AN2528

Base drive circuit design

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Base drive circuit design

Let's have a closer look at the very simple base drive network used in this application.

Normally in applications such SMPS, where the load is variable, the collector current varies

as well. It is very important to provide a base current to the device that is correlated with the

collector current in order to avoid the over saturation of the device at low load and to

optimize its performance in terms of power dissipation. This implies the use of a driving

network which allows getting a base current proportional to the collector current. For

additional information about the ESBT proportional base driving method, refer to AN2131.

Since in our application we must take into account power dissipation and simplicity as well,

we have preferred the simplest and least expensive driving network which is shown in

Figure 4. This choice also satisfies power dissipation requirements.

STC04IE170HP (Figure 5).

Figure 4.

ESBT base driving network

Referring to the calculations in Section 2, the collector peak current is 250 mA. At this

current value the ESBT gain is about 20, so that theoretically just 250 / 20 = 12.5 mA should

be enough to drive the base. This is true for very long conduction time. In the present

example the dynamic phenomenon can occur due to the relatively high switching frequency

(higher than 50 kHz) and even more to the small conduction time.

This concept is illustrated in Figure 6. It is extracted from the STC04IE170HP datasheet and

time is proportional to the collector current amount. That is why a peak base current is

absolutely mandatory to have a low voltage drop during conduction as soon as possible.

For further details about the driving network, please refer to AN2454.