Abstract

This unit was built for a lab course at Oregon State University. The class was Electronics I, where we were introduced to diodes, BJTs and MOSFETs, and their various uses and common configurations.

Theory of Operations

The Transformer
The DC power supply takes in 120VAC (rms) and outputs a positive and negative 2V to 12V DC signal. It starts out by taking the 120VAC (rms) signal and stepping it down with a 120:24 center tapped transformer. This transformer has two primary lines (for the 120VAC rms signal) and three secondary lines. Across the outside lines, there is 24VAC (rms), and the center line being half that, creating two 12VAC (rms) channels.

AC Rectifier
These two signals are then passed through a full-wave center-tapped rectifier. The rectifier is a circuit designed to take in an AC signal and convert it to a smooth DC voltage. The two 12VAC (rms) signals are converted to positive peak waveforms per each half cycle using the diode configuration seen in the power supply schematic. Diodes only allow current to flow in one direction. When using the circuit shown in the schematic, when the AC signal would become negative, it is converted to positive, resulting in a “bumpy” AC signal (entire waveform is positive).

Filter Capacitors
These positive waveforms are then “smoothed out” by the filter capacitors. Capacitors store charge, so when the wave would dip to 0V, the energy stored in the capacitor is used to keep the waveform positive, thereby smoothing out the waveform. When the charge is released from the capacitor, the voltage will dip until the AC signal hits the peak of the wave again. This repetitive action causes a voltage “ripple”. The capacitor value was chosen to keep this ripple under 1V at a 900mA max load.

DC Regulator
At the output, any voltage fluctuations from the load must be mediated. This was accomplished with a voltage regulator circuit. The voltage regulator uses a configuration of transistors, as shown in the schematic. If, say, for the positive channel the output voltage were to fluctuate upwards for some reason, more current would flow to the base of the transistor connected to the base of the darlington pair (explained later) in the circuit. When more current flows to the base of the bottom transistor, it pulls more current from the collector to the emitter to ground. Since the collector of this transistor is connected to the base of the darlington pair, it will pull more current away from the pair, decreasing the collector-emitter current, and thus decreasing the output voltage. This completes the feedback loop of the voltage regulator. The darlington pair is used to handle high currents, i.e. the 1A load this power supply is designed for. The first transistor in the pair is a general purpose transistor, with high gain, capable of sustaining low currents. The top transistor is a power transistor, capable of passing higher currents, but with lower gain. Combining the two in the darlington pair configuration increases the current gain as well as the maximum supported load.

Overcurrent Protection
The power supply’s design constraints are built around a maximum 1A load. This 1A load should not be exceeded, or else the supply will deliver unpredictable results or fail. Thus, an overcurrent protection circuit was added. When the current gets near 1A at any channel, the voltage across the respective 0.68Ω resistor will increase past 0.7V (Vce = 0.68Ω * 1A = ~0.7V), putting the overcurrent protecting transistor into forward active mode. This transistor will pull current away from the base of the darlington pair to the output load, theoretically keeping the current constant at 1A.

Fan Circuit
When nearing the maximum load for the circuit, the resistors and transistors will begin to dissipate heat. If these components get too hot, the components could suffer damage from this prolonged heat. In order to remedy this situation, a fan circuit utilizing a thermistor, 12VDC fan and BJT was built. A network of resistor voltage dividers was implemented, as seen in the circuit, to drive a differential amplifier. One voltage divider is fixed at half the filter capacitor voltage, and the other uses the thermistor. When the heat in the chassis increases, the thermistor’s resistance will decrease, lowering the voltage at the divider and driving the amplifier. The signal passed from the amplifier is sent to the base of an NPN BJT. As the voltage increases, the voltage drop decreases across the BJT, and is dropped instead by the fan.

Capacitor Discharging
As clearly seen by the circuit diagram, the filter capacitors are very large, and thus store a lot of energy. When the power supply is turned off, the capacitors take quite some time to discharge (over an hour). When opening up the chassis to work on it after testing the power supply, this can become quite dangerous, so a discharge circuit was implemented. The circuit uses a configuration of diodes, RC timers and a MOSFET. When the circuit is switched off, the MOSFET is switched to saturation mode. Current begins draining through the 470Ω resistor for as long as the RC circuit attached to the gate remains high enough to keep it in saturation. The MOSFET and resistor heat up significantly, but only briefly, as the filter capacitors are discharged within roughly 5 seconds.

*NOTE:

• RL_POS and RL_NEG in the circuit diagram are not included in the design of the power supply, only the simulation. They are used to simulate the overcurrent protection to its maximum capability.
• R_FAN is used to simulate the fan at max load, when R_THERM is ~8kΩ

Interface Definitions

Top Level Block Diagram

Circuit Diagrams

Power Supply Circuit

Fan Control Circuit

Files

• Full projection specification document:
• Cost Analysis: cost_analysis.pdf
• LTspice – Power Supply Circuit: main_schematic.asc
• LTspice – Fan Control Circuit: fan_circuit.asc
• LTspice – Capacitor Discharge Circuit: cap_discharge.asc