A brief analysis of the working principle of bipolar power supply providing current

How does a bipolar power supply work to provide current? The waveform in the figure below shows the working status of a bipolar power supply circuit. When the input voltage is applied to VIN, the boost converter regulates its output VINTER to 12 V if the input drops below 12 V. If VIN exceeds the 12 V typical value of the nominal 12 V automotive rail, the boost converter goes into Pass-Thru™. In this mode, the top MOSFET Q1 will always be on at 100% duty cycle, so there will be no switching operation; the voltage VINTER applied to the 4-quadrant converter remains relatively stable at VIN.

Figure 1. Waveform when VIN decreases from 14 V to 5 V. VIN = 5 V/div, VOUT = 5 V/div, boost SW = 10 V/div, time scale is 200 µs/div.

This approach significantly improves system efficiency compared to a typical 2-stage device (ie, a boost converter followed by a buck/inverter). This is because efficiency in Pass-Thru mode (where the system is most of the time) can approach 100%, essentially turning the power system into a single-stage converter. If the input voltage drops below the 12 V level (for example, during a cold crank), the boost converter switches to regulate VINTER to 12 V. Using this approach, the 4-quadrant converter is able to deliver ±10 V even if the input voltage drops sharply.

When the control voltage reaches its maximum value (1.048 V in this example), the converter output is +10 V. When the control voltage reaches its minimum value (100 mV), the converter output is –10 V. The relationship between the control voltage and the output voltage is shown in Figure 2, where the control voltage is a sinusoidal signal frequency of 60 Hz and the peak-to-peak amplitude is 0.9048 V. The resulting converter output is a corresponding 60 Hz sine wave with a peak-to-peak amplitude of 20 V. The output changes smoothly from –10 V to +10 V.

Figure 2. Sine wave output waveform as a function of sinusoidal control signal. VCTRL= 0.5 V/div, VOUT = 5 V/div, time scale is 5 ms/div.

In this operating mode, the 4-quadrant converter regulates the output voltage. The output voltage is sensed by U1 through resistor RFB on its FB pin. The voltage on this pin is compared with the control voltage, and the duty cycle of the converter (that is, the gate signal on QN1) is adjusted based on the comparison result to keep the output voltage stable. If VINTER, CONTROL, or VOUT changes, duty cycle modulation occurs, adjusting the output accordingly. MOSFET QP1 and QN1 switch synchronously to achieve synchronous rectification and further fully improve efficiency, as shown in Figure 3.

Figure 3. Efficiency vs. Load Current