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1 INTRODUCTION
In this section we consider the auxiliary supplies often required to power the control circuits in larger power systems. We also consider some common methods used to start such systems. If the auxiliary supply is used only to power the power supply converter circuits, it will not be required when the converter is off. For this special case, the main converter transformer can have extra windings to provide the auxiliary power needs.
However, for this arrangement, some form of start-up circuit is required. Since this start circuit only needs to supply power for a short start-up period, very efficient start systems are possible.
2 DISSIPATIVE (PASSIVE) START CIRCUIT
FIG. 8.1 shows a typical dissipative start system. The high-voltage DC supply will be dropped through series resistors R1 and R2 to charge the auxiliary storage capacitor C3.
FIG. 8.1 Resistive, dissipative start circuit, providing initial low-voltage
auxiliary power needs from the 300-V DC supply.
A regulating zener diode ZD1 prevents excessive voltage being developed on C3. The charge on C3 provides the initial auxiliary power to the control and drive circuits when converter action is first established. This normally occurs after the soft-start procedure is completed.
The auxiliary supply is supplemented from a winding on the main transformer T1 when the converter is operating, preventing any further discharge of C3 and maintaining the auxiliary supply voltage constant.
A major requirement for this approach is that sufficient start-up delay must be pro vided in the main converter to permit C3 to fully charge. Further, C3 must be large enough to store sufficient energy to provide all the drive needs for correct start-up of the converter.
In this circuit, R1 and R2 remain in the circuit at all times. To avoid excessive dissipation the resistance must be high, and hence the standby current requirements of the drive circuit must be low prior to converter start-up. Since C3 may be quite large, a delay of two or three hundred milliseconds can occur before C3 is fully charged. To ensure a good switching action for the first cycle of operation, C3 must be fully charged before start-up, and this requires a low-voltage inhibit and delay on the start-up control and drive circuits.
To its advantage, the technique is very low cost, and resistors R1 and R2 can perform dual duty as the normal safety discharge resistors that are inevitably required across the large storage capacitors C1 and C2.
3 TRANSISTOR (ACTIVE) START CIRCUIT
FIG. 8.2 shows the basic circuit of a more powerful and fast-acting start system, incorporating a high-voltage transistor Q1. In this arrangement, the resistance of R1 and R2 and the gain of Q1 are chosen such that transistor Q1 will be biased into a fully saturated "on" state soon after initial switch-on of the supply.
FIG. 8.2 Lower-dissipation, active transistor start circuit, providing
initial low-voltage auxiliary supply needs from the 300-V DC supply.
As C1 and C2 charge, current flows in R1 and R2 to the base of Q1, turning Q1 fully on. Zener diode ZD1 will not be conducting initially, as the voltage on C3 and the base of Q1 will be low. With Q1 turned on, a much larger current can flow in the low-resistance R3 to charge C3.
In this circuit, resistor R3 can have a much lower value than R1 and R2 in the circuit shown in FIG. 8.1. This will not result in excessive dissipation or degrade the efficiency, as current will flow in R3 only during the start-up period. Transistor Q1 will turn off after C3 has charged and will be operating in the cut-off state after the start-up period; hence its dissipation will also be very low. R3 should be chosen to have a high surge rating (i.e., it should be wirewound or carbon composition).
After switch-on, capacitor C3 will charge up relatively quickly, raising the voltage on Q1 emitter, and the base will track this rising voltage +Vbe until it arrives at the zener volt age ZD1. At this point ZD1 starts to conduct, forcing Q1 into linear operation and reducing the charge current into C3. The voltage and dissipation will now build up across Q1.
However, once converter action is established, regenerative feedback from the auxiliary winding on the main transformer will provide current via D6 and resistor R4 to capacitor C3. Hence the voltage on C3 will continue to increase until the base-emitter of Q1 is reversed-biased and it is fully turned off.
At this point, diode D5 is brought into conduction, and the voltage across C3 will now be clamped by the zener diode ZD1 and diode D5. The dissipation in ZD1 depends on the values of R4 and the maximum auxiliary current. With Q1 off, the current in R3 ceases, and its dissipation and that of Q1 will fall to zero.
As the start-up action is fast, much smaller components can be used for R3 and Q1 than would otherwise be necessary, and heat sinks will not be required. To prevent hazardous dissipation conditions in Q1 and R3 in the event of failure of the converter, the circuit should be "fail safe" and should be able to support continuous conduction. Fusible resistors or PTC thermistors, with their inherent self-protection qualities, are ideal for this application.
This circuit is able to supply considerably more start-up current and gives greater freedom in the design of the drive circuit.
4 IMPULSE START CIRCUITS
FIG. 8.3 shows a typical impulse start circuit, which operates as follows.
Resistors R1 and R2 (normally the discharge resistors for the reservoir capacitors C1 and C2) feed current into capacitor C3 after switch-on. The auxiliary supply capacitor C4 will be discharged at this time.
The voltage on C3 will increase as it charges until the firing voltage of the diac is reached. The diac will now fire and transfer part of the charge from C3 into C4, the transfer current being limited by resistor R3.
The values of capacitors C3 and C4 and the diac voltage are chosen such that the required auxiliary voltage will be developed across C4 and the converter will start via its normal soft-start action.
Once again, by regenerative feedback (via D5 and the auxiliary winding), the auxiliary power is now provided from the main transformer. As C4 is further charged and its voltage increases, the diac will turn off since the voltage across it can no longer reach the firing value (because of the clamping action of ZD1 on C3). This arrangement has the advantage of supplying a high current during the turn-on transient, without excessive dissipation in the feed resistors R1 and R2. In the rare event of the converter failing to start on the first impulse, the start-up action will repeat as soon as capacitor C4 has discharged and C3 recharged to the appropriate firing value for the diac.
The choice of diac is important. It must be able to deliver the required turn-on current, and its firing voltage must be less than V1-Vstart and greater than V1-V2; otherwise lockout can occur after the first impulse. It is possible to replace the diac with a small SCR and an appropriate gate drive circuit.
FIG. 8.3 Diac impulse start circuit, providing initial low-voltage
auxiliary needs from the 300-V DC supply.
Also see: Our other Switching Power Supply Guide