Power
output filter capacitor. The inductor sees a corresponding decrease in current over this part of the cycle. The volt-second balance of the inductor defines the duty cycle, D, of the converter in continuous conduction mode.
Calculating timing and component stresses
Here are the formulas describing the timing and stresses of the power train components.
The duty cycle determines the on/off time of the switches
where RFB(T) is the output voltage sensing resistor.
The average value of the input current, IOUT is input current
,
The feedback circuit presented in Figure 1 is an inexpensive solution, but tolerance of discrete transistors can be affected by the differences in base emitter voltage and temperature variations. To improve accuracy, a matched pair transistor can be used.
The peak value of the inductor current
The voltage stress on the switching MOSFET
Control of the converter power train is left to the LTC7804 boost controller. This chip was selected due to its high efficiency by means of synchronous rectification, easy implementation, high switching frequency operation (if a small inductor size is desired), and low quiescent current.
Test results and topology limitations
The average current through the bottom MOSFET
The average current through the top MOSFET
These expressions are useful for a general understanding of the functionality of the topology and for preliminary selection of the power train components. For final selection and detailed design, please use LTspice® modeling and simulation.1
Converter control description and functionality
Sensing of the output voltage and level shifting of the control voltage are managed by the current mirror based
www.cieonline.co.uk
This solution was meticulously tested and verified. Figure 3 shows that efficiency remains high over a wide range of load currents—reaching 96 per cent. Note that as the absolute value of the input voltage decreases, the input and inductor current increases. At a certain point, the inductor current can exceed the maximum, or saturation current, on the inductor. The derating curve showing this effect is shown in Figure 4. The maximum load current is 6 A in the range from –9 V to –18 V, falling below that for input voltages with absolute values below –9 V. Thermal performance is shown in Figure 5 for the solution board in Figure 6.
Conclusion
This article presents a complete solution for a very efficient and relatively simple design for adding a positive rail to a unipolar negative power supply using a boost controller. It also provides electrical
on the PNP transistors Q3 and Q4. The feedback current IFB (1 mA in this circuit) determines the value of the resistors in the feedback loop.
where VC is the reference voltage of error amplifier.
Figure 5. Thermal image of the converter with VIN cooling with no air flow.
–12 V and VOUT
+12 V at 6 A, using natural convection
Figure 6. Converter photo. “
EVEN THOUGH POWER IS
DISTRIBUTED IN MANY SYSTEMS VIA A NEGATIVE RAIL, THE LOGIC BOARDS AND SENSORS IN THEM STILL REQUIRE A POSITIVE RAIL
”
schematics and calculations for timing, power conversion components, and electrical stress. Test data confirms high efficiency and good thermal performance. Furthermore, the boost topology used in this solution gives the designer the option to use a prequalified boost controller, saving development time and cost. Conversely, qualifying a boost controller for a negative-to-positive converter can prequalify it for future boost applications.
Components in Electronics June 2020 15
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46
