Power adapter routine work

Issue Time:2006-02-03
In the self-oscillating converter considered here, the switching action is maintained by the positive feedback of a winding on the main transformer. The frequency is controlled by the drive clamp action, which corresponds to an increase in the field current during conduction. The input energy is controlled to maintain the output voltage constant by controlling the magnitude of the primary current cutoff. This frequency is often affected by changes in the magnetic properties of the core, the load, or the applied voltage.

After the power adapter switch is closed, there is a voltage across C, current flows through R1, and transistor Q1 begins to conduct. As Q1 begins to conduct, the feedback signal generated via feedback winding P2 enhances the forward drive of the base of Q1. The base current begins to pass through C and flows through D1 after the drive voltage is established. Therefore, Q will turn on quickly, and its maximum drive current is determined by the voltage across resistors R2 and R1 and the feedback winding P2.

Since these circuits operate in the full energy transfer mode, the current in P1 (primary winding of the main transformer) is established from zero when Q1 is turned on, and the rate of change is determined by the primary inductance Lp.

As the collector current of Q1 increases, its emitter current also increases, and the voltage across R increases at the same rate as the Q2 turn-on voltage (about 0.6 V). When Q2 is fully turned on and most of the base drive current of the base of Q1 is transferred, Q1 will begin to turn off. At this time, the collector voltage of Q starts to become positive, and the buffer current flowing in D2, C3, and R provides a regenerative shutdown function. The voltage developed across R helps the turn-on of Q2 and the turn-off of Q1. Further, due to the flyback, all voltages on transformer T1 are reversed, P2 becomes negative, providing an additional regenerative shutdown for Q, and the reverse current flowing through C2 helps Q1 turn off.

The drive system is extremely simple but works well. Tests on the base current of Q1 indicate that the current has an almost ideal drive waveform (see figure). The figure shows the case where the slope of the waveform is turned off. Near the end of the Q turn-on period, Q2 gets a ramp-up base drive voltage, Q2 is gradually turned on, and Q1's base drive current is a very ideal ramp-down waveform. Since the regenerative shutdown does not occur until all of the carriers at the base of Q1 are removed and the collector current begins to drop, this is the ideal drive waveform for most high voltage tubes. This turn-off waveform prevents hot spots and secondary breakdown problems in transistor Q.

This system also has primary power automatic limiting characteristics. Even if the control circuit does not provide the drive, the maximum current of transistor Q: flowing through R4 before being turned on is limited to V=/R4. Therefore, no more current limiting circuits are required, and the system has an automatic over power limit.

In normal operation, the control circuit adds a drive signal to the base of Q2 according to the output voltage, so that the base voltage of Q is positively increased, which reduces the current flowing through R to create a shutdown condition. Therefore, the output power can be continuously controlled to maintain a constant output voltage as the load and input change.

In feedback current limiting applications, the control circuit processes more output voltage and additional information on the current signal to reduce the power limit under short circuit conditions. Note that the constant primary side power limit (its own) has little protection for the output loop because the output current is large at very low output voltages or short circuits.

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