AN2495 datasheet(13/36 Pages) STMICROELECTRONICS

13 / 36 page

AN2495

Base driving circuit design

Doc ID 13127 Rev 5

13/36

Base driving circuit design

In practical applications such as SMPS where the load is variable, the collector current is

variable as well. As a consequence it is very important to provide a base current to the

device which is related to the collector. In this way it is possible to avoid device over-

saturation at low load and to optimize the performance in terms of power dissipation. The

best and simplest way to do this is the proportional driving method provided by the current

transformer in Figure 6. At the same time, it is very useful to provide a short pulse to the

base to make the turn-on as fast as possible and to reduce the dynamic saturation

phenomenon. The pulse is achieved by using the capacitor and the Zener diode in Figure 6.

Figure 6.

Proportional driving schematic and equivalent circuit

The IC/IB ratio is fixed once the current transformer turn ratio has been chosen. From the

ESBT STC04DE170 datasheet, and especially looking at the storage time characterization,

it is clear that a turn ratio equal to 5 is a good value to ensure the right saturation of the

ESBT at IC = 2 A, so that in the current transformer we can first fix:

Equation 20

The core magnetic permeability of current transformer must be as high as possible to

minimize the magnetization current IM (which is not transferred to the secondary side but

only drives the core into saturation). On the contrary, a too high permeability core may lead

the core into saturation even with a very small magnetization current. To avoid saturation, it

is necessary to increase the number of primary turns and the size of the core as well. If

a core with a very small magnetic permeability is chosen, it is possible to reduce the number

of primary turns and the core size. If the permeability is too small, we may not have current

on the secondary side because almost all the collector current becomes magnetization

current. As a compromise, a ferrite material with a relative permeability in the range 4500-

7000 is the best choice.

LTp

-------

---

AN2495 Datasheet(PDF) 13 Page - STMicroelectronics