TECHNICAL FIELD

The present disclosure relates generally to a power supply technical field, and more particularly relates to a Darlington transistor drive circuit, a Darlington transistor drive method, and a constant current switching power supply.

BACKGROUND

FIG. 1has shown an existing Darlington transistor drive circuit, which comprises a drive control circuit, a control switch K1, a switch tube K2and a drive current source. In which, the drive control circuit controls the switching-on and switching-off of the Darlington transistor by switching on or off the control switch K1and the switch tube K2. Combining withFIG. 2, in which Vs represents the voltage of the resistor Rs, Vc represents the voltage at the terminal C, R2represents the equivalent resistance of the switch tube K2, while Ib represents the drive current outputted by the drive current source. When the Darlington transistor needs to be switched on, the drive control circuit controls the switch tube K2to be switched off and the control switch K1to be switched on, thus inputting the drive current to the drive pin of the Darlington transistor. The drive current of the Darlington transistor is Ib0. At this time, the collector and emitter of the Darlington transistor are positively biased, and the Darlington transistor is switched on. When the Darlington transistor needs to be switched off, the drive current is 0, and the drive control circuit controls the switch tube K2to be switched on and the control switch K1to be switched off.

At present, the problem is that when the Darlington transistor is switched off, due to the switching-off delay characteristic of the NPN transistor of the Darlington transistor, just the collector charge and emitter charge are discharged from the terminal B1, while the charges between the terminal B2and the collector remain. Accordingly, the existing scheme has a too long time delay, and there is a large switch loss for the NPN transistor of the Darlington transistor. In additional, when the Darlington transistor is switched on, the current gain is very large and the Darlington transistor would be switched on quickly. Therefore, when the Darlington transistor is switched on, it will cause a large EMI interference, which is reflected in the initial stage of the RS voltage as a large overshoot.

SUMMARY

The present disclosure has provided a Darlington transistor drive circuit, a Darlington transistor drive method, and a constant current switching power supply, aiming at the above mentioned defects.

According to an aspect, a Darlington transistor drive circuit is provided, which comprising:

a drive current circuit connected to a drive pin of a Darlington transistor, used for outputting an output current to the drive pin of the Darlington transistor during a switching-on cycle of the Darlington transistor;

two switch units respectively connected between drive terminals of two transistors of the Darlington transistor and ground;

a drive control circuit connected with the two switch units, used for controlling the two switch units to be switched off during the switching-on cycle of the Darlington transistor and to be switched on during a switching-off cycle of the Darlington transistor, and for changing an equivalent resistance of the two switch units at different stages during the switching-off cycle of the Darlington transistor; wherein each switch unit has a first equivalent resistance in a first stage of the switching-off cycle which is smaller than a second equivalent resistance in a second stage of the switching-off cycle but greater than a third equivalent resistance in a third stage of the switching-off cycle.

Preferably, the two switch units are respectively composed of a plurality of switch tubes connected in parallel, and the drive control circuit can control a state of each switch tube for defining the equivalent resistance of each switch unit by controlling a number of the switch tubes which are switched on in each switch unit.

Preferably, the switch tube is an NPN-type MOS tube.

Preferably, the drive current circuit is connected with the drive control circuit for controlling the output current to gradually increase to a stable value required by the Darlington transistor, and then maintaining the stable value during the switching-on cycle.

Preferably, the output current of the drive current circuit increases from a current value I0to a current value I1in the first stage of the switching-on cycle and then enters the second stage of the switching-on cycle after reaching the current value I1, then increases from the current value I1to a current value I2in the second stage of the switching-on cycle, and then enters the third stage of the switching-on cycle after reaching the current value I2, and maintains at the current value I2during the third stage of the switching-on cycle; wherein a current increase rate of the first stage is less than that of the second stage.

According to a second aspect, a constant current switching power supply is provided, which comprising an input circuit, an output circuit, a transformer and the Darlington transistor drive circuit discussed above; wherein a primary input terminal of the transformer is connected to the input circuit, the Darlington transistor is arranged between a primary output end of the transformer and ground, the output circuit is connected to a secondary side of the transformer.

According to a third aspect, a Darlington transistor drive method implemented based on the Darlington transistor drive circuit discussed above is provided, which comprising implementing following steps repeatedly:

controlling the two switch units to be switched off during the switching-on cycle of the Darlington transistor by the drive control circuit;

controlling the two switch units to be switched on during the switching-off cycle of the Darlington transistor and changing an equivalent resistance of the two switch units at different stages during the switching-off cycle of the Darlington transistor by the drive control circuit; wherein each switch unit has a first equivalent resistance in a first stage of the switching-off cycle which is smaller than a second equivalent resistance in a second stage of the switching-off cycle but greater than a third equivalent resistance in a third stage of the switching-off cycle.

Preferably, the two switch units are respectively composed of a plurality of switch tubes connected in parallel, and the Darlington transistor drive method further comprises defining the equivalent resistance of each switch unit by controlling a number of the switch tubes which are switched on in each switch unit by the drive control circuit.

Preferably, the Darlington transistor drive method further comprises controlling the output current to gradually increase to a stable value required by the Darlington transistor, and then maintaining the stable value during the switching-on cycle, which further comprises:

increasing the output current of the drive current circuit from a current value I0to a current value I1in the first stage of the switching-on cycle;

entering the second stage of the switching-on cycle after the output current reaches the current value I1, and then increasing the output current from the current value I1to a current value I2in the second stage of the switching-on cycle, wherein a current increase rate of the first stage is less than that of the second stage;

entering the third stage of the switching-on cycle after the output current reaches the current value I2, and maintaining the output current at the current value I2during the third stage of the switching-on cycle.

The Darlington transistor drive circuit, a Darlington transistor drive method, and a constant current switching power supply of the present disclosure has following technical effects. When the Darlington transistor is switched off, the equivalent resistances of the two switch units are defined differently in the different stages. Each switch unit has a first equivalent resistance in a first stage which is smaller than a second equivalent resistance in a second stage but greater than a third equivalent resistance in a third stage. In such a way, before entering the final third stage, the time delay and switch loss can be reduced in the first stage and the EMI can be improved in the second stage. In additional, the output current is controlled to gradually increase to a stable value required by the Darlington transistor, and then is maintained the stable value during the switching-on cycle, so the EMI problem caused by the switching-on of the Darlington transistor can be improved. In a word, the switching-off time delay of the Darlington transistor is greatly reduced while achieving the EMI optimization. In additional, the switch loss of Darlington transistor is small when it is switched off, and the efficiency is improved.

