Isolated dc/dc converter, primary side controller, power adapter, and electronic device

Abstract

An isolated DC/DC converter includes a transformer having a first winding and a secondary winding, a switching transistor connected to the primary winding of the transformer, a rectifier element connected to the secondary winding of the transformer, a photocoupler, a feedback circuit configured to drive a light emitting element on an input side of the photocoupler by a forward current corresponding to an error between an output voltage of the DC/DC converter and a target voltage of the DC/DC converter, a conversion circuit configured to convert a collector current flowing in a light receiving element on an output side of the photocoupler into a feedback voltage having a negative correlation with the collector current, a pulse signal generator configured to generate a pulse signal corresponding to the feedback voltage, and a driver configured to drive the switching transistor depending on the pulse signal.

What is claimed is: 
     
         1 . A primary side controller for use in an isolated DC/DC converter, comprising:
 a feedback terminal;   a conversion circuit configured to:
 convert a current flowing at the feedback terminal into a feedback voltage having a negative correlation with the current; and 
 change conversion characteristics from the current to the feedback voltage based on an output current of the isolated DC/DC converter; 
   a pulse signal generator configured to generate a pulse signal corresponding to the feedback voltage; and   a driver configured to generate a driving signal based on the pulse signal.   
     
     
         2 . The primary side controller of  claim 1 , wherein the conversion circuit includes a variable impedance circuit installed between the feedback terminal and a reference voltage line, and
 wherein the conversion circuit is configured such that impedance of the variable impedance circuit is variable based on the output current.   
     
     
         3 . The primary side controller of  claim 2 , wherein the variable impedance circuit includes:
 a first resistor installed between the feedback terminal and the reference voltage line; and   a second resistor and a switch which are installed in series between both ends of the first resistor, and   wherein the variable impedance circuit is configured such that ON/OFF of the switch is switchable based on the output current.   
     
     
         4 . The primary side controller of  claim 2 , wherein the variable impedance circuit includes:
 a resistor installed between the feedback terminal and the reference voltage line; and   a current source and a switch which are installed in series between both ends of the resistor, and   wherein the variable impedance circuit is configured such that ON/OFF of the switch is switchable based on the output current.   
     
     
         5 . The primary side controller of  claim 2 , wherein the variable impedance circuit includes:
 a resistor installed between the feedback terminal and the reference voltage line; and   a variable current source installed between both ends of the resistor and configured to generate a correction current corresponding to the output current.   
     
     
         6 . The primary side controller of  claim 2 , further comprising a current detection terminal,
 wherein the conversion characteristics are controlled based on a voltage drop at the current detection terminal.   
     
     
         7 . The primary side controller of  claim 6 , wherein the conversion circuit further includes a load monitoring circuit configured to monitor the output current and control the impedance of the variable impedance circuit based on the output current. 
     
     
         8 . The primary side controller of  claim 7 , wherein the load monitoring circuit includes:
 a comparator configured to compare the voltage drop at the current detection terminal with a threshold value; and   a timer circuit whose output is changed for a certain period of time using an output of the comparator as a trigger, and   wherein the conversion characteristics are controlled based on the output of the timer circuit.   
     
     
         9 . The primary side controller of  claim 7 , wherein the load monitoring circuit includes:
 a peak hold circuit configured to peak-hold a current detection signal at the current detection terminal; and   a comparator configured to compare a peak value of the current detection signal with a threshold value to control the impedance of the variable impedance circuit, and   wherein the conversion characteristics are controlled based on an output of the comparator.   
     
     
         10 . The primary side controller of  claim 6 , wherein the current detection terminal is connected to a current sense resistor installed in series with a switching transistor of the isolated DC/DC converter, and
 wherein the conversion characteristics are controlled based on the voltage drop of the current sense resistor.   
     
     
         11 . The primary side controller of  claim 1 , wherein the conversion characteristics are discretely changed based on a comparison result between the output current and at least one threshold value. 
     
     
         12 . The primary side controller of  claim 1 , wherein the conversion characteristics are continuously changed based on the output current. 
     
     
         13 . The primary side controller of  claim 1 , wherein the primary side controller is integrated on a single semiconductor substrate. 
     
     
         14 . A primary side controller for use in an isolated DC/DC converter, comprising:
 a feedback terminal;   a current detection terminal;   a conversion circuit configured to:
 convert a current flowing at the feedback terminal into a feedback voltage having a negative correlation with the current; and 
 change conversion characteristics from the current to the feedback voltage based on a voltage drop at the current detection terminal; 
   a pulse signal generator configured to generate a pulse signal corresponding to the feedback voltage; and   a driver configured to generate a driving signal based on the pulse signal.   
     
     
         15 . The primary side controller of  claim 14 , wherein the conversion circuit includes a variable impedance circuit installed between the feedback terminal and a reference voltage line, and
 wherein the conversion circuit is configured such that impedance of the variable impedance circuit is variable based on the voltage drop at the current detection terminal.   
     
     
         16 . The primary side controller of  claim 15 , wherein the variable impedance circuit includes:
 a first resistor installed between the feedback terminal and the reference voltage line; and   a second resistor and a switch which are installed in series between both ends of the first resistor,   wherein the variable impedance circuit is configured such that the switch is capable of being switched based on the voltage drop at the current detection terminal.   
     
     
         17 . The primary side controller of  claim 15 , wherein the variable impedance circuit includes:
 a resistor installed between the feedback terminal and the reference voltage line; and   a current source and a switch which are installed in series between both ends of the resistor, and   wherein the variable impedance circuit is configured such that the switch is capable of being switched based on the voltage drop at the current detection terminal.   
     
     
         18 . The primary side controller of  claim 15 , wherein the variable impedance circuit includes:
 a resistor installed between the feedback terminal and the reference voltage line; and   a variable current source installed between both ends of the resistor and configured to generate a correction current corresponding to the voltage drop at the current detection terminal.   
     
     
         19 . The primary side controller of  claim 15 , wherein the conversion circuit further includes a load monitoring circuit configured to monitor the voltage drop at the current detection terminal and to control the impedance of the variable impedance circuit based on the voltage drop at the current detection terminal. 
     
     
         20 . The primary side controller of  claim 14 , wherein the primary side controller is integrated on a single semiconductor substrate.