Start up switches for switch mode power supplies released
Many topologies of switching power supplies power the control circuits from an auxiliary tap on the main switching transformer or coil.
This method avoids the need for costly house keeping supplies. However, the adoption of this scheme presents the designer with the challenge of creating the initial voltage on the control circuit at start up. Any continuous delivery of current from a high voltage line will cause unwanted dissipation and loss of efficiency. Such a scheme may even take a system out of operational specification for low energy standby applications and may even degrade reliability figures.
The main requirement for the design of this start up supply is to provide a high enough current to raise the control IC supply and additionally charge any associated smoothing capacitance. At a pre determined voltage the controller will initiate the first few pulses into the main power supply switch transistor. Once this occurs the supply will become self sustaining and the start up path can be switched off.
With start up currents of the order of tens of milliamps and high voltages across the device power dissipations can be high for many milliseconds while start up occurs. For AC line operated systems the typical requirement is 450V VDs devices and for 48V DC-DC systems 100 to 200V is usually required.
Zetex manufactures a range of medium voltage MOSFETS and bipolar transistors suitable for use in an improved high voltage MOSFET start up circuit application contained in small outline packages with some offering additional pin spacing for creep age distance compliancy.
5-Dec-2006