The design and wire size of the bias winding is selected to form a single layer completely traversing the width of the transformer bobbin. This is done by selecting a wire size much larger than needed to handle the current delivered by the bias supply. In the case of the transformer for the simple circuit of Figure 1 with only 9 turns on the bias winding, a bifilar construction is used to increase the fill factor. Having the bias winding extend the full width of the bobbin maximizes the coupling to the secondary winding directly beneath it, improving regulation. Since the bias winding fills the width of the bobbin, there is no opportunity for variation in the position of the bias winding, and the unit-tounit consistency is improved.
The bias winding filter resistor (R1) plays an important part in determining the load regulation of the supply. Under ideal conditions, the load regulation of the supply would be determined by the difference in voltage drop between the bias rectifier and output rectifier, as well as the difference in resistance between the bias and output windings. Under actual conditions, the transformer leakage inductance causes a voltage spike to occur when the primary switching MOSFET turns off. With no filter on the feedback winding, this spike peak charges the bias supply and causes the secondary output to droop excessively as the output load is increased. With R1 in place, the turn-off spike is filtered out, and the bias voltage more accurately reflects the final value of the primary reflex voltage, rather than the initial turn off spike. This improves the load regulation of the output. However, this gain in load regulation does have a drawback. The same filtering process that makes the bias winding less sensitive to the turn-off leakage spike also makes the bias supply less sensitive to peak charging of the output at low or zero output load. This means that the output voltage rise at low or zero load will be higher with the filter resistor than without it. These two effects must be traded off when selecting a value for the filter resistor. There is generally an optimum value of resistance, obtainable by experiment, that provides acceptable load regulation without resulting in excessive output voltage rise at zero load. In the case of the circuits of Figure 1 and Figure 2, the optimum value of filter resistance was found by experiment to be 22 Ω. This resistance will vary depending on the transformer leakage inductance, the degree of coupling between the primary and bias windings, and the amount of capacitance after R1. The optimum value will generally be somewhere between 10 to 100 Ω. A reasonable starting value for experimentation in a new design is an intermediate value such as 47 Ω.
In a primary-regulated bias supply, the output voltage is regulated by controlling the voltage of the primary bias winding. Since the output winding and bias winding are coupled together, the output voltage will tend to track the bias voltage. They are related by the turns ratio between the bias winding and the output winding. The output voltage of the supply of Figure 2 can be expressed as follows:
N
VOUT =(5 7. V + VVR2 +
VD2 +
IC(15 + ZVR2))×
S
− VD3 (1)
NB
VOUT is the output voltage of the supply and VVR2 is the VR2 Zener voltage. 5.7 V is the voltage of the U1 internal shunt regulator voltage. IC is the current through VR2 flowing into the CONTROL pin of U1, which provides the operating current for U1, and also establishes its operating duty cycle. 15 Ω is the dynamic impedance of the TOPSwitch internal shunt regulator, and ZVR2 is the dynamic impedance of Zener diode VR2. NS is the number of turns in the T1 secondary winding, and NB is the number of turns in the bias winding. VD3 is the voltage drop of the output rectifier D3. In the case of the circuit of Figure 1,
VR2 is not used, and the terms VVR2 and ZVR2 fall out of the expression, leaving:
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