Power supply circuit and image forming apparatus

Abstract

A power supply circuit includes a thermoelectric conversion element configured to generate electric power when it is differentially heated, an adjustable current circuit configured to draw a current from the thermoelectric conversion element and resultantly output a constant current over a period of time, a voltage conversion circuit configured to output a voltage based on the current output by the adjustable current circuit, and a control circuit configured to control the adjustable current circuit to change a target value of the constant current output by the adjustable current circuit.

What is claimed is: 
     
         1 . A power supply circuit comprising:
 a thermoelectric conversion element configured to generate electric power when it is differentially heated;   an adjustable current circuit configured to draw a current from the thermoelectric conversion element and resultantly output a constant current over a period of time;   a voltage conversion circuit configured to output a voltage based on the current output by the adjustable current circuit; and   a control circuit configured to control the adjustable current circuit to change a target value of the constant current output by the adjustable current circuit.   
     
     
         2 . The power supply circuit according to  claim 1 , wherein the control circuit controls the target value of the constant current based on a change of a value of a current output from the thermoelectric conversion element after the passage of a period of time. 
     
     
         3 . The power supply circuit according to  claim 2 , wherein the adjustable current circuit includes:
 an inductor connected to the thermoelectric conversion element in series;   a first switching element connected to the inductor in series;   a current detection circuit connected to the inductor in series and configured to detect the value of the current from the thermoelectric conversion element;   a second switching element connected in series between a connection point of the inductor and the first switching element and another terminal of the first switching element;   a capacitor connected in series with the second switching element between the second switching element and the another terminal of the first switching element; and   a driver circuit configured to output pulse signals to the first switching element and the second switching element, the pulse signals causing one of the first switching element and the second switching element to turn ON and the other of the first switching element and the second switching element to turn OFF.   
     
     
         4 . The power supply circuit of  claim 3 , wherein the pulse signal supplied to the first switching element has a first portion and a second portion, and the duration of time during which the first and second portions are present in a pulse is adjustable by the driver circuit. 
     
     
         5 . The power supply circuit of  claim 4 , wherein the control circuit is configured to control the driver circuit to change the pulse durations of the first and second portions in the pulse signal supplied to the first switching element based on a value of the current passing through the first switching element. 
     
     
         6 . The power supply of  claim 5 , wherein the current detection circuit measures the value of the current passing through the first switching element. 
     
     
         7 . The power supply circuit of  claim 6 , wherein the control circuit is further configured to generate and transmit a control signal to the driver circuit to:
 vary the duration of the pulse durations of the first and second portions, during a first time period, in the pulse signal supplied to the first switching element; and   in response to the detected current passing through the first switching element in the first time period, in a second time period after the first time period, increase the duration of the first portion of the pulse if the detected current passing through the first switching element increases, maintain the duration of the first portion of the pulse if the detected current passing through the first switching element does not increase or decrease, and reduce the duration of the first portion of the pulse if the detected current passing through the first switching element decreases.   
     
     
         8 . The power supply circuit according to  claim 1 , further comprising:
 a battery connected to the output of the adjustable current circuit;   an AC power supply;   a load circuit;   a first converter configured to connect the AC power supply to a load circuit; and   a second converter configured to connect the battery to the load circuit;   wherein the control circuit is further configured to;
 operate the first converter to supply electric power from the AC power supply to the load circuit and operate the second converter to suspend the supplying of electric power from the battery to the load circuit when an amount of electric power in the secondary battery is smaller than a preset threshold; and 
 operate the first converter to suspend the supplying of the electric power from the AC power supply to the load circuit and operate the second converter to supply the electric power from the battery to the load circuit when the amount of the electric power in the secondary battery is greater than or equal to the preset threshold. 
   
     
     
         9 . The power supply circuit according to  claim 1 , further comprising:
 a thermal fixing device configured to heat a thermal fixing load using electric power from an AC power supply;   wherein the thermoelectric conversion element generates the electric power using heat generated by the thermal fixing device; and   the control circuit operates the adjustable current circuit in accordance with the operation of the thermal fixing device.   
     
     
         10 . An image forming apparatus comprising:
 an image forming unit configured to fix a toner image on a print medium using heat supplied thereto by a thermal fixing load; and   a power supply circuit configured to supply electric power to the thermal fixing load;   wherein the power supply circuit includes;
 a thermal fixing device configured to heat the thermal fixing load using electric energy supplied from an AC power supply or from a battery; 
 a thermoelectric conversion element configured to generate electric power using the heat of the thermal fixing device; 
 an adjustable current circuit configured to receive current from the thermoelectric conversion element and output a current at a constant value for a period of time; 
 a voltage conversion circuit configured to output a constant voltage for a period of time based on the current output by the variable constant current circuit; and 
 a control circuit configured to operate the adjustable current circuit in accordance with an operation of the thermal fixing device and to control the adjustable current circuit so as to change a target value of the current to be drawn from the thermoelectric conversion element. 
   
