Here is a simple, compact circuit for isolating a low power --48V output. The gyrator circuit allows AC signals to ride on the DC output voltage, adding audio signaling capability for communications applications. The deceptively simple feedback circuits provide a surprising degree of accuracy for both voltage and current control.
The schematic of figure 1 shows the basic flyback circuit implemented with an LT1373HV monolithic switching regulator. The transformer used is a standard Midcom part rated for 500VDC isolation. ISO1, an MOC207 optocoupler closes the voltage feedback loop. The output voltage across C2 is set for -56V. This allows for 4V of compliance across each gyrator circuit, producing --48V at the output terminals with the full 30mA load. The output voltage accuracy is set primarily by the tolerance of the Zener diode used for D7. ISO2 closes the current feedback loop. The short circuit current is set for 33mA, and can be adjusted by R5. The total deviation of short circuit current is less than 2mA with temperature step. Heating the optocoupler to 80 C reduces the current to 32.5mA. Cooling it with dry ice increases the current to 34.3mA.
The gyrator circuit acts as a virtual inductor. It uses a single MOSFET and is able to float at any supply potential. Figure 2 shows the performance with a 10K-ohm load. The circuit has almost 4 decades of bandwidth in the audio range, which is better than some of the more complicated transistor circuits. The test circuit in figure 2 uses a single MOSFET gyrator stage with a 10uF gate capacitor, and is comparable to the two series circuits in figure 1. An interesting test is to connect a large aluminum electrolytic capacitor across the load, then step the load current. The voltage will ring, and be damped by the load resistance. The ring frequency follows the relationship f=1/(2p--LC). With a 220uF capacitor and L=200H total, a ring frequency of 0.75Hz will be observed.
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