LINEAR POWER SUPPLY OVERVIEW
Introduction
The basic elements of a linear power supply are shown in the figure below. These are the oscillator, gain control amplifier, push-pull power amplifier, high voltage section (containing the step-up transformer, rectifier or voltage multiplier, output filter and voltage divider), sense amplifier and error amplifier. In addition, there are various protective circuits and signal buffers.
Conversion
The output of a 20 kHz (typical frequency) oscillator is amplified by the gain control amplifier, by an amount determined by the error amplifier (more about this later) and then applied to the push-pull power amplifier. The output of the power amplifier, which is approximately a sine wave, drives the primary of the step-up transformer. Inside the high voltage assembly, the sine wave voltage is multiplied by the transformer and then converted to dc by the rectifier and filter. In many supplies, the rectifier is replaced by a voltage multiplier, which not only rectifies, but, as its name implies, multiplies the voltage by a fixed, integral amount. The output of the filter is the output of the power supply.
Control
The dc output voltage is measured by a precision high voltage divider resistor network. The sense amplifier output voltage is a scaled representation of the dc output high voltage. The typical scale factor is 5 Vdc for maximum output voltage.
The output of the sense amplifier is fed to one input of the error amplifier. There, it is compared to the value of the program input voltage, which is a scaled-down representation of the desired output voltage. If the sense voltage is less than the program voltage, the error amplifier output increases in order to increase the drive output of the gain control amplifier. Similarly, if the sense voltage is greater than the program voltage, the error amplifier output decreases. Equilibrium is reached when the sense voltage and the program voltage are equal. At this point, the error amplifier output is whatever is necessary to maintain the power supply output at the desired voltage. This is the case over the complete range of output voltage, load resistance and line voltage.
SWITCHING POWER SUPPLY OVERVIEW
Introduction
Bertan switching supplies use control and monitoring circuits similar to the linear supplies. Differences occur in the techniques used to generate the output power and in the control circuits after the error amplifier. Switching supplies contain additional protection and other ancillary circuits. However, this is beyond the scope of this discussion. The techniques Bertan uses for switching supplies allow for control over an extremely wide range of output current and voltage.
Conversion
The ac line is rectified and smoothed by a capacitive filter. The dc output is stepped down to a lower dc level by a variable buck regulator, switching at 40 kHz (typical frequency). The power supply's control loop ensures that the buck regulator's dc output is at the level required to maintain constant output high voltage. A half bridge inverter, using a pulse width modulator (PWM), running at 20 kHz with a fixed duty cycle of 50%, converts the buck regulator's dc output to 20 kHz ac. The inverter is quasi-resonant in that switching takes place at approximately zero current to reduce EMI and switching losses. The ac voltage from the inverter is fed to the primary of a step-up transformer. As in the linear supply, the step-up transformer output is rectified or voltage multiplied, and filtered in the high voltage assembly.
The high voltage dc output is sensed by a high voltage divider and fed to a sense amplifier, and its output sent to an error amplifier. The error amplifier compares the programming voltage with the sense voltage, and outputs the necessary voltage to maintain the desired constant output voltage. So far, this is identical to the linear supply. Now, the differences occur. The error voltage is fed to a PWM which changes the on-time of the buck-regulator's switching transistor. The buck regulator output voltage is proportional to the pulse width. Therefore, the high voltage output is controlled by changing the buck-regulator switching transistor on time.
The power supply can work as a voltage source or as a current source, depending on the programming and load conditions. The crossover from current source to voltage source, or vice-versa, is automatic. Additional control loops improve line regulation and output ripple performance. The feed forward loop takes a sample of the rectified ac line and feeds it to the control circuit (PWM) to correct for input line changes.
Control
The high voltage dc output is sensed by a high voltage divider and fed to a sense amplifier, and its output sent to an error amplifier. The error amplifier compares the programming voltage with the sense voltage, and outputs the necessary voltage to maintain the desired constant output voltage. So far, this is identical to the linear supply. Now, the differences occur. The error voltage is fed to a PWM which changes the on-time of the buck-regulator's switching transistor. The buck regulator output voltage is proportional to the pulse width. Therefore, the high voltage output is controlled by changing the buck-regulator switching transistor on time.
The power supply can work as a voltage source or as a current source, depending on the programming and load conditions. The crossover from current source to voltage source, or vice-versa, is automatic. Additional control loops improve line regulation and output ripple performance. The feed forward loop takes a sample of the rectified ac line and feeds it to the control circuit (PWM) to correct for input line changes.
Summary
For both Bertan linear and switching power supplies, amplification and power conversion occur at high frequency. Negative feedback techniques are used to precisely control the output. Since regulation and control of the output is on the primary side of the high voltage transformer, low voltage, and ground referenced signals are used for front panel operation and for remote programming and monitoring.
There are other types of high voltage power supplies that use techniques such as line frequency operation and vacuum tube regulation. None of these have the overall performance capabilities found in standard Bertan High Voltage power supplies.
