention be limited by the exact details of such disclosure. For 
instance, the transistor Q.sub.10 to could be controlled by the primary 
load state signal from the terminal 42, instead of by the output from the 
comparator 16. A variety of other modifications, alterations and 
adaptations of the invention will readily suggest themselves to conform to 
design preferences or to the specific requirements of each application of 
the invention, without departing from the scope of the invention as 
expressed in the claims which follow.