Design Note DN-8. Application Circuits. The circuit descriptions below assume some familiarity with the functioning of TOPSwitch, страница 2

Line and load regulation curves are shown below in Figures 3 and 4.  The minimum parts count supply has looser load regulation than the circuit in Figure 2, with ±4.5% load regulation from 10% to 100% of full load.  Line regulation from 95 to 370 VDC is ±1.25%.  Load regulation for the  circuit in Figure 2 is ±2.6%, from 10% to 100% of full load.  Its output line regulation is the same as that of Figure 1.  If minimum parts count and lowest cost are the most important features, the minimum parts count circuit of Figure 1 can be used.  When output load regulation is paramount, the circuit of Figure 2 should be chosen.

LOAD REGULATION

Load Current (A)

Figure 3.  Load Regulation Performance Comparison.

Design Parameters

Transformer Design

A practical transformer design for bias and housekeeping power supplies must have the following features:

•  Low-cost winding techniques

•  High inductance

•  Low capacitance

•  Adequate electric strength

•  Low radiated emissions (EMI)

•  Bias/feedback winding closely coupled to output winding

LINE REGULATION

Input Voltage (VAC)

Figure 4.  Line Regulation Performance Comparison.

Application Note AN-7 details many techniques for practical high-frequency transformer design.  The transformer used in the design examples of Figures 1 and 2 is shown in Figure 5. The overall design for each transformer is the same, with the exception of the number of turns in the bias winding.  The minimum parts count circuit of Figure 1 requires a bias winding of  9 turns, while the circuit of Figure 2 requires a bias winding of 35 turns.

Design Parameters (cont.)

This transformer has several special design features worthy of note.  The first of these is the use of triple-insulated wire for the transformer secondary. This allows the elimination of tape margins for safety spacing, enabling the full width of the bobbin to be used for the primary and bias windings.  This allows a smaller core to be used for a given power and primary inductance level.  Because of the better utilization of the bobbin width and the consequent reduction of winding build, acceptable values of leakage inductance can be obtained with a conventional two layer primary winding rather than a split primary.

The transformer primary is applied first.  The switching MOSFET is connected to the start of the primary, so that the half of the primary winding that has the largest voltage excursion is shielded from the other windings by the remainder of the primary winding.  This reduces the capacitive coupling from the primary to the secondary, and reduces radiated coupling to other parts of the power supply circuit.  The secondary is  wound on top of the primary using triple-insulated wire, and the bias winding is applied last. This particular winding order is important to the proper functioning of the supply.  Since the bias winding is applied last, it is closely coupled to the secondary, but only loosely coupled to the primary.  This allows the voltage on the bias winding to track the voltage on the output of the supply. Because the bias winding is only loosely coupled to the primary, it is less subject to peak charging due to the primary turn-off leakage spike.  The magnitude of the primary leakage spike is a function of the peak primary current.  At high output currents, peak charging from this leakage spike in the bias supply tends to cause TOPSwitch to reduce its operating duty cycle, resulting in output droop at high output power.  Placing the bias winding last allows the leakage spike to be filtered effectively with a single resistor in series with the bias winding.