Parallel switching power supply

The parallel switching power supply is the most commonly used power supply now. Computer monitors, color TVs, computer power supplies, etc. all use it, so understanding its working principle and mastering its circuit characteristics are necessary for every electronics personnel.

Figure K-3 is the most basic circuit diagram of parallel switching power supply, Q is the switch output tube, T is the pulse transformer, D is the rectifier diode, C is the filter capacitor, R is the load resistor, because the switch tube Q is connected in parallel with the input DC voltage E1, it belongs to the parallel switch circuit.

The pulse transformer coupling switch circuit has two forms: forward excitation and reverse excitation. In the forward excitation mode, during the on-time of the switch tube, the secondary pulse rectifier diode is also on, and during the off-time, the switch tube Q and the diode D are both off.

Reverse excitation mode-D is turned off during the on-time of the switch tube, and D is turned on during the off-time of Q

The working process of this circuit is similar to that of the line output circuit. The switching pulse signal is added to the base of the transistor Q. When the input pulse is positive, Q is saturated. At this time, the voltage characteristic on the primary coil is positive at the top and negative at the bottom, and the secondary induced voltage is negative at the top and positive at the bottom. D is reverse biased and cut off.

When a negative pulse is input to the base of Q, transistor Q is cut off, and the collector potential of Q rises to a high level. At this time, the secondary induced voltage of T is positive at the top and negative at the bottom, D is forward biased and turned on, capacitor C is charged, and a DC output voltage E0 is obtained. T can be regarded as an energy storage element here. When the switching transistor Q is turned on, but the diode D is cut off, the primary coil stores energy. When Q is cut off, T releases energy, and at this time the diode D is turned on.

Here we need to explain a problem. When Q is cut off, the primary current of T jumps to zero and loses the loop. How can the secondary have voltage output? Isn't the current of the coil unable to jump? We can understand this problem from the concept that energy cannot jump, because the energy in the inductor exists in the form of magnetic energy. The general inductor has only one winding, while the pulse transformer has two windings, the primary and the secondary. At the moment when the switching transistor Q changes from on to off, the primary coil current suddenly changes to zero, and T transfers the energy to the secondary. At this time, the diode is turned on, and the secondary coil has an induced current. The magnetic flux generated by the induced current is the same as before the conversion moment, and the magnetic flux remains unchanged. The output voltage E2 has the following relationship:

E2=E1×η2/η1×Tc/T0, η2 and η1 are the primary and secondary turns, Tc is the transistor on time, and To is the cut-off time. For this reason, we can adjust the output voltage E2 by controlling the Tc/To ratio.

Below we will analyze and explain this circuit with a circuit example. Figure K-4 is an actual switching power supply circuit for a color TV.

The circuit works as follows: after starting up, the DC voltage E1 established by the rectifier and filter circuit is added to the base of Q304 through the resistor R302, which then turns on Q304 and generates a collector current. This current generates an induced voltage in the primary winding, with the polarity of positive at pin 8 and negative at pin 1. An induced voltage is formed at pins 9-10 of the secondary winding to make the base potential of Q304 more positive, thereby increasing the collector current. This is a positive feedback process that causes Q304 to enter saturation conduction. This linearly rising current flows through the emitter resistor R313 of Q304 to generate a corresponding linearly rising sawtooth voltage drop. This voltage drop is coupled to the base of Q303 through R312 and capacitor C310 (DC blocking capacitor). At the same time, the square wave pulse output by the winding at pins 11-12 of the transformer is rectified by D306, and C312 filters to establish A sampling voltage En is added to the base of Q301 through R304, VR301, and R305, so that the collector of Q301 maintains the DC voltage related to it, and then added to the base of Q302 through R304 and R309. Therefore, a DC voltage is added to the base of Q302 and a sawtooth voltage is superimposed. Q302 and Q303 are switching frequency control circuits, which work in two states, one is turned on together, and the other is turned off together. During the cut-off period of Q304, the square wave pulse voltage induced by the 10-9 foot winding of T301 is positive at the 10 foot. This voltage is rectified by D307 and charged on C314. During the conduction period of Q304, the sawtooth voltage on R313 turns on Q303, and the voltage on C314 is added between the base and emitter of Q304, so that Q 304 tends to cut off. After Q304 is cut off, the energy stored in the pulse transformer during the conduction period begins to be released through the secondary winding, and D320 is turned on through transformer coupling, and C321 filter output obtains a stable DC output voltage. When the energy of the secondary winding is released to a very small level, both the primary and secondary circuits are not turned on, and the circuit is in a high-resistance state. The parallel resonant circuit composed of the primary winding inductance and the distributed capacitance C resonates. The induced voltage generated by the resonance makes the base of Q304 have a positive potential and turn on through the feedback winding (10-9 pin) of the pulse transformer, thereby entering a saturated conduction state, and the switch circuit enters the next new oscillation cycle.

Voltage regulation control process: When the output DC voltage rises, the corresponding sampling voltage, i.e., the voltage on capacitor C312, rises. After voltage division by R304, VR301, and R305, the base potential of Q301 rises. After comparison and amplification by Q301, the base DC potential of Q302 drops. The base of Q302 is the sum of the DC error voltage and the sawtooth voltage. As the DC error voltage on the base of Q302 drops, the PNP transistor Q302 is more likely to turn on. That is, when the amplitude of the sawtooth voltage is small, Q302 is turned on. This sawtooth wave is related to the linear rising current of the collector of Q304. That is, when the collector current of Q304 rises by a small value, Q302 is turned on, and the base potential of Q303 rises and turns on. The voltage of capacitor 314 is added to the be junction of Q304, causing Q304 to cut off.

The above process reduces the conduction time Tc of Q304, increases the switching oscillation frequency, and the output DC voltage value is proportional to Tc. The reduction of Tc finally causes the output DC voltage to drop, thus achieving the purpose of voltage stabilization.