Voltage Halving Converter
Original Publication Date: 1981-Jul-01
Included in the Prior Art Database: 2005-Feb-11
For a utility input voltage in the 200-240 volt AC range, the circuit shown between the dashed lines in Fig. 1 produces a DC output voltage equivalent to what would be obtained from a full-wave bridge converter operating from 100-127 volts AC.
Voltage Halving Converter
For a utility input voltage in the 200-240 volt AC range, the circuit shown between the dashed lines in Fig. 1 produces a DC output voltage equivalent to what would be obtained from a full-wave bridge converter operating from 100- 127 volts AC.
The well known ""voltage doubler'' which produces a DC output voltage approximately twice the peak amplitude of the AC input voltage is just the reverse action of the circuit described here. Historically, the doubler has been used as the bulk DC source in switching power supplies for the 100-127 volt AC utility voltage range. Primarily, this was to make the supply compatible with a 200-240-volt AC utility source, where a bridge-rectifier converter could be substituted for the doubler to form the same DC bulk voltage as the former case.
Secondarily, with power supplies in excess of 200 watts output, the higher bulk voltage was desirable because it resulted in less current in the supply's switching circuit. However, with the trend toward smaller, less power-consuming data processing equipment, the concern for minimizing switching circuit current is not as great. Further, cost considerations make flyback configurations more attractive, where voltage stresses become prohibitive when a high-voltage bulk source is used. Many systems still need to operate from a 200-240-volt AC utility source, thus requiring a voltage-halving converter for inexpensively reducing the higher utility voltages to be compatible with the 100-127-volt AC class, without 50/60 Hertz transformers and with minimal dissipation.
Attempts to achieve the halving action by capacitor dividers are not as effective because the net DC output current is drawn from only one capacitor, and depending on the configuration, the converter voltage either collapses or expands from the half-value objective.
In Fig. 1, the component values shown may be varied depending on the desired size and performance. The values shown were selected to support a 1- amp load, as an illustrative example. Analysis is best begun at the cycle peak of the input voltage. Diodes D1, D5, D6 and D7 comprise a standard bridge- rectifier which, when the utility input is at a cycle peak, cause charging current to flow through capacitors C1 and C2 (in series) via diode D2. Node 3 experiences a voltage (node 0 being a reference) of approximately 1/2 the full wave rectified peak at node 2. At the cycle peak, D4 is reverse-biased, offering no current path for the emitter of transistor Q2. With transistor Q2 cut off, no base current is available for transistor Q1 via resistor R3; hence, transistor Q1 is also cut off.
As the rectified input begins to descend from the cycle peak, diode D2 becomes reverse-biased. Capacitor C1 holds its charge of approximately 1/2 the peak value, causing the node 4 voltage to begin to descend toward the voltage at node 0. Thus, when node 4 drops to within .5 to 1.5 volts of node 0, transistor Q2 begins to conduct. This s...