High Frequency Saturable Reactor Inderpreet Obhi EE136 12-06-03

What is a HF Saturable Reactor The HF SR is a regulator that is used in high frequency switching regulators It is also known as a saturable core magnetic regulator and a magnetic pulse width modulator SR is used in high frequency switch mode supplies as a flux- saturation controlled power switch. It provides regulation by using secondary pulse width control techniques.

Basic principle of a SR * selection of correct core mat. it must act like a good magnetic on/ off switch it must have high effective inductive reactance in its off state and low effective inductive reactance when it is in the on state Higher the inductance the more it will oppose the change in current

Basic Principle cont. The saturated state is the on state and the non-saturated state will be the off state of the switch. as the core does not saturate there would be a very small current flow as the current does not flow in the non-saturated state, our reactor will act like an off switch

Basic Principle Cont. points s2 and s3--no change in H no change in current wrt change in B more change in B than is seen for H permeability is very high inductor wound with this core characteristic we would see infinite inductance

Basic Switching Analysis On switch Analysis observe the core at point s1 No change in B significant change in H H translates to zero permeability and zero inductance Z equiv to winding.(small) No resistance to current flow Reactor is in its On state Off switch Analysis as long as the core does not saturate very small current flows as long as the current does not flow in the non-saturated state reactor is an off switch

Things to Consider area of ideal B-H loop is very small translates to negligible energy loss negligible loss is important when the ideal core is switching from on to off at high frequencies must exhibit low loss in order to have a very efficient reactor.

Example of a Saturable Reactor power regulator   Single winding SR regulator simple voltage controlled reset transistor Q1 assume the core of the SR is unsaturated at s3

SR example Assuming short on state change in the flux density B is negligible core does not saturate and B moves next to s2 leads to a small magnetization and core loss current secondary winding on T1 goes positive due to a power pulse at the input D1 will conducts voltage drop across the winding SR From s3 accumulated v/swill increase the flux density B from s3 to positive saturation off On

Effect of Q1 on switching If Q1 is on or saturated during the off state lead to a reversed voltage on transformer T1 core will be reset to point s3 the same small loop will be followed by the next power pulse the only current that is passed to the output is the magnetization and loss current of the reactor off switching operation allowing negligible current to flow to the output

SR example. Q1 off Time duration from S2 to saturation td = (N * B * Ac)/ Vs If core was not reset after 1st Fwd V-pulse Core will reset to level with s2 2 nd Fwd V pulse takes core from s2 to saturation Z of reactor is low Large current flows via D1 and L1 to Output

Design of a Saturable Reactor: details to core materials, core size, which in turn depends on the application, frequency and performance that is desired minimum secondary voltage required from the converter transformer must be calculated Vs = Vout ( ton+ toff)/ ( ton) select core size and number of turns last step is to calculate temperature rise, which depends on the core and winding losses

Results Reduction of effective pulse width Reduction of output voltage