Control signals flow from right to left with separate circuits for constant-voltage, constant-current and power-limit control. These three control circuits jointly provide the Autoranging characteristic of Figure 4-lB. AC Turn-on Circuits limit inrush current to the input filter and assure transient free turn-on. Internal Bias Supplies provide five bias and two reference voltages to the unit's circuits and provide input signals to the AC Turn-on Circuits.
The unit is a flyback switching power supply. The power transformer stores energy in its magnetic field while current flows in its primary, and energy transfers to the secondary when current flow in the primary turns off. A pair of PFET switches in series with the primary turns on and off at a 20KHz rate controlling the current flow; and the PWM varies the on-time of the PFET switches to regulate the output voltage or current.
In CV or CC operation the PWM turns the PFET switches on at each clock pulse and turns them off when the IpRAMP VOLTAGE exceeds the CP control-port voltage. The IpRAMP VOLTAGE is derived from a sensing transformer in series with the power transformer primary and is proportional to the primary current. The CP control-port voltage is determined by the CV Control Circuit when the unit is in constant-voltage operation and is determined by the CC Control Circuit when in constant-current operation. Follow the block diagram from right to left to see how the output voltage is regulated during CV mode of operation. The output voltage is monitored both at the output sense terminals + S and--S (OVS outerloop voltage) and also before the two stages of output filter (IVS innerloop voltage). Sensing with output sense terminals provides accurate load-voltage control, and sensing before the output filter stabilizes the supply and permits it to power highly reactive loads.
The CV Monitor Amplifier buffers the OVS outerloop voltage to produce the VMON output monitoring voltage. A buffer amplifier (not shown) monitors the voltage before the output filter to produce the IVS innerloop voltage. CV Error and Innerloop Amplifiers compare V-MON and IVS with the CV PROGRAM Voltage which is set by the front-panel VOLTAGE control or by remote programming to develop the CV CONTROL Voltage. When the CV CONTROL Voltage is lower than the CC CONTROL Voltage, CV determines CP and regulates the output voltage by controlling the turn-off of the PWM.
While the PWM turns off when any of the four inputs shown go low, in CV and CC operation it is controlled by the CONTROL V LIMIT input from the Control Voltage Comparator. When the Ip-RAMP VOLTAGE exceeds CP, CONTROL V LIMIT goes low and the PWM turns off the PFET switches. The next clock pulse causes the PWM to turn on the PFET switches, and thus the cycle repeats at a 20KHz rate. Power is transferred through the transformer as required to produce the output voltage determined by the CV PROGRAM Voltage.
When in CC operation, the output current is regulated in a similar manner. Output current is sensed as the OCS outerloop voltage across a Current Monitoring resistor. OCS is buffered to produce I-MON. IVS is differentiated to produce an innerloop current-sensing voltage; and CC Error amplifier compares these to the CC PROGRAM Voltage from the front- panel CURRENT control or remote programming to develop the CC CONTROL Voltage.
Simplified Schematic
The simplified schematic, Figure 4-3, shows the basic operating circuits of the unit. Detailed descriptions follow for major circuits and components in clockwise order. The circuit names and layout of the simplified schematic are the same as used on the complete schematic in Section 7. The heavy lines are the path of power flow through the unit. Please see Figure 4-5 for the display circuits.
Primary power comes to the Input Rectifier through a resistor which limits turn-on inrush current to the input filter. Jumper A1W1 connects the Input Rectifier and Filter as a voltage doubler for 120Vac mains. This jumper is not used for 220/240Vac; thus the Input Filter develops a dc bus voltage of about 300Vdc for either 120 or 220/240Vac ac mains voltages. Primary power also comes through Mains-Voltage Select switches to the Bias Power Supplies which provide the internal operating voltages for the unit. The Mains-Voltage Select switches connect the primary windings of the bias supplies transformer for operation at 120, 220, or 240Vac.