Linear versus Switch Mode Power Supplies

Linear High Voltage Power Supplies
Linear power supplies were the dominant technology up until the late 1970’s. A linear power supply makes uses a series pass transistor to function as a variable resistor as the main method of regulating the output of the power supply. When full output is required from the power supply, the series pass transistor is turned on such that it had the least resistance and the maximum power flows to the output. If less than full output is required the series pass transistor would be operated as a variable resistor, dissipating the unneeded power as heat to achieve the desired output. 

The pass transistor being operated as a dissipative device requires large heatsinks and often fans to maintain acceptable operating temperatures. Essentially a linear power supply just wastes the power not required as heat. Because of this fact linear power supplies tended to be large, heavy, bulky units with very poor efficiencies. Additionally, linear power supplies are limited in overall power output capability due to the problems of heat dissipation caused by their inherent poor efficiencies. 

Linear power supplies do have a notable benefit, they tend to be very quiet in terms of output noise and ripple since no switching element is used for their regulation. All the standard product Bertan branded high voltage power supplies Spellman sold in the past were linear power supplies. Virtually all these older Bertan units are now obsolete, not being sold or supported by Spellman anymore. 

Switch Mode High Voltage Power Supplies
Switch Mode power supplies are called as such because they use transistors as “switching regulators”. The linear power supply uses a transistor as a variable resistor (the transistor is partially on, dissipating large amounts of power), while the switching regulator uses transistor switch(s) that are either fully ON or fully OFF. When the transistor is fully ON and current is flowing the voltage drop across it is minimal.  When the transistor is OFF, no current flows through it, the power dissipation is essentially zero.

Output regulation of the power supply is achieved by varying the ratio of ON to OFF time done at a high switching frequency (tens to hundreds of kilohertz) as to minimize the size of magnetic and capacitive components used in the power conversion process. Due to these facts switch mode power supplies can be made very small, compact, and highly efficient when compared to their linear counterparts. Extremely high power levels, to hundreds of kilowatts can be very efficiently controlled using switch mode power conversion techniques. 

Because of the switching regulator operating frequency there tends to be more ripple and noise on the output when compared to traditional linear regulator. But due to all the other benefits that switch mode power supplies provide they have become the dominate form of power conversion technology in use today.