Switched mode power supply circuit including a control circuit having

Abstract

In switched-mode power supply circuits the switch is rendered conducting for a longer period (or more frequently), dependent on the load connected to the power supply circuit, so as to ensure that the transformer can transfer enough energy to the secondary side for charging the output capacitor to a sufficient extent. To ensure that the transformer can always supply all energy stored in the transformer to the secondary side, a demagnetizing detection circuit detects the demagnetization of the transformer. If the load decreases (for example, in standby operation) the on-time of the switch is reduced. However, this on-time may achieve a minimum value so that the power supply circuit no longer functions properly. To obtain an efficiently operating power supply circuit, both in normal operation and in a low-load state, the control circuit of the power supply circuit has at least two modes, i.e. a first mode (standby operation) in which the control pulses are applied to the switch at a fixed frequency and the demagnetizing signal has no influence, and a second mode (normal operation) at a variable frequency, in which second mode the frequency of the control pulses is variable and, if necessary, the demagnetizing signal can delay the control pulses applied to the switch when the transformer is not fully demagnetized.