Distributed -48-V power systems often require a positive bias supply rail to power communications and system monitoring circuits. When input-to-output ground isolation isn't necessary, many approaches are available to convert a negative polarity input voltage to a positive polarity output voltage; most are based on the boost topology. Boost converters suffer from high-output ripple voltage and can be difficult to stabilize.
For applications that don't require ground isolation, the circuit shown in Fig. 1 is a good choice. This circuit is based on the push-pull topology, using a dual-winding transformer. The push-pull topology provides a very low output-ripple voltage, reliable overload protection and ease of loop compensation.
The two transformer windings alternate functionality as either a primary input winding or a secondary output winding. To understand the basic operation of the circuit, first assume transistor Q1 is turned on. During this time, a positive potential of Vin is applied across the dot end of the transformer N1 winding as shown. Because the transformer has a 1-to-1 turns ratio, a Vin potential also is on the dot end of the N2 winding. This positive potential will forward bias diode D1 and apply positive voltage to the output LC filter.
This circuit is completely symmetrical. When transistor Q2 is turned on, a positive Vin potential is applied to the non-dot end of the N2 winding. During the time Q2 is conducting, diode D2 will be forward biased and provide power to the output. Fig. 1 indicates the current flow and transformer winding polarities for the cases where either Q1 or Q2 is conducting.
During the off time, when Q1 and Q2 are both off, the output inductor current continues to flow equally through both transformer windings and both output diodes. There is no voltage drop across the transformer windings during the off time, because equal current flows in the opposing windings, canceling the magnetic flux in the transformer core.
For a -48-V input and a 10-V output, the duty cycle of each switch will be approximately 12%. Applications with higher output voltages will require correspondingly higher duty cycles. Taken to the limit, if both power transistors operate alternating at the maximum 50% duty cycle, then the magnitude of the output voltage will be almost equal to the magnitude of the input voltage. The output LC filter completes the power stage and provides a low ripple voltage output.
Another design area of interest is the feedback signal generation. The controller is referenced to the -48-V input while the output voltage is referenced to the system ground return. Some type of level shift circuit is required. The simplest implementation is a current level shifter shown in the main schematic. Once the output is in regulation, the current through R1 is approximately (10 — 0.7) / R1 = 0.823 mA. This same current will flow through Q3 and R2. The LM5030 controller contains an error amplifier with a 1.25-V reference. Resistor R2 has been selected as 1.5 kΩ, so that the Q3 collector current flowing through it will generate 1.25 V, when the output voltage is 10 V. The controller will increase the switching transistors duty cycle until the voltage across R2 is 1.25 V and output voltage regulation is achieved. More output accuracy can be achieved by adding another diode-connected transistor in series with R2. This will increase the accuracy with varying temperature. In this case, R2 should equal R1. The voltage across R2 and the added transistor will equal the output voltage, referenced to -48 V. Another high-impedance voltage divider is required to scale this voltage to the FB pin.
Further accuracy improvements may be made by adding an external reference and error amplifier, and then level shifting the error signal. The loop compensation is accomplished by a zero/pole pair achieved with R3 and C2.
Current-mode control was selected for ease of loop compensation while providing inherent current-limit protection. The 100-to-1 current sense transformer (CS1) provides the current sensing information necessary for current-mode control and overload protection. The controller terminates the on time of the power transistor if the voltage at the CS pin reaches 0.5 V.
A current-sense resistor located in the source of the power transistors could be used instead of the current-sense transformer. The resistor approach generally is used in lower-power applications due to the power loss of the resistor. Additional features used within the LM5030 controller include soft-start slope compensation, clock synchronization, direct gate drive, remote shutdown, bias regulator and thermal shutdown protection.
This circuit was implemented using all surface-mount components with the power stage sized for 10 A of output current. Transformers with a 1-to-1 turns ratio are available as standard catalog products. The power stage transistors and diodes must be rated for a breakdown voltage at least twice that of the maximum input voltage.
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