To keep the amplifier design relatively simple, the amplifier has been designed exculsively for a +/- 30VDC supply voltage. As a result of this design constraint, the power supply must provide the required +/- 30VDC voltage given either a 120VAC or 12VDC power input source. To achieve this, both power sources convert their appropriate voltage to an intermediate 160VDC voltage level that subsequently powers a half-bridge inverter circuit generating the desired output voltage. To achieve the intermediate 160VDC voltage, different techniques are required regarding the AC or DC source. The AC source can be simply rectified while the DC source will require a more complex boost circuit.
DC Boost Circuit
The main purpose of the Boost circuit is to convert 12VDC to 160 VDC. The half-bridge inverter requires 160VDC to achieve a DC output of +/- 30 volts.
The SG3524 pulse-width modulator controls the switching frequency in both the boost and half-bridge inverter circuits. The SG3524 outputs two 180o out of phase square waves at a maximum of 45% pulse width. The "dead gap" between the waves in necessary to prevent the supply from shorting during the MOSFET turn-off period. An external resister and capacitor set the oscillator frequency.
High Voltage MOS Gate
Driver - IR2110
The IR2110 is a high voltage, high-speed two-channel power MOSFET driver. The IR2110 translates the 3524 logic into low impedance outputs capable of driving the MOSFET gates.
The boost converter uses a MOSFET switch and inductor to boost the voltage from 12 to 160VDC. The output voltage is calculated below: Vout = Vin/(1-D) where D=Vout/Vin
AC Rectifier Circuit
The AC rectifier is a simple circuit. It implements a full wave diode bridge and a filtering inductor and capacitor to turn the 170V, 120 Hz wave into DC. The filtering capacitor provides a consistent voltage level while the inductor improves current response. A zener and resistor are used to step the voltage down to 12V for the logic circuitry. A more efficient method may be possible but has not been examined in detail.
The IR2110 and SG3524 ICs control the MOSFET switching for the half-bridge inverter.
The half-bridge inverter converts 160VDC into a +/- 80V square wave. The wave is passed through a LC filter, stepped down through a transformer, rectified and filtered to obtain +/- 30 VDC. The capacitors on the input split the voltage source, creating a voltage difference for the square wave.
Series Resonance Inverter
The series resonant design decreases switching losses in the inverter. This result is due to the switching occurring when zero volts is across the switch. Consequently, lossy snubber networks are not required; instead, a simple capacitor snubber is used across the MOSFET.
Circuit performance is dependent on the resonant frequency, switching frequency, and sensitivity of the output voltage with regards to a variation in frequency or the Q of the circuit.
L = 78.2mh C = 44.0nF wo = 539791r/s fo = 85.9kHz
The required turns on the transformer was obtained by applying Faradays law.
The required turns ratio is approximately 1:2.1
One advantage of the series resonance design is the ability to control the output voltage by varying the frequency. The circuit is designed for maximum output power to occur at 1.1 times resonant frequency or approximately 94.5kHz. The output voltage can be maintained when the load decreases by increasing the switching frequency. The regulation can be realized by implementing a negative feedback control loop.
Implementing the required logic was the first task accomplished in the development of the power supply. The main tasks involved included: reading data sheets, understanding and implementing application notes, and integrating the 3524 and 2110.
The main design issue was achieving an adequate dead gap between the two "out of phase" square waves. The original design used an open collector configuration, but did not allow for a dead gap between the waves. The dead gap was eventually realized by picking the signal from the emitter outputs, allowing for a maximum pulse width of 45%.
The half bridge inverter requires a bootstrapping technique to drive both MOSFETs. The application notes suggested several circuits to accomplish this task. Grounding was another major issue encountered with the 2110. The logic ground and power ground needed to be connected, despite our efforts to separate the two grounds.
The transformer was hand wound specifically for the power supply switching frequency and the required current. During the original testing of the transformer, the secondary voltage did not correlate with the expected results. A large load resistor was used in testing, which limited the amount of current passed through the transformer. The small amount of current was insufficient to generate the required flux. Properly loaded, the transformer operates to specifications.
The boost circuit is designed but is yet to be constructed.
Currently, the power supply operates in the open-loop configuration. The feedback network required to control the output frequency still needs to be examined.
Series Resonant Inverter
Currently the half bridge inverter only operates with a maximum input voltage of 60VDC. It is unknown why the circuit fails to operate above 60VDC. Possible reasons currently being examined are: