Switched mode power supply with a device for limiting the output voltage

A switched mode power supply, comprising:

• a power supply section having output terminals and a capacitor connected between the output terminals;
• a Schottky diode connected in reverse direction between the output terminals in parallel with the capacitor to prevent output voltage from increasing above a threshold value;
• a transformer having a primary winding;
• a first input terminal and a second input terminal to provide the current to the primary winding;
• a first controllable switching device having a control input and a main current path that is connected in series with the primary winding;
• and
• a second controllable switching device having a control input and a main current path that connects the control input of the first controllable switching device to one of the first and second input terminals, the second controllable switching device having a conductivity that depends on a voltage residing at its control input;
• wherein the voltage at the control input of the second controllable switching device is influenced by first, second, and third control voltages, and wherein the first control voltage increases in response to an increase in the current flowing through primary winding, the second control voltage increases in response to an increase in a voltage level between the first and second input lines, and the third control voltage increases in response to an increase in an output voltage at the output terminals.

A switched mode power supply comprising:

• a first input line and a second input line, both of which are connected to an input voltage source during use;
• output terminals to supply an output voltage;
• a Schottky diode connected in reverse direction between the output terminals;
• a transformer having a primary winding;
• a first controllable switching device having a control input and having a main current path;
• a current measuring resistor, wherein the primary winding, the main current path of the first controllable switching device and the current measuring resistor form a series arrangement having one end connected to the first input line and a second end connected to the second input line;
• and
• a second controllable switching device having a control input and having a main current path that connects the control input of the first controllable switching device to one of the first and second input lines, wherein the second controllable switching device has a conductivity that depends on a voltage residing at its control input, which voltage is influenced by a first, second, and third control voltage, and wherein the first control voltage increases in response to an increase in the current flowing through the current measuring resistor, the second control voltage is producible from the input voltage by a primary voltage divider and increases in response to an increase in the input voltage, and the third control voltage increases in response to an increase in the output voltage.