1. High Frequency Saturable Reactor
Inderpreet Obhi
EE136

2. 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.

3. 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

4. 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

5. 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

6. 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

7. 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.

8. 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

9. 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

10. 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

11. 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

12. 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

13. Results
Reduction of effective pulse width
Reduction of output voltage