     
     
         11 . The image forming apparatus according to  claim 10 ,
 wherein the control circuit controls the target value of the current based on a change of the value of the current from the thermoelectric conversion element after the period of time.   
     
     
         12 . The image forming apparatus according to  claim 11 , wherein the adjustable current circuit includes:
 an inductor connected to the thermoelectric conversion element in series;   a first switching element connected to the inductor in series;   a current detection circuit connected to the inductor in series and configured to detect the current from the thermoelectric conversion element;   a second switching element connected in series between a connection point of the inductor and the first switching element and another terminal of the first switching element;   a capacitor connected in series with the second switching element between second switching element and the another terminal of the first switching element; and   a driver circuit configured to output pulse signals to the first switching element and the second switching element, the pulse signals causing one of the first switching element and the second switching element to turn ON and the other of the first switching element and the second switching element to turn OFF.   
     
     
         13 . The image forming apparatus of  claim 12 , wherein the pulse signal supplied to the first switching element has a first portion and a second portion, and the duration of time during which the first and second portions are present in a pulse is adjustable by the driver circuit. 
     
     
         14 . The image forming apparatus of  claim 13 , wherein the control circuit is configured to control the driver circuit to change the pulse durations of the first and second portions in the pulse signal supplied to the first switching element based on a value of the current passing through the first switching element. 
     
     
         15 . The image forming apparatus of  claim 14 , wherein the current detection circuit measures the value of the current passing through the first switching element. 
     
     
         16 . The image forming apparatus of  claim 15 , wherein the control circuit is further configured to generate and transmit a control signal to the driver circuit to:
 vary the duration of the pulse durations of the first and second portions, during a first time period, in the pulse signal supplied to the first switching element; and   in response to the detected current passing through the first switching element during the first time period, and in a second time period after the first time period, increase the duration of the first portion of the pulse if the detected current passing through the first switching element increases, maintain the duration of the first portion of the pulse if the detected current passing through the first switching element does not increase, and reduce the duration of the first portion of the pulse if the detected current passing through the first switching element decreases.   
     
     
         17 . The image forming apparatus according to  claim 10 , further comprising:
 a battery connected to the output of the adjustable current circuit;   an AC power supply;   a load circuit;   a first converter configured to connect the AC power supply to a load circuit; and   a second converter configured to connect the secondary battery to the load circuit;   wherein the control circuit is further configured to:
 operate the first converter to supply electric power from the AC power supply to the load circuit and operate the second converter to suspend the supplying of electric power from the battery to the load circuit when an amount of electric power in the battery is smaller than a preset threshold; and 
 operate the first converter to suspend the supplying of the electric power from the AC power supply to the load circuit and operate the second converter to supply the electric power from the battery to the load circuit when an amount of the electric power in the secondary battery is greater than or equal to the preset threshold. 
   
     
     
         18 . The image forming apparatus according to  claim 10 , further comprising:
 a thermal fixing device configured to heat a thermal fixing load using electric power from an AC power supply;   wherein the thermoelectric conversion element generates the electric power using heat generated by the thermal fixing device; and   the control circuit operates the adjustable current circuit in accordance with the operation of the thermal fixing device.   
     
     
         19 . A circuit for controlling the current drawn from a thermoelectric conversion device and supplied to a load, comprising:
 a pair of input terminals and a pair of output terminals, the pair of input terminals connected to the output terminals of the thermoelectric conversion device and the pair of input terminals connected to the input terminals of the load;   an inductor connected to one of the pair of input terminals;   a first switching element connected in series with the inductor and switchable between a closed position wherein current can flow therethrough and an open position wherein current is blocked from flowing therethrough;   a second switching element connected in series with the inductor and switchable between a closed position wherein current can flow therethrough and an open position wherein current is blocked from flowing therethrough;   a capacitor connected in parallel with the first and second switching elements and across the output terminals;   a current detection circuit connected in series with the first switching element and the other of the input terminals; and   a pulse generator connected to the first and second switching elements and a control circuit, the pulse generator configured to generate pulses capable of opening and closing the first and second switching elements, and the duration of the pulses is selectively variable by the operation of the pulse generator; wherein   the control circuit is configured to change the duration of the pulse generated by the pulse generator received by the first switching element to open the first switching element based on the value of the current flowing through the first switching element in the closed position as detected by the current detection circuit.   
     
     
         20 . The circuit according to  claim 19 , wherein the pulse generator is configured to generate a pulse capable of opening the first switching element and closing the second switching element after the first switching element has been in a closed state for a first time period, and
 during the first time period, thermal energy from the thermoelectric conversion device is stored in the inductor as magnetic energy, and after the first switching device is opened and the second switching device is closed, the magnetic energy stored in the inductor is stored in the capacitor as electric charge energy.