Base Drive Design of power transistor
Base Drive Design:
- The rate of rise of base current in the beginning of the turn on process determines the turn on delay time.
- The magnitude of the base current during turn on decides the values of the voltage fall time, current rise time and VCE (sat) for a given collector current.
- The negative base current during turn off determines the storage time, voltage rise time and current fall time.
- A negative bias at the base also enhances the voltage withstanding capacity of a power transistor.
- The switching characteristics of a BJT it is evident that the base drive voltage source should be bipolar and the base drive resistance should be different during turn on and turn off.
Steps by step procedure:
- From the load current value (to be switched) and desired conduction power loss the desired value of VCE (sat) is determined.
- Using the desired value of VCE (sat) for the given load current, the required value of forward base current (iBP) and the corresponding VBE (sat) is obtained from the manufacturer’s data sheet.
- The forward and reverse base drive voltages (VBB & VBB -) are decided on the basis of the availability of control power supply. These should be kept as low as possible in order to reduce base drive power requirement.
- The forward base drive resistance RBP is given by
- The turn on delay time can be reduced by increasing the rate of rise of iBP at the beginning of the turn ON process. This is achieved by connecting a small capacitor across RBP.
- Once iBP is known the turn on loss is fixed. The allowable turn off loss is determined by subtracting the turn on loss for the desired total switching loss. The required current fall and voltage rise times for the calculated turn off loss is determined for the given load current and VCC.
- A suitable negative base current (iBN) to give the desired voltage rise time is determined from the manufacturer’s data sheet.
- RBN is given
- Once iBN is fixed the storage time (ts) can be determined from the manufacturer’s data sheet.
- The storage time can be reduced by connecting a small capacitor across RBN.