In theory, any isolated power supply can be used as a negative power supply just by tying the high level terminal to system ground and treating the low level terminal as a negative power supply. However, isolated power supply generally has complex topology and requires specific skills associated with power electronics. Among this topology, primary-side regulated (PSR) flyback converter is a simple isolated topology that is fitted to be a negative power supply. Unlike traditional flyback topology, PSR flyback does not require an opto-coupler to transfer feedback signals. The feedback directly goes in the primary side coils during switch off stage and as a result there is a simple circuit with low cost and low size.

The TI LM25184 is a primary-side regulated (PSR) flyback converter with high efficiency over a wide input voltage range of 4.5V to 42V. The LM25184 is a high-density, cost-effective design for industrial systems requiring less than 10W of isolated DC/DC power supply. LM25184 integrated a 65V, 2.5A N-channel power MOSFET. During the MOSFET on-time, the transformer primary current increases from zero with a slope of V_IN / L_MAG (where L_MAG is the transformer primary-referred magnetizing inductance) while the output capacitor supplies the load current. When the MOSFET is turned off by the control logic, the switch (SW) voltage VSW swings up to approximately V_IN + (N_PS × V_OUT), where N_PS = N_P/N_S is the primary-to-secondary turns ratio of the transformer. The magnetizing current flows in the secondary side through the flyback diode, charging the output capacitor and supplying current to the load. To minimize output voltage regulation error, the LM25184 senses the reflected secondary voltage when the secondary current reaches zero. During MOSFET switch off, the output voltage is reflected to the primary side by turns ratio between primary side and secondary side N_PS(V_out +V_D). This voltage is attenuated by the ratio of the resistor between FB pin and SW pin to the resistor at the RSET pin and sets the output voltage. The output voltage can be configure according Equation 15.

Figure 2-10 shows the typical application circuit. See the LM25184 data sheet for more information.

The drawback for PSR flyback converter is designer must design or purchase a customer flyback transformer. This is not convenient for most engineers. The second drawback is the output voltage is not very precise since the diode has a forward voltage drop.