Semiconductor device for controlling switching power supply and switching power supply unit using the same
View Patent ↗A semiconductor device for controlling a switching power supply is provided, which can reduce power consumption at a light load of the switching power supply and improve the efficiency of the power supply. During the intermittent switching operation of a switching element, when a return signal is outputted from a light load detection circuit within the set time of a transformer reset detection time setting circuit during intermittent stop, the switching operation of the switching element is restarted in response to a transformer reset signal from the transformer reset detection circuit after the return signal is outputted. When the return signal is outputted from the light load detection circuit during the intermittent stop after the set time of the transformer reset detection time setting circuit, the switching operation of the switching element is restarted regardless of the transformer reset signal only in response to the output of the return signal.
1. A semiconductor device comprising a switching element for controlling a switching power supply, said switching element for receiving a DC input voltage via a primary winding of a transformer, wherein a switching operation of said switching element for controlling the DC voltage is obtained by rectifying and smoothing an alternating current generated in a secondary winding of such transformer, the semiconductor device controlling the switching operation of the switching element,
said semiconductor device comprising a control circuit, which further comprises:
a transformer reset detection circuit for detecting a reset state of such transformer from an alternating voltage generated in a tertiary winding of such transformer and for outputting a transformer reset detection signal indicating the reset state, the reset state being generated by the switching operation of said switching element,
an IV converter for converting into a voltage a current value of a control current indicating a change of the DC voltage based on the alternating current generated in the secondary winding of such transformer, and
a light load detection circuit for outputting a control signal for controlling an intermittent switching operation of said switching element when detecting a light load according to a change of an output voltage from said IV converter, wherein
said switching element comprises a control electrode for receiving a signal for controlling the intermittent operation, based on the transformer reset detection signal from the transformer reset detection circuit and the control signal from the light load detection circuit,
the control signal from the light load detection circuit outputs the control signal for controlling the intermittent operation and stops the switching operation of said switching element when the output voltage from the IV converter is lower than a lower limit voltage, and restarts the switching operation of said switching element when the output voltage from the IV converter is higher than an upper limit voltage,
said control circuit further comprises:
a transformer reset detection time setting circuit for setting a time of an effective period of the transformer reset detection signal, starting from a stop of the switching operation in response to the control signal from the light load detection circuit, and
a switching-on control unit for controlling timing of restarting the switching operation based on a relationship between the transformer reset detection setting time and the control signal from the light load detection circuit, and
when the control signal indicating the restart of the switching operation is outputted from the light load detection circuit within the transformer reset detection setting time, the switching-on control unit restarts the switching operation in response to the transformer reset detection signal.
2. The semiconductor device for controlling the switching power supply according to claim 1 , wherein when the control signal indicating the restart of the switching operation is outputted from the light load detection circuit after the transformer reset detection setting time, the switching-on control unit only restarts the switching operation when the control signal is outputted.
3. The semiconductor device for controlling the switching power supply according to claim 1 , wherein the transformer reset detection setting time is determined as an optimum time for a load based on a drain voltage waveform of the switching operation during a stop period of the switching operation.
4. The semiconductor device for controlling the switching power supply according to claim 1 , wherein a state kept after the transformer reset detection setting time is reset in response to an output signal for driving the control electrode of the switching element.
5. The semiconductor device for controlling the switching power supply according to claim 1 , wherein the transformer reset detection time setting circuit sets the transformer reset detection setting time according to a capacitance value of a capacitor and outputs timing of the transformer reset detection setting time via an RS flip-flop.
6. The semiconductor device for controlling the switching power supply according to claim 1 , further comprising a time changing terminal for arbitrarily setting the transformer reset detection setting time of the transformer reset detection time setting circuit.
7. The semiconductor device for controlling the switching power supply according to claim 6 , further comprising a capacitor for arbitrarily setting the transformer reset detection setting time of the transformer reset detection time setting circuit, the capacitor being connected between the time changing terminal and a ground.
8. The semiconductor device for controlling the switching power supply according to claim 1 , wherein
the switching element and the control circuit are integrated on the same semiconductor substrate, and
the semiconductor device comprises, as external connection terminals on the semiconductor substrate,
at least a switching element input terminal for inputting the input voltage to the switching element via the primary winding of the transformer,
a switching element output terminal for outputting a switching current obtained by the switching operation of said switching element,
a power supply terminal for supplying a DC voltage to the control circuit in response to the switching operation of said switching element according to a current generated in the tertiary winding of such transformer,
a control terminal for inputting a control signal for controlling an intermittent operation of switching performed by said switching element, and
a transformer reset detection terminal for applying a signal for transformer reset detection to the transformer reset detection circuit.
9. The semiconductor device for controlling the switching power supply according to claim 1 , wherein the DC voltage obtained by rectifying and smoothing the alternating current generated in the secondary winding of such transformer is controlled, and power is supplied to a load, by applying the DC input voltage to said switching element via the primary winding of the transformer, thereby controlling the switching element.